Physiology - The Proximal Tubule and the Loop of Henle

Question 1

Where does the vast majority of reabsorption occur in the nephron?

Answer 1

Proximal convoluted tubule

Question 2

Roughly how much fluid per minute is reabsorbed in the proximal convoluted tubule?

Answer 2

80ml/min

Question 3

How does osmolarity change within the proximal tubule?

Answer 3

It does not change

(fluid reabsorbed in the proximal tubule is iso-osmotic with filtrate)

Question 4

Which substances are reabsorbed from the proximal tubule?

Answer 4

1. Sugars
2. Amino acids
3. Phosphate
4. Sulphate
5. Lactate
6. Na+ and Cl-
7. Water

Useful substances are reabsorbed (out on the tubule and into the blood)

Question 5

Which substances are secreted into the proximal tubule?

Answer 5

1. H⁺
2. Hippurates
3. Neurotransmitters
4. Bile pigments
5. Uric acid
6. Drugs
7. Toxins

Question 6

Which two routes can a substance take in order to be reabsorbed from the proximal tubule?

Answer 6

1. Transcellular
2. Paracellular

Question 7

Which barried must a substance pass in the transcellular route on its path to reabsorption?

Answer 7

Apical membrane

Cytoplasm

Basolateral membrane

Lateral space and extracellular fluid

Endothelial wall of capillary

Question 8

Where does a substance pass, if it exits the nephron tubule via the paracellular route?

Answer 8

Tight junctions

Question 9

What are primary active transport proteins?

Answer 9

Proteins which use hydrolysis of ATP to drive their substrate across its concentration gradient

Question 10

What are secondary active transport proteins?

Answer 10

Proteins which move a substance against its concentration gradient when coupled to another ion by utilising its concentration gradient

Question 11

Which types of diffusion can take place in the nephron?

Answer 11

Diffusion through lipid bilayer (O2, CO2)

Diffusion through channels (Na+)

Facilitated diffusion (glucose)

Question 12

What drives sodium reabsorption in the proximal tubule?

Answer 12

Basolateral Na/K/ATPase

This maintains a concentration gradient out of the proximal tubule and into tubular cells

Question 13

At which membrane is the sodium potassium pump found in the nephron?

Answer 13

Basolateral

Question 14

Why are chloride ions reabsorbed in the proximal tubule?

Answer 14

An electrochemical gradient is set up as sodium is absorbed into the blood

Question 15

Why is water reabsorbed in the peritubular capillary?

Answer 15

1. Salt (is reabsorbed, water follows)
2. There is oncotic drag (due to high protein concentration in capillary)

Question 16

Why does osmolarity not change in the proximal tubule?

Answer 16

Salt and water are reabsorbed in equal proportions

Question 17

By which active transport route does

a) Sodium use
b) Chloride use

Answer 17

a) Sodium - Transcellular
b) Chloride - Paracellular

Question 18

How does glucose become reabsorbed from the proximal tubule?

Answer 18

Transcellular route

Sodium-glucose cotransporter at apical membrane

Faciliated glucose diffusion at basolateral membrane

Question 19

At high plasma glucose concentrations, why does the rate of glucose reabsorption plateau?

Answer 19

Transport mechanisms become saturated

(excess glucose will exit in urine, but under normal circumstances all glucose is reabsorbed in the proximal tubule)

Question 20

What is a summary of all of the main substances reabsorbed by the proximal tubule?

Answer 20

67% of all water and salt

100% of glucose and amino acids

Na+ reabsorption

Cl- reabsorption

Question 21

What is the function of the Loop of Henle?

Answer 21

Generate a cortico-medullary solute concentration gradient

(enabling formation of hypertonic urine)

Question 22

What is meant by the term countercurrent flow?

Answer 22

The opposing directions of flow between the descending and ascending limbs of the loop of Henle

Question 23

Why is a cortico-medullary solute concentration gradient required?

Answer 23

Formation of hypertonic urine

(osmolarity is higher deep in medulla)

Question 24

The process of what sets up a cortico-medullary solute concentration gradient?

Answer 24

Countercurrent multiplication

Question 25

The cells of the descending limb are essentially impermeable to ________ and highly permeable to \_\_\_\_\_\_\_\_\_\_

Answer 25

The cells of the descending limb are essentially impermeable to **salt** and highly permeable to **water**

Question 26

What do the characteristics of the descending limb allow for in terms of permeability to water?

Answer 26

Easy **water reabsorption** (out of descending limb) Since the descending limb is impermeable to salt, the salt stays in limb and salt outside in the medulla also cannot enter This ensures an **osmotic gradient** is presernt

Question 27

Along the length of the ascending limb what can be reasborbed?

Answer 27

Sodium ions Chloride ions (NOT water reabsorption)

Question 28

What do the characteristics of the ascending limb allow for in terms of permeability to salts?

Answer 28

Increases passive diffusion of water at the **descending tubule** Ascending limb has active tranporters which **excrete salt into the medullary space** increasing osmolarity and ensuring water will passively diffuse from the descending tubule into the medulla

Question 29

Describe briefly the key points in countercurrent multiplication

Answer 29

Ascending limb pumps salts into medulla for reabsorption This makes the osmolarity very high in the medulla Since the descending limb is impermeable to salt, water will pass from the inside of the limb to the outside via passive diffusion towards the osmolarity This process means that osmolarity increased down the descending tubule and decreases up the ascending tubule

Question 30

At which value for osmolarity is a stead state reached in the process of countercurrent multiplication?

Answer 30

1200mmol/l

Question 31

The thick ascending limb of henle contains cells which posses which transporter on the apical membrane?

Answer 31

Triple cotransporter

Question 32

How does the triple cotransporter function?

Answer 32

* 1 Sodium * 1 Potassium * 2 Chloride ions Moved into the thick ascending loop of Henle cell from lumen (no electrochemical gradient formed as net neutral charge)

Question 33

In the thick ascending limb of Henle, which transporters can be found at the basolateral membrane?

Answer 33

Sodium/Chloride cotransporters Sodium/Potassium pump (potassium can also passively diffuse across the basolateral membrane)

Question 34

The actions of the triple cotransporter, result in the net reabsorption of which ions into the interstitial fluid?

Answer 34

Sodium and chloride (potassium recycling allows this to happen)

Question 35

Which drug class can act on the triple cotransporter?

Answer 35

Loop diuretetics

Question 36

What is the significance of the actions of the triple cotransporter in the cells of the thick ascending limb of Henle?

Answer 36

Interstitial fluid osmolarity is increased Osmotic gradient is set up from the **descending** limb to the interstital fluid Tubular fluid becomes more concentrated

Question 37

Why does osmolarity progressively decrease up the ascending loop of Henle?

Answer 37

Sodium and chloride ions are actively removed into the interstitial space

Question 38

What are the two factors which set up the corticomedullary gradient?

Answer 38

1. Sodium/chloride 2. Urea

Question 39

What is the urea cycle and what is its purpose regarding the medullary osmolarity?

Answer 39

Collecting duct absorbs roughly 50% of urea under the action of ADH It then functions to add solute to the interstitium Urea can then enter the descending and ascending limbs of Henle forming a cycle This means more water is reabsorbed as the osmolarity gradient is now more stark This reduces urine output

Question 40

What is the purpose of countercurrent multipliction?

Answer 40

Concentrates medullary interstitial fluid Enables kidneys to produce urine of different volumes and concentrations depending on the amount of ADH in circulation

Question 41

What does the vasa recta run alongside?

Answer 41

Long loop of henle of **juxtamedullary** nephrons

Question 42

What is the purpose of the vasa rectae?

Answer 42

The vasa rectae ensures the solute is not washed away (helps maintain the corticomedullary gradient)