Renal Control of Electrolyte:
The kidneys help to regulate the concentrations of plasma electrolytes: sodium, potassium, chloride, bicarbonate, and phosphate by matching the urinary excretion of these compounds to the amounts ingested. The control of plasma Na is important in the regulation of blood volume and pressure; the control of plasma K is required to maintain proper function of cardiac and skeletal muscles.Role of Aldosterone in Na/K Balance
Approximately 90% of the filtered Na and K are reabsorbed in the early part of the nephron before the filtrate reaches the distal tubule. This reabsorption occurs at a constant rate and is not subject to hormonal regulation. The final concentration of Na and K in the urine is varied according to the needs of the body by processes that occur in the late distal tubule and in the cortical region of the collecting duct.Renal reabsorption of Na and secretion of K are regulated by aldosterone, secreted by the adrenal cortex.
Sodium Reabsorption
Although 90% of the filtered sodium is reabsorbed in the early region of the nephron, the amount left in the filtrate delivered to the distal convoluted tubule is still quite large. 8% of the amount filtered is also reabsorbed through the wall of the tubule into the peritubular blood distally without the need of aldosterone. The amount of sodium excreted without aldosterone is thus 2% of the amount filtered. Although this percentage seems small, the actual amount it represents is an impressive 30 g of sodium excreted in the urine each day. When aldosterone is secreted in maximal amounts, by contrast, all of the sodium delivered to the distal tubule is reabsorbed. In this case urine contains no Na at all.Aldosterone stimulates Na reabsorption to some degree in the late distal convoluted tubule, but the primary site of aldosterone action is in the cortical collecting duct. This is the initial portion of the collecting duct, located in the renal cortex, which has different permeability properties than the terminal portion of the collecting duct, located in the renal medulla.
Potassium Secretion
About 90% of the filtered potassium is reabsorbed in the early regions of the nephron (mainly from the proximal tubule). The remaining k will be also reabsorbed in the distal tubules. In order for potassium to appear in the urine, it must be secreted into later regions of the nephron tubule. Secretion of potassium occurs in the parts of the nephron that are sensitive to aldosterone that is, in the late distal tubule and cortical collecting duct.As Na is reabsorbed in these regions of the nephron, the lumen of the tubule becomes more negatively charged (50 mV) compared to the basolateral side. This potential difference then drives the secretion of K into the tubule.
The amount of K secretion into the late distal tubule and cortical collecting duct depends on:
The amount of Na delivered to these regions of the nephron.
The amount of aldosterone secreted.
If the blood concentration of K rises, this will stimulate increased aldosterone secretion from the adrenal cortex. The aldosterone then stimulates increased reabsorption of Na and, as a result, increased secretion of K.
Control of Aldosterone Secretion
Since aldosterone promotes Na retention and K loss, one might predict (on the basis of negative feedback) that aldosterone secretion would be increased when there was a low Na or a high K concentration in the blood. This indeed is the case. A rise in blood K directly stimulates the secretion of aldosterone from the adrenal cortex. A decrease in plasma Na concentration, if it causes a fall in blood volume, also promotes aldosterone secretion. However, the stimulatory effect of a fall in blood volume on aldosterone secretion is indirect.
Atrial Natriuretic Peptide (ANP)
Expansion of the blood volume causes increased salt and water excretion in the urine. This is partly due to an inhibition of aldosterone secretion. However, it is also caused by increased secretion of a natriuretic hormone, a hormone that stimulates salt excretion, an action opposite to that of aldosterone. Atrial natriuretic peptide is produced by the atria of the heart and secreted in response to the stretching of the atrial walls by increased blood volume. In response to ANP action, the kidneys lower the blood volume by excreting more of the salt and water filtered out of the blood by the glomeruli. Atrial natriuretic peptide thus functions as an endogenous diuretic.Relationship between Na, K, and H
The plasma K concentration indirectly affects the plasma H concentration (pH). Changes in plasma pH likewise affect the K concentration of the blood.When the extracellular H concentration increases, some of the H moves into the cells and causes cellular K to diffuse outward into the extracellular fluid. The plasma concentration of H is thus decreased while the K increases, helping to reestablish the proper ratio of these ions in the extracellular fluid. A similar effect occurs in the cells of the distal region of the nephron. In the cells of the late distal tubule and cortical collecting duct, positively charged ions (K and H) are secreted in response to the negative polarity produced by reabsorption of Na.
So in case of acidosis (increase in plasma pH i.e. concentration of H) there will be plasma hyperkalemia and K will thus appear in urine.
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