BIOC34H3 Lecture Notes - Lecture 19: Distal Convoluted Tubule, Nephron, Renal Medulla
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Lecture 19: The Medullary Osmotic Gradient, Urine Formation and Acid-Base Balance
1. The Medullary Osmotic Gradient
1a. Formation of the Gradient: The Loop of Henle
• Medullary osmotic gradient is the osmotic gradient that exists in the peritubular fluid from the outer regions of the
kidney (border between renal medulla and renal cortex) into the inner regions of renal medulla. Osmolarity of peritubular
fluid at cortex/medulla boundary is 300 mOsm. It increases from 300 to 1400 in the inner core of kidney. This
osmotic gradient is what allows kidney to produce a concentrated urine (greater than 300 mOsm).
• Medullary osmotic gradient is established by Loop of Henle which has different permeability to water and ions
between ascending and descending sections. Descending limb is permeable to H2O, but impermeable to ions like,
Na+, K+ or Cl-. The ascending limb is the opposite; permeable to ions and impermeable to water.
• How osmotic gradient is formed (during development and is maintained by the same mechanisms throughout life):
o Starts with kidney without osmotic gradient: in both limbs, and in peritubular fluid, osmolarity is 300 mOsm.
Fluid flows into loop of Henle from proximal tubule (300 mOsm) and enters distal tubule (100 mOsm).
• Key points about the medullary osmotic gradient:
1) Osmolarity in descending limb is always same as the osmolarity in the peritubular fluid.
2) Osmolarity in descending limb and peritubular fluid are always 200 mOsm greater than osmolarity in ascending limb.
3) Fluid enters Loop of Henle with osmolarity of 300 mOsm.
4) Fluid leaves Loop of Henle and enters distal tubule with osmolarity of 100 mOsm (hypo-osmotic to peritubular fluid)
surrounding distal tubule.
5) Medullary osmotic gradient (osmolarity in the peritubular fluid) goes from 300 mOsm at the “top” (border of cortex and
medulla) to 1400 mOsm at the “bottom” (inner core of kidney).
• At any level in kidney (proceeding from outer to inner regions) there’s a
difference of 200 mOsm between fluid in descending and ascending
• There is an 1100 mOsm difference in osmolarity between fluid at “top”
• Loop of Henle is a countercurrent multiplier because it converted
this “horizontal” difference of 200 mOsm between ascending and
descending limbs into 1100 mOsm difference in peritubular fluid
between outer and inner regions of kidney.
• Loop of Henle establishes medullary osmotic gradient, but does not
function in isolation. Blood supply to Loop of Henle is involved in formation and maintenance of medullary osmotic
1b. Maintenance of the Gradient: The Vasa Recta
• Vasa recta is the capillary network that surrounds Loop of Henle. It is a vessel that flows down into core of kidney
and back up again (same as Loop of Henle). It functions as a countercurrent exchanger and helps to maintain medullary
osmotic gradient. It removes some fluid and solutes that were transported across Loop of Henle. This prevents water
and solutes from pooling in peritubular fluid.
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