Reabsorption and secretion in different regions of tubules
- Water transport is by osmotic diffusion.
- Proximal tubule: 65% reabsorption.
- Loop of Henle: 15%.
- Distal tubule: 10%.
- Collecting duct: 9.3%.
- Urine: 0.7%.
- Glucose, proteins, amino acids, vitamins and acetoacetate ions are completely reabsorbed by active process in proximal tubule.
- 50% of urea is reabsorbed.
- Creatinine is not reabsorbed but some quantities are secreted in proximal tubule.
- About 86% of urate ions are reabsorbed.
- Sulphate, phosphate and nitrates are transported similarly like urates.
- K+ ion is secreted in distal tubule and collecting tubule.
- H+ ions are actively secreted in proximal tubule, distal tubule and collecting tubule.
Transport maximum
Substances such as glucose that are actively absorbed by a carrier transport, there is a maximum rate at which they can be reabsorbed. This is known as transport maximum (Tm).
Tm for glucose = 320 mg/min
When Tm is exceeded in the nephron, it appears in the urine, e.g., in Diabetes mellitus, the movement of glucose from the plasma to the cells is impaired because of lack of insulin. Glucose concentration increases causing plasma and tubular loads to increase. When increased tubular load exceeds the availability of the carriers molecules for glucose reabsorption, excess glucose flows through the tubules into to the urine. As glucose is retained within the tubules it contributes to an increase in osmotic pressure and therefore water also remains in the tubular fluid. The point at which the glucose first begins to appear in the urine, 175 mg/dl is known as the renal threshold for glucose. The Tm for the kidney is reached when all the nephrons are reabsorbing to their maximum ability.