S4) Basic Renal Processes

Question 1

Which ion is transport together with Na⁺?

Answer 1

Cl^-

Question 2

How much sodium is reabsorbed in the kidney?

Answer 2

Up to ~99% Na⁺ ions, Cl^- ions and H₂O are reabsorbed in kidney

Question 3

Sodium reabsorption varies with the ingestion of sodium.

Why?

Answer 3

The kidney needs to match excretion of sodium to ingestion to remain sodium balance

Question 4

What happens if the amount of Na⁺ ions in ECF changed due to diet changes?

Answer 4

⇒ Amount of water in the ECF would change

⇒ ECV would change

⇒ BP would change

Question 5

Sodium input only occurs through ingestion of food.

Identify 3 means of sodium output

Answer 5

• Sweat
• Faeces
• Urine

Question 6

What is the main site of absorption in the nephron?

Answer 6

Proximal convoluted tubule – absorbs Na⁺ (67%) and H₂O (65%)

Question 7

What is absorbed by the following:

• Descending LoH
• Ascending LoH

Answer 7

• Descending thin limb of LoH: H₂O (10-15%)

- Ascending thin and thick limb of LoH: Na⁺ (25%)

Question 8

What stimulates proximal tubule reabsorption?

Answer 8

RAAS

Question 9

Which cells are the target for aldosterone?

Answer 9

Principal cells of distal convoluted tubule and collecting duct

Question 10

What happens when renal artery blood pressure increases?

Answer 10

In proximal tubule:

• Number of Na-H antiporter reduces
• Activity of Na-K ATPase reduces

Question 11

What effect does an increased renal artery BP have on reabsorption?

Answer 11

• Pressure natriuresis:

I. Reduction in sodium reabsorption in PCT

II. Increased Na⁺ excretion

• Pressure diuresis:

I. Reduction in water resorption in PCT

II. Increased H₂O excretion

Question 12

What is the significance of the pressure diuresis and natriuresis that follow increased BP?

Answer 12

• ECF volume decreases
• Initial BP rise diminishes

Question 13

Distinguish between paracellular and transcellular reabsorption

Answer 13

• Paracellular reabsorption: lumen → tight junction → capillary
• Transcellular reabsorption: lumen → apical membrane → basal membrane → capillary

Question 14

Distinguish between paracellular and transcellular secretion

Answer 14

• Paracellular secretion: capillary → tight junction → lumen
• Transcellular secretion: capillary → basal membrane → apical membrane → lumen

Question 15

What are aquaporin channels and what do they do?

Answer 15

- Aquaporins are channels which selectively conduct water molecules in and out of the cell

• They prevent the passage of ions/other solutes hence solely increase permeability to H₂O

Question 16

Briefly distinguish between Cl^- and Na⁺ reabsorption

Answer 16

• Na⁺ reabsorption: active, transcellular process, driven by 3Na-2K-ATPase pumps
• Cl^- reabsorption: active, transcellular and also passive paracellular process, coupled to 3Na-2K-ATPase pumps

Question 17

Identify the substances that are reabsorbed in the PCT and how this is achieved

Answer 17

• PCT reabsorbs:

I. 65% H₂O

II. 100% glucose and AA

III. 67% Na⁺

• Proximal tubule is highly permeable to H₂O and reabsorption is isosmotic with plasma

Question 18

Which 3 factors govern sodium uptake into renal capillaries in the PCT?

Answer 18

• Osmotic gradient established by solute absorption (osmolarity in interstitial spaces increases)
• Hydrostatic force in interstitium increases
• Oncotic force in peritubular capillary increases (loss of 20% glomerular filtrate leaving cells & proteins in blood)

Question 19

Identify the 4 different transporters which facilitate sodium reabsorption in the apical membrane in S1 of the PCT

Answer 19

• Na-H (antiporter)
• Na-Glucose (symporter)
• Na-AA (cotransporter)
• Na-Pi

Question 20

Identify the 2 different transporters which facilitate sodium reabsorption in the basolateral membrane in S1 of the PCT

Answer 20

• Basolateral 3Na-2K-ATPase
• NaHCO₃^- cotransporter (acids and bases)

Question 21

Describe the movement of chloride ions in S1 of the PCT

Answer 21

• Urea and Cl^- move down S1 compensating for loss of Glucose
• Increasing Cl^- concentration creates a conc. gradient for chloride reabsorption in S2-3

Question 22

How is water transported in S1 of the PCT?

Answer 22

Using aquaporins

Question 23

How is sodium reabsorbed in the apical and basolateral membranes of S2-S3 in the PCT respectively?

Answer 23

• Basolateral: 3Na-2K-ATPase
• Apical: Na-H exchange

Question 24

How are chloride ions reabsorbed in S2-S3 of the PCT?

Answer 24

Paracellular and transcellular Cl^- transport

Question 25

How is water reabsorbed in S2-S3 of the PCT?

Answer 25

- Aquaporins - ~4mOsmol gradient favours water uptake from lumen

Question 26

Describe sodium and chloride uptake in the late PCT

Answer 26

Question 27

Describe, briefly, how the concentration of the following molecules change to establish an iso-osmotic solution in the PCT: - Cl^- - Pi - Glucose, AA, lactate - HCO₃^-

Answer 27

Question 28

Identify the sodium transporter in the Loop of Henle

Answer 28

NaKCC (symporter)

Question 29

Distinguish between the general actions of the descending and ascending limbs of the Loop of Henle

Answer 29

- **Descending limb** reabsorbs H₂O but not NaCl - **Ascending limb** reabsorbs NaCl but not H₂O

Question 30

Describe the reabsorption that occurs in the descending and ascending limbs of the Loop of Henle

Answer 30

- Increase in [Na⁺]_i allows paracellular reuptake of H₂O from descending limb - Na⁺ and Cl^- are concentrated in the lumen of the descending limb ready for active transport in the ascending limb

Question 31

Describe the reabsorption that occurs in the thin ascending limb of the Loop of Henle

Answer 31

- H₂O reabsorption in descending limb creates a gradient for passive Na⁺ reabsorption in thin ascending limb - This occurs through the paracellular route

Question 32

Describe the 4 active transport processes which occur in the thick ascending limb of the Loop of Henle

Answer 32

- Active transport from lumen to cells via NaKCC transporter - Na⁺ move into interstitium via 3Na-2K-ATPase - K⁺ diffuse via ROMK back into lumen (maintain activity of NKCC2 transporter) - Cl^- move into interstitium

Question 33

What is unique about the thick ascending limb of the Loop of Henle?

Answer 33

- Region uses the most energy in the nephron - Sensitive to hypoxia

Question 34

Describe the osmolarity of the tubule fluid leaving the Loop of Henle

Answer 34

Tubule fluid leaving loop is hypo-osmotic compared to plasma

Question 35

Identify the 2 regions of the distal convoluted tubule

Answer 35

- DCT 1 (early) - DCT2 ( late)

Question 36

Describe the general reabsorption that happens in the DCT as a whole

Answer 36

- Hypo osmotic fluid (from LoH) enters - Active transport of Na⁺ (5-8%) - Low H₂O permeability

Question 37

Identify the sodium transporter in the early DT

Answer 37

NaCC (symporter)

Question 38

Describe the reabsorption that occurs in DCT1

Answer 38

- NaCl enters apical membrane via electro-neutral NCC transporter - NaCl leaves via 3Na-2K-ATPase in basolateral membrane - More hypo-osmotic fluid leaves, so further dilution occurs

Question 39

Identify the sodium transporter in the late DT & CD

Answer 39

ENaC (epithelial sodium channels)

Question 40

Describe the reabsorption that occurs in DCT2

Answer 40

- NaCl enters apical membrane via NCC and ENaC - NaCl leaves 3Na-2K-ATPase in basolateral membrane - Electrochemical gradient from ENaC drives paracellular Cl^- reuptake

Question 41

Describe the action of diuretics on membrane transporters in the DCT

Answer 41

- NCC transporter: sensitive to **Thiazide diuretics** - ENaC transporter: sensitive to **Amiloride diuretics**

Question 42

Describe calcium reabsorption across the DCT

Answer 42

- Cytosolic Ca²⁺ is bound by **calbindin**, shuttling it to the basolateral membrane - It is transported out by **NCX** - Tightly regulated by **hormones** *e.g. PTH and 1,25- dihydroxy vitamin D*

Question 43

How can the collecting duct be divided into segments?

Answer 43

- Cortical region (CCD) - Medullary region (MCD)

Question 44

Identify 2 cell types found in the CCD

Answer 44

- Principal cells (70%) - Intercalated cells

Question 45

Describe reabsorption in the principal cells of the collecting duct

Answer 45

- Reabsorption of Na⁺ via ENaC on apical membrane - Driving force: 3Na-2K-ATPase in basolateral membrane - Lumen(–) charge drives paracellular Cl^- uptake

Question 46

Identify the 2 types of intercalating cells

Answer 46

- **Acid-secreting** Type A intercalating cells - **Bicarbonate-secreting** Type B intercalating cells

Question 47

Describe the different membrane transporters found in Type A intercalating cells of the collecting duct

Answer 47

- Express H⁺-ATPase and H⁺/K⁺-ATPase on apical membrane - Express Cl⁻/HCO₃⁻ exchanger on basolateral membrane