Reabsorption & Secretion Flashcards
describe starling forces in glomerular and peritubular capillaries?
what is a mechanism of reabsorption?
Many substances are reabsorbed by carrier mediated transport systems eg glucose, amino acids, organic acids, sulphate and phosphate ions.
what do carries have in relation to capacity?
Carriers have a maximum transport capacity Tm 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 to cross the membrane.
Capacity is limited by number of carriers.
wat is renal threshold?
plasma threshold at which saturation occurs.
what is the titration curve for glucose?
Glucose is freely filtered, so whatever its [plasma] that will be filtered.
In man for plasma glucose up to 10 mmoles/l, all will be reabsorbed.
Beyond this level of plasma [glucose], it appears in the urine
what is the Rena plasma threshold for glucose?
In man for plasma glucose up to 10 mmoles/l, all will be reabsorbed.
(If plasma [glucose] = 15 mmoles/l, 15 will be filtered, 10 reabsorbed and 5 excreted.)
do the kidneys regulate glucose?
Kidney does NOT regulate [glucose], (insulin and the counter-regulatory hormones responsible for its regulation).
what is the normal plasma glucose?
Normal [glucose] of 5 mmoles/l, so Tm is set way above any possible level of (non-diabetic) [glucose]. Ensures that all this valuable nutrient is normally reabsorbed.
in what type of patients will glucose appear in urine?
diabetic patients = glycosuria, is due to failure of insulin, NOT, the kidney.
N.B. Any patient with glucose in their urine should be followed up.
how is Tm set for amino acids?
set so high that urinary excretion does not occur, regulated by insulin and counter-regulatory hormones.
what substances aren’t regulated by the means of Tm mechanism?
sulphate and phosphate ions
what ions are the most abundant in ECF?
Na+ ions very large amount filtered daily
180 l/day x 142 mmoles/l = 25560 mmoles/day, 99.5% is reabsorbed.
where does the majority of Na+ ion reabsorption occur?
65-75% occurs in the proximal tubule
what is the mechanism of reabsorption for Na+ ions?
Not reabsorbed by a Tm mechanism, but by active transport, which establishes a gradient for Na+ across the tubule wall.
how does the Na+ pump drive the whole process?
Active Na+pumps are located on the basolateral surfaces, where there is a high density of mitochondria.
This decreases [Na+] in the epithelial cells, increasing the gradient for Na+ ions to move into the cells passively across the luminal membrane.
is Na+ permeable to cell membranes?
no
where is there a higher permeability to Na+ ions?
The brush border of the proximal tubule cells has a higher permeability to Na+ ions than most other membranes in the body, 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?
the reabsorption of the other components of the filtrate.
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 create?
followed by Cl- creates an osmotic force, drawing H2O out of the tubules
H2O removed by osmosis from the tubule fluid concentrates all the substances left in the tubule creating outgoing concentration gradients.
what does the rate of reabsorption of non-actively reabsorbed solutes depends on?
a) amount of H2O removed, which will determine the extent of the concentration gradient.
b) the permeability of the membrane to any particular solute.
is tubule membrane permeable to urea?
only moderately permeable to urea, so that only about 50% is reabsorbed, the remainder stays in the tubule.
what substances is the tubular membrane impermeable to?
inulin and mannitol, the tubular membrane is impermeable.
It is the active transport of Na+ that establishes the gradients down which other ions, H2O and solutes pass passively –> what effect would decreasing active transport have on renal function?
Anything which ↓ active transport eg ↓BF → disruption of renal function.
Substances such as glucose, amino acids etc, share the same carrier molecule as Na+ (symport).
High [Na+] in the tubule facilitates and low [Na+] inhibits glucose transport.
Na+ reabsorption also linked to HCO3- ion reabsorption (A/B).
what is tubular secretion?
Third renal process. Secretory mechanisms transport substances from the peritubular capillaries into the tubule lumen and ∴provide a second route into the tubule.
what is tubular secretion particularly important for?
Important for substances that are protein-bound, since filtration at glomerulus is very restricted.
Also for potentially harmful substances, means can be eliminated more rapidly.
give examples of substances secreted at the proximal tubule?
Carrier mechanisms are not very specific so that eg 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).
Similarly, organic base mechanism for choline, creatinine etc, can be used for morphine and atropine.
what is a normal level of K+?
K+ is the major cation in the cells of the body and the maintenance of K+ balance is essential for life.
Normal ECF[K+] ≈ 4mmoles/l.
when is the level of K+ indicative of hyperkalaemia?
If it ↑ to 5.5mmoles/l = hyperkalaemia → ↓resting membrane potential of excitable cells and eventually ventricular fibrillation and death. Remember the Nernst equation!
when is the level of K+ indicative of hypokalaemia?
If [K+] < 3.5 mmoles/l = hypokalaemia →↑ resting membrane potential ie hyperpolarizes muscle, cardiac cells → cardiac arrhythmias and eventually death.
how is K+ handles renally?
Changes in K+ excretion are due to changes in its secretion in the distal parts of the tubule. Any ↑in renal tubule cell [K+] due to increased ingestion will → K+ secretion, while any ↓ in intracellular [K+] → reduced secretion.
what effect does aldosterone have on K+ secretion?
An ↑in [K+] in ECF bathing the aldosterone secreting cells stimulates aldosterone release which circulates to the kidneys to stimulate ↑in renal tubule cell K+ secretion.
Aldosterone also stimulates Na+ reabsorption at the distal tubule but by a different reflex pathway.
H+ secretion?
H+ions are actively secreted from the tubule cells (not the peritubular capillaries) into the lumen A/B Balance.
describe K+ regulation by aldosterone?
