Proximal Tubule Physiology - Rao

Question 1

What is the formula for flow rate into the loop of Henle?

Answer 1

GFR - Reabsorption + Secretion = Rate of Flow to LH

Vloop=(GFR*[P])/TF

Question 2

Is PT reabsorption hypo-, iso-, or hyperosmotic?

Answer 2

Isotonic

Question 3

What is the largest energy requiring process in the proximal tubule?

Answer 3

Active sodium reabsorption using the Na/K ATPase

Actually the major consumer of oxygen by the kidney

Question 4

Explain sodium reabsorption in the proximal tubule

Answer 4

3Na+/2K= ATPase basolateral transporter creates electrical and chemical gradient with low IC sodium concentration and (-) charge. Na can then pass through CHANNEL on luminal side into cell where it is transported via the ATPase into the interstitium.

Question 5

What happens to Cl- in the proximal tubule?

Answer 5

Na migration into cell drives Cl- reabsorption paracellularly through pore, maintaining electroneutrality

Water reabsorption from following sodium also drives Cl reabsorption because of concentration gradient

Question 6

What is the mechanism of water reabsorption in the proximal tubule?

Answer 6

Massive migration of solutes, esp. Na, decreases osmolarity in the tubule fluid, driving water to follow sodium into the interstitium.

It does so via aquaporins: AQP1 on luminal
AQP4/5 on basolateral

Maintains isosmotic reabsorption of filtrate because while ions are being reabsorbed, so is water

Remember that all this water movement is passive and is a byproduct of active sodium movement

Question 7

What are peritubular factors (3)?

Answer 7

Positive interstitial fluid pressure
Low hydrostatic peritubular capillary pressure
High oncotic peritubular capillary pressure

These all favor fluid flow into capillary

Question 8

Why is PAH tubule:plasma concentration ratio higher than inulin?

Answer 8

Because PAH is not only highly filtered in, but also actively secreted

Question 9

Explain reabsorption HCO3-

Answer 9

H+ and HCO3- are made in PT cells from CO2 and H2O: CO2 + H2O (CA) –> H2CO3 –> H+ + HCO3-.

H+ is then secreted into lumen via Na+/H+ exchanger.

Now the process happens again since we have a source for H+. H+ + filtered HCO3- –> H2CO3 (luminal surface CA)–> CO2 + H2O –> those two diffuse back into cell

Net reabsorption of HCO3- but NOT net secretion of H+

Question 10

What is the point called where glucose begins to appear in the urine?

Answer 10

Threshold

Question 11

What is the point called where glucose reabsorption stops increasing?

Answer 11

Tmax

Question 12

Between threshold and Tm, what is happening to excretion and reabsorption?

Answer 12

Reabsorption is slowly rising to a plateau while excretion is rising quickly

Question 13

What are some causes of glucosuria?

Answer 13

Pregnancy
DM
Mutations in SGLT1 or 2

Question 14

What is SGLT1?

Answer 14

Sodium-glucose co-transporter in apical membrane

Question 15

What happens to phosphate concentrations in the tubule in parathyroidectomized patients?

Answer 15

Lack of PTH causes very decreased Tm and increased phosphate reabsorption

Question 16

Tell me about phosphate reabsorption

Answer 16

Coupled to Na+ gradients
Low threshold so always partially excreted

Threshold is very close to plasma concentration so slight changes in plasma concentration will alter urinary excretion

Question 17

What’s the deal with Cl- and HCO3- always being compared

Answer 17

Since so much HCO3- is reabsorbed through active transport (coupled to Na at least), only about 60% of Cl- is reabsorbed

Question 18

Is urea absorbed in the PT

Answer 18

Yeah some, passively and slowly

Question 19

Describe how mannitol flows through and interacts with the tubule

Answer 19

Basically it isn’t reabsorbed or metabolized so it will go through the nephron completely, thereby increasing tubular fluid osmolarity and diuresis. Water just can’t bear to leave it’s MANnitol.

Question 20

What are some clinical situations where you would use mannitol?

Answer 20

To reduce intracranial or intraocular pressure, promote excretion of toxins, edema, etc. Any time you have water overload.

Question 21

What are some things that are secreted in the PT?

Answer 21

Organic acids and bases
Creatinine
PAH
Drugs

Mostly active transport

Question 22

The TDLH is only permeable to one thing - what is it?

Answer 22

WATER

Question 23

The TALH is permeable to one thing - what is it?

Answer 23

SODIUM