Epithelial transport

Question 1

ECF composition

Answer 1

Ca= 1mM
H+ = 0.00004mM
K+ = 4mM
Cl- = 115 mM

Question 2

ICF composition

Answer 2

Ca or H+ = 0.0001mM
Na or K = 140 mM
pure water = 55000 mM
Bicarb (HCO3-) 24mM
Maximum urine osmolarity = 1200 mOSM

Question 3

Apical

Answer 3

Faces “sepcial fluid” like food in gut or urine..

Mucosal, luminal

Question 4

Basolateral

Answer 4

Exposed to interstitial fluid, often has generic transport properties like plasma mems of non-epithelial cells
Serosal, peritubular

Question 5

Endothelial cells

Answer 5

In lungs because do not need transporters for O2 and Co2, they are lipid soluble.
They separate blood from air.

Question 6

Pericellular shunt pathway

Answer 6

Tight junctions b/w epithelial cells are looser in some areas (movement of water and solutes).

Question 7

2 routes through epithelium

Answer 7

Through 2 membranes (apical and basolateral mem) or between cells in pericellular shunt pathway

Question 8

Na/K pump

Answer 8

ALWAYS on basolateral membrane.
Driving force for nearly all transport.
It keeps the intracell [Na] low (protons are universal exception because they have specific primary transporters… esp kidney and stomach).

Question 9

Basolateral mem ion permeability

Answer 9

Low Na permeability, high K permeability.
Also has some Cl channels.
Vm = -70mV

Question 10

Apical mem ion permeability

Answer 10

Relatively highly permeable to sodium not potassium
Depolarized compared to basolateral mem.
Vm= +10mV
Apical solution is negative compared to basolateral (makes sense b/c Na moves out of it that draws Cl- with it).

Question 11

Movement of ions

Answer 11

It is permeable across Na membrane so it crosses here and then is pumped out by basolateral, Cl- follows passively (drawn by electrical force) and H20 follows osmotic gradient into basolateral

Question 12

Replace Cl with SO4 (too large to cross)

Answer 12

Na/K pump would continue, but because no anion could follows the trans-epithelial voltage would increase.
The apical solution would become so negative the net transport would stop.

Question 13

TransPD

Answer 13

Transepithelial potential differenece = Vm (basolateral) - Vm (apical)

Question 14

Leaky epithelium

Answer 14

Moves larger amounts of material.

Also short out the transepithelial potential diff, so you measure a lower transepithelial voltage

Question 15

Apical Cl- channel

Answer 15

Typically closed at resting. When activated the cell will begin to secrete electrolytes and H20 into the lumen (open in digestion and cholera).
Cl- pumped across basolateral, leaks out apical, brings Na (electical draw) and water along. Intracell Cl high because of pump.
Net secretion of NaCl.
At rest absorption wins.

Question 16

Cystic fibrosis

Answer 16

Cl- channel type CFTCR. Channel is mutation and has decreased ability to secrete, get thick mucus secretions.

Question 17

Absorption of nutrients

Answer 17

AA’s and sugars in GI lumen are pumped into blood by GI epithelium. Glucose and Aa are reabsorbed by epithelial cells in kidney.
Na dependence.
Na/Cl leaks from blood to GI through the shunt pathway, and then it can provide drive for glucose uptake. Then nutrients go out basolateral mem by faciliated diffusion.

Question 18

ECF composition

Answer 18

This is regulated by chemical specificity not ECF composition. So we do NOT regulate ECF composition at the input end (kidneys at output end must regulate it).

Question 19

Impermeable to H20

Answer 19

Because H20 follows oslutes …
sweat glands. as fluid travels along sweat duct towards surface NaCl is resorbed by the epithelium lining the duct and returned to the blood.
Because ducts are impermeable to H20, water can’t follow salt, so fluid in lumen gets more dilute.

Question 20

Metabolic waste

Answer 20

Co2 is exhaled (majority of waste)
Most important fx. of kidney is to get rid of non-volatile metabolic waste (urea, protons)…. kind of like lungs in that blood is near tubules and it can move.

Question 21

Kidney and waste

Answer 21

It knows what it likes.
It takes everything and passes it through renal tubules (ultrafiltrate) and resorbs what it wants.
Very energetically expensive, need lots of ATP to drive reabsorbing pumps.

Question 22

Kidney and ECF

Answer 22

it regulates ECF composition by adjusting activity of transporters that do reabsorbing.

Question 23

Kidney and water

Answer 23

The epithelium can become impermeable.
The brain senses extra water as a drop in osmolarity, stops secreting anti-diuretic hormone (vasopressin), and kidney epithelial cells remove aquaporins from apical side.
Water is secreted.

Question 24

4 substances never pumped, always move passively?

Answer 24

Water, O2, Co2, urea