Osmoregulation and Excretion

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Slide 44

Descending Limb of the Loop of Henle

Reabsorption of water continues through channels formed by aquaporin proteins.

Movement is driven by the high osmolarity of the interstitial fluid, which is hyperosmotic to the filtrate.

The filtrate becomes increasingly concentrated.

Ascending Limb of the Loop of Henle

In the ascending limb of the loop of Henle, salt but not water is able to diffuse from the tubule into the interstitial fluid.

The filtrate becomes increasingly dilute.

Slide 45

Distal Tubule

The distal tubule regulates the K+ and NaCl concentrations of body fluids.

The controlled movement of ions contributes to pH regulation.

Collecting Duct

The collecting duct carries filtrate through the medulla to the renal pelvis.

Water is lost as well as some salt and urea, and the filtrate becomes more concentrated.

Urine is hyperosmotic to body fluids.

Slide 46

The Nephron and Collecting Duct: regional functions of the transport epithelium

Key

Active

transport

Passive

transport

INNER

MEDULLA

OUTER

MEDULLA

H2O

CORTEX

Filtrate

Loop of Henle

H2O

K+

HCO3–

H+

NH3

Proximal tubule

NaCl

Nutrients

Distal tubule

K+

H+

HCO3–

H2O

H2O

NaCl

NaCl

NaCl

NaCl

Urea

Collecting duct

NaCl

Slide 47

Solute Gradients and Water Conservation

Urine is much more concentrated than blood.

Cooperative action + precise arrangement of the loops of Henle and collecting ducts are largely responsible for the osmotic gradient that concentrates the urine.

NaCl and urea contribute to the osmolarity of the interstitial fluid, which causes reabsorption of water in the kidney and concentrates the urine.

Slide 48

The Two-Solute Model

In the proximal tubule, filtrate volume decreases, but its osmolarity remains the same

The countercurrent multiplier system involving the loop of Henle maintains a high salt concentration in the kidney.

This system allows the vasa recta to supply the kidney with nutrients, without interfering with the osmolarity gradient.

Considerable energy is expended to maintain the osmotic gradient between the medulla and cortex.

Slide 49

The collecting duct conducts filtrate through the osmolarity gradient, and more water exits the filtrate by osmosis.

Urea diffuses out of the collecting duct as it traverses the inner medulla.