Tubular Function Flashcards
Is transcellular or paracellular more tightly regulated?
transcellular
What are the 2 types of reasborption and secretion?
transcellular and paracellular
Passive transport
Can be controlled by membrane proteins
- lipophilic diffuse straight through
- hydrophilic may need carriers
- with carriers the rate plateaus at high substrate concentrations
Active transport
Active transport can be:
- directly coupled to ATP hydrolysis (primary)
- indirectly coupled to ATP hydrolysis (secondary)
Give an example of primary active transport
vitamin and calcium transport
Na+K+ pump
Give an example of secondary active transport
Na+/glucose (cotransport)
Can water travel transcellularly and paracellularly, and via what?
Yes
paracellular - via tight junctions
transcellular - aquaporins
How are proteins absorbed
receptor mediated endocytosis
What is transport maxima?
Systems are regulated and so may have a basal and stimulated maxima rate of absorption. However they may also have only one state of reabsorption.
Transport maxima of glucose
- more glucose in the blood means a proportional increase in rate of reabsorption
- glucose is reabsorbed up to a point when channels are saturated so plateaus
- up to 15mmol/L of plasma glucose the excretion equals 0 as rate of filtration and reabsorption are the same
- past that the glucose is excreted in urine e.g. in diabetes
How does secretion occur?
Give example of secreted substances
From peritubular capillaries into the tubular lumen
- occurs via diffusion or transcellular transport
- most important secreted substances are H+ and K+
- choline, creatinine (very little) and pencillin are secreted
What is reabsorbed in the PCT?
60-70% all solute
- 100% glucose
- 65% sodium
- 90% bicarbonate
- 60-70% of water
What is reabsorbed in the LoH?
25% sodium
What is reabsorbed in the DCT?
8% sodium
Histology of the PCT, LoH, DCT and CD?
PCT:
- ciliated
- lots of mitochondria
LoH descending:
- no cilia, not much mitochondria
LoH ascending:
- no cilia
- lots of mitochondria
DCT:
- cilia
- some mitochondria
CD:
- no cilia
- not much mitochondria
Function of the PCT?
70% of all filtrate reabsorbed
- driven by Na+/K+ ATPase
- Pump pushes out sodium out basolateral side into capillaries so intracellular sodium is low. Then sodium is drawn in at apical membrane and co-transported with other molecules
- Sodium reabsorption them indirectly leads to bicarbonate absorption
- Sodium acts to excrete protons that buffer bicarbonate
- carbonic anhydrase forms h20 and co2 - co2 taken up into cells and used to remake bicarbonate and secreted into capillaries
Function of LoH?
descending - water passively reabsorbed
ascending - chloride actively reabsorbed, sodium passively with it and potassium too. Bicarbonate reabsorbed and its impermeable to water.
What do loop diuretics do?
Block Na+/K+/Cl- transporter
DCT function? (proximal part)
- Na+ and Cl- co transporter linked to calcium reasborption
DCT function (proximal distal part) and CD?
- fine tuning of the filtrate for homeostasis
- DCT depends on aldosterone
- CD depends on aldosterone too and ADH
- distal part of CD is impermeable to water without ADH
- both use aldosterone sodium channel, linked soidum channel and pH controlled proton ATP channel
Renal tubular acidosis
in early proximal tubule the protons leak back into the cell after being pumped into the lumen so the urine becomes alkaloid
Bartter syndrome - cause and what can it lead to?
Antenatal
- in ascending limb there is too much electrolyte secretion due to mutation in Na+/K+/Cl- transporter on apical membrane
- can cause premature birth, polyhydroamnios, hypokalaemia, metabolic alkalosis, renin and aldosterone hyper secretion
Fanconi syndrome - cause
- increased excretion of uric acid, phosphate, glucose, bicarbonate and low molecular weight protein
- disease in proximal tubule
- AKA renal tubular acidosis: type 1 dent’s disease
- in receptor mediated endocytosis a proton pump is needed to dissociate protein and receptor but there is a mutation
- too many protons in endosome so no receptor dissociation
Principal cell in DCT
important in sodium, potassium and water balance (mediated via na+/k+ pump)
Intercalated cell in DCT
important in acid base balance (via H ATP pump)
How much water is reabsorbed in the PCT, LOH, DCT and CD?
PCT = 60-70% LoH = 10% DCT = 10% CD = variable from less than 1 - 10% excreted
What does the Na+K+Cl- transporter do?
Apical membrane of thick ascending limb
transports them in then sodium out into interstitial space
