Flashcards in Physiology 6 - Microcirculation Deck (31)
Why is the consistency of interstitial fluid more like a gel than a free flowing liquid?
Collagen fibres and proteoglycan filaments (interstitium) are dispersed in the interstitial fluid, increasing viscosity
Why do colloids (plasma proteins) not move out of the capillaries?
In a nutshell, they are too large - barred from crossing by capillary walls. A few however, may escape
Explain the concept behind oncotic pressure.
Large amount of plasma protein (colloid), predominantly albumin, in the capillary
Smaller quantity of albumin in the interstitial space
Oncotic pressure in capillary is therefore high (roughly 28mmHg)
Oncotic pressure in interstitial space is lower (5-8mmHg)
Water is drawn across toward the majority of solute, according to laws of osmosis, so moves into capillary
What is the average hydrostatic pressure at the arterial end of circulation?
Negative hydrostatic pressure in the interstitial space leads to...?
Water drawn out of capillaries
Positive hydrostatic pressure in the interstitial space leads to...?
Water pulled into capillaries
What is the average hydrostatic pressure at the venous end of circulation?
What would be the result of having purely hydrostatic pressure pushing water out?
Give 4 methods which increase venous pressure and hence increase venous return.
Increase blood volume
Increase inspiration movements
Increase skeletal muscle movement (as a pump)
Increase activity of sympathetic nerves to veins
List the chain of events starting with: Venous pressure ^ ---> Venous return ^ --->
Atrial pressure ^
End diastolic ventricular volume ^
Stroke volume ^
Cardiac output ^
What is the interstitium (ECM) composed of?
What are low molecular weight solutes (e.g. Cl-, K+ or Na+) known as?
What are large molecular weight plasma proteins such as albumin known as?
What is bulk flow?
Distribution of the extracellular fluid
What is normal oncotic pressure in the capillary?
What is normal oncotic interstitial pressure?
Why is capillary oncotic pressure higher than interstitial oncotic pressure?
Larger amount of albumin present in the blood than in the interstitium
What are normal arteriole and venule hydrostatic pressures?
What does the hydrostatic pressure do?
Forces fluid out of the capillaries and into the interstitium
What direction does fluid move as interstitial pressure alternates between positive and negative?
Forces fluid in to the capillary when positive
Draws fluid in to the interstitium when negative
What is normal interstitial hydrostatic pressure?
Negligible - 0mmHg
How is fluid built up in the interstitium removed?
Via the lymphatic system
What is the overall movement of fluid at the arteriole end of the capillary?
Hydrostatic pressure > Oncotic pressure – fluid moves out of capillary
What is the overall movement of fluid at the venule end of the capillary?
Hydrostatic pressure < Oncotic pressure – fluid moves into capillary
Overall, do capillaries lose or gain water as blood moves from arteriole to venous end?
Lose more than they gain
What four factors is the lymphatic system responsible for regulating?
Concentration of proteins in interstitial fluids (lymphatics allow movement of large molecules)
Volume of interstitial fluid
Interstitial fluid pressure
What is the pressure range of the systemic venous circulation?
What results from increasing venous return?
Increased EDV and therefore increased CO
Which nervous system controls the capacity of veins?
What four factors increase venous return to the heart?
Sympathetic innervation (smooth muscle contraction)
Inspiratory movements (due to descending diaphragm and decreased thoracic pressure)