Reabsorption and Secretion in the Nephron
Reabsorption
Removes solutes and water from the tubular fluid and returns them to the blood; much of the water, ions, and nearly all of the nutrients that are filtered are reclaimed.
Secretion
Moves solutes from the blood and nephron tubule cells into the tubular fluid; secretion is important for removal of substances that aren't filtered (such as drugs and metabolites) and for fine-tuning the final urine composition.
Transport in the Vasculature:
Efferent arteriole leaves glomerulus, gives rise to peritubular capillaries.
Peritubular capillaries give rise to vasa recta of juxtamedullary nephrons.
Vasa recta drains deoxygenated blood into the interlobular vein.
Reabsorption and Secretion by Segment
Reabsorbed from Proximal Tubule:
Water
Sodium
Chloride
Potassium
Calcium
Phosphate
Urea
Bicarbonate
Glucose, amino acids, and other nutrients.
Secreted into Proximal Tubule:
Hydrogen
PAH (para-aminohippurate)
Ammonium ions
Certain drugs
Organic acids and bases (such as creatinine)
Nephron loop: Concentrates or dilutes urine.
Reabsorbed from Thin Limb:
Water
Secreted into Thin Limb:
Urea
Reabsorbed from Thick Ascending Limb:
Sodium
Potassium
Chloride
Calcium
Bicarbonate
Magnesium
(no water reabsorption)
Distal Tubule reabsorption and secretion are hormonally regulated to fine-tune tubular fluid, to maintain ECF volume and osmolarity homeostasis.
Early Distal Tubule, aka, Diluting Segment:
Sodium
Chloride
Potassium
Calcium
(no water reabsorption).
Reabsorbed from Late Distal Tubule:
Water
Sodium
Chloride
Bicarbonate
Urea.
Secreted into Late Distal Tubule:
Potassium
Secreted or Absorbed in Collecting Duct:
Potassium
Hydrogen
Bicarbonate
Ammonium ions
