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The kidneys serve several essential regulatory roles. They are essential in the regulation of electrolytes, maintenance of acid-base balance, and regulation of blood pressure. They serve the body by filtering blood to remove wastes that are diverted to the urinary bladder for excretion. The kidneys excrete wastes such as urea and ammonium, and are also responsible for the reabsorption of water, glucose, and amino acids. Furthermore, the kidneys also produce hormones, including calcitriol, erythropoietin, and the enzyme renin.
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REGULATION OF SODIUM AND WATER
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There are three major hormones that are involved in regulating Na+ and water balance in the body at the level of the kidney. Antidiuretic hormone (ADH) from the posterior pituitary acts on the kidney to promote water reabsorption, thus preventing its loss in urine. The most important variable in regulating ADH is plasma osmolarity. Reduced volume of extracellular fluid promotes secretion of ADH but is a less sensitive mechanism. Other stimuli for ADH secretion include decrease in systemic arterial blood pressure, stress, nausea, hypoxia, pain, and mechanical ventilation. Aldosterone from the adrenal cortex of the adrenal gland acts on the kidney to promote Na+ reabsorption. It acts mainly on the distal tubules and the collecting ducts of the nephron. Water follows Na+, thereby increasing intravascular volume. K+ levels are the most sensitive stimulator of aldosterone secretion. Atrial natriuretic hormone (ANH) from the atrium of the heart acts on the kidney to promote Na+ excretion to decrease intravascular volume. The main stimulus for ANH secretion is atrial distention.
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REGULATION OF BLOOD PRESSURE
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The renin-angiotensin system (RAS) is a hormone system that regulates blood pressure and fluid balance. A decrease in mean arterial pressure induces juxtaglomerular cell secretion of renin. Renin is responsible for converting angiotensinogen to angiotensin I. Angiotensin converting enzyme (ACE) in the lungs converts angiotensin I to angiotensin II. Angiotensin II is a potent vasoconstrictor resulting in increased blood pressure. It also stimulates aldosterone release from the adrenal cortex that increases Na+ and water reabsorption, increasing total effective circulatory volume.
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The metabolism of amino acids in proteins produces acids referred to as nonvolatile acids that are rapidly buffered, producing CO2 and ammonium salts. The lungs excrete the CO2, whereas the kidneys excrete the ammonium salts. In the process of excreting ammonium, bicarbonate is generated and returned to the blood to replace the bicarbonate lost in titrating the nonvolatile acid. About 85%–90% of the filtered bicarbonate is reabsorbed in the proximal tubule. Cells of the distal tubule and collecting ducts reabsorb the rest. The major factors that control bicarbonate reabsorption are luminal bicarbonate concentration, arterial CO2, and angiotensin II. An increase in any of these factors can cause an increase in bicarbonate reabsorption.