W3006   Physiology    Fall 2002     Problem set #1 - Homeostasis, Thermoregulation, Hormone assays


1.        T/F, Correct if false: 

A. In an organism that maintains homeostasis, blood glucose levels stay constant.

B. On a hot day, blood vessels in the skin constrict to maintain body temperature.

C. The number of active sweat glands in human skin is fixed by our genes.

D. The integrator in homeostatic control mechanisms is located in the hypothalamus.


2.        x>y, x<y, or x=y?

        A.      x. heat lost from body before going into shower (dry skin)

           y. heat lost from body when coming out of shower (wet skin)


        B.       x. increase in body temperature caused by shivering in the cold

           y. increase in body temperature caused by vasoconstriction in the cold  


C.       x. diameter of blood vessels in skin when in hot environment

  y. diameter of blood vessels in skin when in cold environment


        D.       x. temperature in the mouth

           y. temperature in the hypothalamus


3.        A football fan notices that a drink of alcohol makes him feel a sudden flush of warmth, so he drinks a few cans of beer to warm himself at a chilly football game.  Surprisingly, he ends up feeling colder than his non-drinking companions.  Suggest an effect alcohol might have on the circulatory system that would explain both these phenomena: an initial feeling of warmth, followed by cooling of the extremities.


4.        What is the advantage of having thermoreceptors in both the skin and near the internal organs (in the abdominal cavity and brain)?


5.        The temperature of the hypothalamus always remains relatively constant, even though an animal moves from a very hot to a very cold environment.  Explain.


6.        While observing rats in a room that is slowly heating up, you notice that when the temperature reaches 30 C the animals begin to show a new behavior - licking themselves and spreading the saliva over the body, particularly on hairless areas such as the paws, tail and nose.  What might be the function of this behavior?


7.        On a warm day, Guinevere gaily traipses along for 8 hours, producing 50 ml of perspiration each hour.  Arthur loses an equal amount of fluid (400 ml), but he loses it all in half an hour of heavy sweating while strenuously jousting with his knights. In addition, the sweat that Arthur produces has a higher concentration of salt than Guinevere's perspiration.   Why is Arthur's sweat saltier than Guinevere's? (Note: The answer is not a male/female difference.)


8.        It's a hot day and your dog comes running over.  When she reaches you, she is panting (taking rapid shallow breaths through her mouth.

A. How does this help her maintain homeostasis?

B. What do you do instead of panting?  Why do humans and dogs show different responses to increased body temperature?


9.        A traveler to Tibet in the 1930's described a type of yoga in which neophytes would sit on the ground, naked, wrapped in sheets dipped in ice water.  As soon as the sheet was dry, it was again dipped in ice water and wrapped on his body.  Several hours later, the one who had dried the most sheets was declared the winner.  Practitioners said they had learned to change their body temperature. 

More recently, scientists  put thermistors to measure temperature on various parts of the body of three monks while they sat still and practiced this kind of yoga.  They found that during one hour of meditation, skin temperature in fingers and toes increased 3-8 degrees C, but temperature of the skin of the back,  navel and calf did not change much. 

A.  Describe two mechanisms by which skin temperature might increase.  

B.  Which of these two mechanisms is more likely to explain the warm fingers and toes of the meditating monks?


10.     In very rare cases, some individuals develop a life-threatening temperature change when receiving anesthesia.  It has been found that they have an inherited defect in skeletal muscle, such that anesthesia causes Ca++ to flood the muscle cell, causing the muscle to contract and remain in a contracted state for a prolonged period of time.   This should cause body temperature to _____  (increase or decrease) .  Explain why


11.     Claude Bernard knew that for sugar to burn in a dish in the lab, oxygen must be present.  He also knew that after eating a meal, there is an increase in sugar (glucose) in the blood.  He assumed that the blood carries this sugar to the lungs, where it is broken down in the presence of inhaled oxygen, releasing energy in the form of heat, just as occurs in the dish.  He therefore expected the blood that leaves the left  part of the heart would be a little warmer than the blood that enters the right part of the heart.  When he actually measured blood temperature, however, he found that the opposite was true:  Blood entering the right heart was warmer than blood leaving the left heart.  Explain why. Summary of Bernard's first hypothesis: 

INTESTINES -->blood -->RIGHT  HEART -->blood -->LUNGS -->blood -->LEFT HEART--> blood

(sugar absorbed,          (cooler blood enters heart)          (sugar burns w/oxygen,                (warmer blood leaves heart)

transferred to blood)                                          releases heat to blood)


12.     Rudyard Kipling tried to explain "how the rhinoceros got his skin":  "....the Rhinoceros took off his skin and carried it over his shoulder as he came down to the beach to bathe...He waddled straight into the water and blew bubbles through his nose, leaving his skin on the beach."  It probably didn't happen quite like this, but it is true that rhinoceri spend a lot of time wandering through swamps and wallowing in the mud.  A microscopic analysis of rhinoceros skin indicates that sweat glands are totally absent.  Explain why these animals do so much wallowing.


13.     A woman had polio as an infant during the epidemic of the 1950's.  The poliovirus destroyed some of the motor neurons, which innervate the muscles of her arms and legs, leading the muscles to atrophy (degenerate) to the point that she has difficulty walking or lifting anything.  She is most likely to complain when the weather is a. hot b. cold.  Explain why.

14.     Sometimes/Always/Never:    If you have a choice, it's always better to use a radioimmunoassay, rather than a bioassay, to measure blood levels of hormone.

15.     You use a radioimmunoassay to measure the amount of ACTH present in the pituitary gland.  You find more ACTH in pituitary A than in pituitary B as measured by your RIA.  Yet when you inject these pituitary extracts into a test animal, the animal shows identical responses to both.  Explain.

16.     Cells slough off the walls of the vagina, and can easily be removed and examined under the microscope (much like you may have looked at cheek cells under the microscope in high school biology).  A female rat has a 4-day estrous cycle (something like our menstrual cycle), and on each day of the cycle,different types of cells slough off, as shown in the figure below.   Estrogen (produced by the ovary) is high on Day 2, and is responsible for the characteristic appearance of the cells on that day.  Consider this biological response to estrogen, and devise a bioassay for estrogen, using female rats.

17.     . (Answer both parts) You perform a radioimmunoassay of the type described in class.  To determine the amount of hormone in your unknown sample, you'll need to measure the amount of radioactivity in the  A. precipitate  B. supernatant.  and you would expect that higher radioactivity indicates that there is A. more B. less hormone in your sample.

18.     A false positive may occur in the rabbit-based pregnancy test if

A.  it is done in the summer, so the rabbit releases sperm

B.  the rabbit is under stress, so it releases hCG

C.  the rabbit happens to release an ovulatory  LH surge

D.  the rabbit produces antibodies against the human hormone

E.   the woman has androgen-insensitivity syndrome







Answers - Problem set #1 - Homeostasis, Thermoregulation


1.        True/false

A.  F.  Homeostasis does not imply an unchanging situation, but rather one in which changes are kept within narrow limits.

B.  F.  Blood vessels would dilate to increase heat loss from the surface of the body.

C.  F. Climate we're exposed to in early life also plays a role.

D.  Sometimes.  True for thermoregulation, but in other cases (calcium regulation) there are other integrators.


1.        A. y.  As the water on the skin evaporates, it draws heat away from body.

B. x. Shivering actually produces heat,  increases body temp.  Vasoconstriction just prevents a fall in body temp.

C. x. Vasodilation occurs in hot environment.

D. y. Core temperature is highest, decreases slightly as approach the body surface.


2.        Why can't I re-number this?


3.        Alcohol causes dilation of peripheral blood vessels.  Initially, this brings warm blood to the peripheral thermoreceptors, which send a message to the hypothalamus that it is warm, the brain gets the message, and the fan notices he feels warm.  But it's really not warm outside, and there is a large temperature gradient between the body and the air.  With continued dilation of the blood vessels, heat is lost to the cold environment, and he ends up feeling chilly.

4.        It is critical that internal temperature remains fairly constant, so central receptors are critical to provide information about core temperature.  The peripheral receptors allow the hypothalamus to make adjustments before changes in temperature occur centrally.


5.        The hypothalamus is able to sense a decrease in temperature as small as 0.02 degrees C.  When the temperature drops, the hypothalamus brings into play the muscle contraction, vasoconstriction needed to raise the temperature back up to 100 deg F.  So the temperature in the hypothalamus will vary between about 99.98 and 100.02.  Peripheral temperature is another matter; it can vary from 68 to 104 degrees F. 


6.        Thermoregulation.  The rats lack sweat glands, and so spread saliva on the body surface.  As water evaporates from the skin surface, the heat required to transform water from a liquid to a gas is absorbed from the skin, cooling the body. 

7.        The salt is reabsorbed by a carrier protein which is saturable.  Guinevere  and Arthur secreted the same amount of salt into the gland to produce sweat, so they both need to reabsorb that amount of salt out of the gland.  But Arthur tries to reabsorb this salt in a short time interval, saturates his carriers, and some of the salt leaves the duct with the sweat.


8.        a. The quick exhalation blows air over tongue, causing the saliva on the tongue to evaporate more quickly, removing heat from the body.  (Try it.  You'll feel your tongue getting dry.)

b. We sweat.  This spreads water on the surface of the body, which absorbs heat as it goes from liquid to vapor.  For this to work, you need to have the sweat right on the skin, not a few cm away on a layer of hair.  You can't have it both ways -- either you get a fur coat (which is for the dogs) -- or you get to sweat.  Humans, sometimes called naked apes because they lack fur, may be the sweatiest mammals around.


9.        A. Skin temp might increase either because a)muscle contraction (shivering), increases the muscle temperature and the heat is transferred directly to the overlying skin and indirectly, via the blood, to the skin in other regions of the body, or b) vasodilation brings more blood from the warm core to the skin. 

B. Vasodilation is more likely, because the fingers have very little muscle tissue, probably not enough to produce significant heat by contraction alone.  It is usually the contractions of the large skeletal muscles that is responsible for increasing body temp, but if these were involved here, the warmth should be conducted to the entire body, not just the fingers and toes.  So it seems more likely that there is local vasodilation.


10.     Skeletal muscles are the "furnace" for mammals.  During the process of contraction, energy is released, which is used to do the work of moving the muscle, but some of the energy is released as heat. During shivering, the muscle repeatedly goes through this process of contraction, as it contracts, relaxes, and contracts again, over and over again, so large amounts of heat can be produced. In the case described here, the muscle contracts but remains in that contracted state, without relaxing, so it can't contract again, and no more energy can be released, and so the body temperature can drop to dangerous levels.


11.     Bernard concluded that the "burning" of glucose (aerobic respiration) must not occur in the lungs, as had been assumed, but rather occurs  deep in the body's core, and this heat is then transferred to the blood.  The reason blood is warmer in the right heart  is A) Blood in the right heart is coming directly from the core (near the skeletal muscles and the internal organs where most of the body heat is produced) and B) Blood in the left heart has just passed through the lungs, where it is separated from the "outside air" (inhaled into the lungs) by a single layer of epithelial tissue.  This thin layer not only allows rapid transfer of gases, but also allows rapid movement of heat to the outside.  (You can feel this heat leaving your body if you inhale onto your hand.)  So the blood is cooled slightly as it passes through the lungs, decreasing temp of the blood that enters the left heart.


12.     In hot temperatures, sweat on skin evaporates, which requires energy; body's heat provides this energy, and the loss of heat means the body cools off.  Muddy water substitutes for sweaty water, and as it evaporates, it cools the rhino.


13.     Response to cold requires repeated muscle contractions (shivering), and the energy produced during  contractions is released as heat.  Peripheral receptors can sense this decrease in temperature, send a message to the hypothalamus, which sends a message via neurons to the muscles.  So it could be that the motor neurons are less able to get messages to muscles and/or that the decrease in muscle mass means there's less muscle available to produce this heat (a smaller furnace). 

14.     Sometimes. If you're interested in whether the sample has hormone that's biologically active, then a bioassay would be better.

15.     RIA measures the immunoreactivity of the hormone, or how well the hormone binds to the antibody, but doesn't measure the hormone's biological activity.  ACTH is first produced as a prohormone, POMC, which contains additional amino acid sequences.  The ACTH part of this molecule may be active in the RIA, but the POMC molecule itself will not show biological activity.  So pituitary A may contain more POMC than pituitary B, but both may have identical amounts of the bioactive ACTH.  Another answer that would be acceptable is that pituitary A really does have more of the biologically active ACTH.  But in order for a hormone to have an effect in the test animal, it must combine with a receptor.  The amount of hormone in extracts from B may be so much that it saturates the receptors.  Injecting the larger amount of hormone from A may have the same effect as the injection of B's hormone, because it also saturates the receptors;  the excess hormone in A would therefore have no further effect in the test animal.  


16.     A bioassay assays (or tests) for the presence of a hormone by measuring some biological response to the hormone. The first step in the bioassay is constructing a standard curve(dose-response curve) in which increasing doses of estrogen are injected into rats, and the biological response is noted (as % of vaginal cells that are "type A").  To do this, you must ensure that the rat is responding only to the exogenous estrogen that you inject, and not to her own endogenously-produced estrogen, and so you want to use rats whose ovaries had been removed previously.  To analyze the estrogen content of your unknown sample, you inject a certain volume of the sample into an ovariectomized rat, observe the type A cells in the vaginal fluid, and read off your standard curve what estrogen concentration this corresponds to.

17.     A precipitate  B. less

18.     C