A. How ADH Influences Water Reabsorption
1. Antidiuretic hormone (ADH) from the posterior pituitary is secreted in response to a decrease in extracellular fluid; ADH causes the distal tubules and collecting ducts to become permeable to water, which moves back into the blood capillaries.
2. When water intake is excessive, ADH secretion is inhibited; less water is reabsorbed, and thus more is excreted.
3. Caffeine and alcohol are diuretics, a substance that promotes loss of water.
B. How Aldosterone Influences Sodium Reabsorption
1. When sodium levels fall so does the volume of extracellular fluid; this triggers the juxtaglomerular apparatus to secrete renin, which calls forth angiotensins I and II, which act on the adrenal cortex to release aldosterone, which promotes sodium reabsorption.
2. Sodium retention is accompanied by water retention, which can lead to increased blood pressurehypertension, which can affect kidney function.
C. Salt-Water Balance and Thirst
1. When solute concentration in the extracellular fluid rises, the thirst center of the hypothalamus responds by decreasing saliva production.
2. The dry sensation in the mouth causes a liquid-seeking behavior.