Lecture 4 - Reabsorption and Secretion Flashcards
What are mechanisms of reabsorption at the peritubular capillaries?
Carrier mediator transport systems
Active transport
What substances are reabsorbed via carrier mediated transport systems?
Glucose Amino acids Organic acids Sulphate Phosphate
What is Tm?
Maximum transport capacity (carriers have this due to saturation of carriers)
What happens if Tm is exceeded?
Excess substrate enters the urine
What is the renal threshold?
Plasma concentration of a substance above which the kidneys cannot reabsorb it back into the blood and it will go into the urine
I.e. the plasma threshold at which saturation occurs
What is the renal threshold for glucose?
10mmoles/L
Why can we say that glycosuria is not a failure of kidney function?
Kidneys do not regulate glucose levels (insulin does) and normal Tm is set way above any possible level of NON-DIABETIC glucose conc.
Why is the Tm set above the normal level of blood glucose and amino acids?
To ensure nutrients are not lost
Do the kidneys use the Tm mechanism to regulate any substances?
Yes, e.g. phosphate and sulphate ions
Tm set at a level where normal [plasma] causes saturation], so any level above this is excreted achieving regulation
Where does most Na reabsorption occur?
PCT
What is the mechanism by which Na ions are reabsorbed?
Active transport (which establishes a gradient for Na across the tubule wall)
Describe the mechanism by which Na ions are reabsorbed via active transport
Active Na pumps on basolateral surfaces (where there are lots of mitochondria)
This decreases Na in epithelial cells, increasing gradient for Na ions to move into cells passively across the luminal membrane
What Na pump creates a low conc of Na ions in the tubular cells for reabsorption of Na from the PCT?
Na-K-ATPase
Why (unlike most at most cell membranes) is Na permeable at the brush border of the PCT?
Enormous surface area offered by microvilli
Large no of Na ion channels which facilitate passive diffusion of Na
How are Cl ions and water reabsorped at the PCT?
Negative ions, e.g. Cl diffuse passively across the PCT membrane down electrical gradient created by Na transport
Na and Cl moving out creates an osmotic force drawing water out the tubules
Water removed by osmosis concentrates substances left in tubule creating outgoing conc. gradients so permeable solutes can be reabsorbed by diffusion (e.g. K, Ca, urea)
What does the rate of reabsorption of non-actively reabsorbed solutes (e.g. urea, K, Ca) depend on?
Amount of water removed (determines extent of conc. gradient)
Permeability of membrane to slute
Why is only 50% of urea reabsorbed?
Tubule membrane is only moderately permeable to urea
How is glucose reabsorbed from the PCT?
Na moves down its electrochemical gradient using SGLT protein which pulls glucose into cell against its conc. gradient
GLUT protein on basolateral membrane allows it to diffuse into the interstitial fluid
What is the process of tubular secretion?
Transporting substances from peritubular capillaries into the tubule lumen
Where can tubular secretion occur?
PCT, DCT and collecting duct
What substances are actively secreted into the tubules?
Protein bound substances
Potentially harmful substances
What is the major intracellular cation?
K
What is the normal ECF K concentration?
4mmoles/l
What ECF K concentration is considered hyperkalaemia?
> 5.5mmoles/l
What ECF K concentration is considered hypokalaemia?
<3.5mmoles/l
What happens to the resting membrane potential in hyperkalaemia?
Decreased
What happens to the resting membrane potential in hypokalaemia?
Increased–> hyperpolarised muscle + cardiac cells
What can result from hyperkalaemia?
VF and death
What can result from hypokalaemia?
Cardiac arrhythmias + eventually death
Describe the renal handling of K
Filtered K reabsorbed primarily at the PCT
Changes in K excretion are due to changes in secretion at the distal parts of the tubule (inc. K conc –> increased secretion and vice versa)
What hormone regulates K secretion?
Aldosterone
What does aldosterone do?
Release when high levels of K, stimulates K secretion and also stimulates Na reabsorption at the DCT
Where is aldosterone produced?
Adrenal cortex
