Fluid shift across capillary wall

Question 1

What do precapillary sphincters do?

Answer 1

Regulate regional blood flow in few tissues e.g. mesentery.

Question 2

Why is blood flow through the capillary bed very slow?

Answer 2

To allow adequate time for exchange.

Question 3

How are small water-soluble substances, lipid-soluble substances and exchangeable proteins passed across the capillary wall?

Answer 3

Water-soluble: pores.
Lipid-soluble: through endothelial cells.
Exchangeable proteins: vesicular transport.

Question 4

What is ultra-filtration?

Answer 4

Exchange across the capillary wall of essentially protein-free plasma.

Question 5

What is net filtration pressure (NFP) directly proportional to?

Answer 5

Forces favouring filtration - forces opposing filtration.

Question 6

What coefficient also affects net fluid filtration?

Answer 6

A filtration coefficient (Kf).

Question 7

What is another name for the forces involved in transcapillary blood flow?

Answer 7

Starling forces.

Question 8

What are all the forces favouring filtration?

Answer 8

Pc - capillary hydrostatic pressure (biggest force, related to blood pressure).
Piei - interstitial fluid osmotic pressure.

Question 9

What are all the forces opposing filtration?

Answer 9

Piec - capillary osmotic/oncotic pressure (most important).

Pi - intersitial fluid hydrostatic pressure (-ve in some tissues).

Question 10

Give some example values for Pc, piei, Pi and piec at the arteriolar end.

Answer 10

Pc - 35mmHg.
piei - 1mmHg.
Pi - 1mmHg.
piec - 25mmHg.

Question 11

What value changes from the arteriolar to venular end and why?

Answer 11

Pc decreases from arteriolar to venular, due to decrease in blood pressure.

Question 12

What occurs at the arteriolar and the venular end?

Answer 12

Filtration at arteriolar and reabsorption at venular end.

Question 13

How much does filtration exceed reabsorption in a day?

Answer 13

2-4 litres.

Question 14

How does excess fluid get returned to the circulation?

Answer 14

Via the lymphatics as lymph.

Question 15

What helps to prevent the buildup of pulmonary oedema?

Answer 15

Low capillary hydrostatic pressure, same capillary osmotic pressure and efficient lymphatic drainage.

Question 16

How does pulmonary oedema affect the lung compliance?

Answer 16

Will decrease it.

Question 17

What are the 4 things that cause oedema?

Answer 17

1. Raised capillary pressure.
2. Reduced plasma osmotic pressure.
3. Lymphatic insufficiency (non-pitting oedema).
4. Changes in capillary permeability.

Question 18

What can increase capillary pressure?

Answer 18

Arteriolar dilation, raised venous pressure through LVF (pulmonary oedema), RVF (peripheral oedema), prolonged standing (swollen ankles).

Question 19

What is the normal plasma protein concentration and how low does it have to go to cause oedema?

Answer 19

Normal is 65-80g/l. Oedema if less than around 30g/l.

Question 20

What causes reduced plasma osmotic pressure?

Answer 20

Malnutrition, protein malabsorption, excessive renal excretion of protein, hepatic failure (less protein made).

Question 21

What causes lymphatic insufficiency?

Answer 21

Lymph node damage e.g. filariasis (elephantiasis).

Question 22

What can cause changes to capillary permeability?

Answer 22

Inflammation, histamine increases leakage of protein.