Reabsorption and Secretion in the Nephron

Percentage of filtrate reabsorbed

99%.

Location of most reabsorption in nephron

In the proximal convoluted tubule.

Transcellular transport in reabsorption

Transport through the tubule cell: apical membrane → cytosol → basolateral membrane → ECF → endothelium of peritubular capillary → blood.

Paracellular transport in reabsorption

Transport between tubule cells → ECF → endothelium of peritubular capillary → blood.

Na⁺ reabsorption mechanism

By active transport.

Effect of Na⁺ reabsorption on water and solute

Na⁺ reabsorption creates an electrochemical gradient → drives anion reabsorption. Water follows solutes by osmosis. Other solutes are reabsorbed by diffusion as fluid volume decreases.

Na⁺ and glucose reabsorption in proximal tubule

Na⁺ moves down its gradient via SGLT protein, pulling glucose into the cell. Glucose exits via GLUT on basolateral side. Na⁺ is pumped out by Na⁺/K⁺-ATPase.

Substances normally reabsorbed in proximal tubule

All glucose and amino acids/proteins, 65% of Na⁺ and H₂O, 60% of Cl⁻, 55% of K⁺, 50% of urea.

Reabsorption in Loop of Henle

Water can exit the descending limb, but not the ascending limb. Solutes (Na⁺, Cl⁻) exit via the ascending limb.

Percentage of filtered NaCl and water remaining by distal tubule

10% of NaCl and 25% of water.

Determinants of reabsorption in distal tubule and collecting duct

The body's needs, regulated by hormones.

Hormone increasing water reabsorption in collecting duct

Antidiuretic hormone (ADH), by inserting aquaporins in the luminal membrane.

Hormone enhancing Na⁺ reabsorption in collecting duct

Aldosterone.

Substances secreted into nephron tubule

K⁺, H⁺, HCO₃⁻, uric acid, urea, and other compounds.

Direction of secretion movement

Against their concentration gradient.

Type of transport for secretion of organic anions

Indirect active transport on the basolateral membrane, facilitated diffusion on the apical membrane.

Mechanism of nephron secretion of organic anions

Direct active transport: Na⁺/K⁺-ATPase keeps intracellular Na⁺ low. Secondary active transport: Na⁺ cotransporter brings in dicarboxylate. Basolateral transporter: Brings in organic anions using dicarboxylate gradient. Apical exchange: Organic anions exit into lumen in exchange for dicarboxylate.