4.4 ultrafiltration

Question 1

What is ultrafiltration?

Answer 1

• Filtration under pressure of small molecules out of the blood and into Bowman’s (Renal) capsule.
• Formation of glomerular filtrate

Question 2

How is hydrostatic pressure created?

Answer 2

Diameter of afferent arteriole is greater than efferent arteriole resulting in a buildup of pressure in glomerulus.

Question 3

Which 3 parts of Bowmans capsule does ultrafiltration include?

Answer 3

1. Endothelium of blood capillary:
2. Basement membrane
3. Epithelium of renal capsule

Question 4

What is the structure/role of the endothelium of blood capillary in ultrafiltration?

Answer 4

• Very thin
• Perforated with thousands of pores of about 100 nm diameter
• Provides a barrier to cells but not plasma proteins

Question 5

What is the structure/role of the basement membrane in ultrafiltration?

Answer 5

• Meshwork of collagen and glycoprotein fibres
• Water and small molecules can pass through
• Proteins are too large and are repelled by -ive charges on fibres

Question 6

What is the structure/role of the epithelium of Bowmans capsule in ultrafiltration?

Answer 6

• Made of cells which are modified for filtration

Question 7

What are podocytes?

Answer 7

• Each cell has many foot-like extensions projecting from its surface
• The extensions wrap around the capillaries of the glomerulus and interlink with extensions from neighboring cells
• These extensions fit together loosely leaving filtration slits about 25 nm wide
• The filtered fluid passes through the filtration slits

Question 8

What is filtered out of the blood during ultrafiltration?

Answer 8

The glomerular filtrate contains:

• Water
• Amino acids
• Glucose
• Urea
• Inorganic/mineral ions e.g. Na+, Cl-, K+)

Question 9

What is left in the capillary?

Answer 9

• Blood cells
• Proteins
• Some fluid (some of the water and dissolved substances above)

Question 10

What is the glomerular filtration rate?

Answer 10

The rate at which fluid seeps from blood in the glomerular capillaries into the renal capsule.

Question 11

What factors affect the glomerular filtration rate?

Answer 11

• Hydrostatic pressure in glomerular capillaries
• Solute concentration

Question 12

How does hydrostatic pressure in glomerular capillaries affect the glomerular filtration rate?

Answer 12

Caused by afferent artery being wider in diameter than the efferent arterioles – raises water potential of blood plasma (6 KPa)
Water potential of contents of renal capsule (1.5 KPa)
6 - 1.5 = 4.5KPa

Question 13

How does solute concentration affect the glomerular filtration rate?

Answer 13

As water and soluble substances pass out of the glomerular capillaries to renal capsule the blood becomes more concentrated (presence of plasma proteins) and fluid in renal capsule becomes less concentrated
- Osmosis occurs from renal capsule to glomerulus as high water potential in renal capsule compared to low in glomerulus.

Question 14

How to calculate net glomerular filtration rate with hydrostatic and osmotic pressure?

Answer 14

Difference between the two.

Question 15

How can high blood pressure affect ultrafiltration?

Answer 15

Elevated (high) blood pressure will damage capillaries, widen podocyte slits and damage the basement membrane. This allows larger proteins and sometimes erythrocytes (red blood cells) to enter the Bowman’s capsule.

Question 16

Describe how ultrafiltration produces glomerular filtrate? (5 marks)

Answer 16

Hydrostatic pressure (1)
Small molecules (1)
Pass through basement membrane (1)
Proteins are too large to go through (1)
Presence of pores/podocytes in capillaries (1)