Reabsorption and Secretion in the Proximal Convoluted Tubule (PCT)
The largest amount of solute and water reabsorption from filtered fluid occurs in the PCT.
Reabsorbed:
65% of the total filtered water
Na+ and K+
100% of most filtered organic solutes such as glucose, and amino acids 50% of the filtered Cl- 80-90% of the filtered HCO3- 50% of filtered urea variable amount of filtered Ca++, Mg++, and HPO2++ (phosphate), H+ ions, ammonium ions (NH4)
Secreted:
H+
NH4+
drugs & toxins
waste products Most reabsorption in the PCT involves Na+. Na+ transport occurs via symport and antiport mechanisms in the PCT. Normally, glucose, amino acids, lactic acid, water-soluble vitamins, and other nutrients are not lost in the urine but rather reabsorbed in the first half of the PCT by several types of Na+ Symporters located in the apical membrane. Another example of secondary active transport in this area is the Na+/H+ Antiporters which carry Na+ down its concentration gradient into the PCT cell as H+ is moved from the cytosol into the tubule lumen. Solute reabsorption in the PCT promotes osmosis of water. Water moves from tubular fluid into the peritubular capillaries. In other words, reabsorption of solutes creates an osmotic gradient that promotes the reabsorption of water via osmosis.
Reabsorption and Secretion in the Loop of Henle
The chemical composition of tubular fluid is now quite different from glomerular filtrate because glucose, amino acids, and other nutrients are no longer present.
Reabsorbed:
15% of filtered water
20-30% of filtered Na+ and K+
35% of filtered Cl-
10-20% of filtered HCO3
Variable amount of filtered Ca++ and Mg++.
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