Osmoregulation Problem Set

This is similar to the gas exchange problem set that you did earlier. Answer these questions on a Scantron (one for each person). Mark all correct answers; some questions may have more than one answer. You may use books or notes or discuss answers with your classmates. Please write "Osmoregulation" in the subject area of your Scantron. If you don't write this, your score might not be recorded, because I have a big pile of mixed-up Scantrons.

The page numbers given with the answers are for Campbell, 10th edition. I will update them for the 11th edition when I have time.

Osmoregulation & nitrogenous waste

1. Insects conserve water by reabsorbing water from the rectum before waste materials are excreted. Which of the following help create the osmotic gradient necessary for this reabsorption?

  1. Active transport of ions from the hemolymph into the Malphigian tubules.
  2. Active transport of ions from the Malphigian tubules into the hemolymph.
  3. Active transport of ions from the hemolymph into the rectum.
  4. Active transport of ions from the rectum into the hemolymph.
  5. Precipitation of uric acid in the rectum.

Answer: D and E. A also happens, but it draws water into the Malphigian tubules by osmosis; this is secretion, not reabsorption. B and C are going the wrong direction, and don't occur. Precipitation of uric acid is important because it converts dissolved nitrogenous waste into a form that isn't a solute; this reduces the solute concentration in the rectum, helping to create an osmotic gradient that favors reabsorption of water.

2. The urine of earthworms is normally

  1. Hypoosmotic to their cells.
  2. Isoosmotic to their cells.
  3. Hyperosmotic to their cells.

Answer: A. Earthworms cannot make hyperosmotic urine, and they typically need to eliminate excess water that they absorb from their moist environment.

3. Which of these costs the most energy to produce?

  1. Ammonia.
  2. Urea.
  3. Uric acid.

Answer: C. Ammonia (NH3) costs no extra energy to produce, but it is toxic and must be eliminated in a large volume of water. Fish eliminate their waste in this form. Converting ammonia (or ammonium, NH4+) into urea costs some energy, but the benefit is that urea is nontoxic. Also, urea contains two nitrogen atoms, so converting ammonia into urea reduces the osmolarity of the solution. Converting ammonia into uric acid costs more energy, but each uric acid molecule contains four nitrogen atoms, and urea isn't very water soluble, so it can be eliminated in precipitated form using little water.

4. Most marine bony fish are

  1. Osmoregulators.
  2. Osmoconformers.

Answer: A. Marine bony fish maintain their internal osmolarity much lower than that of seawater.

5. Cartilaginous fish such as sharks normally live in an environment that is

  1. Hypotonic to their cells.
  2. Isotonic to their cells.
  3. Hypertonic to their cells.

Answer: A. Sharks accumulate high concentrations of urea and TMAO in their tissues, making their body fluids have a slightly higher osmolarity than the surrounding seawater. See Campbell, p. 973.

Human kidney

6. Where does active transport occur?

  1. In the proximal tubules of the kidney.
  2. In the distal tubules of the kidney.
  3. In the loops of Henle of the kidney.
  4. In the collecting ducts.

Answer: A,B,C,D. See Campbell, p. 983.

7. Which of these components of blood would pass from the blood into the initial filtrate in the glomerulus?

  1. Water
  2. Glucose
  3. Urea
  4. Proteins
  5. Red blood cells

Answer: A,B,C. 

8. How does the kidney help maintain the pH of blood?

  1. The kidney doesn't control pH; it only functions in osmoregulation & excretion.
  2. H+ is reabsorbed.
  3. H+ is secreted.
  4. Active transport of Na+ & Cl- helps regulate pH.
  5. HCO3- is reabsorbed.

Answer: C,E. See Campbell, p.982.

9. The proximal tubule causes a significant change in the volume of the filtrate.

  1. True.
  2. False.

Answer: A. The proximal tubule reabsorbs about 2/3 of the total volume of the initial filtrate.

10. The proximal tubule causes a significant change in the osmolarity of the filtrate.

  1. True.
  2. False.

Answer: B. The proximal tubule is highly permeable to water, so as ions are reabsorbed, the water follows immediately. Water and salt are reabsorbed, so osmolarity does not change. Page 984.

11. The proximal tubule causes a significant change in the urea concentration of the filtrate. (Think carefully!)

  1. True.
  2. False.

Answer: A. The proximal tubule reabsorbs 2/3 of the water from the initial filtrate, but the urea remains in the filtrate. The volume of the filtrate is reduced, but the number of urea molecules remains constant, so the urea concentration is increased. Since the volume is reduced to about 1/3 of the starting volume, the urea concentration increases about threefold.

12. The descending limb of the loop of Henle causes a significant change in the volume of the filtrate.

  1. True.
  2. False.

Answer: A. Page 984. Water is reabsorbed, reducing the volume.

13. The descending limb of the loop of Henle causes a significant change in the osmolarity of the filtrate.

  1. True.
  2. False.

Answer: A. Water is reabsorbed, but solutes remain in the filtrate, so osmolarity increases. Page 984.

14. The descending limb of the loop of Henle causes a significant change in the urea concentration of the filtrate.

  1. True.
  2. False.

Answer: A. This is the same idea as question 12. Reducing water volume concentrates the urea.

15. How does water leave the filtrate in the descending limb of the loop of Henle?

  1. Simple diffusion.
  2. Facilitated diffusion.
  3. Active transport.
  4. It doesn't; the descending limb isn't permeable to water.

Answer: A,B. Facilitated diffusion is more important; the simple diffusion of water is slow. There is no active transport of water.

16. How does Na+ leave the filtrate in the ascending limb of the loop of Henle?

  1. Simple diffusion.
  2. Facilitated diffusion.
  3. Active transport.
  4. It doesn't; the ascending limb isn't permeable to Na+.

Answer: B,C. See p. 984.

17. How does Cl- leave the filtrate in the ascending limb of the loop of Henle?

  1. Simple diffusion.
  2. Facilitated diffusion.
  3. Active transport.
  4. It doesn't; the ascending limb isn't permeable to Cl-.

Answer: B. Sodium is actively reabsorbed, while chloride follows due to the charge gradient that is created.

18. At the glomerulus, what causes water to leave the blood and enter the filtrate?

  1. Blood pressure from the heart and arteries.
  2. Osmosis.

Answer: A. Water and small solutes are pushed out of the blood vessels of the glomerulus by blood pressure, and they primarily leak through small openings between the podocyte cells that make up the glomerulus (not passing through the cell membranes). It can't be osmosis, because there isn't an osmotic gradient in this direction; in fact, because the protein concentration in the blood plasma is high, water would tend to be drawn into the blood vessels. However, the pressure gradient is stronger than the osmotic gradient.

19. How much blood typically flows through a person's pair of kidneys each day?

  1. 1000 to 2000 L.
  2. 180 L.
  3. 1.5 L

Answer: A. Page 982. You might have only 5 liters of blood in your body, but it passes through the kidneys repeatedly.

20. From that amount of blood, how much initial filtrate is usually produced?

  1. 1000 to 2000 L.
  2. 180 L.
  3. 1.5 L

Answer: B. Page 982. The kidneys reduce this huge amount of initial filtrate to a small volume of urine.

21. From that amount of initial filtrate, how much urine is usually produced?

  1. 1100 to 2000 L.
  2. 180 L.
  3. 1.5 L

Answer: C. Page 982. I hope it's obvious that the actual amount of urine produced will vary widely, depending on how much you drink and how much you sweat.

22. What is the osmolarity of the blood before it enters the glomerulus?

  1. 100 mOsm/L.
  2. 300 mOsm/L.
  3. 600 mOsm/L.
  4. 900 mOsm/L.
  5. 1200 mOsm/L.

Answer: B. Page 984.

23. What is the osmolarity of the initial filtrate?

  1. 100 mOsm/L.
  2. 300 mOsm/L.
  3. 600 mOsm/L.
  4. 900 mOsm/L.
  5. 1200 mOsm/L.

Answer: B. The osmolarity of the initial filtrate is almost the same as that of blood. However, the blood contains numerous proteins (which don't pass into the filtrate), so there is some osmotic pressure tending to pull water from the filtrate toward the blood.

24. What is the osmolarity of the filtrate as it passes through the distal tubule?

  1. 100 mOsm/L.
  2. 300 mOsm/L.
  3. 600 mOsm/L.
  4. 900 mOsm/L.
  5. 1200 mOsm/L.

Answer: A.

25. What is the maximum osmolarity of the filtrate (urine) as it leaves the collecting duct?

  1. 100 mOsm/L.
  2. 300 mOsm/L.
  3. 600 mOsm/L.
  4. 900 mOsm/L.
  5. 1200 mOsm/L.

Answer: E. Page 983/984. This is the maximum osmolarity for a typical human, but the actual osmolarity will vary according to your blood volume and osmolarity.

26. What is the osmolarity of the blood as it passes through the lowest point of the vasa recta in the inner medulla?

  1. 100 mOsm/L.
  2. 300 mOsm/L.
  3. 600 mOsm/L.
  4. 900 mOsm/L.
  5. 1200 mOsm/L.

Answer: E. The blood vessels of the vasa recta, like other capillaries, are highly permeable to both water and salt, so the blood will be close to isoosmotic with the surrounding interstitial fluid.

27. The volume of filtrate that passes through the distal tubule each day is

  1. less than the volume of filtrate that passes through the proximal tubule.
  2. greater than the volume of filtrate that passes through the proximal tubule.
  3. approximately equal to the volume of filtrate that passes through the proximal tubule.

Answer: A. Much of the volume of the filtrate is reabsorbed before it reaches the distal tubule.

28. Which of these processes contribute to the difference between the volume of initial filtrate and the volume of urine?

  1. Water passing from the interstitial fluid of the renal medulla into the collecting duct by osmosis.
  2. Water passing from the collecting duct to the interstitial fluid of the renal medulla by osmosis.
  3. Water passing from the interstitial fluid of the renal medulla into the loop of Henle by osmosis.
  4. Water passing from the loop of Henle to the interstitial fluid of the renal medulla by osmosis.
  5. Water actively transported out of the loop of Henle.

Answer: B,D. A and C are going the wrong direction. E is wrong because there is no active transport of water anywhere.

29. In the kidney, some filtrate passes through cortical nephrons and some passes through juxtamedullary nephrons. Can the filtrate that passes through cortical nephrons eventually be turned into hyperosmotic urine?

  1. Yes.
  2. No.

Answer: A. Filtrate is turned into hyperosmotic urine when water is reabsorbed as the filtrate passes through the collecting duct. This is true whether the filtrate came from cortical or juxtamedullary nephrons.

Campbell (page 980) says, "Juxtamedullary nephrons are essential for the production of urine that is hyperosmotic to body fluids." This is true because the juxtamedullary nephrons create the osmotic gradient that can be used to reabsorb water at the collecting duct -- regardless of what type of nephron put the filtrate into the collecting duct.

Regulating mammalian kidneys

30. When blood pressure increases, one of the body's natural responses is to

  1. Increase Na+ reabsorption in the distal tubules.
  2. Decrease Na+ reabsorption in the distal tubules.

Answer: B. If blood volume and pressure increase, urine volume should increase to restore volume and pressure to normal, without changing blood osmolarity. Decreasing reabsorption of NaCl results in decreasing reabsorption of water, so urine volume is increased. Since water and salt both end up in the urine, osmolarity doesn't change. Page 990.

31. An increase in the level of antidiuretic hormone (ADH; also called vasopressin) causes:

  1. An increase in water reabsorption.
  2. A decrease in water reabsorption.
  3. No significant change in water reabsorption.

Answer: A. ADH level is increased when blood osmolarity is high; the kidney responds by reabsorbing more water in the collecting duct. Page 988.

32. An increase in the blood osmolarity would tend to result in:

  1. An increase in the number of aquaporins in the plasma membranes of the cells of the loop of Henle.
  2. A decrease in the number of aquaporins in the plasma membranes of the cells of the loop of Henle.
  3. An increase in the number of aquaporins in the plasma membranes of the cells of the collecting duct.
  4. A decrease in the number of aquaporins in the plasma membranes of the cells of the collecting duct.

Answer: C. This is related to the previous question. Since the collecting duct is the structure that makes concentrated urine (osmolarity can increase from 100 mOsm/liter to 1200 mOsm/liter as filtrate passes through the collecting duct), this is where ADH has a large effect on water reabsorption. The main function of the  loop of Henle is to create the osmotic gradient. This work is relatively constant, so the number of aquaporins and rate of water reabsorption in the descending limb doesn't change in response to ADH. Page 988.

33. When would a vampire bat have higher levels of renin?

  1. Shortly after feeding at night, before flying home to its cave.
  2. During the day, when it is roosting in the cave and digesting its meal.

Answer: B. Renin is released in response to low blood pressure. This would not be the case immediately after the bat consumes a large liquid meal. Pages 985, 990.

34. When would a vampire bat have higher levels of ADH?

  1. Shortly after feeding at night, before flying home to its cave.
  2. During the day, when it is roosting in the cave and digesting its meal.

Answer: B. ADH level increases when blood osmolarity is high, which would be the case when water availability is limited and nitrogenous waste level is high. Note that the ADH system and the RAAS system act in concert. Both affect water reabsorption, but ADH controls water reabsorption only and RAAS controls the reabsorption of water and salt together. Pages 985, 988.

Comparative kidney physiology

35. Suppose a person drinks a liter of seawater and later urinates as a result. The person’s urine would be

  1. Hypoosmotic to his blood.
  2. Hyperosomotic to his blood.

Answer: B. Excess salt must be eliminated.

36. Suppose a person drinks a liter of seawater and later urinates as a result. The concentration of Na+ and Cl- in the person’s urine would be _______ than the concentration of these ions in seawater.

  1. Higher.
  2. Lower.

Answer: B. We can't concentrate the sodium and chloride in our urine to a level as high as that in seawater. The overall osmolarity of human urine can sometimes be higher than the osmolarity of seawater, but urea makes up a large percentage of the solutes.

37. Suppose an elephant seal drinks a liter of seawater and later urinates as a result. The seal’s urine would be

  1. Hypoosmotic to his blood.
  2. Hyperosomotic to his blood.

Answer: B. See the lecture notes for this.

38. Suppose an elephant seal drinks a liter of seawater and later urinates as a result. The concentration of Na+ and Cl- in the seal’s urine would be _______ than the concentration of these ions in seawater.

  1. Higher.
  2. Lower.

Answer: A. See the lecture notes for this.

39. What is the approximate maximum osmolarity of human urine?

  1. Less than 300 mOsm/liter.
  2. 600 mOsm/liter.
  3. 1200 mOsm/liter.
  4. 4600 mOsm/liter.

Answer: C. Page 984. If you look this up in different sources, you might get slightly different numbers; the actual value will vary a little from one person to another.

40. What is the minimum osmolarity of human urine?

  1. Less than 300 mOsm/liter.
  2. 600 mOsm/liter.
  3. 1200 mOsm/liter.
  4. 4600 mOsm/liter.
  5. 9300 mOsm/liter.

Answer: A. It's in the book: "If salt is scarce and fluid intake is high, the kidney can instead get rid of the excess water with little salt loss by producing large volumes of hypoosmotic urine. At such times, the urine can be as dilute as 70 mOsm/liter." We'd be in trouble if we couldn't do this. Page 988.

41. What is the maximum osmolarity of the urine of vampire bats?

  1. Less than 300 mOsm/liter.
  2. 600 mOsm/liter.
  3. 1200 mOsm/liter.
  4. 4600 mOsm/liter.
  5. 9300 mOsm/liter.

Answer: D. Page 986.

42. What is the maximum osmolarity of the urine of Australian hopping mice?

  1. Less than 300 mOsm/liter.
  2. 600 mOsm/liter.
  3. 1200 mOsm/liter.
  4. 4600 mOsm/liter.
  5. 9300 mOsm/liter.

Answer: E. Page 983.

43. Which of these groups of animals can make urine that is hyperosmotic to their body fluids?

  1. Mammals.
  2. Birds.
  3. Lizards.
  4. Freshwater bony fish.
  5. Marine bony fish.

Answer: AB. Page 983.

Kidney physiology: medical aspects

Glomerular filtration rate

Glomerular filtration rate is the rate at which initial filtrate is produced in the kidneys, expressed in ml/minute. Glomerular filtration rate depends on the pressure gradient between the blood in the glomerulus and the fluid in Bowman’s capsule, along with some other factors. For an adult human, 125 ml/min would be a typical glomerular filtration rate; a glomerular filtration lower than 15 ml/min for an extended period could indicate kidney failure.

Renal clearance

Renal clearance describes how quickly a substance passes out of the blood and into the urine. Renal clearance is normally expressed in terms of the volume of blood plasma that is cleared of a particular substance in one minute, and it can be calculated according to this equation:

RC = UV/P

Where:

RC = Renal Clearance

U = concentration of the substance in the urine (mg/ml)

V = rate of urine formation (ml/min)

P = concentration of the substance in blood plasma (mg/ml)

The renal clearance rate (RC) varies from one solute to another. For a solute that freely passes through the glomerular filter from blood into urine and isn’t reabsorbed or secreted, RC is equal to the glomerular filtration rate. In other words, if the kindeys produce 125 ml filtrate per minute, then 125 ml of blood plasma is cleared of the solute. Several factors can affect RC:

  • Filtration: Substances that freely pass through the glomerular filter from blood into urine are called “freely filtered.” If a solute doesn’t pass through the filter, it doesn’t end up in urine. For example, serum albumin, the most abundant protein in blood plasma, doesn’t normally pass into the filtrate, so it would have an RC value close to zero.
  • Reabsorption: If a solute is freely filtered but then completely reabsorbed, it will be returned to blood and won’t be in the urine. Such a substance would have an RC close to zero. This is normally the case for glucose.
  • Secretion: If a solute is freely filtered and also secreted, RC could be greater than the glomerular filtration rate. This is the case for some drug metabolites.

Transport maximum

Most solutes that are reabsorbed must pass though membranes by way of specific transport proteins. Since each protein can transport solutes at a limited rate, there is a maximum rate of reabsorption for each solute. This rate is called the transport maximum, or Tm, expressed in mg/minute. If the solute enters the filtrate via the glomerulus at a rate greater than its Tm, the excess solute won’t be reabsorbed and will remain in the urine.

For the following questions, suppose that the normal glomerular filtration rate is 125 ml/min.

44. The renal clearance rate of inulin, a plant carbohydrate, is usually around 125 ml/min. Which of the following is true?

  1. There is net secretion of inulin.
  2. There is net reabsorption of inulin.
  3. There is no net secretion or reabsorption of inulin.

Answer: C. A freely filtered substance that is neither secreted nor reabsorbed would have a renal clearance rate equal to the glomerular filtration rate. If 125 ml of filtrate is made from the blood, then all the inulin in that amount of filtrate will be eliminated in the urine.

45. The renal clearance rate of creatinine, a nitrogenous waste product, is usually around 140 ml/min. Which of the following is true?

  1. There is net secretion of creatinine.
  2. There is net reabsorption of creatinine.
  3. There is no net secretion or reabsorption of creatinine.

Answer: A. If the clearance rate for creatine is greater than the glomerular filtration rate, there must be net secretion.

46. Globulins are proteins normally found in blood plasma. Globulins are too large to pass through the glomerular filter into the initial filtrate. The renal clearance rate (RC) for globulins would be

  1. Close to zero.
  2. 125 ml/min
  3. 125 mg/min
  4. Greater than 125 ml/min.

Answer: A. Proteins aren't normally cleared in the urine at all. In fact, protein in the urine (proteinuria) can be a sign of kidney damage.

47. The concentration of glucose in urine is normally close to zero. This is because

  1. Glucose does not pass through the glomerular filter.
  2. Glucose is reabsorbed.
  3. Glucose is secreted.

Answer: B. Almost all glucose is normally reabsorbed, so it is not cleared in the urine. Compare this to the previous question: both globulins and glucose normally have clearance rates of zero, but for different reasons.

48. In diabetes mellitus, substantial amounts of glucose may be present in a person’s urine. This would likely be because __________.

  1. The glomerular filtration apparatus is damaged.
  2. The osmotic gradient in the kidney is weaker than normal.
  3. The glomerular filtration rate is too low.
  4. The level of glucose in the blood is so high that the transport maximum (Tm) for glucose in the filtrate is exceeded.

Answer: D. Glucose is reabsorbed by specific transport proteins; if there is an extremely high glucose concentration in the blood, there aren't enough of these proteins to keep up.

49. The concentration of urea is always higher in the urine than it is in the blood plasma. Does this prove that urea is secreted?

  1. Yes
  2. No

Answer: B. No. The urea concentration in urine is high because so much water is reabsorbed, leaving most of the urea to be eliminated in the small remaining volume of urine.

References & further reading

Books

Campbell Biology, Chapter 44: Osmoregulation and excretion. Most of the questions on this page can be answered from this chapter.

Animal Physiology: Adaptation and Environment, Knut Schmidt-Nielsen. This is a classic textbook on animal physiology. Many of the examples you find in general bio texts like Campbell were borrowed from Schmidt-Nielsen. I used this book to find a variety of quantitative data and to learn details that are hard to find elsewhere.

Human Anatomy and Physiology, Elaine N. Marieb and Katja Hoehn. A standard textbook for anatomy & physiology courses, this book includes more detail about kidney function than you can find in a general biology text like Campbell.

Fundamentals of Anatomy & Physiology, Frederic Martini and Judi Nath. Another useful standard text.

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