Reabsorption and Secretion Flashcards
Describe carrier mediated transport systems as a mechanism of reabsorption.
e.g. glucose, amino/organic acids, sulphate/phosphate ions.
Carriers have a maximum transport capacity (Tm), which is due to saturation of the carriers. If Tm is exceeded, then the excess substrate enters the urine.
Carrier protein enables larger molecules to cross the membrane.
Define ‘renal threshold’.
Plasma threshold at which saturation occurs.
Describe glucose reabsorption as an example of renal threshold.
Glucose is freely filtered i.e. all plasma glucose will be filtered.
Up to 10 mmol/L will be reabsorbed. Beyond this, it appears in the urine = renal plasma threshold for glucose.
Does the Tm mechanism regulate the levels of substances in the body.
NO = glucose (regulated by insulin).
Yes = sulphate and phosphate ions.
In these cases, Tm is set at a level whereby the normal [plasma] causes saturation.
Any increased above normal will be excreted, therefore achieving plasma regulation.
(phosphate is also regulated by PTH).
Describe the Tm mechanism for amino acids.
Tm is set so high that urinary excretion does not occur, as amino acids are an invaluable resource to the body.
Describe the mechanism of reabsorption of Na+ ions.
Not reabsorbed by a Tm mechanism, but by active transport, which establishes a gradient for Na+ across the tubule wall.
1) Passive: Na+ moves from high [Na+] in tubule lumen to low [Na+] in proximal tubule cell.
2) Active: Na+ moves from low [Na+] in cell to high [Na+] in ISF through an ATP transporter.
Decrease in [Na+] in cells increases gradient for Na+, allowing cells to passively cross luminal membrane.
How does Na+ cross the membranes of the proximal tubule cells when usually it is not permeable to cell membranes?
“The permeability of a substance is not due to the substance itself but the properties of the cell.” - Dr Scholz.
The brush border of the proximal tubule cells has a higher permeability to Na+ than most other membranes in the body because of;
- Enormous surface area offered by the microvilli
- Large number of Na+ ion channels, which facilitate this passive diffusion of Na+
How are Na+ ions key to the reabsorption of other components of the filtrate?
Negative ions (e.g. Cl-) diffuse passively across the proximal tubular membrane down the electrical gradient established and maintained by the active transport of Na+.
The active transport of Na+ out of the tubule followed by Cl- creates an osmotic force, drawing water out of the tubules.
Less water in tubules = higher conc., which makes it easier for other components to be transported passively.
The rate of reabsorption of passively reabsorbed solutes depends on;
1) Amount of water removed, which will determine the conc. gradient.
2) The permeability of the membrane to the solute.
What types of substances are expelled during tubular secretion?
- Those that are protein-bound, since filtration at glomerulus is very restricted
- Potentially harmful substances, can be eliminated more rapidly
Are Tm-limited carrier-mediated secretory mechanisms specific?
They are NOT specific, and thus allow the excretion of previously unknown substances.
What is the normal ECF[K+[?
4mmol/L.
Describe the renal handling of K+.
COMPLEX!
- K+ filtered at glomerulus is reabsorbed at proximal tubule
- Changes in K+ excretion due to changes in its secretion in distal tubule
e. g. increase in [K+] in tubule –> increased K+ secretion.
Describe the regulation of K+ by aldosterone.
Aldosterone = adrenal cortical hormone.
Increase in ECF[K+] –> stimulates aldosterone release –> circulate kidneys –> increase in renal tubule K+ secretion.
Aldosterone also stimulate Na+ absorption at distal tubule.
