Both marine and terrestrial vertebrates must conserve water

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Some paleontologists believe that the immediate ancestors of the vertebrates lived in fresh water. If that is true, the nephron would have evolved as a structure to excrete excess water. Indeed, the

Uric acid precipitates in rectum and is secreted.

Uric acid precipitates in rectum and is secreted.

Malpighian Tubules

Semisolid wastes (including uric acid)

Hindgut

| Uric acid, Na+, and K+ are transported into the Malpighian tubules; H2O follows.

Semisolid wastes (including uric acid)

Hindgut

| Uric acid, Na+, and K+ are transported into the Malpighian tubules; H2O follows.

51.6 Malpighian Tubules in Insects The blind, thin-walled Malpighian tubules are attached to thejunction of the insect's midgut and hindgut and project into the spaces containing tissue fluid.

nephron can filter large quantities of blood and conserve valuable solutes such as glucose, amino acids, and certain ions while excreting water and nitrogenous wastes. How, then, have vertebrates adapted to environments where water must be conserved and salts excreted? The answer to this question differs among vertebrate groups. Even among the marine fishes, the adaptations of the bony fishes are different from those of the cartilaginous fishes.

marine bony fishes. Marine bony fishes cannot produce urine that is more concentrated than their tissue fluid, but unlike most marine animals, they osmoregulate their tissue fluid at only one-fourth to one-third the osmolarity of sea-water. They prevent excessive loss of water by producing very little urine.

Marine bony fishes take in seawater with their food, which results in large salt loads. The fish handle these salt loads by simply not absorbing some ions (such as Mg2+ or SO42-) from their guts and by actively excreting others (such as Cl-) from the gill membranes and from the renal tubules. Nitrogenous wastes are lost as ammonia from the gill membranes.

cartilaginous fishes. Cartilaginous fishes are osmocon-formers, but not ionic conformers. Unlike marine bony fishes, cartilaginous fishes convert nitrogenous wastes to urea

and another compound called trimethylamine oxide, and they retain large amounts of these compounds in their tissue fluids. As a result, their tissue fluids have an osmolari-ty close to that of seawater, so they do not lose body water to the environment by osmosis. These species have adapted to a concentration of urea in the body fluids that would be toxic to other vertebrates.

Sharks and rays still have the problem of excreting the large amounts of salts they take in with their food. One adaptation to solve this problem is a NaCl-secreting rectal gland.

amphibians. Most amphibians live in or near fresh water and stay in humid habitats when they venture from the water. Like freshwater fishes, most amphibian species produce large amounts of dilute urine and conserve salts. Some amphibians, however, have adapted to habitats that require water conservation.

Amphibians living in very dry terrestrial environments have reduced the permeability of their skin to water. Some secrete a waxy substance that they spread over the skin to waterproof it. Several species of frogs that live in arid regions of Australia burrow deep into the ground and remain there during long dry periods. They enter estivation, a state of very low metabolic activity and therefore low water turnover. When it rains, the frogs come out of estivation, feed, and reproduce. Their most interesting adaptation is an enormous urinary bladder. Before entering estivation, they fill the bladder with dilute urine, which can amount to one-third of their body weight. This dilute urine serves as a water reservoir that is gradually resorbed into the blood during the long period of estivation. Australian aboriginal peoples dig up esti-vating frogs as an emergency source of drinking water.

reptiles. Reptiles occupy habitats ranging from aquatic to extremely hot and dry. Three major adaptations have freed the reptiles from maintaining the close association with water that is necessary for most amphibians. First, reptiles do not need fresh water to reproduce, because they employ internal fertilization and lay eggs with shells that retard evaporative water loss. Second, they have scaly, dry skin that retards evaporative water loss. Third, they excrete nitrogenous wastes as uric acid solids, losing little water in the process.

birds. Birds have the same adaptations for water conservation that reptiles have: internal fertilization, shelled eggs, skin that retards water loss, and uric acid as the nitrogenous waste product. In addition, birds can produce urine that is more concentrated than their tissue fluids.

The nephron is the functional unit of the kidney

To understand how the kidney can fulfill different functions in different animal groups, we need to understand the structure of a nephron and how its parts work together. Vertebrate nephrons generally have three main constituents:

► A dense ball of capillaries called the glomerulus filters a portion of the blood plasma.

► Renal tubules receive and modify the glomerular filtrate.

► Peritubular capillaries bring substances to and take substances away from the renal tubules.

The glomerulus and peritubular capillaries are vascular structures closely associated with the renal tubule (Figure 51.7) The different ways in which these three elements can work together influence the amount and composition of the

3 The glomerulus, a knot of capillaries, is the site of blood filtration.

Site of filtration (glomerulus)

Site of tubular secretion and absorption

Urine processing

Bowman's capsule receives H2O and small molecules filtered from glomerular capillaries.

3 The glomerulus, a knot of capillaries, is the site of blood filtration.

Tubular Capillary

Bowman's capsule receives H2O and small molecules filtered from glomerular capillaries.

Site of filtration (glomerulus)

Site of tubular secretion and absorption

Urine processing

Urine

|The renal venule drains the peritubular capillaries.

|The processed filtrate (urine) of the individual nephrons enters collecting ducts and is delivered to a common duct leaving the kidney.

Urine

Blood is filtered in the glomerulus

The vascular component of the nephron is unusual in that the two capillary beds—the glomerulus and the peritubular capillaries—lie in series between the arteriole that supplies them and the venule that drains them. The glomerulus (plural, glomeruli) is a dense knot of very permeable vessels (Figure 51.8a). Blood enters the glomerulus through an afferent arteriole and exits through an efferent arteriole. The efferent arteriole gives rise to the peritubular capillaries, which surround the tubular component of the nephron.

The tubule component of the nephron—the renal tubule—begins with Bowman's capsule, which encloses the glomerulus. The glomerulus appears to be pushed into Bowman's capsule much like a fist pushed into an inflated balloon. The cells of the capsule that come into direct contact with the glomerular capillaries are called podocytes (Figure 51.8fr). These highly specialized cells have numerous armlike extensions, each with hundreds of fine, fingerlike projections. The podocytes wrap around the capillaries so that their fingerlike projections interdigitate and cover the capillaries completely.

The glomerulus filters the blood to produce a fluid (the renal filtrate) that lacks cells and large molecules. The walls of the capillaries, the basal lamina of the capillary endothelium, and the podocytes of Bowman's capsule all participate in filtration. As we saw in Chapter 49, fenes-trations between the endothelial cells of the capillaries allow water and small molecules to leave them, but are too

51.7 The Vertebrate Nephron The vertebrate nephron consists of a renal tubule closely associated with two capillary beds, the glomerulus and the peritubular capillaries.

Kidney Peritubular Capillaries

51.8 A Tour of the Nephron These scanning electron micrographs (SEMs) show the anatomical bases for kidney function. (a) The blood vessels of the kidney, shown without the tubule components. Each glomerulus (Gl) has an afferent (AAr) and an efferent (EAr) arteriole.The peritubular capillaries (Pt) are looser networks that surround the tubules of the nephron. (b) The podocytes (Po) are in direct contact with the capillaries of the glomerulus. Each podocyte has hundreds of tiny fingerlike projections that create filtration slits between them. Anything passing from the capillaries into the tubule of the nephron must pass through these slits. (c) A cross section of a glomerulus surrounded by the tubule cells that form Bowman's capsule (BC), which collects the filtrate and funnels it into the tubule (T) of the nephron.

small to permit red blood cells to pass through. The mesh-work of the basal lamina is even finer than the fenestrations, and it prevents large molecules from leaving the capillaries. Also smaller than the fenestrations are the narrow slits between the fingerlike projections of the podocytes. As a result of these anatomical adaptations, water and small molecules enter the renal tubule of the nephron (Figure 51.8c), but cells and proteins remain in the capillaries.

The force that drives filtration in the glomerulus is the pressure of the arterial blood. As in every other capillary bed, the pressure of the blood entering the permeable capillaries causes the filtration of water and small molecules. The glomerular filtration rate is high because glomerular capillary blood pressure is unusually high, and because the capillaries of the glomerulus, along with their covering of podocytes, are more permeable than other capillary beds in the body.

The renal tubules convert glomerular filtrate to urine

The composition of the filtrate that first enters the nephron is similar to that of the blood plasma. This filtrate contains glucose, amino acids, ions, and nitrogenous wastes in the

Nitrogenous Wastes

same concentrations as in the blood plasma, but it lacks the plasma proteins. As this fluid passes down the renal tubule, its composition changes as the cells of the tubule actively re-sorb certain molecules from the tubule fluid and secrete other molecules into it. When the tubule fluid leaves the kidney as urine, its composition is very different from that of the original filtrate.

The function of the renal tubules is to control the composition of the urine by actively secreting and resorbing specific molecules. The blood in the peritubular capillaries brings to the renal tubules molecules that the tubule cells will secrete into the urine. It also carries away any molecules that have been resorbed from the urine.

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Responses

  • Aloisio
    What has more density red blood cells, plasma only or platlets?
    8 years ago
  • Simret
    What is filtered out of the bowman's capsule?
    8 years ago
  • ABELA
    Where does absorption, filtration, and secretion?
    8 years ago
  • Anssi Simola
    What are the two capillary beds that are closely associated with the renal tubule of a nephron?
    8 years ago
  • veli
    How do marine bony and cartilagionus fishes conserve water?
    8 years ago
  • paul
    Do fish have adaptations for conserving water?
    8 years ago
  • terenzio
    Is water absorbed, excreted as a part of urine or both from the glomerulus?
    8 years ago
  • jacob
    Why is the nephron unusual?
    8 years ago
  • rufus
    What are malpighian tubules in insects?
    4 years ago
  • frank
    Which of the following vertebrates conserve salts and produce large amounts of dilute urine?
    4 years ago
  • elsa
    What part in the nephron conserves body water?
    2 years ago
  • sesuna
    What is malpigian tubule?
    3 months ago

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