Reabsorption of Glucose

• Glucose Concentration increases in epithelial cells leading to diffusion from them into basal and lateral intercellular spaces (facilitated diffusion), from where glucose enters blood capillaries (simple diffusion; no phospholipid bilayers)
• All glucose is reabsorbed; by Na^+/glucose co-transport protein carriers in microvilli of PCT wall cell’s luminal membranes (symports), each binds one glucose and one Na^+ ion, then a shape change delivers them into cell (facilitated diffusion)
• Carriers need Na^+ diffusion gradient (PCT lumen to cells), ensure low concentration by Na+/K+ pumps on basal and lateral membranes pumping Na^+ into intercellular spaces (active transport; glucose re-absorption is indirectly active process)
• Most glucose is filtered out in ultrafiltration but its body’s main respiratory substrate, also it lowers water potential if it stays in filtrate causing osmotic cell damage and less water re-absorbtion from collecting duct; more lost in urine (dehydration)
• When filtrate reaches PCT base, it’s isotonic with blood plasma (same water potential); diabetics do not reabsorb all filtrate glucose as amount is larger than normal, glucose re-absorption capacity depends on number of Na+/glucose co-transport carrier proteins in microvilli membranes; insufficient for re-absorption of all glucose in diabetic