Tubular Reabsorption: Reabsorbing the Glomerular Filtrate

1. Overview of Reabsorption
1. filtrate - all fluid and its solutes pushed into the capsule
2. urine - filtrate minus reabsorbed substances
3. route of reabsorption (transepithelial process)
• luminal surface of tubule cells >>
• basolateral membrane of tubule cells >>
• interstitial fluid between tubule cells and capillaries >>
• endothelium of the peritubular capillary
4. most sugars and amino acids are reabsorbed
5. water and ion reabsorption depends on hormonal control (see below)
2. Active Tubular Reabsorption
1. glucose, amino acids, lactate, vitamins, ions
• move across luminal surface by diffusion
• actively transported across basolateral membrane
• contransported with Na+
• diffuse into capillary by diffusion
2. Transport maximum (Tm) - when “ carrier proteins” for specific solute becomes saturated and cannot carry the substance across the membrane
• diabetes mellitus – lower Tm (glucose lost)
3. Passive Tubular Resorption
1. Na+ driven into interstitial space actively (above)
2. HCO3- and Cl- follow Na+ into the space
3. obligatory water resoprtion – water follows ions into the interstitial space between tubule & capillary
4. solvent drag – solutes will begin to move into tubule from filtrate, following water (especially some urea and lipid-soluble molecules)
4. Nonreabsorbed Substances
• urea, creatinine, uric acid – most is not reabsorbed because of the following reasons

• no carrier molecules for active transport
• not lipid-soluble
• too large (as with most proteins)
5. Absorption in Different Regions of Renal Tubule
1. proximal tubule – closest to the glomerular capsule

• almost all glucose & amino acids

• 75-80% of water and Na+

• most active transport of ions
2. Loop of Henle – connects proximal & distal tubules
• Regulates Total water retained or lost:

• descending limb – relatively impermeable to solutes but freely permeable to water

• ascending limb – very permeable to solutes, but not to water
3. distal tuble & collecting duct – final passageway

• antidiuretic hormone (ADH) – causes increased permeability of collecting duct to water, resulting in more reabsorption (B.P. ^ )

• aldosterone – stimulated be renin-angiotensin, enhances Na+ reabsorption, resulting in increased water reabsorption (B.P. ^ )
• in response to lower blood pressure
• in response to low blood Na+ concentration (hyponatremia)
• atrial natriuretic factor (protein) (ANF) – reduces Na+ permeability, less water reabsorption (B.P. ↓ )

Category: Physiology Notes