Kidney Function 2

Question 1

What is renal clearance?

Answer 1

Volume of plasma cleared of a substance in a given time

Question 2

How do you calculate renal clearance?

Answer 2

U.V / P

U = concentration in urine

V = volume of urine /min

P = concentration in plasma

Question 3

What is inulin?

Answer 3

Plant polysaccharide

Question 4

Why does inulin clearance reflect GFR?

Answer 4

Freely filtered

Not reabsorbed

Not secreted

Not metabolised

Question 5

What substance is used to estimate GFR clinically?

Answer 5

Creatinine

Question 6

Does creatinine clearance give an over or under estimate of GFR and why?

Answer 6

Overestimate

Slightly secreted as it is an organic base/cation

Question 7

Why is creatinine used instead of inulin to measure GFR clinically?

Answer 7

Present in blood naturally (inulin = i.v.)

Question 8

What is the clearance value of inulin?

Answer 8

120-125ml/min

Question 9

If a substance has a clearance value greater than 120ml/min, what does this suggest?

Answer 9

More excreted than initially filtered (secretion occurs)

Question 10

What substance would have a greater clearance value than inulin?

Answer 10

Para-aminohippuric acid

Question 11

If a substance has a clearance value smaller than 120ml/min, what does this suggest?

Answer 11

Less excreted than initially filtered (reabsorption occurs)

Question 12

Give an example of a substance that shows net reabsorption but is also secreted?

Answer 12

Potassium

Question 13

Why can PAH be used to estimate renal plasma flow?

Answer 13

All plasma that enters kidney per unit time is cleared of PAH

So rate of excretion of PAH must be provided by a volume of plasma which contained that amount of PAH

Question 14

What is the value of effective renal plasma flow?

Answer 14

600ml/min

Question 15

Why is effective renal plasma flow not the same as renal blood flow?

Answer 15

(Underestimate as) some blood is directed to the perirenal fat areas instead of being filtered

Question 16

How do you calculate renal blood flow?

Answer 16

Blood flow = plasma flow / (1 - haematocrit)

Question 17

What is the value of renal blood flow?

Answer 17

(600/0.55 =) 1100ml/min

Question 18

What is osmolality and what are its units?

Answer 18

Measure of water concentration

mOsm/kg

Question 19

What is physiological range of plasma osmolality?

Answer 19

285-295mOsm/kg

Question 20

What is the range of urine osmolality?

Answer 20

50-1400mOsm/kg

Question 21

What is the range of osmolality of concentrated urine?

Answer 21

295-1400mOsm/kg

Question 22

What is the main osmotically active solute in plasma (ECF)?

Answer 22

Na+

Question 23

What is the plasma sodium concentration?

Answer 23

135-145mmol/L

Question 24

How much sodium is filtered at the renal corpuscle (/min)?

Answer 24

(140 x 0.125 =) 17.5mmol/min

Question 25

Where does the majority of sodium reabsorption occur?

Answer 25

PCT

Question 26

Where does the fine hormonal control of sodium reabsorption occur?

Answer 26

DCT

Question 27

In what part of the loop of Henle does passive sodium reabsorption occur?

Answer 27

Thin ascending limb

Question 28

In what part of the loop of Henle does active sodium reabsorption occur?

Answer 28

Thick ascending limb

Question 29

How is sodium reabsorbed in the thin ascending limb?

Answer 29

Paracellularly

Question 30

How much sodium is reabsorbed in the PCT?

Answer 30

~65%

Question 31

Describe the processes involved in sodium reabsorption in the PCT.

Answer 31

Na/K-ATPase pump (BL) maintains low Na concentration in cells - Na/H exchanger NHE3 - Na/nutrient symporter (less Na) BL Na/HCO3 symporter

Question 32

Describe how chloride reabsorption occurs in the PCT.

Answer 32

Movement of positively charged ions into the cell gives a negative charge in the lumen Repels other negative charges (Cl-) to move paracellularly

Question 33

How much sodium is reabsorbed in the thick ascending limb?

Answer 33

25%

Question 34

Describe the processes involved in sodium reabsorption in the thick ascending limb.

Answer 34

Na/K-ATPase pump (BL) maintains low Na concentration in cells Na/K/Cl-cotransporter NKCC2 - Na and Cl move down concentration gradients - K moves against its concentration gradient and leaves via K channels into filtrate BL K/Cl symporter and Cl channel Positively charged filtrate supports paracellular movement of Na+ and other positive ions

Question 35

How much sodium is reabsorbed in the DCT?

Answer 35

2-5%

Question 36

Describe the processes involved in sodium reabsorption in the DCT (basal rates).

Answer 36

Na/K-ATPase pump (BL) maintains low Na concentration in cells Luminal Na/Cl symporter

Question 37

*What are the two main cell types of the collecting duct and what are they mainly involved in the absorption of?

Answer 37

Principal cells - Na Intercalated cells - K, HCO3

Question 38

How much sodium is reabsorbed in the collecting duct?

Answer 38

5%

Question 39

Describe the processes involved in sodium reabsorption in the collecting duct.

Answer 39

Na/K-ATPase pump (BL) maintains low Na concentration in cells Luminal Na channels (ENaC)

Question 40

What three factors does water reabsorption depend on?

Answer 40

Osmosis Na reabsorption Tubule permeability

Question 41

How is water reabsorbed in the PCT?

Answer 41

Coupled to Na reabsorption (filtrate volume decreases but osmolality remains the same) Isotonic reabsorption as tight junctions have high water permeability Membrane expression of AQP1

Question 42

How is the absorption of water and sodium separated when producing concentrated urine?

Answer 42

Henle's loop reabsorbs more salt (25%) than water (10%) Descending limb has no Na reabsorption but expresses AQP1 Thick ascending limb reabsorbs Na (via NKCC2) and is impermeable to water

Question 43

What is the hypothesis of setting up the medullary interstitial gradient?

Answer 43

Initially all osmolality is the same as fed by PCT (equal to plasma) Thick ascending limb pumps salt into interstitium Water reabsorbed at descending limb to equilibrate tubule fluid with interstitium

Question 44

Describe the changes in osmolarity of the tubular fluid in the loop of Henle.

Answer 44

Initially equal to plasma (300mOsm/L) Increases down descending limb as water is reabsorbed Maximum at turn in loop of Henle (1200mOsm/L) Decreases up ascending limb as Na reabsorbed but water remains in lumen (100mOsm/L)

Question 45

How much urea is reabsorbed at the PCT and how?

Answer 45

50% Passive diffusion

Question 46

How much urea is secreted at the loop of Henle and how?

Answer 46

60% Apical secretion via UT-A2

Question 47

How much urea is reabsorbed at the inner medullary collecting duct and how?

Answer 47

70% UT-A1 (luminal) and UT-A3 (BL)

Question 48

How much of the filtered urea is actually excreted?

Answer 48

40%

Question 49

Why do we recycle the urea in the kidney?

Answer 49

Allows medullary interstitium to have an even higher osmolality to drive water reabsorption

Question 50

What does tonicity refer to?

Answer 50

The concentration of non-penetrating substances only

Question 51

When is the filtrate hypertonic to plasma?

Answer 51

Turn of the loop of Henle

Question 52

When is the filtrate hypotonic to plasma?

Answer 52

DCT

Question 53

When is the filtrate isotonic to plasma?

Answer 53

PCT

Question 54

When is the collecting duct permeable to water?

Answer 54

In presence of ADH/vasopressin

Question 55

Describe how ADH affects water reabsorption in the collecting duct.

Answer 55

Binds to V2 receptors cAMP-mediated events/second messenger model (Gs) Causes AQP2 to be inserted luminally so water can enter cells

Question 56

What aquaporins are located on the BL membrane of the collecting duct cells?

Answer 56

AQP3 and AQP4

Question 57

By which transporters can ADH enhance urea reabsorption in the inner medullary collecting duct?

Answer 57

Via UT-A1 and UT-A3

Question 58

What is the major intracellular cation of the body?

Answer 58

Potassium

Question 59

What is the intracellular and plasma concentration of potassium?

Answer 59

140mM (intracellular) 4mM (plasma)

Question 60

How much potassium do we ingest each day?

Answer 60

40-120mmol

Question 61

Why is the potassium gradient important?

Answer 61

Main determinant of resting membrane potential (of excitable cells)

Question 62

What three things are involved in the maintenance of potassium balance in the body?

Answer 62

Renal excretion Gastrointestinal losses (secreted in colon) Cellular shifts/redistribution

Question 63

Where is the vast majority of potassium found in the body?

Answer 63

Intracellular fluid, especially muscles

Question 64

What is the most important factor in maintaining the potassium balance in the body?

Answer 64

Renal excretion (~95mmol/day)

Question 65

How much potassium is filtered at the renal corpuscle per day?

Answer 65

~700-800mmol

Question 66

How much potassium is reabsorbed at the PCT and how?

Answer 66

65% Paracellularly

Question 67

How much potassium is reabsorbed at the thick ascending limb and how?

Answer 67

30% NKCC2

Question 68

How much potassium is reabsorbed at the DCT and how?

Answer 68

5% K/H-exchanger

Question 69

What cells reabsorb potassium in the collecting duct?

Answer 69

Intercalated cells (and distal cells)

Question 70

What cells secrete potassium in the collecting duct?

Answer 70

Principal cells

Question 71

How do principal cells secrete potassium? (3)

Answer 71

Renal outer medullary K channel ROMK Ca-activated big conductance K channel BK K/Cl symporter

Question 72

What affects the potassium secretion of principal cells? (4)

Answer 72

Factors affecting Na entry via ENaC - alters electrochemical gradient (eg. amiloride) Aldosterone Tubular flow rate Acid-base balance

Question 73

How does aldosterone affect the potassium secretion of principal cells?

Answer 73

Increase by upregulating luminal potassium channels

Question 74

How does tubular flow rate affect the potassium secretion of principal cells?

Answer 74

High flow rates favour secretion (constantly removing positive charges)

Question 75

How does acid-base balance affect the potassium secretion of principal cells?

Answer 75

Acidosis inhibits (positive protons repel K+) Alkalosis enhances