Filtration and Reabsorption in the PCT

Question 1

How does filtration occur in the Bowman’s capsule?

Answer 1

• Force fluids and solutes through a membrane by pressure
• Difference in hydrostatic pressure and protein-osmotic pressure between 2 sides of filter
• Fluid is moved along the vessels and across capillary walls by bulk flow

Question 2

How is the renal corpuscle adapted for filtration?

Answer 2

1. Large surface area across which filtration can occur
2. High level of glomerular capillary blood pressure
3. 20 – 25% cardiac output so v high volume of blood
4. Low resistance to the movement of fluid

Question 3

What are the 3 barrier through which substances are filtered through?

Answer 3

• Capillary endothelium
• Basement membrane
• Podocytes

Question 4

How much of the filtrate is reabsorbed in the PCT?

Answer 4

70%

Question 5

What is tubular reabsorption?

Answer 5

The process of returning important substances from the filtrate back into the body

Question 6

Compare paracellular and transcellular reabsorption

Answer 6

• Paracellular reabsorption is a passive process that occurs between adjacent tubule cells
• Transcellular reabsorption is movement through an individual cell.

Question 7

Describe the solvent drag

Answer 7

Bulk movement of water than brings other solutes with it into the capillaries setting up a concentration gradient for passive diffusion of other solutes

Question 8

Give a definition of secondary active transport

Answer 8

Set up a gradient of one ion which then transports another ion with it down its conc gradient e.g. sodium transports glucose

Question 9

What are the 3 methods by which sodium re-absorption occurs?

Answer 9

1) Basolateral sodium potassium pump
2) Na – glucose symporter
3) Glucose facilitated diffusion

Question 10

Is glucose fat or water soluble?

Answer 10

Water soluble

Question 11

What are the 2 main factors that affect reabsorption?

Answer 11

• Rate of flow of the filtrate

- Number of protein transporters

Question 12

What is meant by secretion in the PCT, and what is it used for?

Answer 12

• Secretion provides an additional (or in some cases only opportunity) to increase urinary excretion of specific substances

• always active
• not regulated in the PCT
• substances have to be ionised

Question 13

What results in secretion of hydrogen ions into the filtrate?

Answer 13

Sodium hydrogen exchange

- hydrogen ions bind to a buffer in the filtrate

Question 14

What % of bicarbonate ions are reabsorbed?

Answer 14

80-90%

Question 15

The apical membrane is impermeable to bicarbonate ions so how are they reabsorbed?

Answer 15

• The basolateral membrane is PERMEABLE to bicarbonate
• Sodium hydrogen antiporter secretes H+ which binds to bicarbonate.
• Carbonic acid dissociates and carbon dioxide moves into the tubular cell allowing bicarbonate to be reabsorbed

Question 16

How are the proteins that get into the filtrate reabsorbed?

Answer 16

By endocytosis

• degraded into amino acids
• enter capillaries

Question 17

How is the PCT functionally adapted for reabsorption?

Answer 17

1. Large surface area
2. Single layer of epithelial cell
3. Peritubular capillaries continuous with efferent arteriole have very high oncotic pressure
4. High concentration of Na+ K + ATPase
5. Asymmetrical distribution of Na+ K + ATPase – only on the basolateral side
6. High concentration of carbonic anhydrase

Question 18

Why is it difficult to excrete highly lipid soluble substances via the urine?

Answer 18

Lipid-soluble substances can readily cross cell membranes and as water is absorbed from the filtrate a diffusion gradient is set up promoting reabsorption.

Question 19

What is the main nitrogenous waste product of animals?

Answer 19

Urea

Question 20

Which two substances are 100% reabsorbed in the PCT?

Answer 20

Glucose and amino acids