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Biology, 6/e
Author Dr. George B. Johnson, Washington University
Author Dr. Peter H. Raven, Missouri Botanical Gardens & Washington University
Contributor Dr. Susan Singer, Carleton College
Contributor Dr. Jonathan Losos, Washington University

Maintaining Homeostasis

Answers to Review Questions

Chapter 58 (p. 1194)

1. Homeostasis is the maintenance of constant conditions within the body. A negative feedback loop prevents the departure of a controlled variable such as pH or temperature from its normal, or ideal, value. Changes in the controlled variable trigger responses that oppose the change and restore the variable to its ideal level. An example of negative feedback is the regulation of body temperature. If the body temperature exceeds 37oC, it is detected by sensors in the brain that stimulate effectors like the sweat glands in the skin, lowering the body's temperature. If the temperature is less than 37oC other effectors are stimulated, including those that cause shivering, producing heat in muscles that is transferred through the blood system to the rest of the body. Blood vessels in the extremities constrict as well, reducing heat loss to the surrounding air.

2. An osmoconformer is an organism whose body fluid composition is osmotically similar to its surroundings. Hagfish are the only vertebrates that are strict osmoconformers. Sharks retain urea rather than NaCl, and are osmoconformers when total osmolality is considered. An osmoregulator, on the other hand, maintains a relative constant internal osmotic environment. All other vertebrates are osmoregulators.

3. A freshwater vertebrate has a higher concentration of salt than the environment (hyperosmotic), and therefore water tends to enter its body. To maintain proper body water levels, it must excrete water to prevent self-dilution.

4. The insect system uses an osmotic gradient to pull blood through the Malpighian tubules. Prior to excretion, the wastes are further concentrated through the selective reabsorption of water by the hindgut of the insect.

5. A nephron is the filtration-reabsorption device in the kidney. Bowman's capsule is located at the front of the nephron. The glomerulus is a fine network of capillaries located within Bowman’s capsule. Filtration occurs as blood pressure forces fluid through the capillary walls, retaining proteins, cells, and large molecules and allowing water and small molecules to pass through to the interior of the capsule.

6. Most salt and water is absorbed across the membranes of the proximal convoluted tubule.

7. Water is reabsorbed from the collecting duct because it plunges into the renal medulla with its strong osmotic gradient. Antidiuretic hormone makes collecting ducts more permeable to water by opening more water channels in the plasma membranes.

8. Aldosterone stimulates the kidneys to reabsorb Na+, ultimately causing retention of water. Aldosterone secretion is an indirect response to lowered blood Na+. Lower Na+ results in decreased blood volume and reduced flow past the renal juxtaglomerular apparatus. It responds by secreting rennin into the blood. Rennin catalyzes the reaction angiotensin → angiotensin I. Angiotensin I is converted to angiotensin II which causes constriction of blood vessels and stimulates the adrenal cortex to secrete aldosterone.