Fluid shift across capillary wall Flashcards
What do precapillary sphincters do?
Regulate regional blood flow in few tissues e.g. mesentery.
Why is blood flow through the capillary bed very slow?
To allow adequate time for exchange.
How are small water-soluble substances, lipid-soluble substances and exchangeable proteins passed across the capillary wall?
Water-soluble: pores.
Lipid-soluble: through endothelial cells.
Exchangeable proteins: vesicular transport.
What is ultra-filtration?
Exchange across the capillary wall of essentially protein-free plasma.
What is net filtration pressure (NFP) directly proportional to?
Forces favouring filtration - forces opposing filtration.
What coefficient also affects net fluid filtration?
A filtration coefficient (Kf).
What is another name for the forces involved in transcapillary blood flow?
Starling forces.
What are all the forces favouring filtration?
Pc - capillary hydrostatic pressure (biggest force, related to blood pressure).
Piei - interstitial fluid osmotic pressure.
What are all the forces opposing filtration?
Piec - capillary osmotic/oncotic pressure (most important).
Pi - intersitial fluid hydrostatic pressure (-ve in some tissues).
Give some example values for Pc, piei, Pi and piec at the arteriolar end.
Pc - 35mmHg.
piei - 1mmHg.
Pi - 1mmHg.
piec - 25mmHg.
What value changes from the arteriolar to venular end and why?
Pc decreases from arteriolar to venular, due to decrease in blood pressure.
What occurs at the arteriolar and the venular end?
Filtration at arteriolar and reabsorption at venular end.
How much does filtration exceed reabsorption in a day?
2-4 litres.
How does excess fluid get returned to the circulation?
Via the lymphatics as lymph.
What helps to prevent the buildup of pulmonary oedema?
Low capillary hydrostatic pressure, same capillary osmotic pressure and efficient lymphatic drainage.
How does pulmonary oedema affect the lung compliance?
Will decrease it.
What are the 4 things that cause oedema?
- Raised capillary pressure.
- Reduced plasma osmotic pressure.
- Lymphatic insufficiency (non-pitting oedema).
- Changes in capillary permeability.
What can increase capillary pressure?
Arteriolar dilation, raised venous pressure through LVF (pulmonary oedema), RVF (peripheral oedema), prolonged standing (swollen ankles).
What is the normal plasma protein concentration and how low does it have to go to cause oedema?
Normal is 65-80g/l. Oedema if less than around 30g/l.
What causes reduced plasma osmotic pressure?
Malnutrition, protein malabsorption, excessive renal excretion of protein, hepatic failure (less protein made).
What causes lymphatic insufficiency?
Lymph node damage e.g. filariasis (elephantiasis).
What can cause changes to capillary permeability?
Inflammation, histamine increases leakage of protein.
