PPT #9B - Reabsorption of Nutrients p 526 1 Glucose[NOT...

Reabsorption of Nutrients
p. 526
2. Symport carriers in the proximal tubule transfer both Na+ and the
organic molecule from the lumen into the cell (Na+ passively; organics
actively via symport carrier).
3. Glucose and amino acids passively diffuse down their concentration
gradients across the basolateral membrane through the interstitial fluid into the
capillary plasma.
A
. Cl- passively reabsorbed down the electrical gradient created by the
Na+ entry into the peritubular cell; Cl- passes between, not into the tubular
cells and into the peritubular capillaries following Na+.
4. Not only is secondary active reabsorption of glucose and amino acids linked
to the basolateral Na+/K+ pump, but passive reabsorption of:
(A) Cl-,
(B) H2O (Fig. 14-19 ) and
(C) urea (Fig. 14-20)
also depends on this active Na+ reabsorption mechanism
1. Glucose [ NOT REGULATED BY KIDNEY; normally regulated by endocrine
and liver mechanisms] and amino acids are reabsorbed by Na+-dependent
secondary active transport.

Tubular cell Interstitial fluid
Peritubular
capillary
Lumen
Basolateral
Na+– K+ pump
Lateral space
KEY
= Active transport of ion against concentration gradient
= Passive movement of ion down concentration gradient
Glucose, AAs, water & urea
p. 526
Diffusion
Cl-
-
PROXIMAL
Na+ & glucose
cotransporter (SGLT)
Active T
Passively
p. 524
p.
524

Reabsorption- Water
B. Water is passively reabsorbed throughout the
tubule as it follows reabsorbed Na+.
:
a. 80% of the water reabsorbed is uncontrolled
b. 65% is reabsorbed in the proximal tubule where
Aquaporin (AQP-1) channels are always open (see next
slide).
c. 15% is reabsorbed from the loop of Henle
d. 20% of the water reabsorbed is controlled under
hormonal control of vasopressin (ADH) at the distal tubule
where Aquaporin ( AQP-2 ) channels are located

AQP-1
water
channels
Interstitial fluid
Peritubular
capillary
Lumen Proximal tubular cell
Osmosis
Hydrostatic
pressure
AQP-1 water
channels
Osmosis
Fig. 14-19, p. 527
“Leaky” tight
junctions
Hypertonic regions in the lateral
spaces (osmotic effects)
Always open:
Always open: