Anatomy and Physiology / Unit 10: The Urinary and Reproductive Systems
10.3 Regulation of Water, Urine Concentration and Hormones
Notes
The kidneys adjust urine volume and concentration to maintain fluid balance.
- ADH (from the posterior pituitary) makes the collecting ducts permeable to water, so more water is reabsorbed and urine becomes concentrated. Dehydration increases ADH. Alcohol suppresses ADH and increases urine output. Diuretics increase urine output.
- Aldosterone (adrenal cortex) increases sodium reabsorption (and water follows) and potassium secretion in the distal tubule.
- Renin-angiotensin-aldosterone system: low blood pressure leads the kidneys to release renin, which leads to angiotensin II (vasoconstriction) and aldosterone release, raising blood pressure.
- Atrial natriuretic peptide (ANP) from the heart lowers blood volume by reducing sodium reabsorption.
- The countercurrent mechanism in the loop of Henle creates a salty medulla that allows water to be reabsorbed from the collecting duct.
- Kidney stones form from crystallized minerals; dialysis performs filtration for failing kidneys.
Practice
A hiker is dehydrated after a long walk without water. What is expected?
Dehydration raises ADH, which increases water reabsorption.
Blood pressure falls after a hemorrhage. Which hormone pathway helps restore it?
The renin-angiotensin-aldosterone system raises blood pressure and blood volume.
Alcohol increases urine output because it:
Without ADH, less water is reabsorbed in the collecting duct.
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