Cellular Mechanisms of Reabsorption

Question 1

Describe the transcellular and paracellular pathways of reabsorption.

Answer 1

Transcellular transport: Occurswhena solute passes through both apical and basolateral membranes of a cell when being absorbed into the blood.

Paracellular transport: Occurs when solute moves past tight junctions between lumen cells to be reabsorbed.

Question 2

Describe first phase Na+ (NaCl) transport in the PCT.

Answer 2

• Reabsoprtion of Na+ w/ Glu. AA’s, phosphate, lactate and bicarbonate ions (Symport).
• Na+ Uptake partnered w/ H+ symporter and antiporter proteins (HCO3- reabsorption)
• Na passes to blood via sodium-potassium pump on basolateral membrane.
• Lactate, Glu, AA’s, etc pass through basolateral membrane via facillitated diffusion.
• All the organic solutes cleared by first half of PCT

Question 3

Describe second phase Na+ (NaCl) transport in the PCT.

Answer 3

• Reabsorption of Na+ is transcellular or paracellular (coupled w/ Cl-)
• Paracellular transport occurs due to increase in tubular fluid [Cl-]. Cl- via LIS -> blood. Creats transepithelial voltage whic causes Na+ to follow
• Transcellular transport rout involves the parallel operations of Na+H+ and Cl-Base Antiporters. Base can be OH-; HCO3-, Oxalate etc. H+ and base recombine in tubular luid and passively diffuse into cell.

ON basolateral memebran, Na+ out nd K+ in via SP-ase Pump. K+ Cl- symported allows K+ and Cl+ to travel to ECF

Question 4

Describe water reabsorption in the PCT.

Answer 4

Due to OP gradients. Movement of solutes in LIS creates hyperosmotic gradient which induces pressure that forces fluid from the LIS, containing solutes (K+, Mg2+, Ca2+) to move into blood capillaries. = Solvent drag.

Question 5

Using the terms “renal threshold” and “tubular maximum”, describe glucose reabsorption in the PCT?

Answer 5

There are a limited number of Glu. transport proteins.

When all proteins are occupied, you have reached the tubular maximum = maximum rate of glu. transport. (375mg/min)

When glucose levels exceed 300g/100ml and glu starts appearing in the urine, you have reached renal threshold.

Question 6

Describe fat absorption in the small intestine?

Answer 6

Short chain FA (<10-12C) diffuse directly into the mucosal cell.

Long chain FAs (>12C) require emulsification by bile salt. = FA -> FFA + Glycerol

Transferred via micelles into mucosa of the cell.

Fat soluble vitamins enter cell w/ FAs

Within the mucosa cell: FFA + glycerol = TG

Fat droplets -> chylomicrons

Exocytosis via lacteals -> liver.