Overview of Reabsorption and Secretion in the Nephron

Reabsorption

  • Removes solutes and water from the tubular fluid and returns them to the blood;
  • It reclaims much of the water, ions, and nearly all of the nutrients that are filtered.

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.

Key principles of transport:

  • 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 vien.

Reabsorption and Secretion by Segment:

  • Proximal tubule:
    Reabsorbs:
    Water
    Sodium
    Chloride
    Potassium
    Calcium
    Phosphate
    Urea
    Bicarbonate
    Glucose, amino acids, and other nutrients.
    Secretes:
    Hydrogen
    PAH (para-aminohippurate)
    Ammonium ions
    Certain drugs
    Organic acids and bases (such as creatinine)
  • Nephron loop:
    Concentrates or dilutes urine.
    Thin limb:
    Water is reabsorbed.
    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).
    Late distal tubule:
    Reabsorbs:
    Water
    Sodium
    Chloride
    Bicarbonate
    Urea.
    Secretes:
    Potassium.
  • Collecting duct:
    Secretes OR Reabsorbs
    Potassium
    Hydrogen
    Bicarbonate
    Ammonium ions

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