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