The kidney has evolved so as to enable humans to exist on land where water and salts must be conserved, wastes excreted in concentrated form, and the blood and the tissue fluids strictly regulated as to volume, chemical composition, and osmotic pressure. Under the drive of arterial pressure, water and salts are filtered from the blood through the capillaries of the glomerulus into the lumen, or passageway, of the nephron, and then most of the water and the substances that are essential to the body are reabsorbed into the blood. The remaining filtrate is drained off as urine. The kidneys, thus, help maintain a constant internal environment despite a wide range of changes in the external environment.
The kidneys regulate three essential and interrelated properties of the tissues—water content, acid-base balance, and osmotic pressure—in such a way as to maintain electrolyte and water equilibrium; in other words, the kidneys are able to maintain a balance between quantities of water and the quantities of such chemicals as calcium, potassium, sodium, phosphorus, and sulfate in solution. Unless the concentrations of mineral ions such as sodium, crystalloids such as glucose, and wastes such as urea are maintained within narrow normal limits, bodily malfunction rapidly develops leading to sickness or death.
The removal of both kidneys causes urinary constituents to accumulate in the blood (uremia), resulting in death in 14–21 days if untreated. (The term uremia does not mean that urea is itself a toxic compound responsible for illness and death.) Whenever the blood contains an abnormal constituent in solution or an excess of normal constituents including water and salts, the kidneys excrete these until normal composition is restored. The kidneys are the only means for eliminating the wastes that are the end products of protein metabolism. They do not themselves modify the waste products that they excrete, but transfer them to the urine in the form in which they are produced in other parts of the body. The only exception to this is their ability to manufacture ammonia. The kidneys also eliminate drugs and toxic agents. Thus, the kidneys eliminate the unwanted end products of metabolism, such as urea, while limiting the loss of valuable substances, such as glucose. In maintaining the acid-base equilibrium, the kidneys remove the excess of hydrogen ions produced from the normally acid-forming diet and manufacture ammonia to remove these ions in the urine as ammonium salts.
To carry on its functions the kidney is endowed with a relatively huge blood supply. The blood processed in the kidneys amounts to some 1,200 millilitres a minute, or 1,800 litres (about 475 gallons) a day, which is 400 times the total blood volume and roughly one-fourth the volume pumped each day by the heart. Every 24 hours 170 litres (45 gallons) of water are filtered from the bloodstream into the renal tubules; and by far the greater part of this—some 168.5 litres of water together with salts dissolved in it—is reabsorbed by the cells lining the tubules and returned to the blood. The total glomerular filtrate in 24 hours is no less than 50–60 times the volume of blood plasma (the blood minus its cells) in the entire body. In a 24-hour period, an average man eliminates only 1.5 litres of water, containing the waste products of metabolism, but the actual volume varies with fluid intake and occupational and environmental factors. With vigorous sweating it may fall to 500 millilitres (about a pint) a day; with a large water intake it may rise to three litres, or six times as much. The kidney can vary its reabsorption of water to compensate for changes in plasma volume resulting from dehydration or overhydration.
The kidneys also perform certain nonexcretory functions. They secrete substances that enter the blood. These are of three kinds: renin, which is concerned indirectly with the control of electrolyte balance and blood pressure; erythropoietin, which is important for the formation of hemoglobin and red blood cells, especially in response to anemia or deficiency of oxygen reaching the body tissues; and 1,25-dihydroxycholecalciferol, which is the metabolically active form of vitamin D. Finally, although the kidneys are subject to both nervous and humoral (hormonal) control, they do possess a considerable degree of autonomy; i.e., function continues in an organ isolated from the nervous system but kept alive with circulating fluid. Indeed, if this were not so kidney transplantation would be impossible.
Renal blood circulation
The renal arteries are short and spring directly from the abdominal aorta, so that arterial blood is delivered to the kidneys at maximum available pressure. As in other vascular beds, renal perfusion is determined by the renal arterial blood pressure and vascular resistance to blood flow. Evidence indicates that in the kidneys the greater part of the total resistance occurs in the glomerular arterioles. The muscular coats of the arterioles are well supplied with sympathetic vasoconstrictor fibres (nerve fibres that induce narrowing of the blood vessels), and there is also a small parasympathetic supply from the vagus and splanchnic nerves that induces dilation of the vessels. Sympathetic stimulation causes vasoconstriction and reduces urinary output. The vessel walls are also sensitive to circulating epinephrine and norepinephrine hormones, small amounts of which constrict the efferent arterioles and large amounts of which constrict all the vessels; and to angiotensin, which is a constrictor agent closely related to renin. Prostaglandins may also have a role.
The kidney is able to regulate its internal circulation regardless of the systemic blood pressure, provided that the latter is not extremely high or extremely low. The forces that are involved in maintaining a circulation of the blood in the kidneys must remain constant if the monitoring of the water and electrolyte composition of the blood is to proceed undisturbed. This regulation is preserved even in the kidney cut off from the nervous system and, to a lesser extent, in an organ removed from the body and kept viable by having salt solutions of physiologically suitable concentrations circulated through it; it is commonly referred to as autoregulation.
The exact mechanism by which the kidney regulates its own circulation is not known, but various theories have been proposed: (1) Smooth muscle cells in the arterioles may have an intrinsic basal tone (normal degree of contraction) when not affected by nervous or humoral (hormonal) stimuli. The tone responds to alterations in perfusion pressure in such a way that when the pressure falls the degree of contraction is reduced, preglomerular resistance is lowered, and blood flow is preserved. Conversely, when perfusion pressure rises, the degree of contraction is increased and blood flow remains constant. (2) If the renal blood flow rises, more sodium is present in the fluid in the distal tubules because the filtration rate increases. This rise in the sodium level stimulates the secretion of renin from the JGA with the formation of angiotensin, causing the arterioles to constrict and blood flow to be reduced. (3) If systemic blood pressure rises, the renal blood flow remains constant because of the increased viscosity of the blood. Normally, the interlobular arteries have an axial (central) stream of red blood cells with an outer layer of plasma so that the afferent arterioles skim off more plasma than cells. If the arteriolar blood pressure rises, the skimming effect increases, and the more densely packed axial flow of cells in the vessels offers increasing resistance to the pressure, which has to overcome this heightened viscosity. Thus, the overall renal blood flow changes little. Up to a point, similar considerations in reverse apply to the effects of reduced systemic pressure. (4) Changes in the arterial pressure modify the pressure exerted by the interstitial (tissue) fluid of the kidney on capillaries and veins so that increased pressure raises, and decreased pressure lowers, resistance to blood flow.
The renal blood flow is greater when a person is lying down than when standing; it is higher in fever; and it is reduced by prolonged vigorous exertion, pain, anxiety, and other emotions that constrict the arterioles and divert blood to other organs. It is also reduced by hemorrhage and asphyxia and by depletion of water and salts, which is severe in shock, including operative shock. A large fall in systemic blood pressure, as after severe hemorrhage, may so reduce renal blood flow that no urine at all is formed for a time; death may occur from suppression of glomerular function. Simple fainting causes vasoconstriction and reduced urine output. Urinary secretion is also stopped by obstruction of the ureter when back pressure reaches a critical point.
The importance of these various vascular factors lies in the fact that the basic process occurring in the glomerulus is one of filtration, the energy for which is furnished by the blood pressure within the glomerular capillaries. Glomerular pressure is a function of the systemic pressure as modified by the tone (state of constriction or dilation) of the afferent and efferent arterioles, as these open or close spontaneously or in response to nervous or hormonal control.
In normal circumstances glomerular pressure is believed to be about 45 millimetres of mercury (mmHg), which is a higher pressure than that found in capillaries elsewhere in the body. As is the case in renal blood flow, the glomerular filtration rate is also kept within the limits between which autoregulation of blood flow operates. Outside these limits, however, major changes in blood flow occur. Thus, severe constriction of the afferent vessels reduces blood flow, glomerular pressure, and filtration rate, while efferent constriction causes reduced blood flow but increases glomerular pressure and filtration.
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