The kidneys are organs that precisely maintain the homeostasis of body-fluid composition (water, electrolytes, pH, and waste products). You will study the structure and functions of the nephron and hormonal regulation at an advanced level.
Basic Knowledge
Structure and functions of the nephron (renal unit):
① Glomerulus (ball of capillaries): Filtration by blood pressure → glomerular filtrate (approximately 180 L/day). Red blood cells and proteins are not filtered.
② Bowman's capsule: Receives the glomerular filtrate.
③ Proximal tubule: Reabsorbs 100% of glucose and amino acids. Reabsorbs large amounts of Na and water. Reabsorbs HCO3 (acid-base balance).
④ Loop of Henle: Extends deeply into the medulla. Descending limb = permeable only to water and impermeable to NaCl. Ascending limb = actively reabsorbs NaCl and is impermeable to water. This countercurrent exchange mechanism forms an osmotic gradient in the medulla.
⑤ Distal tubule and collecting duct: Sites of precise hormonal regulation.
・Vasopressin (ADH): Inserts aquaporin (AQP2) into the collecting duct → increases water permeability → increases water reabsorption → concentrates the urine.
・Aldosterone: Increases the expression of Na channels/pumps in the distal tubule and collecting duct → increases Na reabsorption and K excretion.
Final urine volume: Approximately 1.5 L/day (approximately 99% of the glomerular filtrate is reabsorbed).
Answer: During dehydration, osmoreceptors in the hypothalamus detect an increase in blood osmotic pressure, and secretion of vasopressin (ADH) from the posterior pituitary increases. Vasopressin moves AQP2 (aquaporin 2) in collecting-duct epithelial cells to the cell membrane, adding water channels. As a result, water permeability in the collecting duct increases, and water is reabsorbed along the medulla's high osmotic gradient, concentrating and reducing the urine. Effect of furosemide: When active NaCl transport in the ascending limb of the loop of Henle is inhibited, the medullary osmotic gradient can no longer be formed or maintained. Even if vasopressin acts on the collecting duct, water is not reabsorbed without the osmotic gradient, so a large amount of dilute urine is excreted (diuretic effect).
- Nephron = glomerulus → Bowman's capsule → proximal tubule → loop of Henle → distal tubule → collecting duct
- Glomerular filtration = approximately 180 L/day. 99% reabsorbed → urine is 1.5 L/day
- Proximal tubule = 100% reabsorption of glucose and amino acids
- Loop of Henle = formation of the medullary osmotic gradient by countercurrent exchange
- Vasopressin = insertion of aquaporin 2 → increased water permeability of the collecting duct
- Aldosterone = increased Na reabsorption and K excretion in the distal tubule
Practice
- Explain why the countercurrent exchange mechanism of the loop of Henle is important for urine concentration.
- Explain why excess aldosterone (primary aldosteronism) causes hypertension and hypokalemia.
- Explain why glucose appears in the urine in diabetes mellitus, based on the mechanism of glucose reabsorption (SGLT) in the proximal tubule.