Concept 36.1   Kidneys Regulate the Composition of the Body Fluids

• The body fluids of an animal have three characteristics: osmotic pressure, ionic composition, and volume.
• Kidneys are organs composed of tubular structures that produce urine—an aqueous solution derived from the blood plasma—for excretion. The primary function of kidneys is to regulate the composition and volume of the blood plasma by means of controlled removal of solutes and water from the plasma. Review Figure 36.2 and ACTIVITY 36.1
• Kidney function can be expressed in terms of the composition of the urine as a ratio of the composition of the blood plasma. Such a ratio is called a urine/plasma, or U/P, ratio. If the urine is less concentrated in total solutes than the plasma (osmotic U/P < 1), the kidneys are making the plasma become more concentrated. If the urine is more concentrated than the plasma (U/P > 1), the kidneys are making the plasma become more dilute.
• Animals vary in the U/P ratios that can be achieved by their kidneys and thus in how concentrated their urine can be. Mammals, birds, and insects are the only animals that can produce concentrated urine with an osmotic U/P ratio exceeding 1.0.
• Some groups of animals have tissues or organs other than the kidneys that excrete ions at high total concentrations—a process called extrarenal salt excretion. Review Figure 36.3

Concept 36.2   Nitrogenous Wastes Need to Be Excreted

• Metabolism of proteins and nucleic acids produces toxic nitrogenous wastes, which must be eliminated from the body. Review Figure 36.4
• Ammonotelic animals produce ammonia as their primary nitrogenous waste. They are typically water-breathing aquatic animals that eliminate ammonia by diffusion across their gills or other permeable body surfaces.
• Terrestrial animals nearly always excrete nitrogenous wastes that are far less toxic than ammonia. Synthesizing these alternative nitrogenous wastes requires investment of energy.
• Ureotelic animals detoxify ammonia by converting it mostly to urea before excretion. These animals include mammals and most amphibians.
• Uricotelic animals convert ammonia mostly to uric acid or other compounds closely related to uric acid. They include insects, spiders, and birds and some other reptiles. Uric acid and the compounds related to it can be excreted in precipitated form, resulting in little loss of water.

Concept 36.3   Aquatic Animals Display a Wide Diversity of Relationships to Their Environment

• Osmolarity is a measure of the overall solute concentration (osmotic pressure) of a fluid. An animal may have body fluids that have the same osmotic pressure as the animal’s environment (isosmotic) or that have a higher (hyperosmotic) or lower (hyposmotic) osmotic pressure than the animal’s environment.
• Ocean bony fish are hyposmotic to their environment; they tend to lose water by osmosis and gain ions by diffusion. Freshwater fish are hyperosmotic to their environment; they tend to gain water by osmosis and lose ions by diffusion. To regulate the composition of their body fluids, both ocean and freshwater fish employ their kidneys, ion pumping mechanisms in their gills, and drinking behavior. Review Figure 36.5
• Migratory bony fish such as salmon switch between being hyperosmotic regulators in fresh water and hyposmotic regulators in seawater.
• Some marine invertebrates face varying environmental osmolarities. These animals can be osmotic conformers or osmotic regulators. Most marine invertebrates are osmotic conformers and do not survive well in dilute waters. Review Figure 36.6

Concept 36.4   Dehydration is the Principal Challenge for Terrestrial Animals

• Terrestrial animals can readily become dehydrated because of evaporative loss of water from their body fluids.
• Humidic terrestrial animals have outer body coverings (i.e., skin, exoskeleton, or the like) that are highly permeable to water. Such animals lose water so rapidly by dehydration that they can tolerate only limited exposure to dehydrating conditions.
• Xeric terrestrial animals have outer body coverings that highly limit evaporation of their body fluids. These animals can therefore spend indefinite periods in the open air. The low water permeability of their body coverings results from the presence of lipids.
• Metabolic water is the water produced by the oxidation of food materials during metabolism. It sometimes serves as a major source of water for desert animals that have very low total water needs because they conserve water exceptionally well.

Concept 36.5   Kidneys Adjust Water Excretion to Help Animals Maintain Homeostasis

• Vertebrate kidneys consist of many tubules called nephrons.
• The fluid first introduced into a nephron is the primary urine. This fluid is modified as it flows through the nephron by processes of reabsorption and secretion. The fluid that is excreted into the outside environment is the definitive urine.
• Urine formation in a vertebrate nephron begins at the closed end of the nephron, which consists of a Bowman’s capsule that surrounds a glomerulus. The primary urine is formed by ultrafiltration, during which fluid moves out of the blood plasma in the glomerulus and into the lumen of the Bowman’s capsule, driven by the force of blood pressure. The primary urine is similar in composition to the blood plasma in most ways except for lacking blood cells and proteins. Review Figure 36.8 and ACTIVITY 36.2
• The rate at which primary urine is introduced into all the Bowman’s capsules collectively is the glomerular filtration rate (GFR).
• The proximal convoluted tubule of a nephron reabsorbs water, Na⁺, Cl^-, and certain other solutes, reducing the volume of the tubular fluid without changing its osmolarity.
• Antidiuretic hormone (ADH) controls the function of the late kidney tubules: the distal convoluted tubules in amphibians and the collecting ducts in mammals. When the ADH concentration is high, aquaporin molecules are inserted in the cell membranes of the epithelial cells of the late tubules, making the cells water permeable. Consequently, the tubular fluid loses water by osmosis into the surrounding tissue fluids, reducing the volume of the tubular fluid and raising its concentration (an antidiuretic state). When the ADH concentration is low, the epithelial cells are poorly permeable to water. Thus, the volume of the tubular fluid is not reduced as much by osmosis, and the fluid is rendered dilute by removal of solutes (a diuretic state). Review Figure 36.9 and Figure 36.10
• When the concentration of ADH is high, an amphibian makes urine that is isosmotic with the blood plasma because the tissue fluids surrounding the distal convoluted tubules are isosmotic with plasma. A mammal, however, makes urine that has an osmotic pressure higher than the plasma because the tissue fluids surrounding the collecting ducts are hyperosmotic to the plasma.
• In a mammal, the loops of Henle create a concentration gradient in the tissue fluids of the renal medulla by countercurrent multiplication. This is the mechanism by which the tissue fluids surrounding the collecting ducts are made hyperosmotic to the plasma. Review Figure 36.11, ANIMATED TUTORIAL 36.1 and ANIMATED TUTORIAL 36.2
• The excretory system of an insect is composed of the Malpighian tubules (which join the gut at the junction of midgut and hindgut) and the hindgut. Primary urine is formed by a secretory mechanism, often driven by active transport of KCl into the Malpighian tubules. Review Figure 36.12