The proximal tubule and loop of Henle

Question 1

Where does most tubular reabsorption occur?

Answer 1

Proximal tubule

Question 2

Is tubular reabsorption active or passive?

Answer 2

Active

Question 3

Is filtration specific or non specific?

Answer 3

Non-specific

Question 4

Is tubular reabsorption specific or non specific?

Answer 4

Specific

Question 5

Why is fluid reabsorbed in the proximal tubule iso-osmotic with filtrate?

Answer 5

Equal amounts of salt and water are reabsorbed

Question 6

Reabsorption of which substance occurs completely within the proximal tubule?

Answer 6

Glucose

Question 7

What separates the apical and basolateral membranes of tubular epithelial cells?

Answer 7

Tight junctions

Question 8

During transcellular reabsorption which structures must be crossed?

Answer 8

Apical membrane
Cytoplasma
Basolateral membrane 
Interstitium 
Endothelial cells

Question 9

During paracellular reabsorption which structures must be crossed?

Answer 9

Tight junctions
Interstitium
Endothelium

Question 10

What is primary active transport?

Answer 10

Energy required to fuel the carrier protein to transport the substrate against its concentration gradient

Question 11

What is secondary active transport?

Answer 11

The carrier protein transports the substrate coupled to the concentration gradient of an ion (usually sodium)

Question 12

What is facilitated diffusion?

Answer 12

Passive carrier mediated transport of a substance down its concentration gradient

Question 13

Why is sodium commonly utilised during active transport?

Answer 13

Because there are more sodium ions in the ECF and ICF and most transport is occuring into the cell

Question 14

Diffusion can occur passively through ion channels. T/F

Answer 14

True

Question 15

How does sodium reabsorption occur at the kidney?

Answer 15

Sodium enters tubular cells >
Sodium-potassium ATPase transports 3 sodium into the interstitium and 2 potassium into the cell at the basolateral membrane>
Sodium diffuses across the endothelium

Question 16

Is the sodium-potassium pump energy dependent or independent?

Answer 16

Dependent

Question 17

How does sodium enter tubular cells?

Answer 17

Secondary active transport of sodium and:

• Glucose
• Amino acids

Countertransport of sodium into cell and hydrogen ions out

Question 18

How does chlorine reabsorption occur at the kidney?

Answer 18

The positively charged sodium ions in the interstitium set up and electrochemical gradient which attracts chloride ions paracellularly

Question 19

How does water reabsorption occur at the kidney?

Answer 19

Salt absorbed attracts water (paracellular)

Question 20

How is glucose reabsorbed in the kidney?

Answer 20

Secondary active transport with sodium into tubular cells then facilitated diffusion out of tubular cells

Question 21

What is transport maximum for glucose?

Answer 21

Active transport mechanisms (i.e those which use conformational changes in membrane proteins) can only move a set number of molecules per unit time. Transport maximum occurs when these transport mechanisms become saturated (max glucose molecules per unit time)

Question 22

Clearance of reabsorbed or secreted substances is constant once the transport maximum of that substance is reached. T/F

Answer 22

False - it is NOT constant once transport maximum is reached

Question 23

Do secretory or reabsorptive mechanisms reach a transport maximum?

Answer 23

Both do

Question 24

What is the function of the loop of Henle? How does it achieve this?

Answer 24

Producing concentrated urine. Creating a cortico-medullary solute concentration gradient

Question 25

What is countercurrent flow?

Answer 25

Opposing flow in the two limbs of the loop of Henle

Question 26

The loop of Henle and vasa recta are responsible for creating a hyper-osmotic medullary interstitium. Therefore which type of nephron is involved?

Answer 26

Juxto-medullary

Question 27

What is being reabsorbed at the descending loop of Henle? What is it impermeable to?

Answer 27

Water | Salts

Question 28

What is being reabsorbed at the ascending loop of Henle? What is it impermeable to?

Answer 28

Salts | Water

Question 29

How does reabsorption of salts differ within the ascending loop of Henle?

Answer 29

Thick upper portion - active transport | Thin lower portion - passive

Question 30

What is the effect of the differing permeabilities between the limbs of the loop of Henle?

Answer 30

Enable osmotic gradient to be established within the medulla

Question 31

How are ions reabsorbed within the loop of Henle?

Answer 31

Triple cotransporter

Question 32

Explain the triple cotransporter

Answer 32

``` Ions are pumped into the tubular cells: 1 sodium 1 potassium 2 choride (equal charges) ```

Question 33

At which membrane is the triple cotransporter always found?

Answer 33

Luminal

Question 34

How do loop diuretics work?

Answer 34

By blocking the triple cotransporter at the loop of Henle thereby limiting salt reabsorption and thus water reabsorption (water follows salt)

Question 35

How is potassium important in salt reabsorption at the loop of Henle?

Answer 35

Potassium is involved in the active transport of chloride and sodium ions and is continually recycled (i.e continually moves in and out of the cell)

Question 36

What happens to the fluid as it passes through the loop of Henle?

Answer 36

Isotonic fluid enters descending limb > Hypertonic fluid enters the ascending limb > Hypotonic fluid leaves the ascending limb

Question 37

What is countercurrent multiplication?

Answer 37

The effect the loop of Henle has to transform a horizontal concentration gradient into a vertical one within the medulla

Question 38

Describe the concentration gradient produced by the loop of Henle

Answer 38

Osmolarity should be higher deeper within the medulla

Question 39

Apart from the loop of Henle, what is important in creating the corticomedullary concentration gradient?

Answer 39

Urea cycle

Question 40

Hows ADH promote or oppose urea secretion into the proximal tubule?

Answer 40

Promotes

Question 41

Is the distal tubule permeable or impermeable to urea?

Answer 41

Impermeable

Question 42

Why is the cortico-medullary gradient important for urine production?

Answer 42

It allows different concentrations and volumes of urine to be produced

Question 43

What is the countercurrent exchanger?

Answer 43

The vasa recta acts as a countercurrent exchanger for juxtamedullary nephrons allowing blood to equilibrate with the juxtamedullary interstitial gradient

Question 44

What is the countercurrent system?

Answer 44

The combined efforts of the vasa recta and the loop of henle

Question 45

How is the loss of salt and urea from the medulla minimised?

Answer 45

Vasa recta runs in a hairpin loop Vasa recta capillaries freely permeable to salt and water Blood flow to vasa recta is low (minimal juxtamedullary nephrons)

Question 46

Passive exchange of salt and water across the vasa recta helps to maintain the juxtamedullary gradient. T/F

Answer 46

True - since the blood equilibrates

Question 47

What is the function of the vasa recta?

Answer 47

To ensure solute is not washed away from the medulla

Question 48

The high medullary gradient allows the production of hypertonic urine in the presence of ADH. T/F

Answer 48

True