Peripheral Resistance Flashcards
Most important factor in determining resistance
Arteriolar diameter
Factors affecting blood vessel radius
Active tension exerted by smooth muscle
Passive elastic properties of wall (elastin & collagen)
BP Inside vessel
Formula for Distending Pressure
Wall tension/Radius
Formula for wall tension
Pressure x Radius
Compare wall tension of capillaries and aorta and why
Tw is much higher in aorta than capillaries to avoid wall damage
As result of smaller radius (Tw= P x r)
What does an increase in vessel radius cause
An increase in wall tension (e.g. aneurysm)
What part of vessels controls radius
Smooth muscle is responsible for developing tension
Importance of active control of vessel calibre
This allows:
Redistribution of blood flow
Control of pre/post capillary sphincters
Regulation of vascular tone and control of blood pressure
What happens to active tension and passive tension during vasoconstriction
Increased active tension
Decreased passive tension
What happens to active tension and passive tension during vasodilatation
Decreased active tension
Increased passive tension
What does vasoconstriction do to blood vessels
SM contracts and lumen is smaller
Greater resistance
Effect of Sympathetic nervous system on blood vessel dilation
Neurotransmitters released via vesicles (mainly NA)
Alpha1 adrenoceptor activation by NA causes vasoconstriction (most important)
Same with a2
Beta2 on SM stimulation causes vasodilatation
KEY TAKEAWAY - GENERALISED INCREASE IN SYMPATHETIC ACTIVITY, THERE IS VASOCONSTRICTION (a1) IN MOST NERVES; INCREASE IN RESISTANCE AND BP
Hormones affecting vascular smooth muscle contraction
Catecholamines
- Noradrenaline
- Adrenaline
(constrict/dilate)
Peptides
- Vasopressin, Angiotensin (constrict)
- Bradykinin
Intrinsic Mechanisms causing vasorelaxation/vasodilation
PGI2 - Prostacyclin
NO - Nitric Oxide
EDHF - Endothelium-derived hyperpolarising factor (mostly pathological i think)
What changes in CO2 and O2 cause vasodilation and vasoconstriction in systemic vasculature
Low O2 and high CO2 as result of high metabolic activity