Nephron Function: Reabsorption Mechanisms

Reabsorption Mechanisms

• Reabsorption of chloride, urea, and other solutes occurs through passive diffusion, coupled with sodium reabsorption
• 65% of filtered sodium, chloride, bicarbonate, and potassium, and essentially all filtered glucose and amino acids are reabsorbed by the proximal tubule

Proximal Tubule

• Reabsorbs 65% of filtered sodium, chloride, bicarbonate, and potassium, and essentially all filtered glucose and amino acids
• Concentrations of solutes along the proximal tubule can be seen in fig 7
• Secretes organic acids and bases

Loop of Henle

• Composed of three functionally distinct segments: thin descending segment, thin ascending segment, and thick ascending segment
• Thick ascending segment reabsorbs sodium, chloride, and potassium (fig 8-B)

Active Transport

• Primary active transport linked to hydrolysis of ATP (fig 2)
• Secondary active reabsorption through the tubular membrane (fig 3)
• Secondary active secretion into the tubules (fig 3)
• Pinocytosis: an active transport mechanism for reabsorption of proteins
• Transport maximum for substances that are actively reabsorbed (fig 4)

Distal Tubule and Collecting Duct

• Forms part of the juxtaglomerular apparatus (JGA)
• Diluting segment
• Site of action of thiazide diuretics
• Principal cells reabsorb sodium and secrete potassium (fig 10)
• Intercalated cells avidly secrete hydrogen and reabsorb bicarbonate and potassium ions
• Medullary collecting duct reabsorbs only 10% of filtered water and sodium, permeability to water is ADH dependent