## Pharmacokinetics Series – Clearance and Maintenance Dose

Clearance and Maintenance Dose

Clearance can be thought of as the intrinsic ability of the body or its organs of elimination (usually the kidneys and the liver) to remove drug from the blood or plasma. Clearance is expressed as a volume per unit of time. It is important to emphasise that clearance is not an indicator of how much drug is being removed; it only represents the theoretical volume of blood or plasma which is completely cleared of drug in a given period. The amount of drug removed depends on the plasma concentration of drug and the clearance.

As steady state, the rate of drug administration (RA) and the rate of drug elimination (RE) must be equal, so that, RA = RE. Because RA can be described as (S)(F)(Dose/τ), and the RE equals to (Cl)(Css ave), we get the formula for Cl as Cl = (S)(F)(Dose/τ)/(Css ave) [Equation 1].

If an estimate for clearance is obtained from the literature, the clearance formula of [Equation 1] can be rearranged and used to calculate the rate of administration or maintenance dose that will produce a desired average plasma concentration of (Css ave) at steady state: Maintenance Dose = (Cl)(Css ave)(τ)/[(S)(F)] [Equation 2].

Attention must be paid that the units of all factors in these formulas must be consistent.

Factors Affecting Clearance

Body Surface Area/Weight

Most literature values for clearance are expressed as volume/kg/time or as volume/70 kg/time. There is some evidence, however, that drug clearance is best adjusted on the basis of BSA rather than weight. BSA can be calculated using BSA in m2 = (Patient’s Weight in kg/70 kg)0.7(1.73 m2) [Equation 3] or it can be obtained from various charts and nomograms. The value of a patient’s weight divided by 70 taken to the 0.7 power is an attempt to scale or size a patient as a fraction of the average 1.73 m2 or 70-kg individual. Weight divided by 70 taken to the 0.7 power has no units and should be thought of as the fraction of the average-size person.

As an example, a 7-kg patient has a weight ratio relative to 70 kg of 0.1 and, therefore, may be thought of as having a size and thus a metabolic and renal capacity that is one-tenth of the average 70-kg person (7 kg/70 kg = 0.1). If the same weight individual was compared to the 70-kg standard using weight to the 0.7 power, the ratio becomes 0.2 or 20%, (7 kg/70 kg)0.7 = 0.2. Therefore in these two examples, the difference between  0.1 and 0.2 is large. However, when patients do not differ significantly from 70 kg, the difference between using weight versus weight to the power 0.7 (BSA) becomes less significant.

The underlying assumption in using weight or surface area to adjust clearance is that the patient’s liver and kidney size (and hopefully function) vary in proportion to these physical measurements (weight or BSA). However, this may not always be the case; therefore, clearance values derived from the patient population having a similar age and size should be used whenever possible. When a patient’s size is substantially greater or less than the standard 70 kg, or 1.73 m2, a careful assessment should be made to determine if the patient’s body stature is normal, obese, or emaciated. In obese and emaciated patients, neither weight nor surface area is likely to be helpful in predicting clearance, since the patient’s body size will not reflect the size or function of the liver and kidney.

Plasma Protein Binding

For highly protein-bound drugs, diminished plasma protein binding is associated with a decrease in reported steady-state plasma drug concentrations (total of unbound plus free drug) for any given dose that is administered. According to Equation 1, a decrease in the denominator, (Css ave), increases the calculated clearance. This actually would be misleading, however, to assume that because the calculated clearance is increased, the amount eliminated of drug per unit of time has increased. Equation 1 assumes that when (Css ave) changes, the free drug concentration, which is available for metabolism and renal elimination, changes proportionately. In actuality the free or unbound fraction of drug in the plasma generally increases with diminished plasma protein binding. As a result, the amount of free drug eliminated per unit of time remains unchanged. This should be apparent if one considers that at steady state, the amount of drug administered per unit of time (RA) must equal the amount eliminated per unit of time (RE). If RA has not changed, RE must remain the same.

In summary, when the same daily dose of a drug is given in the presence of diminished protein binding, an amount equal to that dose will be eliminated from the body each day at steady state despite a diminished steady-state plasma concentration (Css ave) and an increase in the calculated clearance (Cl). This lower plasma concentration (C bound + C free) is associated with a decreased C bound, no change in C free, and as a result there is an increase in the fraction of unbound drug (fu). Therefore, the pharmacologic effect achieved will be similar to that produced by the higher serum concentration observed, under normal protein binding conditions. This example re-emphasizes the principle that clearance alone is not a good indicator of the amount of drug eliminated per unit of time (RE).

Extraction Ratio

The direct proportionality between calculated clearance and fraction unbound (fu) does not apply to drugs that are so efficiently metabolised or excreted that some (perhaps all) of the drug bound to plasma protein is removed as it passes through the elimination organ. In this situation the plasma protein acts as a “transport system” for the drug, carrying it to the eliminating organs, and clearance becomes dependent on the blood or plasma flow to the eliminating organ. To determine whether the clearance for a drug with significant plasma binding will be influenced primarily by blood flow or plasma protein binding, its extraction ratio is estimated and compared to its (fu) value.

The extraction ratio is the fraction of the drug presented to the eliminating organ that is cleared after a single pass through that organ. It can be estimated by dividing the blood or plasma clearance of a drug by the blood or plasma flow to the eliminating organ. If the extraction ratio exceeds the (fu), then the plasma proteins are acting as a transport system and clearance will not change in proportion to (fu). If, however, the extraction ratio is less than (fu), clearance is likely to increase by the same proportion that (fu) changes. This approach does not take into account other factors that may affect clearance such as red blood cell binding, elimination from red blood cells, or changes in metabolic function.

Renal and Hepatic Function

Drugs can be eliminated or cleared as unchanged drug through the kidney and by metabolism in liver. These two routes of clearance are assumed to be independent of one another and additive. A decrease in the function of an organ of elimination is most significant when that organ serves as the primary route of drug elimination. However, as the major elimination pathway becomes increasingly compromised, the “minor” pathway becomes more significant because it assumes a greater proportion of the total clearance. For example, a drug that is usually 67% eliminated by the renal route and 33% by the metabolic route will be 100% metabolised in the event of complete renal failure; the total clearance, however, will only be one-third of the normal value.

Cardiac Output

Cardiac output also affects drug metabolism. Hepatic or metabolic clearances for some drugs can be decreased by 25% to 50% in patients with CHD. For example, the metabolic clearances of theophylline and digoxin are reduced by approximately one-half in patients with CHD. Since the metabolic clearance for both of these drugs is much lower than the hepatic blood or plasma flow (low extraction ratio), it would not have been predicted that their clearances would have been influenced by cardiac output. The decreased cardiac output and resultant hepatic congestion must, in some way, decrease the intrinsic metabolic capacity of the liver.

## Inflammation Mediators

The mediators of inflammation are the substances that initiate and regulate inflammatory reactions. The most important inflammation mediators include vasoactive amineslipid products (prostaglandins and leukotrienes), cytokines (including chemokines), and products of complement activation. These mediators induce various components of the inflammatory response typically by distinct mechanisms, which is why inhibiting each has been therapeutically beneficial. However, there is also some overlap (redundancy) in the actions of the mediators.

The inflammation mediators have some common characteristics, like

• Mediators are either secreted by cells or generated from plasma proteins. Cell-derived mediators are normally sequestered in intracellular granules and can be rapidly secreted by granule exocytosis (e.g., histamine in mast cell granules) or are synthesised de novo (e.g., prostaglandins and leukotrienes, cytokines) in response to a stimulus. The major cell types that produce mediators of acute inflammation are the sentinels that detect invaders and damage in tissues, that is, macrophages, dendritic cells, and mast cells, but platelets, neutrophils, endothelial cells, and most epithelia can also be induced to elaborate some of the mediators. Plasma derived mediators (e.g., complement proteins) are produced mainly in the liver and are present in the circulation as inactive precursors that must be activated. When activated a series of proteolytic and protein-protein interactions are initiated that ultimately to acquire their biologic properties.
• Ative mediators are produced only in response to various stimuli. These stimuli include microbial products and substances released from necrotic cells. Some of the stimuli trigger well-defined receptors and signalling pathways.
• Most of the mediators are short-lived. They quickly decay, or are inactivated by enzymes, or they are otherwise scavenged or inhibited. There is thus a system of checks and balances that regulates mediator actions.
• One mediator can stimulate the release of other mediators. The secondary mediators may have the same actions as the initial mediators but may also have different and even opposing activities. Such cascades provide mechanisms for amplifying or, in certain instances, counteracting the initial action off a mediator.

Vasoactive Amines: Histamine and Serotonin

The two major vasoactive amines, so named because they have important actions on blood vessels, are histamine and serotonin. They are stored as preformed molecules in cells and are therefore among the first mediators to be released during inflammation. The richest sources off histamine are the mast cells that are normally present in the connective tissue adjacent to blood vessels. It is also found in blood basophils and platelets. Histamine is stored in mast cell granules and is released by mast cell degranulation in response to a variety of stimuli, including 1.physical injury, such as trauma, cold, or heat, by unknown mechanisms;2.binding of antibodies to mast cells, which underlies immediate hypersensitivity (allergic) reactions; and 3.products of complement called anaphylatoxins (C3a and C5a). Antibodies and complement products bind to specific receptors on mast cells and trigger signalling pathways that induce rapid degranulation. In addition, leukocytes are thought to secrete some histamine-releasing proteins but these have not been characterised. Neuropeptides (e.g., substance P) and cytokines (IL-1, IL-8) may also trigger release of histamine.

Histamine causes dilation of arterioles and increases the permeability of venules. Histamine is considered to be the principle mediator of the immediate transient phase of increased vascular permeability, producing interendothelial gaps in venules. Its vasoactive effects are mediated mainly via binding to receptors, called H1 receptors, on microvascular endothelial cells. Histamine also causes contraction of some smooth muscles.

Serotonin is a preformed vasoactive mediator present in platelets and certain neuroendocrine cells, such as in the gastrointestinal tract, and in mast cells in rodents but not humans. Its primary function is as a neurotransmitter in the gastrointestinal tract. It is also a vasoconstrictor, but the importance of this action in inflammation is unclear.

Arachidonic Acid Metabolites

The lipid mediators prostaglandins and leukotrienes are produced from arachidonic acid (AA) present in membrane phospholipids, and stimulate vascular and cellular reactions in acute inflammation. AA does not occur free in the cell but is normally esterified in membrane phospholipids. Mechanical, chemical, and physical stimuli or other mediators (e.g., C5a) release AA from membrane phospholipids through the action of cellular phospholipases, mainly phospholipase A2. The biochemical signals involved in the activation of phospholipase A2 include an increase in cytoplasmic Ca2+ and activation of various kinases in response to external stimuli. AA-derived mediators, also called eicosanoids are synthesised by two major classes of enzymes: cyclooxygenases (for prostaglandins) and lipoxygenases (for leukotrienes). Eicosanoids bind to G protein-coupled receptors on many cell types and can mediate virtually every step of inflammation, including vasodilation (PGI2, PGE1, PGE2 PGD2), vasoconstriction (TxA2/Thromboxane A2, leukotrienes C4/D4/E4), increased vascular permeability (Leukotrienes C4/D4/E4), Chemotaxis, leukocyte adhesion (Leukotrienes B4/HETE or Hydroxyeicosatetraenoic acid).

• Prostaglandins

Prostaglandins (PGs) are produced by mast cells, macrophages, endothelial cells, and many other cell types, and are involved in the vascular and systemic reactions of inflammation. They are generated by the actions of two cyclooxgenases, called COX-1 and COX-2. COX-1 is produced in response to inflammatory stimuli and is also constitutively expressed in most tissues, where it may serve a homeostatic function (e.g., fluid and electrolyte balance in the kidneys, cytoprotection in the gastrointestinal tract). In contrast, COX-2 is induced by inflammatory stimuli and thus generates the prostaglandins that are involved in inflammatory reactions, but it is low or absent in most normal tissues. Prostaglandins include many subtype PGs, such as TxA2, PGI2, PGD2, PGE2, PGF2a etc. These subtype prostaglandins are derived by the action of different enzymes on an intermediate in the pathways, respectively.

TxA2, a potent platelet-aggregating agent and vasoconstrictor is derived by the enzyme thromboxane synthase which locates in the platelets. Prostacyclin synthase in vascular endothelium catalyze the production of PGI2 and PGI2 has functions as vasodilator,  a potent inhibitor of platelet aggregation, and markedly potentiates the permeability-increasing and chemotactic effects of other mediators. PS: a thromboxane-prostacyclin imbalance has been implicated as an early event in thrombus formation in coronary and cerebral blood vessels. PGD2 is the major prostaglandin made by mast cells; along with PGE2 (which is more widely distributed), it causes vasodilation and increases the permeability of post capillary venules, thus potentiating edema formation. Also it has a function of chemoattractant for neutrophils. PGF2a stimulates the contraction of uterine and bronchial smooth muscle and small arterioles.

In addition to their local effects, the prostaglandins are involved in the pathogenesis of pain and fever in inflammation. PGE2 is hyperalgesic and makes the skin hypersensitive painful stimuli, such as intradermal injection of suboptimal concentrations of histamine and bradykinin. It is also involved in cytokine-induced fever during infections.

• Leukotrienes

Leukotrienes are produced by leukocytes and mast cells by the action of lipoxygenase and are involved in vascular and smooth muscle reactions and leukocyte recruitment. There are three different lipoxygenases, 5-lipoxygenase being the predominant one in neutrophils. This enzyme converts AA (arachidonic acid) to 5-hydroxyeicosatetraenoic acid, which is chemotactic for neutrophils, and is the precursor of the leukotrienes. Among leukotrienes, LTB4 is a potent chemotactic agent and activator of neutrophils, causing aggregation and adhesion of the cells to ventral endothelium, generation of ROS (reactive oxygen species), and release of lysosomal enzymes. The LTC4, LTD4, and LTE4 cause intense vasoconstriction, bronchospasm (important in asthma), and increased permeability of venules. Leukotrienes are more potent than is histamine in incresing vascular permeability and causing bronchospasm.

• Lipoxins

Lipoxins are also generated from AA by the lipoxygenase pathway, but unlike prostaglandins and leukotrienes, the lipoxins suppress inflammation by inhibiting the recruitment of leukocytes. They inhibit neutrophil chemotaxis and adhesion to endothelium. They are also unusual in that two cell populations are required for the transcellular biosynthesis of these mediators. Leukocytes, particularly neutrophils, produce intermediates in lipoxin synthesis, and these are converted to lipoxins by platelets interacting with the leukocytes.

Cytokines and Chemokines

• Cytokines

Cytokines are proteins produced by many cell types (principally activated lymphocytes, macrophages, and dendritic cells, but also endothelial, epithelial, and connective tissue cells) that mediate and regulate immune and inflammatory reactions. They include TNF (tutor necrosis factor) and Interleukin-I (IL-1). These cytokines are produced mainly by activated macrophages and dendritic cells; TNF is also produced by T lymphocytes and mast cells, and IL-1 is produced by some epithelial cells as well. The most important roles of these cytokines in inflammation are the following:

1.Endothelial activation. Both TNF and IL-1 act on endothelium to induce a spectrum of changes referred to as endothelial activation. These changes include increased expression of endothelial adhesion molecules, mostly E- and P-selectins and ligands for leukocyte integrins; increased production of various mediators, including other cytokines and cheekiness, growth factors, and eicosanoids; and increased procoagulant activity of the endothelium.

2.Activation of leukocytes and other cells. TNF augments responses of neutrophils to other stimuli such as bacterial endotoxin and stimulates the microbicidal activity of macrophages, in part by inducing production of NO. IL-1 activates fibroblasts to synthesize collagen and stimulates proliferation of synovial and other mesenchymal cells. IL-1 also stimulates TH17 responses, which in turn induce acute inflammation.

3.Systemic acute-phase response. IL-1 and TNF induce the systemic acute-phase responses associated with infection or injury, including fever. They are also implicated in the syndrome of sepsis, resulting from disseminated bacterial infection. TNF regulates energy balance by promoting lipid and protein mobilisation and by suppressing appetite. Therefore, sustained production of TNF contributes to cachexia, a pathologic state characterised by weight loss and anorexia that accompanies some chronic infections and neoplastic disease.

• Chemokines

Cheekiness are a family of small (8 to 10 kD) proteins that act primarily as chemoattractants for specific types of leukocytes. Inflammatory chemokines stimulate leukocyte attachment to endothelium by acting on leukocytes to increase the affinity of integrins, and they stimulate migration (chemotaxis) of leukocytes in tissue to the site of infection or tissue damage. Also, some chemokines are produced constitutively in tissues and are sometimes called homeostatic chemokines. These organize various cell types in different anatomic regions of the tissues.

Complement System

The complement system is a collection of soluble proteins and membrane receptors that function mainly in host defines against microbes and in pathologic inflammatory reactions. This system of complement functions in both innate and adaptive immunity for defines against microbial pathogens. In the process of complement activation, several cleavage products of complement proteins are elaborated that cause increased vascular permeability, chemotaxis, and opsonization.

Complement system acts as the bridge between innate and adaptive immune system. This concept is due to the fact that complement proteins can be activated directly by antigen-antibody complexes.

Primary Functions

There are three main effects of complement: 1.lysis of cells such as bacteria, allografts, and tumor cells; 2.generation of mediators that participate in inflammation and attract neutrophils; and 3.opsonization – enhancement of phagocytosis.

C3b is the central molecule of the complement cascade. It has two core functions: 1.it combines with other complement components to generate C5 convertase, the enzyme that leads to the production of the  membrane attack complex (first it adhere to the surface of the targets); and 2.it opsonises bacteria because phagocytes have receptors for C3b on their surface.

How to activate?

In the classic pathway, antigen-antibody complexes activate C12 to form a protease and thereafter the complement cascade starts. In the lectin pathway,  MBL (mannas-binding lectin/mannose-binding protein) binds to the surface of microbes bearing mannan. This activates proteases associated with MBL that activates complement cascade. In the alternative pathway, many unrelated cll surface substances can initiate the process by binding C3 and factor B. This complex is cleaved by a protease and finally the complement cascade initiates.

Other Mediators

• Platelet-Activating Factor (PAF)

PAF is a phospholipid-derived mediator that was discovered as a factor that caused platelet aggregation, but it is now known to have multiple inflammatory effects. A variety of cell types, including platelets themselves, basophils, mast cells, neutrophils, macrophages, and endothelial cells, can elaborate PAF, in both recreated and cell-bound forms. In addition to platelet aggregation, PAF causes vasoconstriction and bronchoconstriction, and at low concentrations it induces vasodilation and increased ventral permeability.

• Products of Coagulation

Protease-activated receptors (PARs) are activated by thrombin (converting fibrinogen to fibrin), and are expressed on platelets and leukocytes.

• Kinins

Kinins are vasoactive peptides derived from plasma proteins called kininogens, by the action of specific proteases called kallikreins. The enzyme kallikrein cleaves a plasma glycoprotein precursor, high-molecular-weight kininogen, to produce bradykinin, a substance that increases vascualar permeability and causes contraction of smooth muscle, dilation of blood vessels, and pain when injected into the skin. These effects are similar to those of histamine. The action of bradykinin is short-lived, because it is quickly inactivated by an enzyme called kininase.

• Neuropeptides

Neuropeptides are secreted by sensory nerves and various leukocytes, and may play a role in the initiation and regulation of inflammatory responses. These small peptides, such as substance P and neurokinin A, are produced in the central and peripheral nervous systems. Substance P has many biologic functions, including the transmission of pain signals, regulation of blood pressure, stimulation of hormone secretion by endocrine cells, and increasing vascular permeability.

## [Respiratory] Systemic Oxygenation and Parameters

March 2, 2015 Uncategorized No comments

Oxygen may be necessary for life, but it doesn’t prevent death.

Critical care management is dominated by interventions that promote tissue oxygenation, yet there are no direct measurements of the oxygen tension in tissue. Instead, a variety of global, indirect measures of tissue oxygenation are used to guide aerobic support. Note that these oxygenation parameters and measurements are systemic, or global, rather than local.

PS: Blood oxygen partial pressure and oxyhemoglobin saturation

250 mm Hg —> 100%

100 mm Hg —> 97.4%, oxygen content 19.88 mL (chemically bound plus physically dissolved, [Hb] = 15 g/dL)

60 mm Hg —> 90%

40 mm Hg —> 75%, oxygen content 15.2 mL (chemically bound plus physically dissolved, [Hb] = 15 g/dL)

27 mm Hg —> 50%

20 mm Hg —> 32%

Oxygen in Blood

The oxygenation of arterial and venous blood is frequently involved in the evaluation of systemic oxygenation. The relevant measures of oxygen (O2) in blood include the partial pressure of O2 (PO2), the Osaturation of hemoglobin (SO2), the concentrations of hemoglobin-bound Oand dissolved O2, and the total Oconcentration (also called O2 content).

Oxygenation Parameters

Common Oxygenation Parameters

Below are come important oxygenation parameters in daily clinical settings. Normal values are list in Table 1 and the variability is in Table 10.3. Only the values of these parameters exceed the range of inherent variability, you can consider them significant. For example, the variability of the calculated VO2 is ±18%, which is the summed variability of the component measurements. Therefore, the VO2 that is calculated from the modified Fick equation must change by 18% for the change to be considered significant.

The VO2 calculated as above is the Calculated VO2, which is not the whole body VO2 because it does not include the O2 consumption of the lungs. Normally, the VO2 of the lungs accounts for less than 5% of the whole body VO2, but it can make up 20% of the whole body VO2 when there is inflammation in the lungs.

Whole body VO2 is measured by monitoring the O2 concentration in inhaled and exhaled gas. This requires a specialised instrument equipped with an oxygen analyzer (such as the metabolic carts used by nutrition support services). The instrument is connected to the proximal airway (usually intubated patients), and it records the VO2 as the product of minute ventilation (VE) and the fractional concentration of O2 in inhaled and exhaled gas (FIO2 and FEO2).

1.CVP=RAP=RVEDP [mm Hg]

CVP: Central Venous Pressure, RAP: Right Atrial Pressure, RVEDP: Right-ventricular End-diastolic Pressure

2.PAWP=LAP=LVEDP [mm Hg]

PAWP: Pulmonary Artery Wedge Pressure, LAP: Left Atrial Pressure, LVEDP: Left-ventricular End-diastolic Pressure

3.CI=CO/BSA [L/min/m2]

CI: Cardiac Index, CO: Cardiac Output, BSA: Body Surface Area

4.SI=CI/HR [mL/m2]

SI: Stroke Index, CI: Cardiac Index, HR: Heart Rate

5.SVRI=(MAPCVP)/CI [mm Hg/L/min/m2]

SVRI: Systemic Vascular Resistance Index, MAP: Mean Arterial Pressure, CVP: Central Venous Pressure, CI: Cardiac Index

6.PVRI=(PAPPAWP)/CI [mm Hg/L/min/m2]

PVRI: Pulmonary Vascular Resistance Index, PAP: Mean Pulmonary Arterial Pressure, PAWP: Pulmonary Artery Wedge Pressure, CI: Cardiac Index

7.DO2=CI*CaO2=CI*(1.3*Hb*SaO2) [mL/min/m2]

DO2: Oxygen Delivery, CI: Cardiac Index, CaO2: Oxygen Concentration in Arterial Blood, Hb: Hemoglobin Concentration, SaO2: Percent Saturation of Hemoglobin with Oxygen

8.VO2=CI*(CaO2CvO2)=CI*1.3*Hb*(SaO2SvO2) [mL/min/m2] (the modified Fick equation, calculated VO2)

VO2: Oxygen Uptake, CI: Cardiac Index, CaO2: Oxygen Concentration in Arterial Blood, CvO2: Oxygen Concentration in Venous Blood, Hb: Hemoglobin Concentration, SaO2: Percent Saturation of Hemoglobin with Oxygen in Arterial Blood, SvO2: Percent Saturation of Hemoglobin with Oxygen in Venous Blood.

Variability of VO2

Each of the 4 measurements used to derive the VO2 has an inherent variability, which makes the summed variability of the calculated VO2 as high as 18%. Therefore, the VO2 that is calculated from the modified Fick equation must change by at least 18% for the change to be considered significant.

Fick Method vs. Whole Body VO2

The calculated VO2 from the modified Fick equation is not the whole body VO2 because it does not include the O2 consumption of the lungs. Normally, the VO2 of the lungs account for less than 5% of the whole body VO2, but it can make up 20% of the whole body VO2 when there is inflammation in the lungs.

The whole body VO2 is measured by monitoring the O2 concentration in inhaled and exhaled gas. This requires a specialized instrument equipped with an oxygen analyzer (such as the metabolic carts used by nutrition support services). The instrument is connected to the proximal airway (usually in intubated patients), and it records the VO2 as the product of minute ventilation (VE) and the fractional concentration of O2 in inhaled and  exhaled gas (FIO2 and FEO2).

The measured (whole body) VO2 has a variability of + – 5%, which is much less than the variability of the calculated VO2.

9.O2ER=VO2/DO2(*100) [%]

O2ER: Oxygen Extraction Ratio, VO2: Oxygen Uptake, DO2: Oxygen Delivery

Table 1 Hemodynamic and Oxygen Transport Parameters

Oxygenation of Hemoglobin

The oxygenation of haemoglobin is evaluated by the fraction of the hemoglobin in blood that is fully saturated with O2. This is called the O2 saturation (SO2), and is the ratio of fully oxygenated hemoglobin to the total hemoglobin in blood.

SO2 = Oxygenated Hb/Total Hb

This ratio is typically reported as a percentage (the percent saturation of hemoglobin). The SO2 can be measured using spectrophotometry, or it can be estimated using the PO2 of blood.

Oxyhemoglobin Dissociation Curve

The SO2 is determined by the PO2 in blood and the tendency of the iron moieties in hemoglobin to bind O2. The relationship between SO2 and PO2 is described by the oxyhemoglobin dissociation curve like the one shown in Figure 10.1.

The “S” shape of the curve offers two advantages. First, the arterial PO2 (PaO2) is normally on the upper, flat part of the curve, which means that a large drop in Pa O(down to 60 mm Hg) results in only minor changes in the arterial O2 saturation (SaO2). Secondly, the capillary PO2 (which is equivalent to the venous PO2 or PvO2 after equilibration with the tissues) is on the steep portion of the curve, which facilitates the exchange of O2 in both the pulmonary and systemic capillaries.

A number of conditions can alter the affinity of hemoglobin for O2 and shift the position of the oxyhemoglobin dissociation curve. These are list in the boxes in Figure 10.1. A shift of the curve to the right facilitates oxygen release in the systemic capillaries, while a shift to the left facilitates oxygen uptake in the pulmonary capillaries. The position of the curve is indicated by P50, which is the POthat corresponds to an O2 saturation of 50%. The is normally P50 about 27 mm Hg, and it increases when the curve shifts to the right, and decrease when the curve shifts to the left. A decrease in the P50 to 15 mm Hg has been reported in blood that is stored in acid-citrate-dextrose (ACD) preservative for 3 weeks, due to a leftward shift in the oxyhemoglobin dissociation curve from depletion of 2,3-diphosphoglycerate (2,3-DPG) in the red blood cells.

Shifts in the oxyhemoglobin dissociation curve have opposing effects in the pulmonary and systemic capillaries that seem to cancel each other. For example, a rightward shift of the curve caused by acidemia (the Bohr effect) will facilitate Orelease in the systemic capillaries but will hinder O2 uptake in the pulmonary capillaries. So what is the net effect of acidemia on tissue oxygenation? The answer is based on the influence of shitfs in the oxyhemoglobin dissociation curve on different portions of the curve. Shifts in the curve cause less of a change in the flat portion of the curve (where the arterial PO2 and SaO2 reside) than in the steep portion of the curve (where the systemic capillary PO2 and SvO2 reside). Therefore, a rightward shift of the curve from acidemia will facilitate O2 release in the systemic capillaries more than it hinders O2 uptake in the pulmonary capillaries, and the overall effect benefits tissue oxygenation.

Systemic Oxygen Balance

The business of nutrient metabolism is to extract the energy stored in nutrient fuels (which is accomplished by disrupting high-energy carbon bonds) and transfer the energy to storage molecules like adenosine triphosphate (ATP). Oxygen transport has two components: the rate of O2 delivery to the microcirculation (DO2), and the rate of O2 uptake into the tissues (VO2). When the VO2 matches the metabolic rate (MR), glucose is completely oxidized to yield 36 ATP molecules (673 kcal) per mol. When VO2 is less than the metabolic rate, some of the glucose is diverted to form lactate, and the energy yield falls to 2 ATP molecules (47 kcal) per mole.

Types of Hypoxia

The condition where the energy yield of nutrient metabolism is limited by the availability of oxygen is called dysoxia, and the clinical expression of this condition is multiorgan dysfunction progressing to multiorgan failure. Dyslexia can be the result of an inadequate supply of O2, which results in tissue hypoxia, or it can be caused by a defect in oxygen utilization in the mitochondria, which is called cytopathic hypoxia. DO2 and VO2 play an important role in determining the energy yield from nutrient metabolism. Let's discuss further.

VO2 is normally about 25% of the DO2, so the normal O2ER is 0.25 (range from 0.2 to 0.3). Thus, only 25% of the O2 delivered to the capillaries is taken up into the tissues when conditions are normal. The oxygen transport system operates to maintain a constant VO2 in the face of variations in O2 delivery (DO2), and this is accomplished by compensatory changes in the O2 extraction.

The Challenge of VO2

Again, oxygen may be necessary for life, but it doesn’t prevent death. That is, VO2 is necessary for life.

Graph Showing the Relationship Between DO2 and VO2.

As shown in this equation of VO2 = DO2*O2ER, the VO2 will remain constant when DO2 is decreased if there is an equivalent increase in O2ER. However, if O2ER is fixed, a decrease in DO2 will result in an equivalent decrease in VO2. When the SaO2 is above 90%, O2ER = (SaO2 – SvO2). At the normal the (SaO2 – SvO2) is 25%, as shown in Figure 10.5. As DO2 decreases below normal (moving to the left along the curve), the VO2 initially remains unchanged, indicating that the O2ER is increasing. However, a point is eventually reached where the VO2 begins to decrease;at this point, the SvO2 has dropped to 50%, resulting in an increase in (SaO2 – SvO2) to almost 50% (SaO2 >=90%). The point where the VO2 begins to decrease is the point where O2 extraction is maximal (about 50%) and is unable to increase further.

Beyond this point, decrease in DO2 are accompanied by similar decrease in VO2, indicating the onset of tissue hypoxia. Thus, the point where O2ER is maximal is the anaerobic threshold since when VO2 is less than the metabolic rate, some of the glucose is diverted to form lactate, and the energy yield falls to 2 ATP molecules (47 kcal) per mole.

• SaO2 above 90%

The O2ER can be monitored as the (SaO2 – SvO2) as long as the SaO2 is above 90%. The SaO2 is monitored by pulse oximetry and the SvO2 is monitored with pulmonary artery catheters (or central venous catheters). The following general rules can be applied to the interpretation of (SaO2 – SvO2). These interpretations are based on the assumption that the metabolic rate is normal or unchanging: 1.the normal (SaO2 – SvO2) is 20% to 30%;2.an increase in (SaO2 – SvO2) above 30% indicates a decrease in O2 delivery;3.an increase in (SaO2 – SvO2) that approaches 50% indicates either threatened or inadequate tissue oxygenation;4.a decrease in (SaO2 – SvO2) below 20% indicates a defect in O2 utilisation in tissue (where the DO2 is normal but the O2ER is decreased so the VO2 is compromised), which is usually the result of inflammatory cell injury in severe sepsis or septic shock.

• SaO2 appraching 100%

When the SaO2 approaches 100%, O2 extraction can be monitored using only the SvO2. Under this condition, SvO2 = 1 – VO2/DO2, which predicts that the SvO2 will vary inversely with changes in O2 extraction (VO2/DO2). The normal range for SvO2 in pulmonary artery blood is 65% to 75%. Continuous SvO2 monitoring is associated with spontaneous fluctuations that average 5% but can be as high as 20%. A change in SvO2 must exceed 5% and persist for longer than 10 minutes to be considered a significant change. Interpreting the SvO2, 1.the normal SvO2 is 65-75%;2.a decrease in SvO2 below 65% indicate a decrease in O2 delivery;3.a decrease in SvO2 that approaches 50% indicates either threatened or inadequate tissue oxygenation;4.an increase in SvO2 above 75% indicates a defect in O2 utilisation in tissues, which is usually the result of inflammatory cell injury in severe sepsis or septic shock.

The O2 saturation in the superior vena cava, known as the "central venous" O2 saturation (ScvO2), has been proposed as an alternative to the mixed venous O2 saturation (SvO2) because it eliminates the need for a PA catheter (SvO2 correlates the blood sample in pulmonary arteries). However, the ScvO2 is higher than the SvO2 by an average of 7±4% (absolute difference) in critically ill patients. Discrepancies in the two measurements are greatest in patients with heart failure, cariogenic shock, and sepsis. The higher ScvO2 in low output states is attributed to peripheral vasoconstriction with preservation of cerebral blood flow, and the higher ScvO2 in sepsis is attributed to an increase in splanchnic O2 consumption. Despite this discrepancy, changes in ScvO2 generally mirror those in the SvO2, and trends in the ScvO2 are considered more informative than individual measurements. The normal range of ScvO2 in one study was preselected at 70% to 89%, which is consistent with the use of an ScvO2 >70% as one of the early goals of management in patients with severe sepsis or septic shock.

The ScvO2 is monitored with central venous catheters, but the tip of the catheter must be in the superior vena cava.

Update on Aug 3rd 2017

Oxygen and Carbon Dioxide Transport in the Blood

Hemoglobin is a complex molecule with a molecular weight of about 64,500. The protein portion (globin) has a tetrameric strucuture consisting of 4 linked polypeptide chains, each of which is attached to a protoporphyrin (heme) group. Each heme group consists of 4 symmetrically arranged pyrroles with a ferrous (Fe2+) iron atom at its center. The iron atom is bound to each of the pyrrole group and to 1 of the 4 polypeptide chains. A sixth binding site on the ferrous iron atom is freely available to bind with oxygen (or carbon monoxide) to the iron atom in its own heme group, and so the tetrameric hemoglobin molecule can combine chemically with 4 oxygen molecules (or 8 oxygen atoms). Both the globin component and the heme component (with its iron atom in the ferrous state), in their proper spatial orientation to each other, are necessary for the chemical reaction with oxygen to take place – neither heme nor globin alone will combine with oxygen. Each of the tetrameric hemoglobin subunits can combine with oxygen by itself.

Variations in the amino acid sequences of the 4 globin subunites may have important physiologic consequences. Normal adult hemoglobin (HbA) consists of 2 alpha (α) chains, each of which has 141 amino acids, and 2 beta (β) chains, each of which has 146 amino acids. Fetal hemoglobin (HbF), which consists of 2 alpha chains and 2 gamma (γ) chains, has a higher affinity for oxygen than does HbA. For more information about the ontogeny of hemoglobin please refer to thread "Red Blood Cell Analytic Parameters" at http://www.tomhsiung.com/wordpress/2015/12/red-blood-cell-analytic-parameters/.

Chemical Reaction of Oxygen and Hemoglobin

Hemoglobin rapidly combines reversibly with oxygen. It is the reversibility of the reaction that allows oxygen to be released to the tissues; if the reaction did not proceed easily in both directions, hemoglobin would be of little use in delivering oxygen to satify metabolic needs. The reaction is very fast, with a half-time of 0.01 of a second or less. Each gram of hemoglobin is capable of combining with about 1.39 mL of oxygen under optimal conditions, but under normal circumstances some hemoglobin exists in forms such as methemoglobin or is combined with carbon monoxide, in which case the hemoglobin does not bind oxygen. For this reason, the oxygen-carrying capacity of hemoglobin is conventionally considered to be 1.34 mL O2 / g Hb. If the Hb concentration in blood is 15 g/dL, every 100 mL of blood could carries about 20.1 mL of O2.

Influences on the Oxyhemoglobin Dissociation Curve

High temperature, low pH, high PCO2, and elevated levels of 2,3-BPG all "shift the oxyhemoglobin dissocaiton curve to the right." That is, for any particular PO2 there is less oxygen chemically combined with hemoglobin at higher temperatures, lower pHs, higher PCO2s, and elevated levels of 2,3-BPG.

pH and PCO2

The effects of blood pH and PCO2 on the oxyhemoglobin dissociaton curve are: Low pHs and high PCO2s both shift the curve to the right. High pHs and low PCO2s both shitf the curve to the left. Because high PCO2s in blood are often associated with low pHs, these 2 effects often occur together. The influence of pH (and PCO2) on the oxyhemoglobin dissociation curve is referred to as the Bohr effect.

Temperature

High temperatures shift the curve to the right; low temperatures shitf the curve to the left. At very low blood temperatures, hemoglobin has such a high affinity for oxygen that it does not release the oxygen, even at very low PO2s. It should also be noted that oxygen is more soluble in water or plasma at lower temperatures than it is at normal body temperature. At 20 °C about 50% more oxygen will dissolve in plasma.

2,3-BPG

2,3-BPG is produced by erythrocytes during their normal glycolysis and is present in farily high concentrations within red blood cells. 2,3-DPG binds to the hemoglobin in erythrocytes, which decreases the affinity of hemoglobin for oxygen. Higher concentrations of 2,3-BPG therefore shift the oxyhemoglobin dissociation curve to the right. More 2,3-DPG is produced during chronic hypoxic conditons, shifting the dissociation curve to the right and allowing more oxygen to be released from hemoglobin at a particular PO2. Very low levels of 2,3-BPG shift the curve far to the left, which means that blood deficient in 2,3-BPG does not unload much oxygen except at very low PO2s. It is important to note that blood stored at blood banks for as little as 1 week has been shown to have very low levels of 2,3-BPG. Use of banked blood in patients may result in greatly decreased oxygen unloading to the tissue unless steps are taken to restore the normal levels of 2,3-BPG.

Transport of Carbon Dioxide

Carbon dioxide is carried in the blood in physical solution, chemically combined to amino acids in blood proteins, and as bicarbonate ions. About 200 to 250 mL of carbon dioxide is produced by the tissue metabolism each minute in a resting 70-kg person and must be carried by the venous blood to the lung for removal from the body. At a cardiac ouput of 5 L/min, each 100 mL of blood passing through the lungs must therefore unload 4 to 5 mL of carbon dioxide.

## Inflammation – The Beginning and Ongoing

Inflammation is a response of vascularized tissues to infections, foreign invaders, and damaged tissues that brings cells and molecules of host defines from the circulation to the sites where they are needed, in order to eliminate the offending agents. Inflammation generally is a defensive response that is essential for survival, where mediators of this response include phagocytic leukocytes and cytokines/substances produced by them, antibodies, and complement proteins. Most of these mediators normally circulate in the blood, from which they can be rapidly recruited to any site in the body;some of the cells also reside in tissues. The process of inflammation delivers these cells and proteins to damaged or necrotic tissues and foreign invaders, and activates the recruited cells and molecules, which then function to get rid of the harmful or unwanted substances. Without inflammation, infections would go unchecked, wounds would never heal, and injured tissues might remain permanent festering sores.

On the other hand, the primary function of the inflammatory response is to eliminate a pathogenic insult and remove injured tissue components, thus allowing tissue repair to take place. In teleologic terms, the body attempts to contain or eliminate offending agents to protect tissues, organs and, ultimately, the whole body from damage. Specific cells are imported to attack and destory injurious agents, enzymatically digest and remove them, or wall them off. In the process, damaged cells and tissues are digested and removed to allow repaire to occur.

Causes of Inflammation

Inflammatory reactions may be triggered by a variety of stimuli, including:

1.Infections (bacterial, viral, fungal, parasitic, rickettsiaceae and so on) and microbial toxins are among the most common and medically important causes of inflammation. Different infectious pathogens elicit varied  inflammatory responses, from mild acute inflammation that causes little or no lasting damage and successfully eradicates the infection, to severe systemic reactions that can be fatal, to prolonged chronic reactions that cause extensive tissue injury. The outcomes are determined largely by the type of pathogen and, to some extent, by characteristics of the host that remain poorly defined (relative post: http://forum.tomhsiung.com/pharmacy-practice/pharmacotherapy/416-severe-sepsis-septic-shock.html).

PS: In the article of NEJMra1208623, the specific response in any patient depends on the causative pathogen (load and virulence) and the host (genetic characteristics and coexisting illnesses), with differential responses at local, regional, and systemic levels.

2.Tissue necrosis elicits inflammation regardless of the cause of cell death, which may include ischemia (reduced blood flow, the cause of myocardial infarction, etc.) trauma, and physical and chemical injury (e.g., thermal injury, as in burns or frostbite; irradiation; exposure to some environmental chemicals). Several molecules released from necrotic cells are known to trigger inflammation

3.Foreign bodies (splinters, dirt, sutures) may elicit inflammation by themselves or because they cause traumatic tissue injury or carry microbes. Even some endogenous substances can be considered potentially harmful if large amounts are deposited in tissues; such as substances include urate crystals (in the disease gout), cholesterol crystals (in atherosclerosis), and lipids (in obesity-associated metabolic syndrome).

4.Immune reactions (also called hypersensitivity) are reactions in which the normally protective immune system damages the individual's own tissues. The injurious immune responses may be directed against self antigens, causing autoimmune diseases, or may be inappropriate reactions against microbes. Also, the textbook of Drug-Induced Diseases by James E. Tisdale, PharmD, describes the hypersensitivity as reactions mediated by the immune system. In inflammations caused by immune reactions or hypersensitivity, self and environmental antigens act as the triggers of inflammations and since that these stimuli cannot be eliminated, autoimmune and allergic reactions tend to be persistent and difficult to cure, and they are often associated with chronic inflammations.

Tissue Injury

On the other hand, inflammation could be described as the result of tissue injury.

Cell must be able to adapt to fluctuating environmental conditions (e.g., temperature, solute concentrations, oxygen supply, noxious agents, etc.). The evolution of multicellular organisms eased the precarious lot of individual cells by establishing a controlled extracellular environment where the "inner" environmental conditions remain relatively constant. If a change in the environment is too huge, a cell can be injuried; and if the injury exceeds the cell's adaptive  capacity, the cell dies. A cell exposed to persistent sublethal injury has limited available responses, expression of which we interpret as cell injury.

All cells have efficient mechanisms to deal with shifts in environmental conditions. When environmental changes exceed the cell's capacity to maintain normal homeostasis, we recognize acute cell injury. If these stress is removed in time or if the cell can withstand the assault, the damage is reversible, and complete structural and functional integrity is restored. For example, when circulation to the heart is interrupted for less than 30 minutes, all structural and functional alterations prove to be reversible. The cell can also be exposed to persistent sublethal stress, as in mechanical irritation of the skin or exposure of the bronchial mucosa to tobacco smoke. Cell have time to adapt to reversible injury in a number of ways, each of which has a morphologic counterpart. On the other hand, if the stress is sufficiently severe, irreversible injury leads to cell death. The moment when reversible injury becomes irreversible injury, the "point of no return," is not known at present.

Inflammation Mediators

The mediators of inflammation are the substances that initiate and regulate inflammatory reactions. The most important inflammation mediators include vasoactive amines, lipid products (prostaglandins and leukotrienes), cytokines (including chemokines), and products of complement activation. These mediators induce various components of the inflammatory response typically by distinct mechanisms, which is why inhibiting each has been therapeutically beneficial. However, there is also some overlap (redundancy) in the actions of the mediators.

The inflammation mediators have some common characteristics, like

• Mediators are either secreted by cells or generated from plasma proteins. Cell-derived mediators are normally sequestered in intracellular granules and can be rapidly secreted by granule exocytosis (e.g., histamine in mast cell granules) or are synthesised de novo (e.g., prostaglandins and leukotrienes, cytokines) in response to a stimulus. The major cell types that produce mediators of acute inflammation are the sentinels that detect invaders and damage in tissues, that is, macrophages, dendritic cells, and mast cells, but platelets, neutrophils, endothelial cells, and most epithelia can also be induced to elaborate some of the mediators. Plasma derived mediators (e.g., complement proteins) are produced mainly in the liver and are present in the circulation as inactive precursors that must be activated. When activated a series of proteolytic and protein-protein interactions are initiated that ultimately to acquire their biologic properties.
• Ative mediators are produced only in response to various stimuli. These stimuli include microbial products and substances released from necrotic cells. Some of the stimuli trigger well-defined receptors and signalling pathways.
• Most of the mediators are short-lived. They quickly decay, or are inactivated by enzymes, or they are otherwise scavenged or inhibited. There is thus a system of checks and balances that regulates mediator actions.
• One mediator can stimulate the release of other mediators. The secondary mediators may have the same actions as the initial mediators but may also have different and even opposing activities. Such cascades provide mechanisms for amplifying or, in certain instances, counteracting the initial action off a mediator.

A detail about inflammation mediators can be found here, .

The Process of Inflammation

The steps of the inflammatory response can be divided as the five sections: 1.recognition of the injurious agent;2.recruitment of leukocytes;3.removal of the agent;4.regulation (control) of the response;and 5.resolution (repair).

When an individual encounters an injurious agent, as described above, phagocytes that reside in all tissues try to eliminate these agents. At the same time, phagocytes and other sentinel cells in the tissues recognise the presence of the inflammation triggers and react by liberating cytokines, lipid messengers, and other mediators of inflammation. Some of these mediators act on small blood vessels in the vicinity and promote the efflux of plasma and the recruitment of leukocytes (as demonstrated as dilation of small vessels leading to an increase in blood flow [vasodilation/resulting in more blood cells and plasma proteins], increased permeability of the endothelia, and emigration and accumulation of the leukocytes [stasis]) to the site where the offending agent is located.

PS: the following figure shows the inherent differences between exudate and transudate. Edema denotes an excess of fluid in the interstitial tissue or serous cavities; it can be either an exudate or a transudate. Purulent is a inflammatory exudate rich in leukocytes, the debris of dead cells and, in many cases, microbes.

Recognition of Inflammation-Causative Substances

• Cellular Receptors for Microbes

Several cellular receptors and circulating proteins are capable of recognising microbes and products of cell damage and triggering inflammation. Cells express receptors in the plasma membrane, the endosomes (note, it's not the lysosomes), and the cytosol that enable the cells to sense the presence of foreign invaders in any cellular compartment. The most-documentted such receptors are the family of Toll-like receptors (TLRs). These receptors are expressed on many cell types, including epithelial cells, dendritic cells, macrophages, and other leukocytes. Engagement of these receptors triggers production of molecules involved in inflammation, including adhesion molecules on endothelial cells, cytokines, and other mediators.

• Sensors of Cell Damage

All cells have cytosolic receptors that recognise a diverse set of molecules that liberated or altered as a consequence of cell damage. These molecules include uric acid, ATP, reduced intracellular K+ concentrations, even DNA when it is released into the cytoplasm and not sequestered in nuclei, as it should be normally, and many others. These receptors activate a multi protein cytosolic complex called inflammasome which induces the produce of the cytokine interleukin-1 (IL-1).

• Indirect Recognizing

In addition to directly recognising microbes, many leukocytes express receptors for the Fc tails of antibodies and for complement proteins. It is likely that the binding of antibodies and complement proteins by microbes will change the conformation of the Fc tails and activated complement proteins, respectively, and this change of conformation provides the chance of them to bind these leukocyte receptors. These receptors recognise microbes coated with antibodies and complement and promote ingestion and destruction of the microbes as well as inflammation. Some circulating proteins like complements reacts against microbes and produces mediators of inflammation. These proteins act indirectly to the recognition of inflammation-causative substances, with the help of which the cells recognise these substances more effectively.

Reactions of Blood Vessels in Acute Inflammation

The vascular reactions of acute inflammation consist of changes in the flow of blood and the permeability of vessels, both designed to maximise the movement of plasma proteins and leukocytes out of the circulation and into the site of infection or injury. Vasoactive mediators originating from plasma and cells are generated at sites of tissue injury. These vasoactive molecules bind specific receptors on vascular endothelial and smooth muscle cells, causing vasoconstriction or vasodilation. Vasodilation of arterioles increases blood flow and exacerbates fluid leakage into the tissue. Vasoconstriction of postcapillary venules increases capillary bed hydrostatic pressure, further stimulating edema formation. Also vasoactive mediators bind specific receptors on endothelial cells, causing reversible endothelial cell contraction and gap formation.

Changes in Vascular Flow and Caliber

Changes in vascular flow and caliber begin early after injury and consist of the following. Vasodilation is induced by the action of several mediators, notably histamine, on vascular smooth muscle. It is one of the earliest manifestations of acute inflammation. Vasodilation first involves the arterioles and then leads to opening of new capillary beds in the area. The result is increased blood flow, which is the cause of heat and redness.

PS: Critical Closing Pressure & Active/Inactive Capillaries

In rigid tubes, the relationship between pressure and flow of homogeneous fluids is liner, but in thin-walled blood vessels in vivio it is not. When the pressure in a small blood vessel is reduced, a point is reached at which no blood flows, even though the pressure is not zero. This is because the vessels are surrounded by tissues that exert a small but definite pressure on them, and when the intraluminal pressure falls below the tissue bpressure, they collapse. The threshold press is called critical closing pressure.

So in resting tissues, most of the capillaries are collapsed, these capillaries are inactive cappliaries. In active tissues, the metarterioles and the precapillary sphinctersdilate. The result is that the intracapillary pressure rises, overcoming the critical closing pressure of the vessels, and blood flows through all of the capillaries. Relaxation of the smooth muscle of the metarterioles and precapillary sphincters is due to the action of vasodilator metabolites formed in active tissue.

Vasodilation is quickly followed by increased permeability of the microvasculature, with the outpouring of protein-rich fluid into the extravascular tissues. The loss of fluid caused by increased permeability and the increased vessel diameter lead to slower blood flow, concentration of red cells in small vessels, and increased viscosity of the blood. These changes result in engorgement of small vessels with slowly moving red cells, a condition termed stasis, which is seen as vascular congestion and localised redness of the involved tissue.

As stasis develops, blood leukocytes, principally neutrophils, accumulate along the vascular endothelium. At the same time endothelia cells are activated by mediators produced at sites of infection and tissue damage, and express increased levels of adhesion molecules. Leukocytes then adhere to the endothelium, and soon afterward they migrate through the vascular wall into the interstitial tissue.

Increased Vascular Permeability (Vascular Leakage)

Several mechanisms are responsible for the increased permeability of post capillary venules, a hallmark of acute inflammation. These mechanisms of increased vascular permeability are described separately, all probably contribute in varying degrees in responses to most stimuli. For example, at different stages of a thermal burn, leakage results from chemically mediated endothelial contraction and direct and leukocyte-dependent endothelia injury. The vascular leakage induced by these mechanisms can cause life-threatening loss of fluid in severely burned patients.

Contraction of endothelial cells resulting in increased inter endothelial spaces is the most common mechanisms of vascular leakage. It is elicited by histamine, bradykinin, leukotrienes, and other chemical mediators. It is called the immediate transient response because it occurs rapidly after exposure to the mediator and is usually short-lived (15 to 30 minutes). In some forms of mild injury (e.g., after burns, irradiation or ultraviolet radiation, and exposure to certain bacterial toxins), vascular leakage begins after a delay of 2 to 12 hours and lasts for several hours or even days;this delayed prolonged leakage may be caused by contraction of endothelial cells or mild endothelial damage. Late-appearing sunburn is a good example of this type of leakage.

Endothelial injury, resulting in endothelia cell necrosis and detachment. Direct damage to the endothelium is encountered in severe injuries, for example, in burns, or is induced by the actions of microbes and microbial toxins that target endothelial cells. Neutrophils that adhere to the endothelium during inflammation may also injure the endothelial cells and thus amplify the reaction. In most instances leakage starts immediately after injury and is sustained for several hours until the damage vessels are thromboses or repaired.

Increased transport of fluids and proteins, called transcytosis, through the endothelial cell. This process may involve intracellular channels that may be stimulated by certain factors, such as vascular endothelial growth factor (VEGF), that promote vascular leakage. However, the contribution of this process to the vascular permeability of  acute inflammation is uncertain.

PS: In addition to blood vessels, lymphatic vessels also participate in acute inflammation. The system of lymphatics and lymph nodes filters and polices the extravascular fluids. Lymphatics normally drain the small amount of extravascular fluid that has seeped out of capillaries. In inflammation, lymph flow is increased and helps drain deem fluid that accumulates because of increased vascular permeability. In addition to fluid, leukocytes and cell debris, as well as microbes, may find their way into lymph. Therefore the lymphatics may become secondarily inflamed (lymphangitis), as may the draining lymph nodes (lymphadenitis). Inflamed lymph nodes are often enlarged because of hyperplasia of the lymphoid follicles and increased numbers of lymphocytes and macrophages. This constellation of pathologic changes is termed reactive, or inflammatory, lymphadenitis. For clinicians the presence of red streaks near a skin would is telltale sign of an infection in the wound. This streaking follows the course of the lymphatic channels and is diagnostic of lymphangitis;it may be accompanied by painful enlargement of the draining lymph nodes, indicating lymphadenitis.

Leukocyte Recruitment to Sites of Inflammation

The changes in blood flow and vascular permeability are quickly followed by an influx of leukocytes into the tissue. The most important leukocytes in typical inflammatory reactions include neutrophils and macrophages, both termed phagocytosis. These leukocytes (but no limited to) ingest and destroy bacteria and other microbes, as well as necrotic tissue and foreign substances. Leukocytes also produce growth factors that aid in repair.

But a price that is paid for the defensive potency of leukocytes is that, when strongly activated, they may induce tissue damage and prolong inflammation, because the leukocyte products that destroy microbes and help "clean up" necrotic tissues can also injure normal bystander host tissues.

The journey of leukocytes from the vessel lumen to the tissue is a multistep process that is mediated and controlled by adhesion molecules and cytokines called chemokines. Briefly, the recruitment of leukocyte to sites of inflammation is a multistep process, which can be divided into adhesion to endothelium, migration through endothelium, and chemotaxis.

The whole process includes the margination, rolling, and adhesion of leukocytes to endothelium. In normal and unactivated status, vascular endothelium does not bind circulating cells or impede their passage. In inflammation, the endothelium is activated and can bind leukocytes as a prelude to their exit from the blood vessels, which follows those mechanisms below.

In normally flowing blood in venues, red cells are confined to a central axial column, displacing the leukocytes toward the wall of the vessel. Because blood flow slows early in inflammation (stasis), hemodynamic conditions change (wall shear stress decreaes), and more white cells assume a peripheral position along the endothelial surface. This process of leukocyte redistribution is called margination. Subsequently, leukocytes adhere transiently to the endothelium, detach and bind again, thus rolling on the vessel wall. Finally the cells come to rest at some point where they adhere firmly (adhesion).

The attachment of leukocytes to endothelial cell is mediated by complementary adhesion molecules on the two cell types (leukocytes and endothelium) whose expression is enhanced by cytokines. The two major families of molecules involved in leukocyte adhesion and migration are the selections and interns, and their ligands.

Table 1 Endothelia and Leukocyte Adhesion Molecules

The initial rolling interactions are mediated by a family of proteins called selectins. There are three types of selectins: L-selectin expressed on leukocytes, E-selectin expressed on endothelium, and P-selectin expressed in platelets and on endothelium. There are ligands for each selectin, which are expressed on leukocytes and endothelium, respectively. The expression of selecting and their ligands is regulated by cytokines produced in response to infection and injury. Within 1 to 2 hours the endothelial cells begin to express E-selectin and the ligands for L-selectin. Other mediators such as histamine and thrombin stimulate the redistribution of P-selectin from its normal intracellular stores in endothelial cell granules (called Weibel-Palade bodies) to the cell surface.

The interactions between selections and their ligands are low-affinity with a fast off-rate, and they are easily disrupted by the flowing blood. As a result, the bound leukocytes bind, detach, and bind again, and thus begin to roll along the endothelial surface.

These weak rolling interactions slow down the leukocytes and give them the opportunity to bind more firmly to the endothelium. Firm adhesion is mediated by a family of heterodimeric leukocyte surface proteins called integrins such as VLA-4, LFA-1 etc, where they are expressed on leukocytes with ligands such as VCAM-1, ICAM-1, etc. Leukocytes normally express interns in a low affinity state. Chemokines that were produced a the site of injury bind to endothelial cell proteoglycans, and are displayed at high concentrations on the endothelial surface. Meanwhile, these chemokines bind to and activate the rolling leukocytes which induces the conversion of integrins from low-affinity state to hight-affinity state. Finally, high level of ligand on endothelial surface and high -affinity of integrin on leukocytes results in firm integrin-mediated binding of these two cell types at the site of inflammation. The leukocytes stop rolling, their cytoskeleton is reorganised, and the spread out on the endothelial surface.

Leukocyte Migration Through Endothelium

Transmigration of leukocytes occurs mainly in postcapillary venules. Chemokines act on the adherent leukocytes and stimulate the cells to migrate through interendothelial spaces toward the chemical concentration gradient, that is, toward the site of injury or infection where the cheekiness are being produced. Several adhesion molecules present in the intercellular junctions between endothelia cells are involved in the migration of leukocytes, including a member of the immunoglobulin superfamily called CD31 or PECAM-1 (platelet endothelial cell adhesion molecule). After traversing the endothelium, leukocytes pierce the basement membrane, probably by secreting collagenases, and enter the extravascular tissue. The cells then migrate toward the chemotactic gradient created by cheekiness and other chemoattractants and accumulate in the extravascular site.

Chemotaxis of Leukocytes

After exiting the circulation, leukocytes move in the tissue word the site of injury by a process called chemotaxis, which is defined as locomotion along a chemical gradient. Both exogenous and endogenous substances can act as chemoattractants. The most common exogenous agents are bacterial products, including peptides that possess an N-formylmethionine terminal amino acid and some lipids. Endogenous chemoattractants include several chemical mediators like: 1.cytokines, particularly those of the chemokine family (e.g., IL-8);2.components of the complement system, particularly C5a;and 3.arachidonic acid (AA) metabolites, mainly leukotriene B4 (LTB4). All these chemotactic agents bind to specific seven-transmembrane G protein-coupled receptors on the surface of leukocytes. Signals initiated from these receptors result in activation of second messengers (check thread "G Protein-Coupled Receptors" at http://www.tomhsiung.com/wordpress/2014/09/g-protein-coupled-receptors/) that increase cytosolic calcium and activate small guanosine triphosphatases of the Rac/Rho/cdc42 family as well as numerous kinases. These signals induce polymerization of actin, resulting in increased amounts of polymerized actin at the leading edge of the cell and localization of myosin filaments at the back. The leukocyte moves by extending filopodia that pull the back of the cell in the direction of extension, much as an automobile with front-wheel drive is pulled by the wheels in front. The net result is that leukocytes migrate toward the inflammatory stimulus in the direction of the locally produced chemoattractants.

The Nature of the Leukocyte Infiltrate

The nature of the leukocyte infiltrate varies with the age of the inflammatory response and the type of stimulus. In most forms of acute inflammation neutrophils predominate in the inflammatory infiltrate during the first 6 to 24 hours and are replaced by monocytes in 24 to 48 hours. There are several reasons for the early preponderance of neutrophils: they are more numerous in the blood than other leukocytes, they respond more rapidly to chemokines, and they may attach more firmly to the adhesion molecules that are rapidly induced on endothelial cells, such as P- and E-selectins. After entering tissues, neutrophils are short-lived;they undergo apoptosis and disappear within 24 to 48 hours. Monocytes not only survive longer but may also proliferate in the tissues, and thus they become the dominant population in prolonged inflammatory reactions.

There are, however, exceptions to this stereotypic pattern of cellular infiltration. In certain infections like those produced by Pseudomonas bacteria, the cellular infiltrate is dominated by continuously recruited neutrophils for several days;in viral infections, lymphocytes may be the first cells to arrive; some hypersensitivity reactions are dominated by activated lymphocytes, macrophages, and plasma cells;and in allergic reaction, eosinophils may be the main cell type.

Complement System

The complement system is a collection of soluble proteins and membrane receptors that function mainly in host defines against microbes and in pathologic inflammatory reactions. This system of complement functions in both innate and adaptive immunity for defines against microbial pathogens. In the process of complement activation, several cleavage products of complement proteins are elaborated that cause increased vascular permeability, chemotaxis, and opsonization.

Complement system acts as the bridge between innate and adaptive immune system. This concept is due to the fact that complement proteins can be activated directly by antigen-antibody complexes.

Primary Functions

There are three main effects of complement: 1.lysis of cells such as bacteria, allografts, and tumor cells; 2.generation of mediators that participate in inflammation and attract neutrophils; and 3.opsonization – enhancement of phagocytosis.

C3b is the central molecule of the complement cascade. It has two core functions: 1.it combines with other complement components to generate C5 convertase, the enzyme that leads to the production of the  membrane attack complex (first it adhere to the surface of the targets); and 2.it opsonises bacteria because phagocytes have receptors for C3b on their surface.

How to activate?

In the classic pathway, antigen-antibody complexes activate C12 to form a protease and thereafter the complement cascade starts. In the lectin pathway,  MBL (mannas-binding lectin/mannose-binding protein) binds to the surface of microbes bearing mannan. This activates proteases associated with MBL that activates complement cascade. In the alternative pathway, many unrelated cll surface substances can initiate the process by binding C3 and factor B. This complex is cleaved by a protease and finally the complement cascade initiates.

Negative Feedback Mechanisms

Innate and inflammatory responses are regulated by either enhancing or inhibiting mechanisms. The inhibiting mechanisms controls the degree of inflammation and terminate it when appropriate so that the causative substances of inflammation are eliminated while harmful effects to body could be limited to minimize the tissue damage.

In part, inflammation declines after the offending agents are removed simply because the mediators of inflammation are produced in rapid bursts, only as long as the stimulus persists, have short half-lives, and are degraded after their release. On the other hand, as inflammation develops, the process itself triggers a variety of stop signals that actively control and inhibit the inflammatory reaction. Some substances like lipoxins derived from arachidonic acid (AA), transforming growth factor-β (TGF-β), and IL-10 act as anti-inflammatory mediators to obtain the purpose of controlling and inhibiting the inflammation.

A Price Paid to Inflammation

General Symptoms and Signs of Inflammation

Although inflammation serves to protect and control infections and other harmful insults, it can also cause further tissue damage, which is manifested as the disease symptoms of redness, swelling, heat, and pain. The increased blood flow due to vasodilation results in redness and increased temperature in the area. The increased vascular permeability causes blood fluids to leak out of the vessels as the phagocytes transmigrate and thereby also cause edema (swelling) of the surrounding tissue. The source of the pain is still not clearly understood, but it is probably due to the combined effects of cytokines (e.g.,, prostaglandins) and coagulation cascade components on nerve endings in the inflamed region. Bradykinin also appears to increase sensitivity to pain. Pus, a common sign of infection, is composed mainly of dead PMNs and tissue cells.

Although phagocytic cells are effective killers of bacteria and are essential for clearing the invading bacteria from an infected area, the body can pay a high price for this service. During active killing of a bacterium, lysosomal enzymes are released into the surrounding area, as well as into the phagolysosome. Released lysosomal enzymes damage adjacent tissues and can be the main cause of tissue damage that results from a bacterial infection. Also, PMNs kill themselves as a result of their killing activities, and lysosomal granules released by dying PMNs contribute further to tissue destruction.