Exchange Across Capillaries and Venous Return Flashcards
(32 cards)
In what vessels does capillary take place?
capillaries
Capillaries are simple tubes of what cells?
endothelial
What is exchanged between capillaries?
- solutes - oxygen, glucose, amino acids etc and also waste products and metabolites such as CO2 and K
- by diffusion
- fluid - plasma minus plasma proteins pass across endothelial cells to interstitial space and back by filtration
What are three characteristics of capillaries?
- endothelium (no vascular smooth muscle)
- 3-6 micrometres diameter
- discontinuous blood flow - can become more or less continuous
Arteriolar constriction leads to what in terms of arteriolar resistance and pressure in capillaries?
increased arteriolar resistance
pressure at capillary opening - lost when going through
Compare capillaries to arteries and veins in terms of surface area and speed of flow.
capillaries have the highest surface area and the lowest speed of flow
this means theres more time for exchange in the capillaries
What is the formula for rate of diffusion?
rate = P x (C1-C2) x A
C conc gradient
A surface area of capillaries
P permeability - increased during inflammation
Why does rate of diffusion increase when arteries dilate?
tissue blood flow increases
conc gradient steeper and more capillaries are better perfused with blood
eg functional hyperaemia
What forces does filtration across capillaries depend on?
hydrostatic pressure - fluid pressure
osmotic pressure - mainly exerted by proteins called oncotic pressure
Explain the starling forces across capillaries.
dependent on four different pressures
- capillary hydrostatic pressure
- tissue hydrostatic pressure
- plasma oncotic pressure
- tissue oncotic pressure
What is plasma oncotic pressure always?
What about tissue oncotic pressure?
POP 25mmHg
TOP 2-3mmHg
What happens when hydrostatic pressure is higher than oncotic pressure?
water pushed out
Why would there be changes in capillary hydrostatic pressure?
arteriolar dilatation/constriction
What happens when arteriolar dilatation occurs in terms of oncotic pressure?
Give an example of when this would occur
hydrostatic pressure higher along vessels, as theres less resistance meaning more energy to the blood
HP = OP point not reached, HP > OP along capillary instead
this means water keeps on being pushed out
net fluid out > net fluid in
eg cutaneous dilatation when hot - can cause oedema
What happens when arteriolar constriction occurs in terms of oncotic pressure?
Give an example of when this would occur
hydrostatic pressure lower along vessels, as theres more resistance meaning less energy to the blood
HP = OP reached early, HP > OP along capillary instead
net fluid in > net fluid out
eg in muscle, skin, GIT during haemorrhage
What happens when plasma oncotic pressure is increased?
Give an example of when this would occur
due to more plasma proteins
net fluid in > net fluid out
eg in dehydration helps to restore blood volume
What happens when plasma oncotic pressure decreases?
net fluid out > net fluid in = oedema
eg in malnutrition, liver failure
What happens when permeability to proteins is increased? When would this occur?
same as when plasma oncotic pressure decreased
eg in inflammation, infection, allergic responses (anaphylaxis)
proteins leak out - tissue oncotic pressure increased
gradient reduced between POP and TOP
oedema caused
What is the net loss from CVS in terms of capillaries?
over 24 hours:
20 litres fluid filtered out of capillaries
16 litres filtered into capillaries
net loss 4L/24 hours
NB: lymphatic system returns excess fluid
What do lymphatic collecting vessels drain back into?
back into CVS
via thoracis ducts which drain into subclavian vessels
What lymph vessels enter the subclavians veins?
right lympatic duct enters right subclavian vein
thoracic duct enter left subclavian vein
When does oedema occur physiologically and pathologically?
physiologically: outward filtration > lymph drainage
pathologically: vascular permeability increased
An example of pathological oedema is lymphatics blocked. When would this occur?
consequence of surgical removal of lymph nodes = lymphodaema
How do venules and veins have a reservoir function?
theyre normally elliptical when supine; can fill more and become more circular to store blood
determines filling of ventricles in diastole (EDV)
