Kidney Function 2

Question 1

What is the equation for renal clearance?

Answer 1

concentration of urine * volume of urine / concentration in plasma

Question 2

Why does the clearance of insulin measure GFR?

Answer 2

It is:

• freely filtered
• not reabsorbed
• not secreted
• not metabolised
• easily measured

Question 3

What is used in the clinic to test for glomerular filtration rate?
Is this an over or under estimate and why?

Answer 3

Creatinine

Over because some creatinine is slightly secreted.

Question 4

What is the value for insulin clearance?

What does it mean if the value for a molecule below or above this?

Answer 4

120ml/min

<120ml means clearance
= 120ml means no reabsorption or secretion
>120m means secretion

Question 5

What is the value for the renal plasma flow approximation?

How much of this do blood cells occupy in %? How about plasma?

Answer 5

600ml/min

Blood cells - 45%
Plasma - 55%

Question 6

What is the formula for blood flow?

Answer 6

(plasma flow)/ (1-haematocrit)

If haematocrit is a % then you do 100 - haematocrit instead.

Question 7

What is osmolarity and explain this?

Answer 7

It is a measure of water concentration. The higher the solution osmolarity, the lower the water concentration.

Question 8

Which one of these has a wider range? Plasma osmolarity or urine?

Answer 8

Urine

Question 9

If urine has an osmolarity over 300mosm/l, what does this mean?

Answer 9

It is concentrated

Question 10

Why do we use osmolality instead of osmolarity?

Answer 10

It is not affected by temperature.

Question 11

What is the most osmotically active solute in plasma?
What is plasma sodium concentration?
Where is sodium freely filtered?

Answer 11

Sodium

135-145mmol/l

At the renal corpuscle

Question 12

What is sodium balance linked to in the body?

Answer 12

Blood pressure

Question 13

Where does reabsorption mainly occur?

Answer 13

In the PCT

Question 14

What are 4 places in the nephron for sodium reabsorption have lots of mitochondria?

Answer 14

1) Proximal tubule
2) Thick ascending limb
3) Distal tubule
4) Collecting duct

Question 15

Why does the descending limb of the loop of Henle have few mitochondria?

Answer 15

No sodium reabsorption.

Sodium moves between cells in thick ascending limb passively. Energy is not required for this process.

Question 16

Explain the 4 steps of sodium reabsorption

Answer 16

1) Proximal tubule - sodium moved into the cell for exchange of H+. Positive charged inside ell and negative charges repel chloride on outside. (65% of NaCl absorbed here)
2) Thick ascending limb - potassium moves against its concentration gradient into the cell. High concentration inside the cell so it can leave and return to filtrate here. Positive charge inside loop supports movement of sodium between cells. (25% of NaCl absorbed here
3) Distal tubule - Has a NaCl co-transporter. (2-5% of NaCl reabsorbed here)
4) Collecting duct - sodium channel on luminal membrane, cells in collecting duct include intercalated cells. (5% of NaCl reabsorbed)

Question 17

What is the role of intercalated cells in the kidney?

Answer 17

Intercalated cells are epithelial cells traditionally associated with the regulation of acid-base homeostasis in distal segments of the kidney tubule

Question 18

What 3 things does water reabsorption depend on?

Answer 18

• Osmosis
• Sodium reabsorption
• Tubule permeability

Question 19

Give details on the coupling of water reabsorption to sodium reabsorption?

Answer 19

Isotonic reabsorption - tight junctions have a high water permeability.
Also membrane expression of aquaporin water channels.
Filtrate volume reduced but not osmolarity.

Sodium moves into epithelial cells and this decreases osmolarity so water moves out he tubule lumen and then is reabsorbed into the capillary.

Question 20

What two things produce concentrated urine?

Explain them

Answer 20

1. Separating Na+ and water reabsorption
2. Generate a renal medulla interstitial fluid with high osmolarity to drive water reabsorption

1=Two limbs are separated by the interstitial fluid. Flow is said to be countercurrent. Ascending limb is where sodium is reabsorbed and water will not follow as ascending is impermeable to water. Water leaves the descending loop as this is permeable to water.

2 = Setting up a medullary interstitial gradient. Osmolarity of 300 inside the loop. The ascending loop is activated and water will leave the descending loop.

Question 21

Where is the highest osmolarity in the loop?

Answer 21

Around the turn

Question 22

What are vasa recta and explain some of their properties?

Answer 22

Blood vessels running alongside the loop of Henle.

They have the same shape running along so blood can fully supply. They supply blood without washing the gradient away.

Question 23

Does plasma leaving the vasa recta have a higher or lower osmolarity?

Answer 23

Higher

Question 24

What are the 3 places that urea goes to when being recycled by the kidney?

Answer 24

1. Proximal tubule (passive reabsorption of urea by moving across plasma membrane).
2. Loop of Henle (apical secretion via urea transporters - secreting urea into filtrate)
3. Inner medullary collecting duct (apical reabsorption via UT-A1)

40% of that filtered is excreted

Question 25

What does secretion and reabsorption of urea allow for?

What does the amount of urea excreted depend on?

Answer 25

Higher osmolarity

Hydration status

Question 26

What are the two urea transporters in the inner medullary collecting duct called?
What are they enhanced by?

Answer 26

UTA1 and UTA3

ADH

Question 27

What are diuretics and what do they target?

Answer 27

Diuretics are medications deigned to increase the volume of salt and water expelled from the body as urine.
They target sodium transporters/channels.

Question 28

Where does urea secretion occur?

Answer 28

Thin descending limb of loop of Henle

Question 29

Fluid leaving the ascending loop is ——- (lower tonicity than —–). Why is this?

Answer 29

Hypotonic, plasma

No reabsorption occurs here.

Question 30

Which tubule is the diluting tubule?

Answer 30

The distal tubule in the thick descending tubule.

Question 31

Explain the mechanism of ADH

Answer 31

ADH bind to V2 receptor which works via cyclic AMP causing aquaporin 2 water channel to be inserted to the tubule membrane epithelium.
Aquaporin 3 and 4 is where water leaves the collecting duct.

Question 32

What does concentrated urine depend on?

Answer 32

• Renal medulla interstitial fluid high osmolarity

- Collecting duct (pass through concentrated zone, water permeable)

Question 33

Give some details on potassium in the body and what 3 elements are involved in maintenance of potassium balance

Answer 33

K+ is a major intracellular cation in the body.
The gradient of it is maintained to maintain resting potential.
Higher concentration in intracellular fluid than extracellular.
1) Renal excretion
2) Gastrointestinal loses
3) Cellular shifts

Question 34

Explain renal excretion of potassium

Answer 34

• Involves filtration, reabsorption and secretion
• 95% of the potassium is reabsorbed (65% of this at the PT)
• No potassium transporter on the luminal membrane so potassium is reabsorbed passively.

Question 35

Explain potassium pathways in thick ascending loop

Answer 35

Potassium reabsorption is outweighed by secretion. K+i inside the cell leaves the cell and enters the filtrate.
30% of potassium is reabsorbed here.

Question 36

Explain potassium pathways in collecting duct

Answer 36

K+ is reabsorbed by intercalated discs and distal cells in exchange for H+.

This is outweighed by K+ secretion by principal cells.

2 potassium exits:

• K+ channels
• K+ Cl- cotransporter

Question 37

What are 4 factors affecting K+ secretion by principal cells?

Answer 37

1. Factors affecting Na+ entry through epithelial Na+ channels.
2. Aldosterone stimulates K+ channels
3. Tubular flow rate. High flow rate favours secretion
4. Acid-base balance. Acidosis inhibits and alkalosis enhances.