Hormonal Control: ADH and Aldosterone - Anatomy and Physiology
- What is the primary stimulus for ADH secretion?
- A rise in plasma osmolality detected by hypothalamic osmoreceptors; a large fall in blood volume or pressure also stimulates it.
- A patient with central diabetes insipidus lacks ADH. What urine and thirst changes are expected?
- Large volumes of dilute urine with intense thirst, because the collecting ducts stay impermeable to water.
- What are the two main stimuli for aldosterone secretion?
- Angiotensin II and an elevated plasma K+ concentration.
- Why does drinking alcohol increase urine output?
- Alcohol inhibits ADH release, so the collecting ducts reabsorb less water and more dilute urine is produced.
- How does ADH increase water reabsorption in the kidney?
- It binds V2 receptors on collecting duct principal cells, causing insertion of aquaporin-2 water channels into the apical membrane.
- What does aldosterone do in the principal cells of the distal nephron and collecting duct?
- It increases Na+ reabsorption (more ENaC channels and Na+/K+ pumps) and increases K+ secretion into the urine.
- An adrenal tumor secretes excess aldosterone. What change in plasma potassium is expected?
- Hypokalemia (low K+), because aldosterone increases K+ secretion; Na+ retention also tends to raise blood pressure.
- A patient has excessive ADH secretion (SIADH). What happens to plasma sodium and urine concentration?
- Plasma Na+ falls (dilutional hyponatremia) from water retention, while the urine is inappropriately concentrated.
- Where is ADH synthesized, and where is it released into the blood?
- It is synthesized by neurons of the hypothalamus (supraoptic and paraventricular nuclei) and released from the posterior pituitary.
- Which region of the adrenal cortex secretes aldosterone?
- The zona glomerulosa.
- How does spironolactone act, and why is it called potassium-sparing?
- It blocks aldosterone receptors, reducing Na+ reabsorption and K+ secretion, so less K+ is lost in urine.
- What is nephrogenic diabetes insipidus?
- Dilute polyuria caused by kidneys that do not respond to ADH, even though ADH levels are normal or high.