Month: June 2016

Regulation of Sodium Excretion

June 25, 2016 Cardiology, Critical Care, Nephrology, Physiology and Pathophysiology No comments , , , , , , , , , , , , , , , , , , , , , , , , ,

Percentage of Sodium Reabsorbed

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The Goals of Regulation

The overriding goals of regulating sodium and water excretion are to support the requirements of the cardiovascular system. This is manifested in 3 ways: 1.the kidneys maintain a sufficient ECF volume to fill the vascular space (mean circulatory filling pressure); 2.keep the osmolality of the ECF at a level consistent with cellular health; and 3.limit the changes in renal blood flow (RBF) and GFR that might otherwise reach deleterious levels. The kidneys and the CV system work cooperatively to ensure that peripheral tissue is sufficiently perfused. An adequate circulating volume is one of the essential requirements for tissue perfusion and it is the kidneys that control this volume. Osmolality is the ratio of solute content to water content. Sodium and chloride together account for 80% of the normal extracellular solute; thus the excretion of sodium and water by the kidneys regulates osmolality in the tight range that is needed for the health of tissue cells. There is a separate goal of regulation that differs from those stated above. Variations in RBF and GFR are major means of regulating sodium excretion. However, the kidney cannot change blood flow and filtration to such extreme values that they compromise the metabolic health of the kidneys or interfere with the excretion of substances other than sodium, particularly organic waste.

Formulas for ECF Volume

There are some formulas showing the relationship between ECF solute content, ECF osmolality, and ECF volume. Since almost all of the ECF solute is accounted for by sodium and an equivalent number of anions (mostly chloride and bicarbonate), the amount of ECF solute is approximately twice the sodium content.

ECF osmolality = ECF solute content / ECF volume (Equation 7-1)

ECF volume = ECF solute content / ECF osmolality (Equation 7-2)

ECF volume ≈ 2 x Na content / ECF osmolality (Equation 7-3)

Therefore, in the face of tightly controlled ECF osmolality, ECF volume varies directly with sodium content. But how do the kidneys know how much sodium there is in the ECF? The detection of sodium content is indirect, based on a combination of assessing sodium concentration and vascular pressures. Glial cells in regions of the brain called the circumventricular organs have sensory Na+ channels that respond to and act as detectors of extracellular sodium concentration. The glial cells modulate the activity of nearby neurons involved  in the control of body sodium. There are also neurons in the hypothalamus contain the sensory Na+ channels that respond to the sodium concentration in the cerebrospinal fluid. Thus cells in or near the hypothalamus monitor extracellular sodium concentration.

The volume affects pressure in different regions of the vasculature. It is the presssure baroreceptors in these regions of the vasculature detect the vascular pressures.

Major Controllers of Sodium Excretion

Sympathetic Stimulation 

Vascular pressures are so important in regard to sodium excretion and because volume affect pressure in different regions of the vasculature, so the changes in ECF affects pressures (arterial and/or venous) and changes in pressure affect sodium excretion (Thread "Regulation of Arterial Pressure" at http://www.tomhsiung.com/wordpress/2016/06/physiology-regulation-of-arterial-pressure/ and thread "Mean Circulatory Filling Pressure and CVP" at http://www.tomhsiung.com/wordpress/2016/06/mean-circulatory-filling-pressure-and-cvp/).

The vasculature and tubules of the kidney are innervated by postganglionic sympathetic neurons that release norepinephrine. In most regions of the kidney, norepinephrine is recognized by alpha-adrenergic receptors. In the renal vasculature activation of alpha1-adrenergic recpetors causes vasoconstriction of afferent and efferent arterioles. This reduces RBF and GFR.

GFR is a crucial determinant of sodium excretion. However, except in body emergencies such as hypovolemic shock, GFR is kept within rather narrow limits due to autoregulatory processes (detail for vascular autoregulatory regulation http://www.tomhsiung.com/wordpress/2015/07/arteriolar-tone-and-its-regulation-local-mechanisms/). Thus although neural control does affect GFR, this component of sympathetic control is probably less important in normal circumstances than its effect on sodium reabsorption. Neural control of the renal vasculature is exerted primarily on blood flow in the cortex, allowing preservation of medullary perfusion even when cortical blood flow is reduced.

The proximal tubule epithelial cells are innervated by alpha1- and alpha2-adrenergic receptors. Stimulation of these receptors in the proximal tubule by norepinephrine activates both components of the main transcellular sodium reabsorptive pathway, that is, the sodium-hydrogen antiporter NHE3 in the apical membrane and the Na-K-ATPase in the basolateral membrane. The effects of sympathetic stimulation on cells in the distal nephron are less straightforward. However, the overall outcome of sympathetic stimulation of the kidney is clearly reduced sodium excretion.

The Renin-Angiotensin System

AII's function

  • Reduces the RBF and GFR
  • Stimulation of sodium tubular reabsorption
  • Stimulation of the CNS: salt appetite, thirst, and sympathetic drive
  • Stimulation of aldosterone secretion

The major determinant of circulating AII is the amount of renin available to form angiotensin I.

PS: "Control of the Circulating RAAS" is ready at http://www.tomhsiung.com/wordpress/2016/06/control-of-the-circulating-raas/

AII is a potent vasoconstrictor, acting on the vasculature of many peripheral tissues, the effect of which is to raise arterial pressure. It also vasoconstricts both cortical and medullary vessels in the kidney. This reduces total RBF and reduces GFR, thus decreasing the filtered load of sodium.

AII stimulates sodium reabsorption in both the proximal tubule and distal nephron. In the proximal tubule it stimulates the same transcellular transport pathway as does norepinephrine, namely NHE3 sodium/hydrogen antiporter in the apical membrane and the Na-K-ATPase in the basolateral membrane. In the distal tubule and connecting tubule, it stimulates the activity of sodium/chloride symporters and sodium channels (ENaC) that reabsorb sodium.

AII stimulates behavioral actions in response to fluid loss that increase salt appetite and thirst. AII acts on the circumventricular organs in the brain. These function as detectors of many substances in the blood and convey information to various areas of the brain. In situations of volume depletion and low blood pressure, when circulating levels of AII are high, a key effect, in addition to vascular and tubular actions is increased thirst and salt appetite. These pathways also increase sympathetic drive.

Aldosterone is a major stimulator of sodium reabsorption in the distal nephron, that is, regions of the tubule beyond the proximal tubule and loop of Henle. We focus here on the role of aldosterone in sodium reabsorption, but aldosterone has many other important actions, including stimulation of potassium excretion and acid excretion. The most important physiological factor controlling secretion of aldosterone is the circulating level of AII, which stimulates the adrenal cortex to produce aldosterone. But keep in mind that elevated plasma potassium concentration, atrial natriuretic factors are other stimulators of aldosterone secretion. The aldosterone has enough lipid character to freely cross principal cell membrane in the collecting ducts, after which it combines with mineralocorticoid receptors (aldosterone receptors) in the cytoplasm. After being transported to the nucleus, the receptor acts as a transcription factor that promotes gene expression of specific proteins. The effect of these proteins is to increase the activity or number of luminal membrane sodium channels (ENaCs) and basolateral membrane Na-K-ATPase pumps.

Dopamine

Dopaimine inhibits sodium reabsorption in the kidney. The dopamine that acts in the kidney is not released from neurons; rather it is synthesized in proximal tubule cells from the precursor l-DOPA. l-DOPA is taken up from the renal circulation and glomerular filtrate and converted to dopamine in the proximal tubule epithelium, and then released to act in a paracrine manner on nearby cells. Although the signaling path is not clear, it is known that increases in sodium intake lead to increased production of intrarenal dopamine. Dopamine has 2 actions, both of which reduce sodium reabsorption. First, it causes retraction of NHE antiporters and Na-K-ATPase pumps into intracellular vesicles, thereby reducing transcellular sodium reabsorption. Second, it reduces the expression of AII receptors, thereby decreasing the ability of AII to stimulate sodium reabsorption.

Other Controllers of Sodium Excretion

ADH

When ADH binds to V2 reecptors in tubular cells, it increases the production of c-AMP. This results in increased activity of the NKCC multiporter in the thick ascending limb and increased sodium channel (ENaC) presence in principal cells of the distal nephron, thereby increasing the uptake of sodium that, in both regions, is actively transported into the interstitium by the Na-K-ATPase. Interestingly, in the distal nephron the mechanism proceeds, not by moving ENaCs into the membrane, but rather by decreasing their removal and degradation.

Glomerulotubular Balance

Glomerulotubular balance (not to be confused with TG feedback described previously) refers to the phenomenon whereby sodium reabsorption in the proximal tubule varies in parallel with the filtered load, such that approximately two thirds of the filtered sodium is reabsorbed even when GFR varies. The mechanism by which reabsorption varies with filtered load appears to be via mechanotransduction by the microvilli on the apical surface of the proximal tubule cells, similar in principle to mechanotransduction by primary cilia in the macula densa. As flow changes, the amount of bending of the microvilli changes, and this is converted by cellular mechanisms into changes in transport.

Pressure Natriuresis and Diuresis

Because the kidneys are responsive to arterial pressure, there are situations in which elevated blood pressure can lead directly to increased excretion of sodium. This phenomenon is called pressure natriuresis, and because natriuresis is usually accompanied by water, it is often called pressure diuresis. This is an intrarenal phenomenon, not requiring external signaling. However, external signals normally override pressure natriuresis.

Natriuretic Peptides

Several tissues in the body synthesize members of a hormone family called natriuretic peptides. Key among these are atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP). The main source of both natriuretic peptides is the heart. The natriuretic peptides have both vascular and tubular actions. The relax the afferent arteriole, thereby promoting increased filtration, and act at several sites in the tubule. They inhibit release of renin, inhibit the actions of AII that normally promote reabsorption of sodium, and act in the medullary collecting duct to inhibit sodium absorption. The major stimulus for increased secretion of the natriuretic peptides is distention of the atria, which occurs during plasma volume expansion. This is probably the stimulus for the increased natriuretic peptides that occurs in persons on a high salt diet.

Control of the Circulating RAAS

June 23, 2016 Cardiology, Critical Care, Nephrology, Physiology and Pathophysiology No comments , , , , , , , , ,

The activity of the circulating RAAS is governed by the amount of renin secreted by the granular cells of the jg (juxtaglomerular) apparatus. There are 3 major controllers of renin secretion.

PS: Look at the RAAS, plasma angiotensinogen is synthesized in the liver and plasma angiotensinogen levels are normally high therefore do not limit the production of AII. Furthermore, ACE is expressed on the endothelial surfaces of the vascular system, particularly the pulmonary vessels, and avidly converts most of the angiotensin I into AII. Therefore, the major determinant of circulating AII is the amount of renin available to form angiotensin I.

The first contoller is sympathetic input. Norepinephrine released from postganglionic sympathetic neurons acts on beta1-adrenergic receptors in the granular cells. This activates a c-AMP-mediated pathway that causes the release of renin. The granular cells are quite sensitive to norepinephrine and respond to low levels of sympathetic activity that may have minimal direct effect on the renal vasculature or sodium transport.

The second controller of renin secretion is pressure in the afferent arteriole. The granular cells not only respond to vascular pressures indirectly via adrenergic stimulation, they respond directly to changes in afferent arteriolar pressure. When pressure in the afferent arteriole decreases, renin production increases. Except in cases of major renal arterial blockage, pressure in the arteriolar lumen at the granular cells is close to systemic arterial pressure and changes in parallell with it. Because the granular cells respond to vascular pressure, they are acting as baroreceptors. In fact, the granular cells are the intrarenal baroreceptors. Even though they are not neurons and do not send afferent feedback, they are baroreceptors nevertheless. Consider what happens when arterial pressure drops. The intrarenal baroreceptors (the granular cells) sense the drop in pressure and increase their secretion of renin. Simultaneously, the drop in pressure is also sensed by the arterial baroreceptors in the carotid arteries and aorta. The fall in their afferent signaling allows the vasomotor center to increase sympathetic drive to the granular cells, resulting in a huge combined stimulation of renin secretion.

The third contoller of renin release originates from another component of the jg apparatus; namely the macula densa. The operation of the macula densa is somewhat complicated, but serves as a fascinating example of negative feedback in biological systems. The meacula densa is a detection system and initiator of feedback that helps regulate renin secretion and GFR (tubuloglomerular feedback/TG feedback). For the regulation of GFR please refer to thread "Factors That Affect GFR" at http://www.tomhsiung.com/wordpress/2014/04/factors-that-affect-gfr/. The macula densa is located at the end of the loop of Henle where the tubule passes between the afferent and efferent arterioles of Bowman's capsulre. It is able to sense flow and salt content in the tubular lumen that are the net result of filtration and reabsorption in tubular elements preceding it, that is, it sense "everything done so far." Flow is sensed by cilia that project into the tubular lumen from macula densa cells. Bending of the cilia initiates intracellular signaling that leads to release of paracrine mediators. Tubular sodium chloride is sensed by uptake via Na-K-2Cl multiporters whose action changes ionic concentrations within the macula densa cells and also causes release of paracrine mediators.

When tubular flow and sodium content are high it is as if "the body has too much sodium" and "GFR is too high." The mediators released by the macula densa reduce the secretion of renin (thereby allowing more sodium excretion) and decrease GFR (restoring GFR to an appropriate level). The immediate mediators is ATP, which is converted extracellularly to adenosine. One or both bind to purinergic receptors on the nearby granular cells. This has the effect of increasing intracellular calcium and reducing the release of renin. In turn, the reduction in renin secretion reduces the levels of AII and allows the kidneys to excrete more of the filtered sodium. Simultaneously, the adenosine binds to purinergic receptors on afferent arteriole smooth muscle. The subsequent rise in calcium in these cells stimulates contraction, thus reducing pressure and flow through the glomerular capillaries and reducing GFR.

What happens in the opposite case? Now "the body has too little sodium" and "GFR is too low." This initiates the release of different mediators, specifically prostaglandins and nitric oxide. In the granular cells the prostaglandins stimulate or prolong the lifetime of c-AMP, thereby stimulating the release of renin. In the afferent arterioles NO is a dilator of smooth muscle. The effect is to raise flow and pressure in the glomerular capillaries, and restore GFR to an appropirate level.

Urinary Concentration – The Medullary Osmotic Gradient

June 22, 2016 Anatomy, Critical Care, Hemodynamics, Histology, Nephrology, Physiology and Pathophysiology No comments , , , , , , , , , , , , , ,

The kidneys can produce a range of urine osmolalities depending on the levels of ADH. The production of hypo-osmotic urine is an understandable process: The tubules (particularly the thick ascending limb of Henle's loop) reabsorb relatively more solute than water, and the dilute fluid that remains in the lumen is excreted. The production of hyperosmotic urine is also straightfoward in that reabsorption of water from the lumen into a hyperosmotic interstitium concentrates that luminal fluid, leaving concentrated urine to be excreted.

The Mechanism to Generate Medullary Osmotic Gradient

There is a gradient of osmolality (hyperosmotic), increasing from a nearly iso-osmotic value at the corticomedullary border to a maximum of greater than 1000 mOsm/kg at the papilla. The peak osmolality is variable depending on dehydration or overhydration, where highest during periods of dehydration and lowest (approximately half of that during excess hydration) during excess hydration.

In the steady state there must be mass balance, that is, every substance that enters the medulla via tubule or blood vessel must leave the medulla via tubule or blood vessels. However, during development of the gradient there are transient accumulations of solute, and during washout of the gradient there are losses. To develop the osmotic gradient in the medullary interstitium, there must be deposition of solute in excess of water. It is reabsorption of sodium and chloride by the thick ascending limb in excess of water reabsorbed in the thin descending limbs that accomplishes this task. At the junction between the inner and outer medulla, the ascending limbs of all loops of Henle, whether long or short, turn into thick regions and remain thick all the way back until they reach the original Bowman's capsules. As they reabsorb solute without water and dilute the luminal fluid, they simultaneously add solute without water to the surrounding interstitium. This action of the thick ascending limb is absolutely essential and is the key to everything else that happens. If transport in the thick ascending limb is innhibited, the lumen is not diluted and the interstitium is not concentrated, and the urine becomes iso-osmotic.

Solutes

  • For thick ascending limbs in the cortex, reabsorbed solute is taken up by abundant cortex blood flow so intersititum osmolality in the cortex approximately equal plasma
  • High concentratiion sodium level in outer medulla interstitium makes them diffuse into DVR and AVR
  • Hyperosmotic sodium in AVR can diffuse into nearby DVR – the countercurrent exchange

For those portions of the thick ascending limbs in the cortex, the reabsorbed solute simply mixes with material reabsorbed by the nearby proximal convoluted tubules. Because the cortex contains abundant peritubular capillaries and a hight blood flow, the reabsorbed material immediately moves into the vasculature and returns to the general circulation. However, in the medulla, the vascular anatomy is arranged differently and total blood flow is much lower. Solute that is reabsorbed and deposited in the outer medullary interstitium during the establishment of the osmotic gradient is not immediately removed, that is, it accumulates. The degree of accumulation before a steady state is reached is a function of the arrangement of the vasa recta, their permeability properties and the volume of blood flowing within them.

Imagine first a hypothetical situation of no blood flow. Sodium would accumulate in the outer medulla without limit, because there would be no way to remove it. But, of course the outer medulla is perfused with blood, as are all tissues. Blood enters and leaves the outer medulla through parallel bundles of descending and ascending vasa recta (DVR and AVR). These vessels are permeable to sodium. Therefore sodium enters the vasa recta driven by the rise in concentration in the surrounding interstitium. Sodium entering the ascending vessels returns to the general circulation, but sodium in the descending vessels is distributed deeper into the medulla, where it diffuses out across the endothelia of the vassa recta and the interbundle capillaries that they feed, thereby raising the sodim content throughout the medulla.

Later, the interbundle capillaries drain into ascending vasa recta that lie near descending vasa recta. The walls of the ascending vasa recta are fenestrated, allowing movment of water and small solutes between plasma and interstitium. As the sodium concentration of the medullary interstitium rises, blood in the ascending vessels also takes on an increasingly higher sodium concentration. However, blood entering the medulla always has a normal sodium concentration (approximtely 140 mEq/L). Accordingly, some of the sodium begins to re-circulate, diffusing out of ascending vessels and reentering nearbydescending vessels that contain less sodium (countercurrent exchange). So sodium is entering descending vasa recta from 2 sources – re-circulated sodium from the ascending vasa recta, and new sodium from the thick ascending limbs. Over time, everything reaches a steady state in which the amount of new sodium entering the interstitium from thick ascending limbs matches the amount of sodium leaving the interstitium in ascending vasa recta. At its peak, the concentration of sodium in the medulla may reach 300 mEq/L, more than double its value in the general circulation. Since sodium is accompanied by an anion, mostly chloride, the contribution of salt to the medullary osmolality is approximately 600 mOsm/kg.

Water

  • While solute can accumulate without a major effect on renal volume, the amount of water in the medullary interstitium must remain nearly constant; otherwise the medulla would undergo significiant swelling or shrinking.
  • Because water is always being reabsorbed from the medullary tubules into the interstitium (from descending thin limbs and medullary colelcting ducts), that water movement must be matched by equal water movement from the interstitium to the vasculature.
  • Blood entering the medulla has passed through glomeruli, thereby concentrating the plasma proteins. While the overall osmotic content (osmolality) of this blood is essentially isosmotic with systemic plasma, its oncotic pressure is considerably higher.

The challenge for the kidneys is to prevent dilution of the hyperosmotic interstitium by water reabsorbed from the tubules and by water diffusing out of the iso-osmotic blood entering the medulla. The endotheial cells of descending vasa recta contain aquaporins. So water is drawn osmotically into the outer medullary interstitium by the high salt content in a manner similar to water being drawn out of tubular elements. At first glance it seems that this allows the undesired diluting effect to actually take place. But, of course, solute is also constantly being added from the nearby thick ascending limbs. The loss of water from descending vasa recta in the outer medulla serves the useful purpose of raising the osmolality of blood penetrating the inner medulla and decreasing its volume, thereby reducing the tendency to dilute the inner medullary interstitium. The ascending vasa recta have a fenestrated endothelium, allowing free movement of water and small solutes. Since the oncotic pressure is high, water entering the interstitium of the outer medulla from descending vasa recta is taken up by ascending vasa recta and removed from the medulla. In addition, water reabsorbed from tubular elements (descending thin limbs and collecting ducts) is also taken up by ascending vasa recta and removed, thereby preserving constancy of total medullary water content.

The magnitude of blood flow in the vasa recta is a crucial variable. The peak osmolality in the interstitium depends on the ratio of sodium pumping by the thick ascending limbs to blood flow in the vasa recta. If this ratio is high (meaning low blood flow), water from the isosmotic plasma entering the medulla in descending vasa recta does not dilute the hyperosmotic interstitium. In effect the "salt wins" and osmolality remains at a maximum. But in conditions of water excess, this ratio is very low (high blood flow) and the diluting effect of water diffusing out of descending vasa recta is considerable. In part the tendency to diulte is controlled by ADH (due to its vasoconstriction effect to limit the blood flow of DVR).

Urea

The peak osmolality in the renal papilla reaches over 1000 mOsm/kg. Approximately half of this is accounted for by sodium and chloride, and most of the rest is (500-600 mOsm/kg) accounted for by urea. Urea is a very special substance for the kidney. It is an end product of protein metabolism, waste to be excreted, and also an important component for the regulation of water excretion. For the handle of urea in the kidneys: 

1.There are no membrane transport mechanisms in the proximal tubule; instead, it easily permeates the tight junctions of the proximal tubule where it is reabsorbed paracellularly.

2.Tubular elements beyond the proximal tubule express urea transporters and handle urea in a complex, regulated manner.

The gist of the renal handling of urea is the following: it is freely filtered. About half is reabsorbed passively in the proximal tubule. Then an amount equal to that reabsorbed is secreted back into the loop of Henle. Finally, about half is reabsorbed a second time in the medullary collecting duct. The net result is that about half the filtered load is excreted.

Urea does not permeate lipid layer because of its highly polar nature, but a set of uniporters transport urea in various places beyond the proximal tubule and in other sites within the body. Because urea is freely filtered, the filtrate contains urea at a concentration identical to that in plasma. In the proximal tubule when water is reabsorbed, the urea concentation rises well above the plasma urea concentration, driving diffusion through the leaky tight junctions. Roughly, half the filtered load is reabsorbed in the proximal tubule by the by the paracellular route. As the tubular fluid enters the loop of Henle, about half the filtered urea remains, but the urea concentration has increased somewhat above its level in the filtrate because proportionally, more water than urea was reabsorbed. At this point, the process becomes fairly complicated.

The interstitium of the medulla has a considerably higher urea concentration than does plasma. The concentration increases from the outer to the inner medulla. Since the medullary interstitial urea concentration is greater than that in the tubular fluid entering the loop of Henle, there is a concentration gradient favoring secretion into the lumen. The tight junctions in the loop of Henle are no longer permeable, but the epithelial membranes of the thin regions of the Henle's loops express urea uniporters, members of the UT family. This permits secretion of urea into the tubule. In fact, the urea secreted from the medullary interstitium into the thin regions of the loop of Henle replaces the urea previously reabsorbed in the proximal tubule. Thus, when tubular fluid enters the thick ascending limb, the amount of urea in the lumen is at least as large as the filtered load. However, because about 80% of the filtered water has now been reabsorbed, the luminal urea concentration is now several times greater than in the plasma. Beginning with the thick ascending limb and continuing all the way to the inner medullary collecting ducts (through the distal tubule and cortical collecting ducts), the apical membrane urea permeability (and the tight junction permeability) is essentially zero. Therefore, an amount of urea roughly equal to the filtered load remains within the tubular lumen and flows from the cortical into the medullary collecting ducts.

During the transit through the cortical collecting ducts variable amounts of water are reabsorbed, significantly concentrating the urea.We indicated eariler that the urea concentration in the medullary interstitium is much greater than in plasma, but the luminal concentration in the medullary collecting ducts is even higher (up to 50 times its plasma value), so in the inner medulla the gradient now favors reabsorption and urea is reabsorbed a second time via another isoform of UT urea uniporter. It is this urea reabsorbed in the inner medulla that leads to the high medullary interstitial concentration, driving urea secretion into the thin regions of the loop of Henle.

Effect of Respiratory Activity on Circulation

June 21, 2016 Cardiology, Critical Care, Physiology and Pathophysiology No comments , , , , , , , , ,

The physical processes associated with inhaling air into and exhaling air out of the lungs can have major effects on venous return and cardiac output. During a normal inspiration, intrathoracic pressure falls from approximately negative 2 mm Hg to approximately negative 7 mm Hg as the diaphragm contracts and the chest wall expands. It rises again by an equal amount during expiration. These periodic pressure fluctuations not only promote air movement into and out of the lungs but also are transmitted through the thin walls of the great veins in the thorax to influence venous return to the heart from the periphery. Because of the unidirectional nature of venous valves, venous return is increased more by inspiration than it is decreased by expiration. The net effect is that venous return from the periphery is generally facilitated by the periodic fluctuations in central venous pressure caused by respiration. This phenomenon is often referred to as the "respiratory pump."

Cyclical alterations in intrathoracic pressure with normal breathing evoke primary disturbances in blood flow and distribution within the cardiovascular system. Some of these disturbances and compensatory responses are illustrated in Figure 10-1. Filling of the right side of the heart is transiently increased during inspiration (due to decreased CVP and increased venous return which increases the RVEDV) and stroke volume and thus cardiac output are transiently increased. In addition, the reduction in pulmonary vascular resistance that accompanies inspiration reduces the right ventricular afterload, which contributes to a transient increase in right ventricular stroke volume. Because changes in output of the right side of the heart induce changes in output of the left side of the heart within a few beats, the net effect of inspiration will be a transient increase in stroke volume and cardiac output from the left ventricle. This will lead to a transient increase in arterial pressure and a transient increase in firing of the arterial baroreceptors. The short-term cardiovascular reflex results via arterial baroreceptors are increased parasympathetic nerve activity, decreasing sympathetic nerve activity, and a decrease in heart rate.

The inspiration-induced decrease in intrathoracic pressure will also stretch low-pressure (volume) cardiopulmonary baroreceptors in the vascular and cardiac walls and will increase their firing rate (which results in a drop in BP set point). These low-pressure baroreceptor inputs will add to the information from the high-pressure baroreceptors and promote similar pressure-lowering ouputs from the medullary cardiovascular centers.

Lung mechanoreceptors located primarily within the airways are also stretched during normal inspiration. Unlike the first two mechanisms, their input into the medullary centers results in an inhibition of the normal tonic vagal activity to the sinoatrial node, causing a transient increase in the heart rate.

[Physiology] Regulation of Arterial Pressure

June 21, 2016 Uncategorized No comments

Appropriate systemic arterial pressure is the single most important requirement for proper operation of the cardiovascular system. Without sufficient arterial pressure, the brain and the heart do not receive adequate blood flow, no matter what adjustments are made in their vascular resistance by local control mechanisms. In contrast, unnecessary demands are placed on the heart by excessive arterial pressure (afterload, see thread "Afterload and Its Components" at http://www.tomhsiung.com/wordpress/2015/10/afterload-and-its-components/). Thus, although dramatic changes in peripheral resistance and cardiac function can and do occur normally during the course of our normal daily activities, mean arterial pressure is maintained within a narrow range and is tightly regulated.

Arterial pressure is continuously monitored by various sensors located within the body. Whenever arterial pressure varies from normal, multiple reflex responses are initiated, which cause the adjustments in cardiac output, and total peripheral resistance needed to return arterial pressure to its normal value.


Short-Term Regulation of Arterial Pressure

Arterial Baroreceptor reflex

The arterial baroreceptor reflex is the single most important mechanism providing short-term regulation of arterial pressure. The efferent pathways of the arterial baroreceptor reflex are the cardiovascular sympathetic and cardiac parasympathetic nerves. The effector organs are the heart and peripheral blood vessels.

Efferent Pathways

In the sympathetic pathways, the cell bodies of the preganglionic fibers are located within the spinal cord (T1-L2). These pregangllionic neurons have spontaneous activity that is modulated by excitatory and inhibitory inputs, which arise from centers in the brainstem and descend in distinct excitatory and inhibitory spinal pathways.

In the parasympathetic system, the cell bodies of the preganglionic fibers are located within the brainstem and spinal cord (S2-S4). Their spontaneous activity is modulated by inputs from adjacent centers in the brainstem.

Afferent Pathways

Screen Shot 2016-06-06 at 9.11.08 PMSensory receptors, called arterial barorecetpros, are found in abundance in the walls of the aorta and carotid arteries. Major concentrations of these receptors are found near the arch of the aorta and at the bifurcation of the common carotid artery into the internal and external carotid arteries on either side of the neck. The receptors themselves are mechanoreceptors that sense arterial pressure indirectly from the degree of stretch of the elastic arterial walls. In general, increased stretch causes an increased action potential generation rate by the arterial baroreceptors. Baroreceptors actually sense not only absolute stretch but also the rate of change of stretch. For this reason, both the mean arterial pressure and the arterial pulse pressure affect baroreceptor firing rate. If arterial pressure remains elevated over a period of several days for some reason, the arterial baroreceptor firing rate will gradually return toward normal. Thus, arterial baroreceptors are said to adapt to long-term changes in arterial pressure. For this reason, the arterial baroreceptor reflex cannot serve as a mechanism for the long-term regulation of arterial pressure.

Action potentials generated by the carotid sinus baroreceptors travel through the carotid sinus nerves (Hering's nerve), which join with the glossopharyngeal nerves (cranial nerve IX) before entering the CNS. Afferent fibers from the aortic baroreceptors run to the CNS in the vagus nerves (cranial nerve X).

Central Integration

Much of the central integration involved in reflex regulation of the cardiovascular system occurs in the medulla oblongata in what are traditionally referred to as the medullary cardiovascular centers. The neural interconnections between the diffuse structures in this area are complex and not completely mapped. Moreover, these strucutures appear to serve multiple functions including respiratory control, for example. What is known with a fair degree of certainty is where the cardiovascular afferent and efferent pathways enter and leave the medulla. And the intermediate processes involved in the actual integration of the sensory information into appropriate sympathetic and parasympathetic responses are not well understood at present. Although much of this integration takes place within the medulla, higher centers such as the hypothalamus are probably involved as well.

The major external influence on the cardiovascular centers comes from the arterial baroreceptors which supply a tonic input to the central integration centers. Increased inputs from the arterial baroreceptors tends to simultaneously 1).inhibit the activity of the spinal sympathetic excitatory tract; 2).stimulate the activity of the spinal sympathetic inhibitory tract, and 3).stimulate the activity of parasympathetic preganglionic nerves. Thus, an increase in the arterial baroreceptor discharge rate (via increased arterial pressure and/or pulse pressure) causes a decrease in the tonic activity of cardiovascular sympathetic nerves and a simultaneous increase in the tonic activity of cardiac parasympathetic nerves.

Operation of The Arterial Baroreceptor Reflex

The arterial baroreceptor reflex is a continuously operating control system that automatically makes adjustments to prevent primary disturbances on the heart and/or vessels from causing large changes in mean arterial pressure. The arterial baroreceptor reflex mechanism acts to regulate arterial pressure in a negative feedback manner that is analogous in many ways to the manner in which a thermostatically controlled home heating system operates to regulate inside temperature despite disturbances such as changes in the weather or open windows. One should recall that nervous control of vessels is more important in some areas such as the kidney, the skin, and the splanchnic organs than in the brain and the heart muscle. Thus, the reflex response to a fall in arterial pressure may, for example, include a significant increase in renal vascular resistance and a decrease in renal blood flow without changing the cerebral vascular resistance or blood flow. The peripheral vascular adjustments associated with the arterial baroreceptor reflex take place primarily in organs with strong sympathetic vascular control.

Other Cardiovascular Reflexes and Responses

Seemingly in spite of the arterial baroreceptor reflex mechanism, large and rapid changes in mean arterial pressure do occur in certain physiological and pathological situatons. These reactions are caused by influences on the medullary cardiovascular centers other than those from the arterial baroreceptors. These inputs on the medullary cardiovascular centers arise from many types of peripheral and central receptors as well as from "higher centers" in the CNS such as hypothalamus and the cortex.

As discussed before, the analogy was made that the arterial baroreceptor reflex operates to control arterial pressure somewhat as a home heating system acts to control inside temperature. Such as system automatically acts to counteract changes in temperature caused by such things as an open window or a dirty furnace. It does not, however, resist changes in indoor temperature caused by someone's resetting of the thermostat dial – infact, the basic temperature regulating mechanisms copperate wholeheartedly in adjusting the temperature to the new desired value. The temperature setting on a home thermostat's dial has a useful conceptual analogy in cardiovascular physiology often referred to as the "set point" for arterial pressure. Most (but not all) of the influences that are about to be discussed influence arterial pressure as if they changed the arterial baroreceptor reflex's set point for pressure regulation. Consequently, the arterial baroreceptor reflex does not resist most of these pressure disturbances but actually assists in producing them.

Reflexes From Receptors in the Heart and Lungs/Cardiopulmonary Receptors (+)

A host of mechanoreceptors and chemoreceptors that can elicit reflex cardiovascular responses have been identified in atria, ventricles, coronary vessels, and lungs. The role of these cardiopulmonary receptors in neurohumoral control of the cardiovascular system is, in most cases, incompletely understood. One general function that the cardiopulmonary receptors perform is sensing the pressure (or volume) in the atria and the central venous pool. Increased central venous pressure and volume cause receptor activation by stretch, which elicits a reflex decrease in sympathetic activity.

Chemoreceptor Reflexes (+)

Low PO2 and/or high PCO2 levels in the arterial blood cause reflex increases in respiratory rate and mean arterial pressure. These responses appear to be a result of increased activity of arterial chemoreceptors, located in the carotid arteries and the arch of the aorta, and central chemoreceptors, located somewhere within the CNS. Chemoreceptors probably play little role in the normal regulation of arterial pressure because arterial blood PO2 and PCO2 are normally held very nearly constant by respiratory control mechanisms.

  • Arterial PO2 and PCO2
  • Cerebral ischemic response

Reflexes From Receptors in Exercising Skeletal Muscle

Reflex tachycarida and increased arterial pressure can be elicited by stimulation of certain afferent fibers from the skeletal muscle. These pathways may be activated by chemoreceptors responding to muscle ischemia (more accurately, low PaO2 and/or high PaCO2), which occur with strong, sustained static (isometric) exercise. This input may contribute to the marked increase in blood pressure that accompanies such isometric efforts. It is uncertain as to what event this reflex contributes to the cardiovascular responses to dynamic (rhythmic) muscle exercise.

Dive Reflex

Aquatic animals respond to diving with a remarkable bradycardia and intense vasoconstriction in all systemic organs except the brain and the heart. A similar but less dramatic dive reflex can be elicited in humans by simply immersing the face in water (Cold water enhances the response). The response involves the unusual combination of bradycardia produced by enhanced cardiac parasympathetic activity and peripheral vasoconstriction caused by enhanced sympathetic activity. This is a rare exception to the general rule that sympathetic and parasympathetic nerves are activated in reciprocal fashion.

Cardiovascular Responses Associated with Emotion (+)

Cardiovascular responses are frequently associated with certain states of emotion. These responses include blushing, fight or flight, vasovagal syncope, etc., which originate in the cerebral cortex and reach the medullary cardiovascular centers through corticohypothalamic pathways.

Central Command (+)

The term central command is used to imply an input from the cerebral cortex to lower brain centers during voluntary muscle exercise. In the absence of any other obvious causes, central command is at present the best explanation as to why both mean arterial pressure and respiration increase during voluntary exercise.

Reflex Responses to Pain (+)

Pain can have either a positive or a negative influence on arterial pressure. Generally, superficial or cutaneous pain causes a rise in blood pressure in a manner similar to that associated with the alerting response and perhaps over many of the same pathways. Deep pain from receptors in the viscera or joints, however, often causes a cardiovascular response similar to that which accompanies vasovagal syncope, that is, decreased sympathetic tone, increased parasympathetic tone, and a serious decrease in blood pressure. This response may contribute to the state of shock that often accompanies crushing injuries and/or joint displacement.

  • Cutaneous pain
  • Deep pain

Temperature Regulation Reflexes

Temperature regulation responses are controlled primarily by the hypothalamus, which can operate through the cardiovascular centers to discretely control the sympathetic activity to regulate vasoconstriction of cutaneous vessels and thus skin blood flow. The sympathetic activity to cutaneous vessels is extremely responsive to changes in hypothalamic temperature.


Long-Term Regulation of Arterial Pressure

Long-term regulation of arterial pressure is a topic of extreme clinical relevance because of the prevalence of hypertension in our society. The most long-standing and generally accepted theory of long-term pressure regulation is that it crucially involves the kidneys, their sodium handling, and ultimately the regulation of blood volume. This theory is sometimes referred to as the "fluid balance" model of long-term arterial blood pressure control. In essence, this theory asserts that in the long term, mean arterial pressure is whatever it needs to be to maintain an appropriate blood volume through arterial pressure's direct effects on renal function.

Circulating blood volume can influence arterial pressure because:

Blood volume (decreases) –> peripheral venous pressure (down) –> left shift of venous function curve –> CVP (down) –> SV (decreases) –> CO (decreases) –> arterial pressure (down)

A fact yet to be considered is that arterial pressure has a profound influence on urinary output rate and thus affects total body fluid volume. Because blood volume is one of the components of the total body fluid, blood volume alterations accompany changes in total body fluid volume. The mechanisms are such that an increase in arterial pressure causes an increase in urinary output rate (due to decreased ADH secretion) and thus a decrease in blood volume. But decreased blood volume tends to lower arterial pressure. Thus, the complete sequence of events that are initiated by an increase in arterial pressure can be listed as follows:

Arterial pressure (increases) –> urinary output rate (up) –> fluid volume (decreases) –> blool volume (decrease) –> cardiac output (down) –> arterial pressure (decreases)

Note the negative feedback nature of this sequence of events: increased arterial pressure leads to fluid volume depletion, which tends to lower arterial pressure. Conversely, an initial disturbance of decreased arterial pressure would lead to fluid volume expansion, which would tend to increase arterial pressure. Because of negative feedback, these events constitute a fluid volume mechanism for regulating arterial pressure.

Remember that the arterial baroreceptor reflex is very quick to counteract disturbances in arterial pressure, but for fluid volume mechanism hours or even days may be required before a change in urinary output rate produces a significant accumulation or loss of total body fluid volume. Whatever this fluid volume mechanism lacks in speed, however, it more than makes up for that in persistence. As long as there is any inequality between fluid intake rate and the urinary output rate, fluid volume is changing and this fluid volume mechanism has not completed its adjustment of arterial pressure. The fluid volume mechanism is in equilibrium only when the urinary output rate exactly equals the fluid intake rate. In the long term, the arterial pressure can only be that which makes the urinary output rate equal to the fluid intake rate.