Page 1034 - Fundamentals of anatomy physiology

Chapter 26 The Urinary System 1021

Figure 26–13 Countercurrent Multiplication and Urine Concentration.

Proximal Distal Tubular fluid Urea
convoluted convoluted
tubule
tubule
Glomerulus Cells of 2 Cl– K+ This plasma
thick membrane is
Glomerular ascending impermeable
capsule limb to water

Nephron loop Na+
2 Cl– K+
KEY
Water Collecting KEY
reabsorption duct Cotransport
Solute Exchange pump
reabsorption Peritubular Cl– K+ Na+ K+ Reabsorption
fluid Secretion

Diffusion

Urine storage a The mechanism of sodium and chloride ion transport involves the
and elimination Na+–K+/2 Cl– carrier at the apical surface and two carriers at the basal
surface of the tubular cell: a potassium–chloride cotransport pump and
a sodium–potassium exchange pump. The net result is the transport of
sodium and chloride ions into the peritubular fluid.

300 300 100 Na+Cl– Renal 100 DCT and
mOsm/L Na+Cl– 300 cortex 300 collecting
ducts
Thin Na+Cl– 300 (impermeable
descending 600 600 to urea;
Na+Cl– variable
limb Na+Cl– permeability
(permeable Thin descending 300 H2O 300 to water)
limb (permeable H2O
to water; 600
impermeable to water;
impermeable
to solutes)
to urea)
Na+Cl–

H2O 900 Thick H2O H2O H2O 26
ascending limb
900 (impermeable 900 900
to water;
H2O 900 active solute H2O 1200 Papillary duct
H2O 1200 transport) (permeable to
1200 urea)
1200 1200
Na+
Renal medulla Renal medulla Cl–
Urea

b Transport of NaCl along the ascending thick limb results c The permeability characteristics of both the loop and the collect-
in the movement of water from the descending limb. ing duct tend to concentrate urea in the tubular fluid and in the
medulla. The nephron loop, DCT, and collecting duct are imperme-
KEY Impermeable to urea; able to urea. As water reabsorption occurs, the urea concentration
Impermeable to water variable permeability to water increases. Papillary duct permeability to urea makes up nearly
Impermeable to solutes one-third of the solutes in the deepest portions of the medulla.
Permeable to urea