Epithelial Transport

Question 1

Salt Absorption

Answer 1

Apical membrane is permeable to Na but Basolateral mmbr is not. So Na leaks down conc. gradient across AP and is then pumped across BL. Chloride passively diffuses across both membranes and follows the opposite charge of Na into blood. Water is osmotically drawn with these ions.

Question 2

Glucose and AA absorption

Answer 2

Both glucose and AAs utilize secondary active cotransport with sodium to move across AP mmbr. They then passively cross BL mmbr down concentration gradients.

Question 3

Tight vs Leaky epithelia

Answer 3

Tight epithelia have impermeable tight junctions between epithelial sheet cells. These maintain higher energy gradients and allow for highly regulated absorption/secretion.
Leaky epithelia do not have these tight junctions so most solutes can passively move from lumen to interstitium at slow rate with minimal selectivity. This is common in high-volume absorption areas.

Question 4

Epithelial Secretion Mechanism

Answer 4

Chloride co-transporter in BL mmbr pumps 3:1:2 Na:K:Cl into epithelial cell from interstitium. The increased Cl concentration causes Cl to diffuse out of AP mmbr. Na follows opposite charge through intercellular shunt and osmotically draws water into lumen.
This is usually governed by Ca signaling.

Question 5

Substances that are never pumped (4)

Answer 5

Water, Urea, O2, CO2
Water moves passively through aquaporins.
Urea leaks through proximal tubule into filtrate and is not drawn back into blood
O2 and CO2 are volatile and passively move through alveoli without need of receptors.

Question 6

Excretion of metabolic wastes (general)

Answer 6

15M of metabolic waste made per day. 14.5 is volatile CO2. Remaining is 0.4M urea, 0.1M hydrogen

Question 7

Excretion of CO2

Answer 7

CO2 is volatile and passively diffuses out of lungs

Question 8

Excretion of urea

Answer 8

Urea is collected in ultra-filtrate in proximal kidney where it passively moves through leaky junctions. Only selected solutes are pumped back to blood and urea is excluded

Question 9

GI vs. Kidney excretion

Answer 9

GI excretes 30mM/day of RBC breakdown products from liver. Kidney takes care of 0.5M of waste daily in ultrafiltrate

Question 10

Main function of kidney

Answer 10

Excrete non-volatile metabolic waste. All solutes passively enter ultrafiltrate in proximal tubule through leaky junctions. Only desired solutes are pumped back in to blood. The rest is excreted.

Question 11

Maximum Urine Concentration?

Answer 11

1200 mosM