Sodium Reabsorption

Although 90% of the filtered sodium is reabsorbed in the early region of the nephron, the amount left in the filtrate delivered to the distal convoluted tubule is still quite large. In the absence of aldosterone, 80% of this remaining amount is reabsorbed through the wall of the tubule into the peritubular blood; this represents 8% of the amount filtered. The amount of sodium excreted without aldosterone is thus 2% of the amount filtered. Although this percentage seems small, the actual amount it represents is an impressive 30 g of sodium excreted in the urine each day. When aldosterone is secreted in maximal amounts, by contrast, all of the sodium delivered to the distal tubule is reabsorbed. In this case urine contains no Na+ at all.

Aldosterone stimulates Na+ reabsorption to some degree in the late distal convoluted tubule, but the primary site of aldo-sterone action is in the cortical collecting duct. This is the initial portion of the collecting duct, located in the renal cortex, which has different permeability properties than the terminal portion of the collecting duct, located in the renal medulla.

Test Yourself Before You Continue

1. Define renal plasma clearance and describe how this volume is measured. Explain why the glomerular filtration rate is equal to the clearance rate of inulin.

2. Define the terms reabsorption and secretion. Using examples, describe how the renal plasma clearance is affected by the processes of reabsorption and secretion.

3. Explain why the total renal blood flow can be measured by the renal plasma clearance of PAH.

4. Define transport maximum and renal plasma threshold. Explain why people with diabetes mellitus have glycosuria.

Fox: Human Physiology, I 17. Physiology of the I Text I I © The McGraw-Hill

Eighth Edition Kidneys Companies, 2003

Physiology of the Kidneys

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