Percentage of Sodium Reabsorbed
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
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
- 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.
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
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 (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.
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.