Page 1032 - Fundamentals of anatomy physiology

Chapter 26 The Urinary System 1019

proteins. For this reason, these individuals are unable to recover reaches the nephron loop. In the process, useful organic sub-
specific solutes from tubular fluid. In renal glycosuria (glı.-ko. -SYU. - strates and many mineral ions have been reclaimed. The nephron
re.-uh), for example, a defective glucose carrier protein makes it loop reabsorbs about half of the remaining water and two-thirds
impossible for the PCT to reabsorb glucose from tubular fluid. of the remaining sodium and chloride ions. This reabsorption
takes place efficiently according to the principle of countercur-
The Nephron Loop and Countercurrent rent exchange. We introduced this principle in Chapter 25 in our
Multiplication discussion of heat conservation mechanisms. p. 991

Approximately 60–70 percent of the volume of filtrate produced The thin descending limb and the thick ascending limb of
by the glomerulus has been reabsorbed before the tubular fluid the nephron loop lie very close together. They are separated only
by peritubular fluid. The exchange between these segments is

Figure 26–12 Transport Activities at the PCT. Sodium ions may enter a tubular cell from the filtrate

by diffusion, cotransport, or countertransport. The sodium ions are then pumped into the peritubular fluid
by the sodium–potassium exchange pump. Other reabsorbed solutes may be ejected into the peritubular
fluid by separate active transport mechanisms. The absorption of bicarbonate is indirectly associated with
the reabsorption of sodium ions and the secretion of hydrogen ions.

Lumen containing H+ + HCO3– H2CO3 CO2 + H2O
tubular fluid
Cuboidal Tubular fluid

epithelial cells

Cells of
proximal
convoluted
tubule

Na+ Na+ Na+ CO2
H2O
Proximal
convoluted Distal Glucose H2CO3
convoluted and other H+ + HCO3– Osmotic
tubule tubule organic
Glomerulus water
solutes flow

Glomerular Peritubular 2 K+
capsule fluid 3 Na+
Peritubular
Collecting capillary Na+ 26
duct HCO3–

Nephron loop Na+ H2O
H+ HCO3–
3 Na+

KEY Urine storage KEY
and elimination Leak channel
Water reabsorption Countertransport
Exchange pump Diffusion
Solute reabsorption Reabsorption
Cotransport Secretion
Variable solute reabsorption
or secretion