The Proximal Tubule and the Loop of Henle

Question 1

What do the kidneys reabsorb?

Answer 1

• 99% of fluid
• 99% of salt
• 100% glucose
• 100% amino acids
• 50% urea
• 0% creatinine

Question 2

Compare reabsorption and filtration?

Answer 2

Reabsorption is specific (relies on specific transporters)

Filtration is relatively non-specific

Question 3

What is glomerular filtrate?

Answer 3

Modified filtrate of the blood, i.e: contains ions and solutes at plasma concentrations but lacks RBCs and large plasma proteins)

Question 4

Describe reabsorption of fluid in the PCT

Answer 4

~80 ml/min of filtered fluid is reabsorbed

The fluid reabsorbed in the PCT is iso-osmotic with the filtrate, i.e: the tubular fluid does not change in osmolarity as it passes along the PCT

Question 5

Which substances are reabsorbed and secreted in the PCT?

Answer 5

Reabsorbed:
• Sugars
• Amino acids
• Phosphate
• Sulphate
• Lactate

Secreted:
• H+
• Hippurates
• Neurotransmitters
• Bile pigments
• Uric acid
• Drugs
• Toxins

Question 6

2 routes of tubular reabsorption?

Answer 6

Transcellular - from the tubular lumen across the epithelial cell and then into the interstitial fluid and then the peritubular capillary

Paracellular - between the epithelial cells

Question 7

3 types of carrier-mediated transport? Describe each

Answer 7

Primary active transport - energy is directly required to operate the carrier and move the substrate against its conc. gradient

Secondary active transport - carrier molecule is transported coupled to the conc. gradient of an ion (usually Na+)

Facilitated diffusion - passive carrier-mediated transport of a substance down its conc. gradient

Question 8

Example of diffusion through a lipid bilayer?

Answer 8

O2 and CO2

Question 9

Example of diffusion through channels?

Answer 9

Na+

Question 10

Example of facilitated diffusion?

Answer 10

Glucose

Question 11

Example of primary active transport?

Answer 11

Na+/K+ ATPase brings Na+ out and K+ in

Question 12

Example of secondary active transport?

Answer 12

Na+/glucose transporter (on the apical membrane)- both Na+ and glucose are transported in

Question 13

How does Na+ reabsorption occur?

Answer 13

Via the energy-dependent Na+/K+ ATPase at the basolateral membrane

This brings Na+ out of the tubular cell and into the interstitial fluid; from here, it can diffuse into the peritubular capillary

Question 14

Why is there iso-osmotic fluid reabsorption?

Answer 14

This occurs across a “leaky” proximal tubule epithelium due to:

1. Standing osmotic gradient
2. Oncotic pressure gradient

There is passive water reabsorption down the NaCl osmotic gradient, i.e: water follows Cl- ions paracellularly (AKA oncotic drag of peritubular plasma); this means that osmolarity does not change because water and salt are reabsorbed in equal proportions

Question 15

Which transporters are present on the luminal end of the tubular cells?

Answer 15

Na+/glucose (brings both in)

Na+/amino acid (brings both in)

Na+/H+ (brings Na+ in and pumps H+ out)

Question 16

How is glucose reabsorbed?

Answer 16

On the luminal side of the tubular cell:
• Na+/glucose

On the basolateral side of the tubular cell:
• Facilitated diffusion allows glucose to enter interstitial fluid

Normally, 100% of glucose in the filtrate is reabsorbed in the PCT

Question 17

What is the transport maximum for glucose?

Answer 17

2 mmol/min

Transport systems inv. with reabsorption and secretion are saturable; once the Tm is reached, clearance of reabsorbed or secreted substances is not constant

Question 18

Function of the loop of Henle?

Answer 18

Generates a cortico-medullary solute conc. gradient, which enables the formation of HYPERTONIC URINE

Question 19

Fluid flow in the loop of Henle?

Answer 19

Opposing flow in the two limbs (ascending and descending) is termed countercurrent flow; the entire loop functions as a counter-current multiplier

Together, the loop and vasa recta establish a hyper-osmotic medullary interstitial fluid

Question 20

Reabsorption in the ascending limb?

Answer 20

Along the entire length:
• Na+ and Cl-

However, the ascending limb is relatively impermeable to water and so little/no water follows salt reabsorption

Question 21

Reabsorption in the descending limb?

Answer 21

Does not reabsorb NaCl and is highly permeable to water

Question 22

HOW is Na+ reabsorbed in the thick ascending limb?

Answer 22

Via the Na+/K+/Cl- (triple) co-transporter

This region is impermeable to water

Question 23

How can the triple co-transporter be blocked?

Answer 23

Loop diuretics

Question 24

Steps of reabsorption in the loop of Henle?

Answer 24

1. Solute removed from lumen of ascending limb (water cannot follow)
2. Tubular fluid is diluted and osmolality of interstitial fluid is raised
3. Interstitial solute cannot enter the descending limb
4. Water leaves the descending limb by osmosis
5. Fluid in the descending limb is concentrated

ADD IMAGE

This means that fluid enters the descending limb and is concentrated (becomes hyperosmotic compared to the blood) before moving into the ascending limb; dilute/hypotonic fluid them moves onto the distal tubule

Question 25

Why is the loop of Henle horse-shoe/hairpin shaped?

Answer 25

ADD IMAGE

Question 26

Why is urea important?

Answer 26

Urea cycle contributes ~1/2 of the medullary osmolarity

The collecting ducts absorb ~50% urea (ADH promotes this); this passively diffuses into the loop

ADD IMAGE

NOTE: the distal tubule is impermeable to urea

Question 27

Function of countercurrent multiplication?

Answer 27

To concentrate the medullary interstitial fluid (high medullary osmolarity) in order to enable the kidney to produce urine of different volume and conc. according to the amount of circulating ADH

Question 28

What is the countercurrent exchanger

Answer 28

Vasa recta acts as this and it runs alongside the long loop of Henle of the juxtamedullary nephrons

Capillary blood euilibriates with interstitial fluid across the “leaky” endothelium:
• Blood osmolarity rises as it dips down into the medulla, i.e: water loss, solute gained
• Blood osmolarity falls as it rises back up into the cortex, i.e: water gained, solute lost

Question 29

What does the countercurrent system consist of?

Answer 29

Vasa recta (countercurrent exchanger)

Loop of henle (countercurrent flow)

Question 30

How is washing away of NaCl and urea in the medulla, by essential blood flow, minimised?

Answer 30

1. Vasa recta capillaries follow hairpin loops
2. Vasa recta capillaries are freely permeable to NaCl and water; passive exchange across the endothelium preserves medullary gradient and ensures solute is not washed away
3. Blood flow to the vasa recta is low (there are few juxtamedullary nephrons)

Question 31

2 components in the creation of the medullary osmotic gradient?

Answer 31

Countercurrent multiplier

Urea cycle