Kidney Questions for discussion plus Answers EEOB 410
1. What is the functional unit of the kidney?
Nephron
2. What factors determine glomerular filtration pressure?
Net filtration pressure = Glomerular Hydrostatic Pressure – Bowman’s Capsule Hydrostatic
Pressure – Glomerular Oncotic Pressure (Colloid Osmotic Pressure)
3. What portion of the nephron is most important in urine concentration?
Loop of Henle, because it creates a countercurrent multiplier, which concentrates solutes in the
inner medulla.
4. What processes contribute to urine composition?
glomerular filtration, reabsorption of solutes in tubules, secretion of solutes in tubules
5. Is water permeable in the descending limb of the loop of Henle? The ascending limb? The
distal tubule? The proximal tubule? The collecting duct?
6. What is the counter-current mechanism?
Descending Limb – Yes
Ascending Limb – No
Distal Tubule – No
Proximal Tubule – yes
Collecting Duct – Yes, but depends on levels of ADH. If ADH is high, very permeable. If ADH
is low, much less permeable.
7. What is the importance of the counter-current mechanism in urine concentration?
The counter-current mechanism allows for the interstitial fluid of the medulla to become
increasingly concentrated as ions are actively transported into the interstitial space. This high
concentration of ions allows the urine to become concentrated as it passes through the collecting
duct, as water moves by osmosis out of the collecting duct and into the interstitial space.
8. What factors determine the volume of urine produced?
Level of ADH
Concentration of the interstitial fluid of the medulla.
9. What does hyperosmotic mean? hypoosmotic? isoosmotic?
Hyperosmotic – Solution (such as urine) is more concentrated with solutes, and has a higher
osmotic pressure than blood plasma.
Hypoosmotic – Solution is less concentrated with solutes, has a lower osmotic pressure than
blood plasma.
Isoosmotic – Solution has an equal concentration of solutes with plasma. Osmotic pressures are
identical.
10. What regions of the nephron actively transport NaCl? What regions passively transport
NaCl?
The thick ascending tubule, Distal convoluted tubule, Proximal Convoluted Tubule, and the
Collecting duct actively transport NaCl. The walls of these tubules are composed of cells with
large amounts of mitochondria, which facilitate active transport.
11. Draw a nephron. Label Bowman's Capsule, glomerulus, proximate tubule, distal tubule, loop
of Henle and collecting duct.
12. Plasma osmolality in mammals does not vary by more than a few mOsm, yet in birds it can
vary by 100mOsm, from 300mOsm to 400mOsm when the bird is dehydrated. If the plasma of a
mammal varied by such extremes, the red blood cells would begin to crenate. Why can birds
vary the osmolality of their plasma and not mammals? (you might have to look up the
differences in RBC between birds and mammals).
Red blood cells in birds have mitochondria. These mitochondria are able to generate ATP, a
source of energy for active transport. Hence, RBC of birds can regulate intracellular ion
concentration even though plasma osmolality increases.
13. What is a renal corpuscle?
The term for the glomerulus and Bowman’s capsule, as a single unit.
14. A 50-year old man with a history of a duodenal ulcer is admitted to the hospital after several
days of intermittent vomiting. On physical examination he is noted to have orthostatic ( when he
stands up) changes in his blood pressure and pulse, no visible jugular venous distension, and
marked decrease in his skin turgor. Laboratory tests reveal the following:
serum [Na+] = 138 mmole/L
serum [K+] = 2.4 mmole/L
serum [Cl- ] = 88 mmole/L
serum [HCO3- ] = 40 mmole/L
arterial pH = 7.52
arterial pCO2 = 50 mm Hg
Urine [Na+] = 38 mmole/L
Urine [K+] = 60 mmole/L
What disorder does this man have?
Gastric secretions normally contain about 10mmole/L potassium. How do you account for the
low serum potassium in this patient?
See other sheet.
15. Why can lizards not produce a urine that is more concentrated than mammals?
Lizards do not have loops of Henle.
16. Why can mountain beavers not concentrate their urine?
Mountain beavers only have short loops of Henle, which do not reach into the medulla of the
kidney. Thus, these short loops are not able to create a large osmotic gradient around loops of
Henle.
17. Explain how urea is involved in concentrating the urine of mammals?
Urea is a major osmolyte that is found in the interstitial fluid of the medulla. Thus urea
contributes to the 1200mOsm of the gradient
18. How does the vasa recta act as a countercurrent diffusion exchanger?
By making a loop through the medulla, the blood does not dilute the interstitial fluid of the
medulla. As it enters the medulla, the blood loses water and takes up ions. On its way out, it
gains water and loses ions. The vasa recta acts to transport some water out of the medulla
because of the oncotic pressure of the plasma. By this process, the interstitial fluid of the
medulla is kept concentrated. Also fluid flow in the vasa recta is slow, hence slow movement of
ions.
19. Please explain the results of the graph below. Why do cells deep in the medulla have higher
concentrations of osmolytes?
The graph shows increasing concentrations of organic
osmolytes in the cells of the kidney with increasing depth.
Cells in the inner medulla have much higher
concentrations than cells in the cortex. This is because
the cells must match the osmotic pressure of the
interstitial fluids. The osmotic pressure of the interstitial
fluids increases toward the inner medulla as a result of
counter-current multiplication.