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Inquiry Questions

FIGURE 49.4
Thermoregulation in insects. Some insects, such as the sphinx moth, contract their thoracic muscles to warm up for flight.
Why does muscle temperature stop warming after two minutes?
Answer: After two minutes of shivering, the thoracic muscles have warmed up enough to engage in full contractions. The muscle contractions that allow the full range of motion of the wings utilizes kinetic energy in the movement of the wings, rather than releasing the energy as heat, which occurred in the shivering response.

Self Test

1). Which of the following is not a method used in maintaining homeostasis in the body?
    a). behavioral changes
    b). negative feedback loops
    c). hormonal actions
    d). positive feedback loops
Answer: d

2). Which of the following describes an osmoconformer?
    a). mammals
    b). hagfish
    c). an animal that maintains a relatively constant blood osmolarity independent of its surroundings
    d). all of these
Answer: b

3). Which of the following animals uses Malpighian tubules for excretion?
    a). ants
    b). birds
    c). mammals
    d). earthworms
Answer: a

4). A shark's blood is isotonic to the surrounding seawater because of the reabsorption of ___________ in its blood.
    a). ammonia
    b). uric acid
    c). urea
    d). NaCl
Answer: c

5). Which of the following animals has the least concentrated urine relative to its blood plasma?
    a). bird
    b). freshwater fish
    c). human
    d). camel
Answer: b

6). Which of the following is a function of the kidneys?
    a). The kidneys remove harmful substances from the body.
    b). The kidneys recapture water for use by the body.
    c). The kidneys regulate the levels of salt in the blood.
    d). All of these are functions of the kidneys.
Answer: d

7). The longer loops of Henle and the collecting ducts are located in the
    a). renal cortex.
    b). renal medulla.
    c). renal pelvis.
    d). ureter.
Answer: b

8). Selective reabsorption of components of the glomerular filtrate occurs where?
    a). Bowman's capsule
    b). glomerulus
    c). loop of Henle
    d). collecting duct
Answer: c

9). Humans excrete their excess nitrogenous wastes as
    a). uric acid crystals.
    b). compounds containing protein.
    c). very toxic ammonia.
    d). relatively nontoxic urea.
Answer: d

10). Which of the following statements is not true?
    a). ADH makes the collecting duct more permeable to water.
    b). Guano contains high concentrations of uric acid.
    c). Aldosterone is produced by the hypothalamus in response to high levels of sodium ions in the blood.
    d). Uric acid is the least soluble of the nitrogenous waste products.
Answer: c

Test Your Visual Understanding

1). Indicate the areas of the nephron that the following hormones target, and describe when and how the hormones elicit their actions.
    a). antidiuretic hormone
    b). aldosterone
    c). atrial natriuretic hormone
Answer:
    1a). Antidiuretic hormone (ADH) is produced in the hypothalamus and is secreted by the posterior pituitary. ADH targets the collecting duct of the nephron and stimulates the reabsorption of water from the urine by increasing the permeability of water in the walls of the duct. The primary stimulus for ADH secretion is an increase in the osmolarity of blood plasma, which occurs when a person is dehydrated or when a person eats salty foods. Osmoreceptors in the hypothalamus detect the increased osmolarity and stimulate the release of ADH.
    1b). Aldosterone is produced and secreted by the adrenal cortex in response to a drop in blood Na+ concentration. Aldosterone stimulates the distal convoluted tubules to reabsorb Na+, decreasing the excretion of Na+ in the urine. The reabsorption of Na+ is followed by Cl and water, and so aldosterone has the net effect of retaining both salt and water. Aldosterone secretion however, is not stimulated by a decrease in blood osmolarity, but rather by a decrease in blood volume. A group of cells located at the base of the glomerulus, called the juxtaglomerular apparatus, detect drops in blood volume that then stimulates the renin-angiostensin-aldosterone system.
    1c). Atrial natriuretic hormone (ANH) is produced and secreted by the right atrium of the heart, in response to an increase in blood volume. The secretion of ANH results in the reduction of aldosterone secretion. With the secretion of ANH, the distal convoluted tubules reduce the amount of Na+ that is reabsorbed, and likewise reduces the amount of Cl and water that is reabsorbed. The final result is the reduction in blood volume.

Apply Your Knowledge

1). John's doctor is concerned that John's kidneys may not be functioning properly due to a circulatory condition. The doctor wants to determine if the blood volume that is flowing through the kidneys (called renal blood flow rate) is within normal range. Calculate what would be a "normal" renal blood flow rate based on the following information:
John weighs 90 kg. Assume a normal total blood volume is 80 ml/kg body weight, and a normal heart pumps the total blood volume through the heart once per minute (cardiac output). Also assume that the normal renal blood flow rate is 21% of cardiac output.
Answer: Under normal conditions, John's total blood volume would be 80 ml/kg body weight or 90 x 80 = 7,200 ml or 7.2 liters
His cardiac output would be 7.2 l/min
His normal renal blood flow rate would be 21% of cardiac output or 7.2 l/min x 0.21 = 1.5 l/min
If John's kidneys are not affected by his circulatory condition, his renal blood flow rate should be about 1.5 l/min.

2). The glomeruli filter out a tremendous amount of water and molecules needed by the body, which must later be reclaimed by the energy-requiring process of reabsorption. The Malpighian tubules of the insects might seem to function more logically, secreting molecules and ions that need to be excreted. What advantages might a filtration-reabsorption process provide over a strictly secretion process of elimination?
Answer: The Malpighian tubules work very well for insects that may not require the "fine-tuning" of electrolyte balance that is required in mammals and other animals that utilize a filtration-reabsorption process for excretion. The advantages of a filtration-reabsorption system allows for the maintenance of electrolyte balance within a very narrow range, which affects blood volume, blood pressure, and many other vital body functions. Without the regulation of Na+ reabsorption (increasing when necessary and decreasing when necessary), which affects water reabsorption, the body could not function properly. This is evident by the absolute necessity of aldosterone in the body. Without aldosterone, a person would die.







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