W3006 '00 Physiology Problem Set #5 - Urinary system ANSWERS

1. swelling.  Water would naturally move from the water into the more concentrated fish fluid, causing it to swell. The urinary system must work to constantly remove dilute fluids. 

2. b   3. d.

4. inhibiting.  Alcohol inhibits ADH release from the posterior pituitary.  Usually, ADH binds membrane receptors in collecting duct cells, which (via cAMP, protein kinase activation) causes water channels to be inserted into luminal membrane of cells, so that 99% of water in filtrate will be reabsorbed.   With decreased ADH, there's decreased water reabsorption and very dilute urine produced.  The amount of water lost in urine is actually greater than the amount contained in the alcoholic drink, and so the body has a net loss of water and becomes dehydrated.

5. Nephrons cannot produce such a concentrated salt solution.  To excrete that much salt, they'd have to dilute the salt in water, and would loose too much water.  (Crocodiles also have these salt glands near their eyes, so the drops of salt solution appear to be tears.  Hence the phrase "crying crocodile tears" to refer to someone who is crying when there's no real problem.)

6. A. secondary active transport of glucose.  Expt 1 shows that.  B. The protein that transports glucose and sodium into the epithelial cell.  C. Could be the protein that's used for facilitated diffusion of glucse from inside the epithelial cells out of the basolateral embrane.  But experiment two shows that the glucose never got into those cells in the first place.  D. There's an excess of glucose that stays in the intestine, can't be reabsorbed, increases the osmotic pressure there, less water is reabsorbed.  E. In the proximal tubule of the nephron, secondary active transport of glucose also occurs.  

7. Sometimes.  Only for metabolic alkalosis, not respiratory alkalosis. 

8. Sometimes.  This is true if ADH is maximally secreted, with less ADH, a greater percentage of water leaves in the urine.

9. Metabolism of proteins (but not carbohydrates) produces urea, which stays in the filtrate and is mostly excreted in the urine.  The higher the concentration of urea, the higher will be the osmolarity of the filtrate and the less water will be reabsorbed, increasing the volume of urine produced. 

10. False.  Glomerular filtration never stops, even if a person abstains from all food and water.  Toxic byproducts of metabolism must still be gotten rid of, and acid-base balance must be maintained.  There is no "off" switch to filtration -- the porosity of the capillaries can't be closed.

11. f   12. a  13. A. x  B. y   C. x

14.  After drinking salt water, he would excrete a volume of urine larger than the volume of water he drank, so he would become even more dehydrated.  The increased Na+ concentration in extracellular fluids would cause water to osmose out of cells, causing dehydration.  The adrenal cortex would secrete less aldosterone, so that more salt would be excreted, and more ADH would be produced, to increase water retention to dilute the salt.  But even excreting as much salt as possible, the kidney can still produce a urine with a maximum of 1200 mOsm.  So if you drink, say, 1L of 2400 mOsm salt solution, you can get rid of all that salt only by producing 2 L of 1200 mOsm urine.  So the more he drinks, the more dehydrated and thirsty he becomes. 

15. a. Renin from the kidney stimulates conversion of angiotensinogen to angiotensin I, which (after conversion to angiotensinII) stimulates the adrenal cortex to release aldosterone, which in turn promotes Na+ reabsorption in the collecting duct/distal tubule.  So if ANP promotes NaCl excretion, it must be decreasing renin production

b. Another way to promote NaCl excretion is to promote excretion in general.  If ANP acts on the efferent arteriole to promote excretion of NaCl, it must be causing constriction of this arteriole.  Constricting the efferent relative to the afferent increases blood pressurei n the glomerular capillaries and increases GFR, so that more Na+ enters the filtrate and can be excreted

c. If ANP increases NaCl excretion, it must be decreasing secretion of aldosterone from the adrenal cortex, since this hormone decreases salt excretion (ie, increases salt retention).

16. A diuretic is a substance that increases the production of urine.
a. Excess glucose in blood would enter the glomerular filtrate.  The large amount of glucose would saturate the carriers for glucose re-uptake, so some glucose stays in filtrate, raising the osmolarity of the filtrate.  Less water would leave the filtrate in the medullary regions, and more water would be excreted in the urine. 
b. Aldosterone binds to nuclear receptors in the collecting duct and stimulates Na+ reabsorption (by increasing the number of sodium channels in the membrane, and increasing enzymes involved in energy production for Na/K pump).   If aldosterone receptors are blocked, less Na+ is reabsorbed, and more remeains in the filtrate.  Under normal circumstances, some ADH is present, and water leaves the collecting duct by osmosis, so that the amount of water reabsorbed depends on filtrate osmolarity.  With more Na+ in the filtrate, more water stays in the filtrate, and so more water leaves the body in urine.