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58.1 The regulatory systems of the body maintain homeostasis.
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• Negative feedback loops maintain nearly constant extracellular conditions in the internal environment of the body, a condition called homeostasis.
• Antagonistic effectors afford an even finer degree of control.
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1. What is homeostasis? What is a negative feedback loop? Give an example of how homeostasis is maintained by a negative feedback loop.
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58.2 The extracellular fluid concentration is constant in most vertebrates.
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• Osmoconformers maintain a tissue fluid osmolality equal to that of their environment, whereas osmoregulators maintain a constant blood osmolality that is different from that of their environment.
• Insects eliminate water by secreting K+ into Malpighian tubules and the water follows the K+ by osmosis.
• The kidneys of most vertebrates eliminate water by filtering blood into nephron tubules.
• Freshwater bony fish are hypertonic to their environment, and saltwater bony fish are hypotonic to their environment; these conditions place different demands upon their kidneys and other regulatory systems.
• Birds and mammals are the only vertebrates that have
loops of Henle and thus are capable of producing a hypertonic urine.
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2. What is the difference between an osmoconformer and an osmoregulator? What are examples of each?
3. How does the body fluid osmolality of a freshwater vertebrate compare with that of its environment? Does water tend to enter or exit its body? What must it do to maintain proper body water levels?
4. In what type of animal are Malpighian tubules found? By what mechanism is fluid caused to flow into these tubules? How is this fluid further modified before it is excreted?
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58.3 The functions of the vertebrate kidney are performed by nephrons.
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• The primary function of the kidneys is homeostasis of blood volume, pressure, and composition, including the concentration of particular solutes in the blood and the blood pH.
• Bony fish remove the amine portions of amino acids and excrete them as ammonia across the gills.
• Elasmobranchs, adult amphibians, and mammals produce and excrete urea, which is quite soluble but much less toxic than ammonia.
• Insects, reptiles, and birds produce uric acid from
the amino groups in amino acids; this precipitates, so that little water
is required for its excretion.
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5. What drives the movement of fluid from the blood to the inside of the nephron tubule at Bowman's capsule?
6. In what portion of the nephron is most of the NaCl and water reabsorbed from the filtrate?
7. What causes water reabsorption from the collecting duct? How is this influenced by antidiuretic hormone?
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58.4 The kidney is regulated by hormones.
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• Antidiuretic
hormone is secreted by the posterior pituitary gland in response to an
increase in blood osmolality, and acts to increase the number of water
channels in the walls of the collecting ducts.
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8. What effects does aldosterone have on kidney function? How is the secretion of aldosterone stimulated?
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