The excretory system of mammals centres on the kidneys, which are also the principal site of water balance and salt regulation. Mammals have a pair of kidneys. Each kidney, bean–shaped and about 10 cm long in humans, is supplied with blood by a renal artery and drained by a renal vein.
Blood flow through the kidneys is voluminous. In humans, the kidneys account for less than 1% of body weight, but they receive about 20% of resting cardiac output. Urine exits each kidney through a duct called the ureter, and both ureters drain into a common urinary bladder. During urination, urine is expelled from the urinary bladder through a tube called the urethra, which empties to the outside near the vagina in females or through the penis in males. Sphincter muscles near the junction of the urethra and the bladder, which are under nervous system control, regulate urination.
The mammalian kidney has two distinct regions, an outer renal cortex and an inner renal medulla. Packing both regions are microscopic excretory tubules and their associated blood vessels. The nephron—the functional unit of the vertebrate kidney—consists of a single long tubule and a ball of capillaries called the glomerulus. The blind end of the tubule forms a cup–shaped swelling, called Bowman′s capsule, which surrounds the glomerulus. Each human kidney contains about a million nephrons, with a total tubule length of 80 km.
Filtration of the Blood
Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowman′s capsule. The porous capillaries, along with specialised cells of the capsule called podocytes, are permeable to water and small solutes but not to blood cells or large molecules such as plasma proteins. Filtration of small molecules is nonselective, and the filtrate in Bowman′s capsule contains salts, glucose, amino acids, and vitamins; nitrogenous wastes such as urea; and other small molecules—a mixture that mirrors the concentrations of these substances in blood plasma.
Pathway of the Filtrate
From Bowman′s capsule, the filtrate passes through three regions of the nephron: the proximal tubule; the loop of Henle, a hairpin turn with a descending limb and an ascending limb; and the distal tubule. The distal tubule empties into a collecting duct, which receives processed filtrate from many nephrons. This filtrate flows from the many collecting ducts of the kidney into the renal pelvis, which is drained by the ureter.
In the human kidney, approximately 80% of the nephrons, the cortical nephrons, have reduced loops of Henle and are almost entirely confined to the renal cortex. The other 20%, the juxtamedullary nephrons, have well–developed loops that extend deeply into the renal medulla. Only mammals and birds have juxtamedullary nephrons; the nephrons of other vertebrates lack loops of Henle. It is the juxtamedullary nephrons that enable mammals to produce urine that is hyperosmotic to body fluids, an adaptation that is extremely important for water conservation.
The nephron and the collecting duct are lined by a transport epithelium that processes the filtrate to form the urine. One of this epithelium′s most important tasks is reabsorption of solutes and water. Between 1,100 and 2,000 L of blood flows through a pair of human kidneys each day, a volume about 275 times the total volume of blood in the body. From this enormous traffic of blood, the nephrons and collecting ducts process about 180 L of initial filtrate, equivalent to two or three times the body weight of an average person. Of this, nearly all of the sugar, vitamins, and other organic nutrients and about 99% of the water are reabsorbed into the blood, leaving only about 1.5 L of urine to be voided.
Blood Vessels Associated with the Nephrons
Each nephron is supplied with blood by an afferent arteriole, a branch of the renal artery that subdivides into the capillaries of the glomerulus. The capillaries converge as they leave the glomerulus, forming an efferent arteriole. This vessel subdivides again, forming the peritubular capillaries, which surround the proximal and distal tubules. More capillaries extend downward and form the vasa recta, the capillaries that serve the loop of Henle. The vasa recta also form a loop, with descending and ascending vessels conveying blood in opposite directions.
Although the excretory tubules and their surrounding capillaries are closely associated, they do not exchange materials directly. The tubules and capillaries are immersed in interstitial fluid, through which various substances diffuse between the plasma within capillaries and the filtrate within the nephron tubule. This exchange is facilitated by the relative direction of blood flow and filtrate flow in the nephrons.
Secretion and reabsorption in the proximal tubule substantially alter the volume and composition of filtrate. For example, the cells of the transport epithelium help maintain a relatively constant pH in body fluids by the controlled secretion of H+. The cells also synthesise and secrete ammonia, which neutralises the acid and keeps the filtrate from becoming too acidic. The more acidic the filtrate, the more ammonia the cells produce and secrete, and the urine of a mammal usually contains some ammonia from this source (even though most nitrogenous waste is excreted as urea). The proximal tubules also reabsorb about 90% of the important buffer bicarbonate (HCO3−). Drugs and other poisons that have been processed in the liver pass from the peritubular capillaries into the interstitial fluid, and then are secreted across the epithelium of the proximal tubule into the nephron′s lumen. Conversely, valuable nutrients, including glucose, amino acids, and potassium (K+), are actively or passively transported from the filtrate to the interstitial fluid and then are moved into the peritubular capillaries.
One of the most important functions of the proximal tubule is reabsorption of most of the NaCl (salt) and water from the huge initial filtrate volume. Salt in the filtrate diffuses into the cells of the transport epithelium, and the membranes of the cells actively transport Na+ into the interstitial fluid. This transfer of positive charge is balanced by the passive transport of Cl− out of the tubule. As salt moves from the filtrate to the interstitial fluid, water follows by osmosis. The exterior side of the epithelium has a much smaller surface area than the side facing the lumen, minimizing leakage of salt and water back into the tubule. Instead, the salt and water now diffuse from the interstitial fluid into the peritubular capillaries.
Reabsorption of water continues as the filtrate moves into the descending limb of the loop of Henle. Here the transport epithelium is freely permeable to water but not very permeable to salt and other small solutes. For water to move out of the tubule by osmosis, the interstitial fluid bathing the tubule must be hyperosmotic to the filtrate. The osmolarity of the interstitial fluid does in fact become progressively greater from the outer cortex to the inner medulla of the kidney. Thus, filtrate moving downward from the cortex to the medulla within the descending limb of the loop of Henle continues to lose water to interstitial fluid of greater and greater osmolarity, which increases the solute concentration of the filtrate.
The filtrate reaches the tip of the loop, deep in the renal medulla in the case of juxtamedullary nephrons, then moves back to the cortex within the ascending limb. In contrast to the descending limb, the transport epithelium of the ascending limb is permeable to salt but not to water. The ascending limb has two specialized regions: a thin segment near the loop tip and a thick segment adjacent to the distal tubule. As filtrate ascends in the thin segment, NaCl, which became concentrated in the descending limb, diffuses out of the permeable tubule into the interstitial fluid. This movement increases the osmolarity of the interstitial fluid in the medulla. The exodus of salt from the filtrate continues in the thick segment of the ascending limb, but here the epithelium actively transports NaCl into the interstitial fluid. By losing salt without giving up water, the filtrate is progressively diluted as it moves up to the cortex in the ascending limb of the loop.
The distal tubule plays a key role in regulating the K+ and NaCl concentration of body fluids by varying the amount of the K+ that is secreted into the filtrate and the amount of NaCl reabsorbed from the filtrate. Like the proximal tubule, the distal tubule also contributes to pH regulation by the controlled secretion of H+ and reabsorption of bicarbonate (HCO3−).
The collecting duct carries the filtrate through the medulla to the renal pelvis. By actively reabsorbing NaCl, the transport epithelium of the collecting duct plays a large role in determining how much salt is actually excreted in the urine. Though its degree of permeability is under hormonal control, the epithelium is permeable to water. However, it is not permeable to salt or, in the renal cortex, to urea. Thus, as the collecting duct traverses the gradient of osmolarity in the kidney, the filtrate becomes increasingly concentrated as it loses more and more water by osmosis to the hyperosmotic interstitial fluid. In the inner medulla, the duct becomes permeable to urea. Because of the high urea concentration in the filtrate at this point, some urea diffuses out of the duct and into the interstitial fluid. Along with NaCl, this urea contributes to the high osmolarity of the interstitial fluid in the medulla. This high osmolarity enables the mammalian kidney to conserve water by excreting urine that is hyperosmotic to the general body fluids.