Reabsorption and Filtration Flashcards
What mechanisms of reabsorption are there?
- Carrier mediated proteins
- Reabsorption of Na ions
- Tubular secretion
Give examples of substances reabsorbed by carrier mediated proteins.
- Glucose
- Amino acids
- Organic acids
- Sulphate
- Phosphate ions
What is Tm?
The maximum transport capacity of carriers which is due to saturation of the carriers
What happens if Tm is exceeded?
The excess substrate enters the urine
What do carrier proteins enable?
Larger molecules such as glucose can cross the membrane
What is capacity limited by?
The number of carriers
What is renal threshold?
The plasma threshold at which saturation occurs
What is one of the most important substances to consider in transport?
Glucose
Glucose is freely filtered so…
Whatever its [plasma] that will be filtered
In man, all glucose will be reabsorbed up to what level/
10mmoles/l
What will happen to plasma [glucose] of 15mmoles/l?
- 15 will be filtered
- 10 will be reabsorbed
- 5 will be excreted
What is the renal plasma threshold for glucose?
10mmoles/l, beyond this level of plasma [glucose] it will appear in the urine
What does the kidney NOT regulate?
[Glucose]
What is responsible for [glucose] regulation?
Insulin and the counter-regulatory hormones
Glycosuria
The appearance of glucose in the urine typically associated with diabetes
What is glycosuria due to?
Failure of insulin not the kidney
Why is the Tm for amino acids set high?
So that urinary excretion does not occur (also regulated by insulin and counter-regulatory hormones)
Give examples of substances that the kidney does regulate via the Tm mechanism.
- Sulphate ions
- Phosphate ions
how are sulphate and phosphate ions regulated via the Tm mechanism?
Tm (maximum transport capacity) is set at a level whereby normal [plasma] causes saturation. any increase above the normal level will be excreted, achieving its plasma regulation
What are the most abundant ions in the ECF?
Na ions
How much Na is filtered and reabsorbed each day?
- 25,560 mmoles/day filtered
- 99.5% reabsorbed
- 65-75% of reabsorption occurs in the proximal tubule
How are Na ions reabsorbed?
active transport establishes a gradient for Na across the tubule wall
Where are active Na pumps located?
On the basolateral surfaces, where there is a high density of mitochondria
How does active transport of Na ions allow Na ions to cross the luminal membrane passively?
The pumps decrease the [Na] in the epithelial cells, which increases the gradient for Na ions to move into the cells passively across the luminal membrane
Why does the brush border of the proximal tubule cells have a higher permeability to Na ions?
Partly because of the enormous surface area offered by the microvilli and the large number of Na+ ion channels, which facilitate this passive diffusion of Na+.
What is the reabsorption of Na ions key to?
Reabsorption of the other components of the filtrate.
How do Cl ions cross the membrane?
Negative ions such as Cl diffuse passively across the proximal tubular membrane down the electrical gradient established and maintained by the active transport of Na
What does the active transport of Na out of the tubule followed by Cl create?
An osmotic force, drawing H2O out of the tubules.
What does H2O removed by osmosis from the tubule fluid do?
Concentrates all the substances left in the tubule creating outgoing concentration gradients
What does rate of reabsorption of the non-actively reabsorbed solutes depend on?
- Amount of H2O removed, which will determine the extent of the concentration gradient.
- The permeability of the membrane to any particular solute.
Give examples of substances than the tubular membrane in impermeable to.
- Inulin
- Mannitol
Why is only about 50% of urea reabsorbed?
Tubule membrane is only moderately permeable to urea, so that only about 50% is reabsorbed, the remainder stays in the tubule.
What establishes the gradient down which ions. H2O and solute pass passively?
The active transport of Na
Why is all the mannitol and inulin filtered excreted in the urine?
Despite a concentration gradient being established favouring their reabsorption, they cannot gain access through the tubule membrane so that all that is filtered stays in the tubule and passes out in the urine
What will anything that decreases active transport result in?
Disruption of renal function
What substances share the same carrier molecule as Na?
- Glucose
- Amino acids
What facilitates glucose transport?
High [Na]
How does low [Na] affect glucose transport?
Inhibits glucose tranpsport
What is Na reabsorption linked to in terms of the acid base balance?
HCO3 ion reabsorption
What does SGLT stand for?
Sodium dependent glucose transporter
Where do secretory mechanism transport substances from?
From the peritubular capillaries into the tubule lumen
Where is there no secretion?
Loop of Henle
What is secretion especially important for?
- protein-bound substances (filtration at glomerulus is very restricted)
- potentially harmful substances (can be eliminated more rapidly)
Give an example of how carrier mechanisms are not very specific?
- Organic acid mechanism, which secretes lactic and uric acid can also be used for substances such as penicillin, aspirin and PAH (para-amino-hippuric acid).
- Organic base mechanism for choline, creatinine can be used for morphine and atropine
Where are substances such as morphine and atropine secreted?
Proximal tubule
What is the major cation in the cells of the body?
Potassium
What is the normal ECF [K]?
4mmoles/l
When does hyperkalaemia occur?
If [K] > 5.5mmoles/l
When does hypokalaemia occur?
If [K] < 3.5mmoles/l
What does hyperkalaemia cause?
Decrease in resting membrane potential of excitable cells and eventually ventricular fibrillation and death
What does hypokalaemia cause?
Increase in resting membrane potential ie hyperpolarizes muscle, cardiac cells leading to cardiac arrhythmias and eventually death
Where is K filtered?
Glomerulus
Where is K primarily reabsorbed?
Proximal tubule
What are changes in K excretion due to?
Changes in its secretion in the distal parts of the tubule
What will cause an increase K secretion?
Any increase in renal tubule cell [K] due to increased ingestion
What will cause a decrease in K secretion?
A decrease in intracellular K
What is K secretion regulated by?
By the adrenal cortical hormone aldosterone
What stimulates aldosterone release?
An increase in [K] in ECF bathing the aldosterone secreting cells stimulates the release of aldosterone
What effect does aldosterone have on the kidneys?
It stimulates an increase in the renal tubule cell K secretion
What effect does aldosterone have on Na?
It stimulates Na reabsorption
Where are H ions actively secreted?
Into the lumen (not the peritubular capillaries)
