L2: Bulk Reabsoprtion In The PCT Flashcards
LO: Define tubular reabsorption
Tubular reabsorption = the process of returning important substances from the filtrate back to the body/blood stream
LO: Define tubular secretion
Tubular secretion = movement of waste materials form the body to the filtrate
How are the tubular cells of the PCT adapted for reabsorption?
- Microvilli/brush border on apical membrane to increase SA
- Large amount of NaK-ATPase on basolateral membrane
- Contain large amount of carbonic anhdyrase enzyme
What are the effects of active transport of Na+ out of the tubular cell?
Sets up a sodium gradient - drives Na+ movement acros apical membrane
Cl- follows sodium down the electrochemical gradient
Sets up and osmotic gradient causing water to follow by osmosis
*bulk movement of after causes:
-> solvent drag - brings other solutes from filtrate unto capillaries
-> diffusion - sets up conc gradient for passive diffusion of solutes through selective ions/protein channels
How is reabsorption in the PCT “selective”?
Presence/absence of ion channels helps control diffusion
Not regulated by hormones
How is glucose reabsorbed from the filtrate
-> Secondary active transport
Leaves filtrate with Na+
Dependent upon presence of specific membrane proteins
- 2 Na+ and glucose travel through apical membrane using an Na+-glucose symporter
- Glucose diffuses across basolateral membrane through a glucose facilitated diffusion transporter
Is there a limit to how much glucose can be filtered into the PCT?
No - freely filtered and does not saturate
- filtration depends on plasma concentration
Is there a limit t how much glucose can be reabsorbed from the PCT?
Yes - reabsorption is dependent on:
- rate of flow of the filtrate
- number of protein transporters
How can we increase reabsorption of glucose from the PCT?
- Decrease rate of flow of filtrate
- Increased number of protein transporters
What is the result of renal threshold for glucose reabsorption being reached?
Glycosuria - some glucose remains in the filtrate and is passed into the urine
How is phosphate reabsorbed from the PCT?
Linked to Na co-transport in PCT (similarly to glucose)
Unlike most reabsorption in PCT it is hormonally regulated
- regulated by parathyroid hormone (PTH)
- PTH reduces phosphate reabsorption in the PCT & increases phosphate excretion
What is the advantage of most filtrate being immediately reabsorbed back into the blood in the PCT?
This is a protective mechanism
Quickens secretion of toxic substances/waste - freely filtered at the renal corpuscle
Why are a large number of environmental toxins highly lipid soluble?
Lipid soluble substances cross membranes readily
Sets up diffusion gradient promoting reabsorption
Liver converts many but not all foreign substances into water soluble substances which are much easier to secrete
What are the features of tubular secretion? (3)
- Always active process (usually inked to Na+ transport)
- Not hormonally regulated (in PCT)
- Substances must be ionised
What is the mechanism for secretion of H+ ions?
Protons secreted into filtrate via secondary active transport
- sodium hydrogen exchanger in apical membrane
Some protons bind to non-bicarbonate buffers and are excreted in urine
Can also be secreted via NH4 sodium anti-porter on apical membrane
