From the nephrons the urine enters the final 15 or 20 collecting tubules that open on to each papilla of the renal medulla, projecting into a minor calyx. These open into two or three major calyxes, and these in turn open into the renal pelvis, the upper expanded portion of the ureter.
Urine is passed down the channel of the renal pelvis and ureter by a succession of peristaltic waves of contraction that begin in the muscle fibres of the minor calyxes, travel out to the major calyxes and then along the ureter every 10–15 seconds. Each wave sends urine through the ureteric orifice into the bladder in discontinuous spurts; these can be seen through a cystoscope if a dye is injected into the bloodstream. Gravity aids this downward flow, which is faster when one is standing erect. Though the overall picture suggests that there is a pacemaker (a set of specialized cells capable of rhythmic contractions) near the pelviureteric junction, this has never been satisfactorily demonstrated in the tissue. The pressure in the renal pelvis is normally low, but the smooth muscle coat of the ureter is a powerful one and the pressure above an obstructed ureter may rise as high as 50 millimetres of mercury. The ureters are doubly innervated from the splanchnic nerves above and the hypogastric network below.
Certain hormones and hormonelike substances are intimately related to renal function. Some of these, such as ADH (or vasopressin), are produced outside the kidney and travel to the kidney via the blood as chemical messengers. Others are produced within the kidney and appear to exert only a local effect. The role of ADH in controlling diuresis has already been discussed. ADH regulates water excretion by increasing the permeability of the collecting ducts to water and salt and by accelerating water and ion transfer in a direction determined by the osmotic gradient. The receptors at the base of the brain form part of the feedback mechanism that (1) stimulates ADH output if the osmotic concentration of extracellular fluid (ECF) is high, so as to concentrate the urine, and (2) reduces ADH output and so dilutes the urine if osmotic concentration of ECF and of plasma falls.
The hormones of the adrenal cortex are also important in influencing renal function, directly or indirectly. In stress situations, as after an injury or a surgical operation, the output of hydrocortisone and other corticosteroids is increased because the adrenals are stimulated by adrenocorticotropin ( ACTH), a secretion of the pituitary gland. Hydrocortisone increases protein breakdown, and consequently the output of nitrogen in the urine, and affects water metabolism; lack of hydrocortisone reduces the power of the kidney to deal with normal water loads. The hormone also promotes sodium retention and loss of potassium and hydrogen ions by the kidney. Aldosterone influences electrolyte metabolism by facilitating the reabsorption of sodium ions at the distal tubules, also at the expense of hydrogen and potassium excretion. The action of aldosterone has been described as priming the sodium reabsorption pump; it is the adrenal hormone most important to tubular function. It also influences the ability of the bowel to absorb sodium, and thus its level of production profoundly influences overall sodium balance. Deficiency of aldosterone allows a steady loss of sodium in the urine, causing a fall in blood pressure that may result in fainting.
The action of the parathyroid glands is to increase blood calcium by mobilizing calcium from the bones and other sources; if this hormone functions to excess, as in tumours of the glands, the urinary loss of calcium is much increased and calcium stones tend to form in the kidneys and the bladder. Parathyroid hormone also increases the renal excretion of phosphate and accelerates the conversion of hydroxylated vitamin D to the dehydroxylated form in the kidney. The pituitary growth hormone facilitates protein synthesis and decreases the urinary loss of nitrogen. The sex hormones estrogen and progesterone exert an ill-defined activity as regards salt and water metabolism.
The juxtaglomerular apparatus (JGA), consisting of an asymmetrical cuff of large granular cells in the wall of the afferent arteriole near its entry into the capsule of the nephron, contains renin in the granules in the cells. Renin is a true internal secretion of the kidney. Entering the plasma, it acts as an enzyme that induces one of the plasma globulins to yield angiotensin I, which is inactive, and which gives rise in turn to angiotensin II, the most potent agent for constricting the blood vessels and raising the blood pressure. The formation of renin at the JGA is induced by a fall in blood pressure and inhibited by a rise. When the pressure falls, the output of angiotensin II raises the pressure and also excites the release of aldosterone from the adrenal cortex. This process is another example of a feedback mechanism analogous to that controlling the output of ADH.
Among the prostaglandins, a group of hormonelike fatty acids synthesized throughout the body, the ones found in the kidney tissues appear to exert local influence on various aspects of renal function. Unlike true hormones, prostaglandins are not transported away from their site of origin by the blood. The interstitial and collecting duct cells of the kidney produce a characteristic prostaglandin, PGE2, and the renal cortex produces PGI2, or prostacyclin. Renal prostaglandins interact with the renin–angiotensin system in several ways. The renal cortex prostaglandin PGI2 mediates the increased release of renin in response to decreases in renal blood flow. The angiotensin subsequently formed in the plasma stimulates production of the interstitial and duct cell prostaglandin (PGF2), which itself inhibits angiotensin-induced vasoconstriction. For this reason the renal cortex prostaglandin is thought to be an important vasodilator, maintaining renal blood flow when this is threatened (for example, after blood loss). Prostaglandins may also inhibit the action of ADH on the distal tubule and collecting ducts, and the interstitial and duct cell prostaglandin may have a direct effect in inhibiting renal tubular sodium reabsorption; however, the relative importance of these different actions in the healthy human is not known.
Another substance that causes the dilation of blood vessels, the enzyme kallikrein, may also exert an influence on renal blood flow. Kallikrein is secreted by renal tubules and is added to the urine in the distal tubules. It activates the conversion of kininogen to bradykinin, which is also a powerful vasodilator. Bradykinin is inactivated by a kininase, which also converts angiotensin I to angiotensin II, a substance that causes the constriction of blood vessels. Thus the same enzyme that inactivates the vasodilator bradykinin catalyzes the production of the vasoconstrictor angiotensin II. This relationship again suggests a delicately balanced internal control system.
Dopamine is a putative renal hormone that may affect salt balance. The sympathetic nerves that travel to the kidney, the terminals of which release catecholamines such as norepinephrine, are not believed to be important in controlling tubular salt reabsorption. Transplanted human kidneys function adequately despite the lack of any nerve supply and so renal nerves are not essential. However, because dopamine (also a catecholamine released at sympathetic nerve endings) is present in urine in amounts far in excess of the amount that might be filtered from the blood, it may be deduced that some dopamine is formed within the kidney. It is now believed that dopamine is formed enzymatically within the kidney from its precursor, L-dopa, which freely circulates in the blood, and that only small amounts are released by sympathetic nerve endings. Dopamine is a powerful natriuretic substance (i.e., one capable of increasing urinary salt loss) and renal vasodilator. Its role in salt balance, renal function, and blood pressure control remains speculative.
The most recently identified hormone that influences renal function is secreted by special “stretch receptor” cells in the atria of the heart in response to a rise in atrial pressure, as during heart failure. This hormone, called atrial natriuretic peptide (ANP), exerts a vasodilator effect on the kidney and also reduces tubular reabsorption of sodium. Both actions result in increased urinary elimination of salt and water and tend to restore atrial pressure toward the normal. It is probably an important hormone controlling the volume of the extracellular fluid.
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