Rivka is an active 21-year-old who decided to take a day off from her university classes. The weather was hot and the sun bright, so she decided to go down to the beach

Question

When she arrived, she found a few people playing beach volleyball, and they asked if she wanted to join in. She put down her school bag and began to play. The others were well prepared for their day out and stopped throughout the game to have their power drinks and soda pop. Several hours after they began to play, however, Rivka was not feeling so good. She stopped sweating and was feeling dizzy. One player noted she had not taken a washroom break at all during the day. They found a shaded area for her, and one of the players shared his power drink with her. Rivka was thirstier than she realized and quickly finished the drink.

In pronounced dehydration, hypotension can occur. How would this affect the glomerular filtration rate of the kidney? What actions by the juxtaglomerular apparatus would occur to restore GFR?

What is the effect aldosterone has on the distal convoluted tubule? Why would the actions of aldosterone be useful to Rivka in her situation?

What does a specific gravity test measure? If someone tested the specific gravity of Rivka's urine, what might it indicate?

Answer 1

The glomerular filtration rate would decrease with a decline in blood pressure. The juxtaglomerular apparatus participates in renal autoregulation and, when activated, increases GFR. The process involves the release of renin from the juxtaglomerular cells found in the afferent arteriole. Renin enters the bloodstream and converts inactive angiotensinogen to angiotensin I. Angiotension-converting enzyme (from the lungs) converts angiotensin I to angiotensin II, a potent vasoconstrictor of the efferent arteriole, which in turn increases GFR. Angiotensin II also encourages sodium reabsorption by the proximal convoluted tubule and stimulates aldosterone release by the adrenal glands.

Aldosterone secretion causes sodium reabsorption and potassium secretion at the distal convoluted tubule. Aldosterone release by the adrenal cortex is stimulated by angiotensin II and would therefore be a component of water conservation in the dehydrated individual; when sodium is reabsorbed, an osmotic gradient is created and allows for the passive reabsorption of water.

The specific gravity (or osmolality) urine test determines the concentration of solutes in a urine sample. It can therefore measure both hydration status of the patient and renal function. Rivka's urine would be concentrated, and her urine specific gravity would be high (perhaps 1.030 to 1.040). This would reflect a decrease in hydration and water conservation by the kidneys.