Physiology 4: Proximal Convoluted Tubule

Question 1

why do we actually need re-absorption

Answer 1

because the glomerular filtration rate is 125ml/min which would produce 180 litres of urine a day which for obvious reasons is not feasible

Question 2

how many times does plasma get filtered through the glomerulus per day

Answer 2

65

Question 3

how much fluid is re-absorbed

Answer 3

99%

Question 4

how much salt is re-absrbed

Answer 4

99%

Question 5

how much glucose is re-absorbed

Answer 5

100%

Question 6

how much urea is re-absorbed

Answer 6

50%

Question 7

how much creatinine is re-absorbed

Answer 7

0%

Question 8

what is the glomerular filtrate

Answer 8

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

Question 9

moving a substance from the blood into the tubule is called

Answer 9

tubular secretion

Question 10

moving a substance from the tubule into the blood is called

Answer 10

tubular re-absorption

Question 11

how much filtered fluid is re-absorbed in the proximal convoluted tubule

Answer 11

80ml/min therefore, the flow into the descending loop of hence is 45 ml/min (125-80)

Question 12

fluid re-absorbed in the proximal convoluted tubule is

Answer 12

iso-osmotic with the filtrate

Question 13

what is re-absorbed in the proximal convoluted tubule

Answer 13

electrolytes,sugars, amino acids, phosphate, sulphate, lactate

Question 14

what is secreted into the proximal tubule

Answer 14

H+, hippurtes, neurotransmitters, bile pigments, uric acid, drugs and toxins

Question 15

tubular re-absorption in the proximal convoluted tubule: SODIUM AND GLUCOSE

Answer 15

• 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

Question 16

Also on the apical surface of the proximal convoluted tubule

Answer 16

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)

Question 17

this process also occurs with

Answer 17

lactate

Question 18

what happens once the glucose, amino acids and lactate get into the cell

Answer 18

there are specific transporters on the basolateral membrane which transports them into the blood stream via diffusion

Question 19

normally how much glucose amino acids and lactate get re-absored

Answer 19

100%

Question 20

in the blood there is

Answer 20

• 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

Question 21

as the sodium is moving into the tubular cells

Answer 21

water follows sodium by the process of OSMOSIS from the tubules into the blood which is called obligatory water re-absorption

Question 22

how much sodium and water is re-absorbed in the proximal convoluted tubule

Answer 22

65%

Question 23

in the proximal convoluted tubule

Answer 23

potassium, magnesium and calcium in the filtrate move between the cells in a process called para-cellular transport

Question 24

in the proximal convoluted tubule how much potassium and chloride is re-absorbed

Answer 24

50% and very little calcium and magnesium is re-absorbed

Question 25

what is the other mechanism of re-absorbeding chloride

Answer 25

there is a sodium/ chloride symporter on the apical surface which re-asbrobed sodium and chloride

Question 26

lipids are re-absorbed

Answer 26

straight through the rubulear cells because there can pass straight through the phospholipid membrane

Question 27

what is a type of lipid soluble solute

Answer 27

urea

Question 28

there is a channel on the proximal convoluted tubule

Answer 28

• 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

Question 29

The more glucose in the plasma then

Answer 29

• 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

Question 30

for para-amino hipputric acid

Answer 30

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

Question 31

the clearance of

Answer 31

re-absorbed or secreted substances is ont constant once tre trasporto maximum has been reached