5 - The Microcirculation Flashcards
What is the arrangement of vessels in the microcirculation?
- 1st order arterioles
- terminal arterioles
- capillaries
- pericytic (post capillary) venules
- venule
What is meant by ‘microcirculation’?
individual circulation for any individual tissue
What is blood flow rate?
the volume of blood passing through a vessel per unit time
Give the equation relating blood flow rate with pressure gradient and vascular resistance
F = ∆P/R
i.e. blood flow rate = pressure gradient / vascular resistance
Define resistance
hindrance to blood flow due to friction between moving fluid and stationary vascular walls
Which of the 3 factors impacting resistance has the major influence? Why is this?
vessel radius has the major influence because:
- blood viscosity and vessel length remain mostly constant (blood flow is predetermined by these factors_
- the resistance is indirectly proportional to the radius^4, so altering the radius has a huge effect on resistance
What is meant by the pressure gradient?
the difference between the BP of the blood entering the vessel and that of the blood leaving the vessel
F = ∆P/R ∆P= FxR
What would an increase in BP cause?
An increase in the pressure gradient —–> an increase in flow
F = ∆P/R ∆P= FxR
What would arteriolar vasoconstriction cause?
Decrease in radius —–> increase in resistance —–> decrease in flow
In which vessels is the biggest drop in pressure seen?
Why is the result of this?
the arterioles
its allows the blood to flow through the capillary bed passively/slowly, too allow exchange of nutrients
What happens to the pressure of the blood in the arteries?
not much change from the pressure generated by the heart
What is MAP?
Mean Arterial Pressure
(the average pressure in a patient’s arteries during one cardiac cycle)
NOTE: in any (1°) artery in the body, the blood pressure is around MAP
Overall, what is the major determinant of blood flow?
the resistance of the arterioles in the organ/tissue
Why is the arteriolar smooth muscle usually in a state of partial constriction?
it allows for the changing of blood flow up AND down by contracting or dilating
What is active hyperaemia?
an increase in organ blood flow that is associated with increased metabolic activity of an organ or tissue
Which receptors facilitate constriction and dilation?
adrenoreceptors
Which 2 hormones are used to control the heart rate?
adrenaline and noradrenaline
Which 2 hormones control Bp by acting on the arterioles?
Vasopressin and angiotensin II
How are capillaries perfectly designed for exchange?
Which Law does this come under?
- minimise diffusion distance - very narrow walls (only 1 micrometer thick)
(no capillary is more than a few micrometers away from a tissue cell) - maximise SA - extensive branching
via Fick’s law
Why does the myocardium and brain need a high capillary density?
They are particularly vulnerable to hypoxia
What is the capillary density like in skeletal muscles?
What happens to blood flow at rest and during exercise?
has a high capillary density
- a large number of these capillaries are shut off at rest (by pre-capillary sphincters)
- during exercise and active hyperaemia, the sphincters relax, so a larger amount of the cardiac output travels to here
What are the 3 main types of capillaries?
Which is the most common?
- continuous - most common
- fenestrated
- discontinuous
Where are there fenestrated capillaries in the body?
in the kidney
Where are there discontinuous capillaries in the body?
close to the bone marrow, where white cells have to get into the blood
What does the term ‘bulk flow’ refer to?
a volume fo protein free plasma filters out of the capillary, mixes with the surround interstitial fluid and is reabsorbed
What is oncotic pressure?
an osmotic force due to protein in the capillary drawing water back in
NOTE: it doesn’t really change down a capillary since the protein conc doesnt really change
Describe some characteristics of the lymphatic system.
- NOT a closed loop
- Consists of blind-ended lymphatic capillaries (not once to induce flow-uses forces from the diaphragm as a pump)
- Valves - prevent backflow (once the fluid has drained into the lymph, it cannot drain back out)
- All but the right upper quadrant of the body drains via the thoracic duct into the left subclavian vein
- The right upper quadrant drains into the left subclavian vein
