Tubular Reabsorption

Tubular Reabsorption

Movement of solute, water from renal tubule to peritubular capillaries include glucose, fructose, Na+, amino acid, K+, Ca+2, HCO3-,
Cl-…etc.
As glomerular filtrate inter renal tubule, it flows sequentially through different segment of nephron.
GF 125 ml/min. approximately 124 ml/min. are reabsorbed by tubules to peritubular capillaries leaving 1 ml/min. to pass to urine.
1. Quantitively large:
Process of glomerular filtrate and reabsorption are large relative to urinary excretion. this means that a small change in glumerular filtrate or tubular reabsorption can cause a relative large change in urinary excretion.
2. Highly selective process:
In contrast to GF (non-selective). eg. Glucose and amino acid are almost completely reabsorbed by tubules, yet excretion nearly zero – high threshold substances Na+, Cl-, HCO3- are also highly reabsorbed with variable excretion on body needs.
Urea, uric acid, phosphate are poorly reabsorbed, thus excreted largely – low threshold substances.
Creatinine is not reabsorbed at all, excretion is 100% - non threshold substances.
Tubular reabsorption include passive, active mechanism.
For a substance to be reabsorbed, it transport:
1. Across tubular epithelial membrane to renal interstitial fluid by passive, active transport by para-cellular or trans-cellular.
2. Through low pressure peritubular capillary membrane back to blood by ultrafiltration (bulk flow) mediated by hydrostatic, oncotic pressure by .
• Primary active transport:
Solute move against an electrochemical gradient require energy source, eg. Na+ through Na+/K+ ATpase pump.
• Secondary active transport:
Two or more substances interact with specific membrane protein (carrier molecule) and cotransported together across membrane.
Transport coupled indirectly to an energy source, eg. ion gradient, glucose, amino acid, PAH.