49.1 The regulatory systems of the body maintain homeostasis.
The Need to Maintain Homeostasis
• Homeostasis refers to dynamic constancy of the internal environment around set points. (p. 1040)
• A negative feedback loop occurs as the activity of effectors is negatively influenced by the effects they produce. (p. 1040)
• Endothermic organisms can maintain a relatively constant body temperature independent of the environmental temperature. The body temperature of ectothermic organisms is basically dependent on the environmental temperature. (p. 1042)
Antagonistic Effectors and Positive Feedback
• Control by antagonistic effectors occurs when the increasing activity of one effector is accompanied by the decreasing activity of an antagonistic effector, affording a fine degree of control. (p. 1043)
• In a positive feedback loop, perturbations cause the effector to drive the value of the controlled variable even farther from the set point. (p. 1043)
49.2 The extracellular fluid concentration is constant in most vertebrates.
Osmolality and Osmotic Balance
• An animal's body must be able to take water from its environment or excrete excess water into its environment in order to maintain osmolality (total moles of solute per kilogram of water). (p. 1044)
• An animal cell placed in a hypertonic solution tends to lose water, while an animal cell placed in a hypotonic solution tends to gain water. (p. 1044)
• Osmoconformers have body fluids with osmolality equal to that of the surrounding environment (seawater). But osmoregulators maintain a relatively constant blood osmolality despite a different concentration in the surrounding environment. (p. 1044)
Osmoregulatory Organs
• Many invertebrates filter fluids into tubules where water and ions are reabsorbed, leaving waste products to be eliminated. (p. 1046)
• Insects create an osmotic gradient by secreting K+ into Malpighian tubules, causing water to enter the tubules by osmosis. Water and K+ are reabsorbed through the epithelium of the hind gut. (pp. 1046-1047)
• Vertebrates create a tubular fluid, urine, by pressurized blood filtration in the kidney. (p. 1047)
Evolution of the Vertebrate Kidney
• The kidney is made of thousands of repeating nephrons. (p. 1048)
• Freshwater fish excrete large amounts of dilute urine, while marine bony fish (teleosts) drink seawater and excrete isotonic urine. (p. 1048)
• Multiple modifications of the nephron, including the loop of Henle, allow birds and mammals to reabsorb water and produce hypertonic urine. (p. 1048)
Ammonia, Urea, and Uric Acid
• Metabolic breakdown of amino acids and nucleic acids produces ammonia in the liver. (p. 1051)
• Reptiles, birds, and insects excrete nitrogenous wastes as low-toxicity uric acid, which is only slightly soluble in water and can be excreted using small amounts of water. (p. 1051)
• Most mammals eliminate nitrogenous wastes in the form of urea, which is water soluble. (p. 1051)
49.3 The functions of the vertebrate kidney are performed by nephrons.
The Mammalian Kidney
• Each kidney receives blood from a renal artery and produces urine, which then drains from each kidney through a ureter and is carried to the urinary bladder. (p. 1052)
• Each kidney contains about one million nephrons, and is divided into the cortex and medulla. (p. 1052)
• Excretion of water in urine eliminates a variety of potentially harmful substances and helps maintain blood volume and pressure. (p. 1054)
Transport Processes in the Mammalian Nephron
• The loop of Henle produces a hypertonic renal medulla due to NaCl extrusion from the ascending limb and interaction with the descending limb. (p. 1055)
• The hypertonic medulla draws water osmotically from the distal convoluted tubule and collecting duct, which are permeable to water under the influence of antidiuretic hormone. (p. 1056)
Hormones Control Osmoregulatory Functions
• The kidneys excrete a hypertonic urine when the body needs to conserve water and a hypotonic urine if the body contains too much water. (p. 1057)
• ADH stimulates the reabsorption of water and the excretion of hypertonic urine. (p. 1058)
• Aldosterone promotes NaCl and water reabsorption across the distal convoluted tubule and collecting ducts, as well as K+ secretion into the tubules. (pp. 1058)