Biology 42 – Human Biology (Spring 2007)
[Students and professors, please read.]
1. Name the parts of the nephron, and tell how the structure of the convoluted tubules suits their function.
A nephron is made up of a glomerular capsule (Bowman’s capsule) (a tuft of capillaries), the proximal convoluted tubule, the loop of the nephron (loop of Henle), the distal convoluted tubule, and the collecting duct.
Cuboidal epithelial cells lining the proximal convoluted tubule are anatomically adapted for active reabsorption from the tubule to the blood. They have a lot of microvilli which increase the surface area for reabsorption, and a lot of mitochondria for ATP production for active transport (selective reabsorption). The cuboidal epithelial cells of the distal convoluted tubule have many mitochondria, but no microvilli, because they are not specialized for reabsorption, but participate in tubular excretion when they move molecules from the blood into the tubule, to enter collection ducts which carry urine to the renal pelvis.
2. Where in particular is water and salt reabsorbed along the length of the nephron? Describe the contribution of the loop of the nephron.
Water and salt are reabsorbed from the loop of the nephron (loop of Henle) as well as the collecting duct.
The loop of the nephron (loop of Henle) consists of a descending limb that allows water and salt (NaCl) to passively diffuse out into the inner renal medulla, and an ascending limb (impermeable to water) that extrudes salt by active transport into the outer renal medulla (establishing the solute gradient that increases toward the inner medulla). The water in the renal medulla is then returned to the cardiovascular system.
3. How do the kidneys maintain the pH of the blood within normal limits?
The kidneys usually reabsorb bicarbonate ions and excrete hydrogen ions as needed to maintain the normal pH (7.4) of blood. However, if blood is acidic, hydrogen ions are not excreted by the kidneys, and bicarbonate ions are not reabsorbed.