Shear Stress

Arteriolar Tone and Its Regulation (Local Mechanisms)

July 17, 2015 Cardiology, Physiology and Pathophysiology No comments , , , , , , , , , , , , , , , , ,

jesus-christ-0202

I.Arteriolar Tone

A.Basal tone

B.ANS

C.Adrenal Glands

D.Local

1.Metabolic substances

2.Endothelial cells secretion

3.Other local chemical influences

4.Transmural pressure (myogenic response)

II.Venous Tone

A.Basal tone (little)

B.ANS

C.Adrenal glands

D.Internal pressure (recall deltaV/deltaP = C)

E.External compression (muscle pump)


Because the body's needs are continually changing, the cardiovascular system must continually make adjustments in the diameter of its vessels. The purposes of these vascular change are 1.to efficiently distribute the cardiac output among tissues with different current needs (the job of arterioles) and 2.to regulate the distribute of blood volume and cardiac filling (the job of veins). So besides central regulatory mechanisms for vascular system (CNS, autonomic nerves system) and hormonal regulatory mechanisms (RAAS/angII and vasopressin, natriuretic hormone, insulin resistance and hyperinsulinemia, circulating catecholamines), there are another vascular regulatory mechanism – peripheral regulatory mechanisms/local mechanisms.

Total peripheral resistance (TPR) is determined by resistances of each primary organs and tissues, whereas resistance of an single organ or tissue region is primarily determined by resistances of arterioles that distribute within this organ or tissue. Therefore, TPR is determined primarily by resistance of arterioles. According to the famous Hagen–Poiseuille equation, resistance to flow is inversely and directly related to the radius of the vessel.

(Note: Q = ΔP/R, and R is resistance of the vessel)

Because resistances of arterioles are so important for TPR and the resultant blood flow (Q), we need to study the characteristics of arteriolar resistance carefully. Vascular tone is a term commonly used to characterize the general contractile state (so the radius of the vessel) or a vascular region. The "vascular tone" of a region can be taken as an indication of the "level of activation" of the individual smooth muscle cells in that region. Because the blood flow through any organ is determined largely by its vascular resistance, which dependent primarily on the diameter of its arterioles, thus an organ's flow is controlled by factors that influence the arteriolar smooth muscle tone.

Arterioles remain in a state of partial constriction even all external influences on them are removed; hence, they are said to have a degree of basal tone. The understanding of the mechanism is incomplete, but basal arteriolar tone may be a reflection of the fact that smooth muscle cells inherently and actively resist being stretched as they continually are in pressurized arterioles. Another hypothesis is that the basal tone of arterioles is the result of a tonic production of local vasoconstrictor substances by the endothelial cells that line their inner surface. Nevertheless, the arterioles have basal tone, and several factors externally influence it, including local influences, neural influences, and hormonal influences.


Autoregulation

The capacity of tissues to regulate their own blood flow is referred to as auto regulation. Most vascular beds have an intrinsic capacity to compensate for moderate changes in perfusion pressure by change in vascular resistance, so that blood flow remains relatively constant. The ability of vascular autoregulation is probably due in part to the intrinsic contractile response of smooth muscle to stretch (myogenic theory of autoregulation). That is, as the perfusion pressure rises, the blood vessels are distended and the vascular smooth muscle fivers that surround the vessels contract, which increases the vascular resistance so that the blood flow remains constant (Q = ΔP/R). At the last section of this thread you can find more detail information for the mechanisms and rationales about vascular autoregulation.


General Mechanisms for Activation of the Vascular Smooth Muscle

The task of the vascular smooth muscle is unique, because to maintain a certain vessel diameter in the face of the continual distending pressure of the blood within it, the vascular smooth muscle must be able to sustain active tension for prolonged periods. Compared with other muscle types, smooth muscle cells have these different characteristics, including:

1.Contract and relax much more slowly;

2.Can change their contractile activity as a result of either action potentials or changes in resting membrane potential;

3.Can change their contractile activity in the absence of any change in membrane potential;

4.Can maintain tension for prolonged periods at low energy cost;

and

5.Can be activated by stretch.


Local Influences on Basal Tone

Local factors influencing arteriolar basal tone (and the diameter of arterioles) include metabolic influences, endothelial cells, other chemical influences, and transmural pressure.

Metabolic Substances. The arterioles that control flow through a given organ lie within the organ tissue itself. Thus, arterioles and the smooth muscle in their walls are exposed to the chemical composition of the interstitial fluid of the organ they serve. The interstitial concentrations of many substances reflect the balance between the metabolic activity of the tissue and its blood supply. Exposure to low oxygen, and metabolic substances such as high H+, high K+, high CO2, high osmolarity, and adenosine, cause reduced arteriolar tone and vasodilation. By contrary, exposure to high oxygen and low metabolic substances induce increased arteriolar tone and vasoconstriction. When metabolic activity is over the blood supply, oxygen pressure in that tissue gets lower and the metabolic wastes accumulate in the tissue, which cause vasodilation of arterioles. As a result of arteriolar vasodilation, the blood supply to that tissue is improved and oxygen pressure gets back to normal or even higher, whereas increased amount of metabolic wastes are washed away by the improved blood flow therefore the accumulation of metabolic wastes is resolved. Finally, the basal tone gets back to normal.

Endothelial cells cover the entire inner surface of the cardiovascular system. A large number of studies have shown that the blood vessels respond very differently to certain vascular influences when their endothelial lining is missing. In the case of the vasodilator effect of infusing acetylcholine through intact vessels, the vasodilator influence produced by endothelial cells has been identified as nitric oxide. Nitric oxide is produced within endothelial cells from the amino acid, L-arginine, by the action of an enzyme, nitric oxide synthase. Nitric oxide synthase is activated by a rise in the intracellular level of the Ca2+. And nitric oxide is a small lipid-soluble molecule that, once formed, easily diffuses into adjacent smooth muscle cells where it causes relaxation by stimulating cGMP production.

Acetylcholine and several other agents such as bradykinin, vasoactive intestinal peptide, and substance P stimulate endothelial cell nitric oxide production because their receipts on endothelial cells are linked to receptor-operated Ca2+ channels. Probably more importantly from a physiological standpoint, flow-related shear stresses on endothelial cells stimulate their nitric oxide production presumably because stretch-sensitive channels for Ca2+ are activated. Such flow-related endothelial cell nitric oxide production may explain why, for example, exercise and increased blood flow through muscles of the lower leg can cause dilation of the blood-supplying femoral artery at points far upstream of the exercising muscle itself.

One general unresolved issue with the concept that arteriolar tone is regulated by factors produced by arteriolar endothelial cells is how these cells could know what the metabolic needs of the downstream tissue are. This is because the endothelial cells lining arterioles are exposed to arterial blood whose composition is constant regardless of flow rate or what is happening downstream. One hypothesis is that there exists some sort of communication system between vascular endothelial cells. That way, endothelial cells in capillaries or venules could telegraph upstream information about whether the blood flow is indeed adequate.

Other local chemical influences. Many specific locally-produced and locally-reacting chemical substances have been identified that have vascular effects and therefore could be important in local vascular regulation in certain instances. In most cases, however, definite information about the relative importance of these substances in cardiovascular regulation is lacking. Prostaglandins are a group of several chemically related products of the cyclooxyrgenase pathways of arachidonic acid metabolism, which have vasoactive effects. Certain prostaglandins are potent vasodilators, while some are potent vasoconstrictors. However, despite the vasoactive potency of the prostaglandin and the fact that most tissues are capable of synthesizing prostaglandins, it has not been demonstrated convincingly that prostaglandins play a crucial role in the normal vascular control.

Histamine is synthesized and stored in high concentrations in secretory granules of tissue mast cells and circulating basophils. When released, histamine produces arteriolar vasodilation (via the cAMP pathway) and increases vascular permeability (by causing separations in the junctions between the endothelial cells that line the vascular system), which leads to edema formation and local tissue swelling. Other effects that histamine plays include stimulation of sensory nerve endings to produce itching and pain sensation.

Bradykinin is a small polypeptide that has approximately ten times the vasodilator potency of histamine on a molar basis. It also acts to increase capillary permeability by opening the junctions between endothelial cells. Bradykinin is formed from certain plasma globulin substances by the action of an enzyme, kvllikrein, and is subsequently rapidly degraded into inactive fragments by various tissue kinases.

Transmural pressure. The effect of transmural pressure on arteriolar diameter is more complex because arterioles respond both passively and actively to changes in transmural pressure. For example, a sudden increases in the internal pressure within an arteriole produces: 1.first an initial slight passive mechanical distention, and 2.then an active constriction that, within seconds, may completely reverse the initial distention. A sudden decrease in transmural pressure elicits essentially the opposite response, that is, an immediate passive decrease in diameter followed shortly by a decrease in active tone, which returns the arteriolar diameter to near that which existed before the pressure change. The active phase of such behavior is referred to as a myogenic response, because it seems to originate within the smooth muscle itself. The mechanism of the myogenic response is not known for certain, but stretch-sensitive ion channels on arteriolar vascular smooth muscle cells are likely candidates for involvement.


Examples of Local Regulation

Active Hyperemia – In organs with a highly variable metabolic rate, such as skeletal and cardiac muscles, the blood flow closely follows the tissue's metabolic rate. For example, skeletal muscle blood flow increases within seconds of the onset of muscle exercise and returns to control values shortly after exercise ceases. This phenomenon, which is illustrated in Figure 7-3A, is known as exercise or active hyperemia. Active hyperemia could be explained by mechanisms related to local metabolic theory and to local flow-related shear stresses theory.Screen Shot 2015-07-17 at 8.11.32 PM

Reactive Hyperemia – In this case, the higher-than-normal blood flow occurs transiently after the removal of any restriction that has caused a period of lower-than-normal blood flow and is sometimes referred to as post occlusion hyperemia. The phenomenon is illustrated in Figure 7-3B. For example, flow through an extremity is higher than normal for a period after a tourniquet is removed from the extremity. Both local metabolic and myogenic mechanisms may be involved in producing reactive hyperemia.

Autoregulation talks about the arterioles' reaction to the changes of the perfusion pressure. Except when displaying active and reactive hyperemia, nearly all organs tend to keep their blood flow constant despite variations in arterial pressure – that is, they autoregulate their blood flow. For example, an abrupt increase in arterial pressure is normally accompanied by an initial abrupt increase in organ blood flow that then gradually returns toward normal despite the sustained elevation in arterial pressure. The later autoregulation that returns the flow toward the normal level is caused by a gradual increase in active arteriolar tone and resistance to blood flow. Ultimately, a new steady state is reached with only slightly elevated blood flow because the increased driving pressure is counteracted by a higher-than-normal vascular resistance. The mechanisms for autoregulation are believed to be both local metabolic feedback theory and myogenic theory. Also, tissue pressure hypothesis of blood flow auto regulation for which it is assumed that an abrupt increase in arterial pressure causes transcapillary fluid filtration and thus leads to a gradual increase in interstitial fluid volume and pressure. Presumably the increase in extravascular pressure would cause a decrease in vessel diameter by simple compression. This mechanism might be especially important in organs such as the kidney and brain whose volumes are constrained by external structures.

 

Pathophysiology of Selected Vascular Disorders

June 2, 2014 Cardiology, Physiology and Pathophysiology 1 comment , , , ,

CaduceusAtherosclerosis

A condition that afflicts the large and medium-sized arteries of almost every human, at least in societies in which cholesterol-rich foodstuffs are abundant and cheap, is atherosclerosis. This condition begins in childhood and, in the absence of acelerating factors, develops slowly until it is widespread in old age. However, it is accelerated by a wide variety of genetic and environmental factors.

Atherosclerosis is characterized by localized fibrous thickenings of the arterial wall associated with lipid-infiltrated plagques that may eventually calcify. Old plaques are also prone to ulceration and rupture, triggering the formation of thrombi that obstruct flow.

Risk Factors Accelerating Atherosclerosis

First let see the risk factors that accelerate the progression of atherosclerosis, since treating the accelerating conditions that are treatable and avoiding those that are avoidable should reduce the incidence of myocardial infarctions, strokes, and other complications of atherosclerosis.

Detail mechanisms of lipid metabolism can be found here http://forum.tomhsiung.com/physiology-and-pathophysiology/402-the-pathway-of-lipids.html

Estrogen

Estrogen increases cholesterol removal by the liver, and the progression of atherosclerosis is less rapid in premenopausal women than in men. In addition, epidemiologic evidence shows that estrogen replacement therapy protects the cardiovascular system in postmenopausal women. But, in several studies, estrogen treatment of postmenopausal women failed to prevent second heart attacks (note here, it was secondary prevention, not primary prevention).

Homocysteine and related molecules

The effect of increased plasma levels of homocysteine and related molecules such as homocystine and homocysteine thiolactone, a condition sometimes called hyperhomocystinemia, deserves emphasis. These increases are assoicated with accelerated atherosclerosis, and the magnitude of the plasma elevation is positively correlated with the severity of the atherosclerosis. Markedly elevated levels resulting from documented mutations of relevant genes are rare, but mild elevations occur in 7% of the general population.

The mechanism responsible for the accelerated vascular damage is unsettled, but homocysteine is a significant source of H2O2 and other reactive forms of oxygen, and this may accelerate the oxidation of LDL.

Cholesterol and triglyceride

Evidence is now overwhelming that lowering plasma cholesterol and triglyceride levels and increasing plasma HDL levels slows, and in some cases reverses, the atherosclerotic process. For the ways to lowering serum cholesterol and triglyceride (including how to elevate HDL) please refer the thread describing the management of dyslipidemia.

Smoking

Men who smoke a pack of cigarettes a day have a 70% increase in death rate from ischemic heart disease compared with nonsomokers, and there is also an increase in women. The deleterious effects of smoking include endothelial damage caused by carbon monoxide-induced hypoxia. Other factors may also be involved. In general, these deleterious increase the risk of atherosclerosis.

Blood pressure

Because of the increased shear stress imposed on the endothelium by an elevated blood pressure, hypertension is another important modifiable risk factor for atherosclerosis.

Diabetes

In diabetes, there are microvascular complications and macrovascular complications. These complications are shown below:

Others

The nephrotic syndrome and hypothyroidism also accelerate the progression of atherosclerosis.

Pathogenesis of Atherosclerosis

The initial event in atherosclerosis is infiltration of LDLs into the subendothelial region. The endothelium is subject to shear stress, the tendency to be pulled along or deformed by flowing blood. This is most marked at points where the arteries brach, and this is where the lipids accumulate to the greatest degree.

The LDLs are oxidized or altered in other ways. Thus, altered LDLs activate various components of innate immune system including macrophages, natural antibodies, and innate effector proteins such as C-reactive protein and complement. Altered LDLs are recognized by a family of scavenger receptors expressed on macrophages. These scavenger receptors mediate uptake of the oxidized LDL into macrophages and the formation of foam cells. The foam cells form fatty streaks.

The streaks appear in the aorta in the first decade of life, in the coronary arteries in the second decade, and in the cerebral arteries in the third and fourth decades.

Oxidized LDLs have a number of deleterious effects, including stimulation of the release of cytokines and inhibition of NO production. Vascular smooth muscle cells in the vicinity of foam cells are stimulated and move from the media to the intima, where they proliferate, lay down collagen and other matrix molecules, and contribute to the bulk of the lesion. Smooth muscle cells also take up oxidized LDL and become foam cells.

Lipids accumulate both intracellularly and extracellularly.Screen Shot 2015-10-18 at 1.31.03 PM

As the atherosclerotic lesions age, T cells of the immune system as well as macrophages are attracted to them. The intercellular “soup” in the plaques contains a variety of cell-damaging substances, including ozone. Overally, the lesions have been shown to have many of the characteristics of a low-grade infection.

Growth factors and cytokines involved in cell migration and proliferation are also produced by smooth muscle cells and endothelial cells, and there is evidence for shear stress response elements in the flanking DNA of relevant genes in the endothelial cells. Major investigations found bacteria in plaques – Chlamydophila pneumoniae, whereas other organisms have also been found.

As plaques mature, a fibrous cap forms over them. The plaques with defective or broken caps are most prone to rupture. The lesions alone may distort vessels to the point that they are occluded, but it is usually rupture or ulceration of plaques that triggers thrombosis, blocking blood flow.

A characteristic of atherosclerosis that is currently receiving considerable attention is its association with deficient release of NO and defective vasodilation. As noted, oxidized LDLs inhibit NO production. If acetylcholine is infused via catheter into normal coronary arteries, the vessels dialte; however, if it is infused when atherosclerosis is present, the vessels constrict. This indicates that endothelial secretion of NO is defective.

PS: Acetylcholine (ACh) can effect vasodilation by several mechanisms, including activation of endothelial nitric oxide (NO) synthase and prostaglandin (PG) production[1].

References:

1.Kellogg DL Jr1, Zhao JL, Coey U, Green JV. Acetylcholine-induced vasodilation is mediated by nitric oxide and prostaglandins in human skin. J Appl Physiol (1985). 2005 Feb;98(2):629-32. [PMID: 15649880]


Hypertension

Current guidelines of Joint Natinal Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure define normal blood pressure as systolic pressure of <120 mm Hg and diastolic pressure of <80 mm Hg. Hypertension is defined as an arterial pressure greater than 140/90 mm Hg in adults on at least three consecutive visits to the doctor’s office.

The most common cause of hypertension is increased peripheral vascular resistance. However, because blood pressure equals total peripheral resistance times cardiac output, prolonged increase in cardiac output can also cause hypertension. These are seen, for example, in hyperthyroidism and beriberi.

In addition, increased blood volume causes hypertension (see below), especially in individuals with mineralocorticoid excess or renal failure; and increased blood viscosity (blood resistance increases with viscosity), if it is marked, can increase arterial pressure.

PS: Cardiac output is a function of stroke volume, heart rate, and venous capacitance. Increased blood volume increase cardiac preload, which causes increase in stroke volume, and finally the cardiac output and arterial pressure.

Pathogenesis of Hypertension