PS%209_solutions

PS%209_solutions - BIOE 322/BIOS 332 Fundamentals of...

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Page 1 of 3 BIOE 322/BIOS 332 Fundamentals of Systems Physiology Spring 2008 Problem Set #9 Solutions 1. Urine osmolarity has no single “normal” value. It can be as low as 50 mOsm/L, as high as 1200 mOsm/L, or any value in between. Normal urine osmolarity depends on the person’s plasma osmolarity and water status. For example, in a person who is dehydrated, the kidneys should concentrate the urine; in this case, “normal” urine osmolarity is higher than plasma osmolarity (hyperosmotic). In a person who is drinking water, the kidneys should dilute the urine; in this case, “normal” urine osmolarity is lower than plasma osmolarity (hypoosmotic). 2. Without the normal active transport of Na+, the concentration of Na+ and ions symported with them remains elevated in the nephron. Movement of water by osmosis out of the nephron into the interstitial spaces is decreased, resulting in an increased volume of urine. 3. Inhibition of ADH secretion is one of the numerous effects alcohol has on the body. The lack of ADH secretion causes the distal convoluted tubules and the collecting ducts to be relatively impermeable to water. The water cannot, therefore, move by osmosis from the distal nephrons and collecting ducts and remains in the nephrons to become urine. In addition, because other fluids are normally consumed with the alcohol, the increased water intake also results in an increase in dilute urine production. 4. Low urea clearance indicates that the amount of blood cleared of urea, a metabolic waste product, per minute is lower than normal. It is consistent with the reduction in the number of functional nephrons that occurs in advanced cases of renal failure. In addition, a low urea clearance is an indication that the GFR is reduced and that the blood levels of urea are increasing. 5. Urea is partially responsible for the high osmolality of the interstitial fluid in the medulla of the kidney. Since a high osmolality of the interstitial fluid must exist for the kidney to produce concentrated urine, a small amount of urea in the kidney results in the production of dilute urine by the kidney. 6.
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PS%209_solutions - BIOE 322/BIOS 332 Fundamentals of...

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