11. Regulation of Arterial Resistance Flashcards
What is the equation for Darcy’s law?
flow= (difference in pressure)/ resistance
What is the equation for Poisieuille’s law?
resistance= nL8/ r^4xpie
Poisesieuille said that varying radius is a powerful way of controlling resistance
What does varying the radius of resistance vessel control?(2)
- flow of blood
- redirect blood
(as well as TPR and regulates MAP)
What is the equation for pressure?
pressure=flow x resistance (therefore radius of arterioles (TPR) affects pressure (MAP)
What is the equation for MAP (mean arterial pressure)?
MAP= CO x TPR
What does varying the radius of arterioles allow?
allows us to control flow through individual vascular beds
What effect does reducing the resistance of a vascular bed have on flow?
Increases flow through the vascular bed (flow= difference in pressure- resistance)
What effect does reducing TPR have on mean arterial pressure? (MAP)
reduces MAP (difference in pressure= flow x resistance)
What 2 things does arterial radius affect?
- flow through individual vascular beds
2. MAP
What is found around arterioles that controls its radius?
smooth muscle
What are 2 levels of control over the smooth muscle surrounding the arterioles?
- intrinsic mechanism
2. extrinsic mechanism
What is intrinsic mechanism?
concerned with meeting the selfish needs of each individual tissue
What is extrinsic mechanism?
concerned with ensuring that the total peripheral resistance (TPR) of the whole body stays in the right ball park
What are 2 ways in which extrinsic control is regulated?
- hormonal
2. neural
What neves control the extrinsic control? What do they release?
sympathetic nerves; release norepinephrine
What receptors does norepinethrine act on? What action does it induce?`
alpha 1 receptors; cause arteriolar constriction
What effect does arteriole constriction have on flow and TPR?
- decreases flow through that tissue
- tends to increase TPR
What effect on the extrinsic control of arterioles does the parasympathetic system have?
usually no effect
What main hormone is released for extrinsic control?
epinephrine
What part of the brain releases epinephrine?
adrenal medulla
What receptors does epinephrine act on and what does it induce?
acts on alpha 1 receptors and induces arteriolar constriction
What effect does arteriolar constriction have on:
- flow
- TPR
- decreases flow
2. increases TPR
In which tissues does epinephrine activate B2 receptors also? (2)
- skeletal
2. cardiac
What do Beta 2 receptors in cardiac and skeletal tissues cause?
arteriolar dilation
What effect does arteriolar dilation have on:
- flow
- TPR
- increase in flow
2. decrease in TPR
What 2 hormones is produced which causes arteriolar constriction?
- Angiotensin II
- Vasopressin ( antidiuretic hormone)
What are angiotensin II and vasopressin produced in response to?
produced in response to LOW blood volume
What effect on TPR do angiotensin and vasopressin have?
increases TPR (because it causes vasoconstriction)
What is another name for vasopressin?
antidiuretic hormone
What hormones are produced that cause arteriolar dilation?
- atrial natriuretic peptide
2. brain natriuretic peptide
What are atrial natriuretic peptide and brain natriuretic peptides released as a response to?
as a response to HIGH blood volume
What effect on TPR do atrial natriuretic peptide and brain natriuretic peptide have?
decrease TPR (because causes vaodilation)
In summary, what 5 hormones are produced for extrinsic arteriolar control? What effect do they have?
- epinephrine (constricts)
- angiotensin II (constricts)
- vasopressin (constricts)
- atrial natriuretic peptide (dilates)
- brain natriuretic peptide (dilates)
What are 4 local intrinsic controls of arterioles?
- active (metabolic) hyperaemia
- pressure (flow) autoregulation
- reactive hyperaemia
- the injury response
What occurs during active metabolic hyperaemia?
- increase in metabolic activity
- increase in conc. of metabolites
What hormone is released during active (metabolic) hyperaemia?
EDRF: endothelium-derived relaxing factor from the paracrines
What effect does EDRF have on arterioles?
causes arteriolar dilation
What does dilation allow arterioles to do in active metabolic hyperaemia?
-increase in flow to wash out metabolites
(an adaptation to match blood supply to the metabolic needs of that tissue
What effect does decrease in MAP have on flow? What does this cause in terms of metabolites?
decrease in flow; causes metabolites to build up
What hormone is release when metabolites build up during pressure/ flow autoregulation?
triggers release of EDRF; endothelium derived relaxing factor
What effect does EDRF have in pressure (flow) autoregulation?
causes arterioles to dilate and flow is restored to normal (or could be myogenic; originating in the smooth muscle)
Why is pressure (flow) autoregulation necessary?
it’s an adaptation to ensure that a tissue maintains its blood supply despite changes in MAP
What is reactive hyperaemia?
- extreme version of pressure autoregulation
- occlusion of blood supply causes a subsequent increase in blood flow
What is the injury response? What does it cause?
- aids delivery of blood born leucocytes/ WBCs to injured area
- mast cells release cytokines and histamine to stimulate WBCs
- causes arteriolar dilatation (increased blood flow and increased permeability)
What are 4 special areas (circulations) that have specific conditions when it comes to arteriolar resistance regulation?
- coronary circulation
- cerebral circulation
- pulmonary circulation
- renal circulation
Why is coronary circulation regarded as “special”? What control mechanism does it use?
- its blood supply is interrupted by systole but still has to cope with increased demand during exercise
- shows excellent HYPERAEMIA
How does excellent hyperaemia happen in coronary circulation which allows it to function efficiently? (what receptors allows this?)
- expresses many beta 2 receptors
- they swamp/overrun any sympathetic arteriolar constriction
What special control mechanism is used in cerebral circulation? What does it ensure?
- shows excellent pressure autoregulation
- keeps circulation stable
What mechanism occurs in pulmonary circulation when there are hypoxic conditions? (decrease in O2)
- causes arteriolar constriction
- opposite response to most tissues
- ensures blood is redirected to the best ventilated parts of the lungs
What mechanism occurs in renal circulation when changes in MAP occur? Why is it essential?
- shows excellent autoregulation
- changes in MAP have BIG effects on blood volume
- main function is filtration so depends highly on MAP
What are 2 main components of extrinsic effects?
- neural
2. hormonal
What do extrinsic effects regulate? (2)
- regulating TPR of whole body
2. trying to maintain adequate MAP
What do local effects regulate?
trying to match blood flow to the selfish needs of that specific tissue
