Physiology 4: Proximal Convoluted Tubule Flashcards
why do we actually need re-absorption
because the glomerular filtration rate is 125ml/min which would produce 180 litres of urine a day which for obvious reasons is not feasible
how many times does plasma get filtered through the glomerulus per day
65
how much fluid is re-absorbed
99%
how much salt is re-absrbed
99%
how much glucose is re-absorbed
100%
how much urea is re-absorbed
50%
how much creatinine is re-absorbed
0%
what is the glomerular filtrate
modified filtrate of blood which contains electrolytes, water, glucose, calcium, lips, vitamins, amino acids but lacks red blood cells and large proteins as they cannot get into the filtrate
moving a substance from the blood into the tubule is called
tubular secretion
moving a substance from the tubule into the blood is called
tubular re-absorption
how much filtered fluid is re-absorbed in the proximal convoluted tubule
80ml/min therefore, the flow into the descending loop of hence is 45 ml/min (125-80)
fluid re-absorbed in the proximal convoluted tubule is
iso-osmotic with the filtrate
what is re-absorbed in the proximal convoluted tubule
electrolytes,sugars, amino acids, phosphate, sulphate, lactate
what is secreted into the proximal tubule
H+, hippurtes, neurotransmitters, bile pigments, uric acid, drugs and toxins
tubular re-absorption in the proximal convoluted tubule: SODIUM AND GLUCOSE
- Na/K+ ATPase transport mechanism present on the base-lateral membrane which pumps 3 Na+ ions out of the cell and 2 K+ ions into the cell
- this requires ATP because it is pumping sodium against its concentration gradient out of the cell into an area where it is already in high concentration and potassium is also getting pumped against its concentration gradient into the cell where the concentration is already high
- the whole point of this is to reduce the sodium concentration within the cell to set up an osmotic gradient which allows sodium to flow via passive diffusion down its concentration gradient from the proximal convoluted tubule into the cell because on the apical surface of the cell there is a transporter which transport sodium BUT the glucose concentration inside the cell is higher than in the tubule so glucose does not wanto flow via passive diffusion so instead it uses the energy provided by sodium going down its concentration gradient to go against its concentration gradient and move into the ell this is called SECONDARY ACTIVE TRANSPORT
Also on the apical surface of the proximal convoluted tubule
there is a sodium amino acid co-transporter, so because the Na/K+ ATPase pump is moving sodium out of the cell and potassium into the cell, sodium moves into the cells from the proximal convoluted tubule via passive diffusion, which provides the energy to drive amino acids from the proximal convoluted tubule into the cell which is secondary active transport (again without the sodium the amino acids would not be re-absorbed because they are in higher concentrations in the cell than in the tubule)
this process also occurs with
lactate
what happens once the glucose, amino acids and lactate get into the cell
there are specific transporters on the basolateral membrane which transports them into the blood stream via diffusion
normally how much glucose amino acids and lactate get re-absored
100%
in the blood there is
- carbon dioxide with can move into the tubular cells where it combines with water to form carbonic acid via an enzyme called carbonic anhydrase
- the carbonic acid then dissociates into protons and bicarbonate
- there is a Na/H+ ante- porter on the apical membrane so as sodium moves into the cell a proton (H+) is pumped out via secondary active transport
- the hydrogen ion then combines with the bicarbonate ion in the tubule to form carbonic acid
- the carbonic acid in the tubule then dissociates via reaction with carbonic anhydrase to form protons (H+) and bicarbonate
- THE BICARBONATE ION FORMED WITHIN THE CELL IS THEN MOVED INTO THE BLOOD SO IT IS ACTUALLY INDIRECTLY ABSORBED NOT DIRECTLY RE-ABSORBED
as the sodium is moving into the tubular cells
water follows sodium by the process of OSMOSIS from the tubules into the blood which is called obligatory water re-absorption
how much sodium and water is re-absorbed in the proximal convoluted tubule
65%
in the proximal convoluted tubule
potassium, magnesium and calcium in the filtrate move between the cells in a process called para-cellular transport
in the proximal convoluted tubule how much potassium and chloride is re-absorbed
50% and very little calcium and magnesium is re-absorbed
what is the other mechanism of re-absorbeding chloride
there is a sodium/ chloride symporter on the apical surface which re-asbrobed sodium and chloride
lipids are re-absorbed
straight through the rubulear cells because there can pass straight through the phospholipid membrane
what is a type of lipid soluble solute
urea
there is a channel on the proximal convoluted tubule
- on the apical surface which bring in sodium and phosphate from the tubule into the cell
- there is a PTH receptor on the basolateral membrane of the proximal convoluted tubule and when PTH binds to it is produces protein Kinase A which adds phosphate onto the transporter and inhibits phosphate re-absoprtion causing phosphate excretion
The more glucose in the plasma then
- the more glucose being filtered therefore, more glucose is being re-absorbed in the proximal convoluted tubule
- however, at a certain point at around 10-12mmol/litre of glucose glucose reaches its transport maximum and no more can be re-asbsorbed therefore, some glucose ends up in the urine causing a glycosuria
for para-amino hipputric acid
none of it should be re-absorbed therefore, the amount of PAH being excreted should be equal to amount being filtered and the amount being secreted
- so the transport maximum for PAH is where the amount of PAH being secreted plateaus as no more can be secreted
the clearance of
re-absorbed or secreted substances is ont constant once tre trasporto maximum has been reached
