Summary

Basic Mechanisms of Kidney Function

• Primary urine is formed by ultrafiltration or by active solute secretion.
• During ultrafiltration, fluid is driven by elevated hydrostatic pressure from the blood plasma into the kidney tubules through intervening epithelia and basement membranes that act as a filter. The filtrate, which is the primary urine, is almost identical to blood plasma in its composition, except that it lacks high-molecular-weight solutes such as plasma proteins.
• In cases in which primary urine is formed by active solute secretion, the process that initiates and drives primary-urine formation is the active transport of one or more solutes into the kidney tubules. Water then follows by osmosis, and other solutes enter by diffusion, following electrochemical gradients set up by the active solute transport and osmosis.
• As primary urine flows through the kidney tubules, it undergoes exchange with the blood plasma by active or passive transport of solutes and by osmosis of water across the epithelial walls of the tubules. These processes are the predominant regulatory processes in the kidney tubules: They determine the ways in which the production of urine ultimately alters the composition and volume of the blood plasma. The urine produced by the kidneys is sometimes (as in mammals) the definitive urine, but in many animals, further regulatory exchange between urine and blood plasma occurs by postrenal processing.

Urine Formation in Amphibians

• A primary function of the proximal convoluted tubule of the amphibian nephron is the return of both water and solutes to the body fluids by the isosmotic reduction of urine volume. NaCl is actively reabsorbed from the tubular fluid. Because the epithelial wall of the proximal tubule is permeable to water, water exits the tubular fluid by osmosis, keeping the tubular fluid isosmotic to the blood plasma.
• Glucose and amino acids are actively reabsorbed from the tubular fluid in the proximal tubule, returning them to the body fluids.
• The distal convoluted tubule differentially returns water and solutes to the body fluids; in the process it helps regulate plasma composition and determines the volume and osmotic concentration of the definitive urine produced by the kidney. An important mechanism by which control of distal-tubule function is exercised is that the epithelial wall of the distal convoluted tubule can have high or low permeability to water, depending on blood levels of antidiuretic hormone (ADH) secreted by the neurohypophysis (posterior pituitary).
• When ADH levels are low, the distal-tubule epithelium is poorly permeable to water. Active reabsorption of NaCl returns NaCl to the body fluids and dilutes the tubular fluid. However, relatively little water is returned to the body fluids because water cannot readily move out of the tubular fluid by osmosis. The volume of the tubular fluid remains high, and both the osmotic pressure and the NaCl concentration of the fluid become progressively lower as the tubular fluid flows through the tubule.
• When ADH levels are high, aquaporins are believed to be inserted into cell membranes in the distal-tubule epithelium, causing the water permeability of the epithelium to become high. As active reabsorption of NaCl takes place, osmosis carries water out of the tubular fluid. Thus relatively high amounts of water are returned to the body fluids. The volume of the tubular fluid is reduced, and the fluid remains similar to the blood plasma in its osmotic pressure and NaCl concentration.

Urine Formation in Mammals

• The loops of Henle, collecting ducts, and vasa recta form parallel arrays in the medulla of the mammalian kidney, creating the structural basis for the ability to form urine hyperosmotic to the blood plasma. Among species of mammals of a particular body size, the species with long loops of Henle tend to be able to produce more concentrated urine than those with shorter loops.
• The proximal convoluted tubule reabsorbs—and returns to the body fluids—much of the NaCl and water from the filtrate by processes that do not alter the osmotic pressure of the tubular fluid. It also fully reabsorbs glucose and amino acids, returning them to the body fluids.
• After the tubular fluid passes through the loop of Henle, it is less concentrated than when it entered. Nonetheless, processes in the loop of Henle create the gradients of osmotic pressure and NaCl concentration in the medullary interstitial fluid that are responsible for the ultimate concentration of the urine. In the part of the loop where the ascending limb is thick, active NaCl transport creates a single-effect difference in osmotic pressure and NaCl concentration between adjacent parts of the ascending and descending limbs. By acting as a countercurrent multiplication system, the loop generates a difference in osmotic pressure and NaCl concentration from end to end that is much larger than the single effect.
• During antidiuresis, as tubular fluid makes its last pass through the medulla in the collecting ducts, nonurea solutes are concentrated because the collecting-duct walls are freely permeable to water, permitting osmotic equilibration between the tubular fluid and the medullary interstitial fluid. The high permeability of the collecting-duct epithelial walls to water results from insertion of aquaporin-2 molecules into cell membranes in response to ADH (vasopressin).
• During diuresis, the collecting-duct walls are poorly permeable to water, so tubular fluid is osmotically isolated from the medullary interstitial fluid and can be diluted by solute reabsorption.

Urine Formation in Other Vertebrates

• Freshwater teleost fish have nephrons structurally similar to amphibian nephrons. Marine teleost fish, however, usually lack the distal convoluted tubule and have a relatively poorly developed glomerular filtration apparatus that seems often to be supplemented by active solute secretion. A few marine fish are aglomerular and depend entirely on secretion.
• Birds and other reptiles have nephrons structurally similar to amphibian nephrons. Birds, in addition, have mammalian-type nephrons (with loops of Henle) organized into parallel arrays—the medullary cones—in which urine hyperosmotic to blood plasma can be made.

Urine Formation in Insects

• Primary urine is introduced into the Malpighian tubules by a secretory process usually based on active transport of KCl into the tubular fluid. As the primary urine flows down the Malpighian tubules, it may be modified by reabsorption or secretion, but typically remains isosmotic to the blood.
• The Malpighian tubules empty into the hindgut at the junction of the midgut and hindgut.
• The rectum modifies the volume, composition, and osmotic pressure of the urine in ways that help regulate the volume, composition, and osmotic pressure of the blood. The production of hyposmotic urine occurs by reabsorption of solutes in excess of water. Two of the known mechanisms of producing hyperosmotic urine, however, enable insects to reabsorb water in excess of solutes. Some of the insects that produce hyperosmotic urine in this way do so by local osmosis and solute recycling in rectal pads or papillae; others do so with a cryptonephridial complex. Saline-water insects may form hyperosmotic urine by secretion of solutes into the rectum.

Nitrogen Disposition and Excretion

• Animals that synthesize ammonia or urea as their primary nitrogenous end product are termed, respectively, ammonotelic or ureotelic. Animals that synthesize mainly uric acid or urates are uricotelic.
• Ammonotelism is the primitive condition and is seen in most water-breathing aquatic animals. Ammonia has the advantage of costing no extra ATP to produce. It is toxic, however. Thus, for an animal to be ammonotelic, the animal must have a means to void ammonia reliably as fast as it is produced so that blood levels are kept low. Aquatic animals void ammonia into the ambient water across their gills or general body surfaces.
• Ureotelism is more costly than ammonotelism because producing urea has an ATP cost. Urea is far less toxic than ammonia, however. Ureotelism has evolved principally in certain groups of vertebrates, in which it usually serves one or more of three possible functions: reducing the water requirement of routine nitrogen excretion (e.g., terrestrial amphibians and mammals), adjusting the blood osmotic pressure in advantageous ways (e.g., elasmobranch fish), and detoxification of waste nitrogen during periods when water-stressed animals cease urine production.
• Although uricotelism is even more costly per nitrogen atom than ureotelism, uric acid and related compounds have the advantage that they are so poorly soluble that they are low in toxicity, can be excreted in little water, and can be accumulated in the body indefinitely. Most groups of terrestrial animals, including invertebrates (e.g., insects) and vertebrates (e.g., birds, lizards, and snakes), are uricotelic or primarily produce other purines (e.g., guanine) or purine derivatives.

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