Reabsorbs 25 of the filtered Na

- contains a Na+-K+-2C1" cotransporter in the luminal membrane.

- is the site of action of the loop diuretics (furosemide, ethacrynic acid, bumetanide), which inhibit the Na+-K+-2C1~ cotransporter.

Peritubular

Lumen Cell of the thick ascending limb capillary blood

Furosemide//

Peritubular

Lumen Cell of the thick ascending limb capillary blood

Furosemide//

Figure 5-10. Mechanism of ion transport in the thick ascending limb of the loop of Henle.

- is impermeable to water. Thus, NaCl is reabsorbed without water. As a result, tubular fluid [Na+] and tubular fluid osmolarity decrease to less than their concentrations in plasma (i.e., TF/PNa+ and TF/Posra < 1.0). This segment, therefore, is called the diluting segment.

- has a lumen-positive potential difference.

3. Distal tubule and collecting duct

- together reabsorb 8% of the filtered Na+.

a. Early distal tubule—special features

- is called the cortical diluting segment.

- reabsorbs NaCl by a Na+-Cl~ cotransporter.

- is the site of action of thiazide diuretics.

-is impermeable to water, as is the thick ascending limb. Thus, reabsorption of NaCl occurs without water, which further dilutes the tubular fluid.

b. Late distal tubule and collecting duct—special features

- have two cell types.

(1) Principal cells

- Aldosterone increases Na+ reabsorption and increases K+ secretion. Like other steroid hormones, the action of aldosterone takes several hours to develop because new protein synthesis is required. About 2% of overall Na+ reabsorption is affected by aldosterone.

- Antidiuretic hormone (ADH) increases H20 permeability by directing the insertion of H20 channels in the luminal membrane. In the absence of ADH, the principal cells are virtually impermeable to water. -K+-sparing diuretics (spironolactone, triamterene, amiloride) decrease K+ secretion.

(2) «-Intercalated cells

- secrete H+ by a H+-adenosine triphosphatase (ATPase).

- Aldosterone increases H+ secretion by stimulating the H+-ATPase (in addition to its actions on the principal cells).

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