Year : 2009 | Volume
: 20 | Issue : 4 | Page : 560--569
Sodium balance-an integrated physiological model and novel approach
Salford Royal Foundation Trust, Manchester, United Kingdom
13, Heathfield Close, Sale M33 2PQ
Various physiological mechanisms regulate sodium and water balance in the human body. These processes achieve acute and chronic sodium regulation and the simultaneous or sequential changes can be explained using a single physiological model. Steady intracellular water and osmolality is necessary for cell membrane integrity and cellular processes. Body fluids protect circulatory blood volume by altering Na + and water balance. This is the most vital homeostatic function of the body. Changes in ECF volume are sensed by various cardinal sensors. Physiologically, the main aim of Na + and water balance is to permit variable salt and water intake without large fluctuations in blood pressure or volume status. Homeostatic processes act in an integrated fashion to protect against any perturbations. Characteristically, these mechanisms are sequential as well as parallel. These may be synergistic or antagonistic to each other. Rapidity, sensitivity and potency of these powerful feedback systems differ. Various physiological and pathological insults determine the magnitude of response of these systems.
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Patel S. Sodium balance-an integrated physiological model and novel approach.Saudi J Kidney Dis Transpl 2009;20:560-569
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Patel S. Sodium balance-an integrated physiological model and novel approach. Saudi J Kidney Dis Transpl [serial online] 2009 [cited 2014 Oct 31 ];20:560-569
Available from: http://www.sjkdt.org/text.asp?2009/20/4/560/53242
Various physiological mechanisms regulate sodium and water balance in the human body. These processes achieve acute and chronic sodium regulation by simultaneous or sequential changes. Sodium homeostasis can be explained by using a single physiological model.
Water is an important biological solvent that provides an ideal environment for biochemical reactions. Intracellular (IC) water is 2/3 of total body (TB) water. The remaining 1/3 of TB water is extracellular (EC) of which plasma is 1/4 and interstitial (IS) is 3/4. The main barrier between intravascular (IV) and IS fluid is the capillary endothelium and between the IS and IC fluid compartment is cell membrane. Water moves freely across these membranes from low to high solute concentration.
Sodium (Na + ) is restricted mainly to extracellular fluid (ECF), and is a major determinant of its osmolality and volume. Normal body Na + content is essential to maintain ECF homeostasis. The fraction of ECF volume in the vascular system partly determines pressure on both arterial and venous circulations. Adequate cardiac output and mean arterial blood pressure are major determinants of Na + and water homeostasis.  Thus, mechanisms that regulate Na + , water, and blood pressure form a loop, which is completed on one side by "sensors" and on the other by "effectors" [Figure 1].
The physiological factors that protect balance of Na + , water, adequate blood pressure and cardiac output interact in an integrated and interdependent fashion, [Figure 2]. 
Cellular Water and Sodium Control
Steady intracellular water and osmolality are necessary to ensure normal cell membrane integrity and cellular processes. In health IC osmolality is constant so IC water movement is usually due to changes in EC osmolality. water redistribution occurs if there is an osmotic disequilibrium across the cell membrane, i.e. an increase or decrease in Na + concentration in ECF. As a result, cells shrink or swell in order to adapt to changes in osmolality of the environment.  On the other hand, if osmolality is increased equally in all body fluids due to urea, cellular volume does not change because an osmotic gradient does not develop.
Acute osmolar stresses elicit electrolytes transportation across the cell membrane and return the cell volume back to normal within minutes. Cells adapt to chronic changes in tonicity by altering their osmolal constituents. Chronic ECF tonicity causes the accumulation or loss of small IC organic molecules termed osmolytes (ex. sorbitol) and amino acids. Both compensatory mechanisms occur over a different time scale. Clinically, correction of Na + imbalance should be carried out gradually to allow time for reversal of these compensatory processes. Rapid reversal of serum Na + level back to normal can cause serious cellular water, and volume changes particularly in the neuronal cells of the brain.
Role of Na + -K + -ATPase pump
The Na + -K + -ATPase pump is a membrane bound enzyme that carries out the active electrogenic translocation of Na + and K ions across the plasma membrane of most cells. In a normal isoosmolar environment, the cellular volume is maintained by Na + -K + -ATPase.  It indirectly controls ionic balance and epithelial transport. There are hundreds to thousands of Na + -K +ATPase pumps per cell bound to its membrane. Na + -K + -ATPase pumps indirectly control the cell water at a "local" level. The activity of this enzyme is under multifactorial control. At the cellular level, changes in cytoplasmic concentration of Na + , K and calcium can modulate the activity of the pumps. Various hormones, notably aldosterone, increase the activity and numbers of these pumps. Their function is also affected by diseases and drugs.
One of the main functional activities of the Na + -K + -ATPase pumps is to create an electrochemical gradient in luminal cells of the intestine and nephron.
Role of the Cardiovascular System
Body fluids protect circulatory blood volume by altering Na + and water balance. This is the most vital homeostatic function of the body. Changes in ECF volume are sensed by various cardinal sensors. These sensors in turn send signals, which lead to various effective changes via neural, hormonal and physical responses. In fact, as mentioned earlier, the mean arterial blood pressure is a major determinant of Na + output [Figure 2].
Plasma and ISF are continuously interchanged because of starling forces (SF) operating at the capillary level. These forces are hydrostatic and osmotic within the vascular and interstitial compartments. Dynamically, the balance between these two opposite forces determines distribution of water between the interstitial fluid and plasma.
Single layer capillary endothelial cells permit free movement of water and other solutes. Outward movement of fluid from capillaries at the arteriolar end occurs at one liter per hour because hydrostatic forces are greater than osmotic forces. About 85% of this filtered fluid is returned directly at the venule end as hydrostatic pressure falls. The remaining almost 15% (nearly 3.5 to 4.0 L per day) returns to the circulation via the lymphatic system.
Fluid exchange in capillary beds varies in different tissues and under different conditions. Capillaries can be opened up and closed off by precapillary sphincters. The balance of these forces across the capillaries can be disturbed by diseases such as cardiac failure and nephrotic syndrome. Consequently, fluid accumulates abnormally in the interstitial spaces. Starling forces and their importance in renal function are discussed later.
Effective Circulatory Blood Volume (ECBV)
This physiological concept, which cannot be exactly quantified and measured, is related to "effective" perfusing pressure within tissues. ECBV normally relates to ECF volume, systemic blood pressure and cardiac output.  If ECBV is low and tissue perfusion inadequate, various Na + and water retaining mechanisms are activated in order to "normalize" the ECBV. For example, in diseases such as cirrhosis and congestive cardiac failure, excess Na + and water is retained in the body. The reduction in ECBV may be due to a pump problem (ex. cardiac failure) or disturbances in starling forces (ex. cardiac failure, nephrotic syndrome) [Figure 2].
Low pressure (volume) sensors
Right and left atrial myocytes release atrial natriuretic peptide (ANP) in response to stretch or distension.  This endogenous peptide hormone is released if there is 5 to 10% increase in BV. ANP has various physiologic effects. Low pressure sensors signal the medulla and result in a decrease in sympathetic activity in response to the increased BV. The opposite occurs if hypovolemia develops. Volume sensors are also present in pulmonary vessels. Additionally, in the presence of hypovolemia and hypotension, low pressure sensors also mediate ADH release via vagal afferents to the medulla [Figure 2].
High Pressure sensors
In response to fluctuations in the mean arterial blood pressure, baroreceptors located in the carotid sinus and aortic arch send signals to medulla via the glossopharyngeal and vagus nerves, respectively. This reflex acts to modulate autonomic nervous system activity rapidly. These high pressure receptors also influence ADH release if ECF loss is severe enough to affect the blood pressure.
It is clear that pathways, which mediate hemodynamic regulation of ADH release via baroreceptors, are entirely different from those in 6 volved in osmoregulation [Figure 2].
Renal and Neuroendocrine Mechanisms
The kidney has two important functions for Na + and water balance: filtration and reabsorption. Multiple local and systemic mechanisms directly or indirectly achieve these functions. Normally filtration is autoregulated, so it is the reabsorptive mechanisms that adjust to variable input and output.
Every minute 125 mL (180 L/day) of filtrate containing 17 mmoL of Na + (daily 25,000 mmoLs) enters the proximal tubule (PT); 99% is reabsorbed and 1% excreted. The kidney can handle large variation in salt and water intake with enormous efficiency. Daily, it can excrete 0.5 to 25 L of urine with osmolality varying from 40-1400 mosm/L. Thus, depending on the demands from the body for conservation or excretion, urine volume can vary 50-fold and urine osmolality 35-fold.
Reabsorption of filtered Na + load varies quantitatively and qualitatively in the different parts of the nephron; 65%, 20%, 10% and 4% of filtered Na + is reabsorbed from the PT, thick ascending part of loop of Henle (aLOH), the distal tubule (DT), and collecting duct (CD) respectively. There is an abundance of Na + -K +ATPase pumps at the basolateral membrane of all cells in the nephron, which actively shift Na + out of the cells. At the luminal border an electrochemical gradient drives Na + into the cells via Na + -cotransporters in the PT, Na+-K+-2 Cl¯cotransporters in the thick aLOH, Na+- Cl¯cotransporters in the DT and epithelial Na + channels in the CD [Figure 2]. Na + is also reabsorbed in a significant amount via paracellular pathway in later part of proximal tubule. Na + is also exchanged with H + in via Na + - H + exchanger in the PT, which are stimulated by angiotensin II.  Water diffuses passively from all parts of the nephron except the LOH.
In the kidney, starling forces at the glomeruli control glomerular filtration rate. The hydrostatic and colloid osmotic forces of the renal interstitium provide an important link between circulatory function and renal tubular reabsorption.  Hydrostatic and oncotic forces within the peritubular capillaries (PTC) also influence reabsorption.  Conversely, these reabsorptive mechanisms alter the fluid in the medulla by changing its osmolality.
Rapid and large volume solute reabsorption in the early part of the PT is favored by high oncotic pressures in the peritubular capillaries (PTC), which originate from the efferent arteriole. PTC in this part of nephron has a high concentration of proteins because they are not filtered. Relative afferent and efferent arterioles (AA/EA) vosoconstriction or vasodilatation can alter hydrostatic forces both within glomeruli and PTC. Such changes in arterioles modify the filtration into and reabsorption from the PT. For example, selective EA constriction increases both filtered (by an increase of hydrostatic force in glomeruli) and reabsorbed (by a decrease of hydrostatic force in the PTC) load. Moreover, both filtered and reabsorbed loads increase in response to selective afferent arteriolar vasoconstriction (hydrostatic forces are reduced both within glomeruli and PTC).
Medullary blood flow (MBF) is also a regulatory mechanism for water and Na + reabsorption.  For example, if MBF is increased due to increased ECF volume, medullary hypertonicity is lost, which results in less water reabsorption. Na + is diluted and is absorbed less in the thick aLOH and thus more Na + and water is excreted.
Renin is released from modified fenestrated and granular endothelial cells  in the AA in response to a decrease in actual (e.g., hemorrhage) or effective (e.g. hypotension due to peripheral vasodilatation) blood volume. These baroreceptors detect perfusion pressure in the AA. Increased sympathetic activity or catecholamines also stimulate renin release, which leads to increased angiotensin II levels in blood.
Renal Sympathetic Aactivity
Increased renal sympathetic activity causes (1) a decrease of renal blood flow and GFR (2) a decrease of perfusion pressure in the AA and indirect release of renin (3) a direct release of renin via 01 receptors located on JG cells in the AA (4) an increase of absorption of Na + in the PT via a1 receptors located at basolateral membrane. 
The distal tubule has specialized epithelial cells (macula densa) situated directly opposite the JG cells. These cells act as chemoreceptors. They sense the Na + and Cl¯ load delivered in the DT.  Depending on this signals are conveyed to JG cells in the AA. Accordingly, AA tone is altered and the amount of Na + filtered is changed. This is a continuous feedback mechanism at the individual nephron level. For example, if Na + delivery in the DT is less, vasodilatation in the AA will occur and more Na + will pass into the PT. Thus, there is a negative feedback relationship between the afferent arterioles and the volume of fluid filtrate that enters the DT.
Angiotensin II (AT II)
In addition to increased SNS activity and de creased Na + delivery in the DT, low perfusion pressure in the AA directly releases renin from the JG cells. Renin converts angiotensiongen to angiotensin I, which is further cleaved by ACE in vascular endothelium to AT II. AT II has a variable effect on GFR.  It increases GFR by EA vasoconstriction. It decreases GFR by reducing the filtration surface area due to mesangial cell contraction. It also reduces medullary blood flow. AT II increases Na + reabsorption in the PT by stimulating Na + -H + exchanger.  AT II is a potent stimulus for synthesis of aldosterone. It has a negative feedback mechanism on its own actions. Inhibition of the Na + retaining actions of A II is clinically useful in various pathological conditions such as hypertension and cardiac failure [Figure 2], [Table 1].
It is a potent hormone for Na + reabsorption. Low oral Na + intake or plasma Na + level increases the synthesis and release of aldosterone. Na + reabsorption by aldosterone is a slow process and may take hours to be effective. Aldosterone is responsible for reabsorption of 2% of filtered load of Na + . In other words, absence of aldosterone results in renal loss of Na + in excess of 500 mmols/day. On the other hand, if aldosterone is secreted in excess amounts, final Na + concentration in urine will be close to nil [Figure 2], [Table 1].
ECF osmolality is delicately and continuously sensed by osmoreceptors (OR). These receptors increase or inhibit release of ADH immediately resulting in an increase or decrease in plasma osmolality respectively.  Despite a wide variation in water intake the body preserves plasma osmolality within 1-2% of normal range. Supraoptic (SO) and paraventricular (PV) nuclei in the hypothalamus synthesize ADH which is transported along the axons to the posterior pituitary where it is stored [Figure 2], [Table 1]. ADH is released linear proportion to increasing plasma osmolality. A 10-15% decrease in BV will also stimulate ADH secretion but in an exponential fashion.
The thick ascending loop of juxtamedullary nephrons (the counter-current multiplier) and vasa recta (counter-current exchanger) create hypertonicity of medullary interstitium. This is essential for water reabsorption from the CD by ADH. In certain clinical conditions, increased ADH release may occur despite normal ECF osmolality. This causes water retention and dilutional hyponatremia.
Atrial Natriuretic Peptide (ANP)
ANP is a counter-regulatory mechanism to the renin angiotensin system. It has an important role in regulation of Na + and water balance, blood volume, and blood pressure.  ANP exhibits diverse direct and indirect physiological actions [Figure 2], [Table 1] on the kidney and cardiovascular systems thereby protecting against volume overload. It also decreases sympathetic tone. ANP has a specific role in volume overload conditions such as CCF, where increased levels may prevent the development of complications due to volume over load. Thus, ANP is an endogenous diuretic.
PGs (e.g. PGE2) have an important role in maintaining renal blood flow particularly under conditions of stress. Their production is increased in response to hypotension and renal ischemia. PGs play a defensive role by promoting renal vasodilatation and preserving intrarenal hemodynamics. They have also been shown to influence water and Na + excretion.
Nitric Oxide (NO)
In the kidney, NO is synthesized in vascular endothelium as well as tubular cells. It increases GFR and medullary blood flow. NO has also been shown to inhibit Na + reabsorption in the CD.  It opposes the renal vasoconstrictor effects of AT II and the sympathetic nervous system and decreases renal vascular resistance.
Gastrointestinal and Other Factors
For a 70 kg adult daily Na + intake is 100-150 mmols in addition to 1.5-2.5 L of oral daily intake of fluids. Approximately 8 L more are produced and secreted by various parts of GIT. These secretions contain 1200-1400 mmol of Na + . 6 to 6.5 L is reabsorbed in the small intestine and the remainder passes to the large intestine where further absorption occurs. Only 100200 mL of fluid and 4-5 mmol of Na + are excreted in the stool. Similarly to the kidney, Na +K + -ATPase pumps at the basolateral membrane of intestinal epithelial cells plays a crucial role which appears to be stimulated by aldosterone.
Water is also lost insensibly through the skin and lungs (approximately 400 mL via each) but partly compensated by water gain due to metabolic processes (nearly 400 mL).
ECF Volume vs Osmoregulation
Water regulation is dependent on (1) appropriate ADH synthesis, transport, release, and function, (2) counter-current multiplier and exchanger system, and (3) functioning thirst mechanism.
Three discrete areas are located in hypothalamus (1) osmoreceptors (OR) (2) thirst center and (3) SO and PV nuclei responsible for ADH synthesis [Figure 2]. Afferent input from OR to SO and PV nuclei is essential for a normal ADH response.
Maintenance of ECF osmolality is necessary to keep the relative ratio of Na + to water normal. If either the osmolality or volume of ECF are individually disturbed independent mechanisms for osmoregulation or volume control are activated. For example, 1 L of drinking water is readily absorbed from GIT into plasma and then distributed to interstitial fluid and cells. It reduces plasma osmolality and secretion of ADH from the posterior pituitary. Within the next hour of drinking this excess water dilute urine is excreted without altering its solute concentration. However, with 1 L of saline in-fusion, ECF osmolality is not changed, but an increase in its volume will lead to activation of baroreceptors, [Figure 2]. Consequently, ANP is released and sympathetic activity is reduced; while AT II and aldosterone formation is inhibited. Ultimately, more Na + and water excretion occurs to adjust for the extra Na + and water load.
In contrast to the above scenario, hypovolemia severe enough to drop blood pressure can activate volume restoring processes where ANP release is inhibited. Even if there is a relative increase in osmolality in this condition, volume mediated release of ADH will inhibit water excretion from distal parts of nephron. Hemodynamic responses override those of osmolality in an attempt to restore tissue perfusion [Figure 2].
Hypo and Hypernatremia
Pathophysiology and corrective measures are summarized in [Table 2] and [Figure 3].
Physiologically, the main aim of Na + and water balance is to permit variable salt and water intake without large fluctuations in blood pressure or volume status. Homeostatic processes act in an integrated fashion to protect against any disturbances. Characteristically, these mechanisms are sequential as well as parallel. As feedback systems, they may be synergistic or antagonistic, and their speed, sensitivity, and potency may differ. Various physiological and pathological insults determine the magnitude of response of these systems.
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