regulation of arteriolar resistance Flashcards
what are extrinsic effects and what are they governed by?
are concerned with keeping total peripheral resistance of the body within the right range
-governed by neural and hormonal mechanisms
what are intrinsic effects?
concerned with satisfying the needs of the individual tissue
what does darcy’s law state?
flow = difference in pressure/ resistance
what does it follow from darcys’s law?
MAP= CO x TPR
in which organs is it first noticeable if MAP is too low?
brain and veins
what does poiseuille’s law state?
the resistance is directly proportional to the length of the tube and the viscosity of the fluid
what follows from poiseuille’s law?
since length of systemic circulation and viscosity of blood are relatively constant, it is the radius of the arterioles which is the primary determinant of resistance
what does varying radius of arterioles have an effect on in total?
- blood flow and redirection of flow
- TPR & hence MAP
what does extrinsic neural control involve ?
sympathetic - noradrenaline acts on A1 receptors and constricts smooth muscle –> increased TPR
parasympathetic - no effect
what does extrinsic hormonal control involve?
adrenaline - released from adrenal medulla, acts on a1 receptors, arteriolar constriction, increased TPR
BUT in some tissues aka cardiac & skeletal, binds to B2 receptors and causes vasodilation, increased flow, reduced TPR
angiotensin II- released in response to low blood volume, causes arteriolar constriction & increase in TPR
Vasopressin- released in response to low blood volume, causes arteriolar constriction, increase in TPR & increase in BP
Atrial & Brain natriuretic peptide- released in response to high blood volumes. Induces sodium release from kidneys by taking in more water. Causes vasodilation & reduce TPR
what are the 4 effects of local (intrinsic) control?
- active (metabolic) hyperaemia
- pressure auto regulation
- reactive hyperaemia
- injury response
what is the effect of active hyperaemia?
concentration of metabolites is constant while muscle is working at a constant rate.
When skeletal muscle starts working, metabolites start building up. endothelium detects this and releases hormone EDRF which causes vasodilation. Increased blood flow washes away the metabolites.
what is the effect of pressure auto regulation?
-decerase in MAP = decrease in flow
metabolites accumulate , release of EDRF, vasodilation, increased blood flow returns blood flow to normal
what is the effect of reactive hyperaemia?
decrease in tissue blood flow due to occlusion.
metabolic vasodilators accumulate in ECF
arterioles dilate but occlusion prevents blood flow
decreased resistance creates increased blood flow
occlusion removed
as vasodilators wash away, arterioles constrict and blood flow returns to normal
what is the effect of the injury response?
nociceptors in skin detect a stimulus, they are attached to C fibres. AP reaches C fibre terminal and triggers release of a peptide called substance P which acts on mast cells to release histamine, which makes smooth muscle relax and arterioles dilate, and increases permeability e.g. inflammation/infection
