Basic tubular function

Question 1

Osmolarity

Answer 1

A measure of the osmotic pressure exerted by a solution across a perfect semi-permeable membrane
All the concentrations of the different solutes (measured in mmol/l) added together. Each ion is “counted” separately

Question 2

Osmolarity dependent on?

Answer 2

Dependent on the number of particles in a solution and NOT the nature of the particles

Question 3

Any solute present at equal concentrations either side of a semi-permeable membrane…?

Answer 3

No net effect on water movement

Question 4

Types of transport in tubules

Effect of rate as solute concentration increases for each of these?

Answer 4

Passive movement- protein independent transport (lipophilic)
Incease conc.= increase rate (directly proportional)

Passive movement- protein dependent transport (hydrophilic molecules)
Increase conc= increase rate but then plateaus

Active movement- directly coupled to ATP hydrolysis
Increase conc= increase rate but then plateaus

Active movement- indirectly coupled to ATP hydrolysis
Increase conc= increase rate but then plateaus

Question 5

2 types of H2O transport

Answer 5

Paracellular

Transcellular

Question 6

Regulation of passive uptake system?

Answer 6

Storage of protein channels inside cell- moved to surface when needed

Question 7

Protein reabsorption that gets into primary urine

Answer 7

1. Receptors have low specificity+ high capacity for binding to protein
2. Receptor binds to protein+ gets endocytosed
3. Receptor dissociates from protein by dropping pH of vesicle
4. Releases receptor + protein from each other= receptor can be used again
5. Pump protons in endosome= decreased pH= increased +ve charge
6. Harder to pump H+ in because of positive charge so Cl- transporter required to decreased charge so more H+ can be pumped in (2Cl- exhcnaged for one H+)

Question 8

Example of transport maxima?

Answer 8

Glucose

After a certain solute conc, limit to how much solute can be in the reabsorbed solution= increase glucose in urine

Question 9

What causes glucose in urine?

Answer 9

Diabetes mellitus

Ingestion of large quantities of vitamins B and C

Question 10

Where does most of reabsorption occur?

Answer 10

Proximal convoluted tubule

Largest brush border

Question 11

Where is reabsorption regulated the most?

Answer 11

Collecting duct

Question 12

Least active part of the nephron?

Answer 12

Descending limb of loop of Henle

Water passively reabsorbed because osmolarity of IF fluid= high; Draws in Sodium and Potassium

Question 13

Bicarbonate absorption process?

Answer 13

Reabsorption of bicarbonate= indirectly coupled to Na transport

1. Pumping proton out in exchange for Na in neutralizes bicarbonate
2. Bicarbonate in the presence of carbonic anhydrase outside cell turns to H20 and CO2 which cross membrane
3. Inside the cell carbonic anhydrase activity converts the H20 and CO2 back into H2CO3
4. H2CO3 dissociates into H+ and HCO3-
5. H+ is used on Na+/H+ exchange to start the process again
6. HCO3- moves out of the basolateral membrane by simple diffusion.

Question 14

Why is net secretion in PCT important?

Answer 14

Route of excretion for some substances

Some drugs enter tubular fluid here + act further down nephron.

Question 15

Ascending limb?

Answer 15

Chloride actively reabsorbed
Sodium passively reabsorbed with it (because pumping on basolateral side)
Bicarbonate reabsorbed
Impermeable to water (very tight junctions+ no aquaporins

Question 16

Thiazide mechanism?

Answer 16

Blockage with thiazide= increased plasma Ca2+

1. Under normal conditions when pumping out sodium= low Na conc. In cell
2. Na passively enters into cell either through Na/Cl- co transport on apical membrane or 3Na+/ Ca2+ exchange (counter transport) on basolateral membrane
3. Thiazide blocks Na/Cl- co transport protein= only way to bring Na in is through countertransport
4. Decreased Ca in cell
5. More Ca comes in through passive system on apical membrane

Question 17

Types of cells in collecting duct + distal part of distal convoluted tubule
Dependent on?

Answer 17

Principal Cell: important in sodium, potassium and water balance (mediated via Na/K ATP pump)
Dependent on vasopressin

Intercalated Cell: important in acid-base balance (mediate via H-ATP pump)- imp in regulating acid-base balance

Question 18

Single gene defects that affect tubular function
Caused by?
Leads to?

Answer 18

Renal tubule acidosis
Caused by acidosis is in blood (metabolic acidosis) because of failure of getting protons out into lumen (not excreting protons) OR carbonic anhydrase problems
Less acidic urine + Leads to impaired growth+ hypokalaemia

Antenatal Bartter syndrome
Caused by excessive electrolyte secretion, mutation in Na/2Cl-/K+ uptake process
Premature birth, polyhydramnios (increased aminiotic fluid)
Severe salt loss
Moderate metabolic alkalosis
Hypokalemia
Renin and aldosterone hypersecretion

Fanconi syndrome (Dent’s disease)
Cause= can’t separate protein from receptor because can’t acidify endosome (mutation in 2Cl-/ H+ exchanger)= can’t recirculate carrier/ receptor
Increased excretion of uric acid, glucose, phosphate, bicarbonate
Increased excretion of low MW protein

Question 19

Which ion transporter is found on the apical side of the ascending limb of the loop of Henle?

Answer 19

Na+/ 2Cl-/ K+ triple transporter