Passage 43
Homeostasis, an animal’s maintenance of certain internal variables within an acceptable range,
particularly in extreme physical environments, has long interested biologists. The desert rat and
the camel in the most water-deprived environments, and marine¹ vertebrates in an all-water
environment, encounter the same regulatory problem: maintaining adequate internal fluid balance.
For desert rats and camels, the problem is conservation of water in an environment where standing
water is nonexistent, temperature is high, and humidity is low. Despite these handicaps, desert rats
are able to maintain the osmotic pressure of their blood, as well as their total boy-water content, at
approximately the same levels as other rats. One countermeasure is behavioral: these rats stay in
burrows during the hot part of the day, thus avoiding loss of fluid through panting or sweating,
which are regulatory mechanisms² for maintaining internal body temperature by evaporative
cooling. Also, desert rats’ kidneys can excrete a urine having twice as high a salt content as sea
water.
Marine vertebrates experience difficulty with their water balance because though there is no
shortage of seawater to drink, they must drink a lot of it to maintain their internal fluid balance.
But the excess salts from the seawater must be discharged somehow, and the kidneys of most
marine vertebrates are unable to excrete a urine in which the salts are more concentrated than in
seawater. Most of these animals have special salt-secreting organs outside the kidney that enable
them to eliminate excess salt.
 
261. Which of the following most accurately³ states the purpose of the passage?
(A)   To compare two different approaches to the study of homeostasis
(B)    To summarize the findings of several studies regarding organisms’ maintenance of internal variables in extreme environments
(C)    To argue for a particular hypothesis regarding various organisms’ conservation of water in desert environments
(D)    To cite examples of how homeostasis is achieved by various organisms (D)
(E)     To defend a new theory regarding the maintenance of adeuate fluid balance
 
262. According to the passage, the camel maintains internal fluid balance in which of the
following ways?
I. By behavioral avoidance of exposure to conditions that lead to fluid loss
II. By an ability to tolearte high body temperatures
III. By reliance on stored internal fluid supplies
(A)   I only
(B)    II only
(C)    I and II only
(D)    II and III only (B)
(E)     I, II, and III
 
263. It can be inferred from the passage that some mechanisms that regulate internal body
temperature, like sweating and panting, can lead to which of the following?
(A)   A rise in the external body temperature
(B)    A drop in the body’s internal fluid level
(C)    A decrease in the osmotic pressure of the blood
(D)    A decrease in the amount of renal water loss (B)
(E)     A decrease in the urine’s salt content
 
264. It can be inferred from the passage that the author characterizes the camel’s kidney as
“entirely unexceptional” (line 24) primarily to emphasize that it
(A)   functions much as the kidney of a rat functions
(B)    does not aid the camel in coping with the exceptional water loss resulting from the extreme conditions of its environment
(C)    does not enbale the camel to excrete as much salt as do the kidneys of marine vertebrates
(D)    is similar in structure to the kidneys of most mammals living in water-deprived environments (B)
(E)     requires the help of other organs in eliminating excess salt

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