Brent Cornell

    
Selective reabsorption is the second of the three processes by which blood is filtered and urine is formed

• It involves the reuptake of useful substances from the filtrate and occurs in the convoluted tubules (proximal and distal)
• The majority of selective reabsorption occurs in the proximal convoluted tubule, which extends from the Bowman’s capsule

The proximal convoluted tubule has a microvilli cell lining to increase the surface area for material absorption from the filtrate

• The tubule is a single cell thick and connected by tight junctions, which function to create a thin tubular surface with no gaps

There are also a large number of mitochondria within these tubule cells, as reabsorption involves active transport

• Substances are actively transported across the apical membrane (membrane of tubule cells facing the tubular lumen)
• Substances then passively diffuse across the basolateral membrane (membrane of tubule cells facing the blood) 

The tubules reabsorb all glucose, amino acids, vitamins and hormones, along with most of the mineral ions (~80%) and water

• Mineral ions and vitamins are actively transported by protein pumps and carrier proteins respectively
• Glucose and amino acids are co-transported across the apical membrane with sodium (symport)
• Water follows the movement of the mineral ions passively via osmosis

Selective Reabsorption in the Proximal Convoluted Tubule