Control of arterial blood pressure 1 (CVS7) Flashcards
blood pressure
-blood pressure is the outwards/hydrostatic pressure exerted by the blood on the blood vessel walls
how is blood pressure often measured clinically
by measuring the systemic arterial blood pressure
what can systemic arterial blood pressure be expressed as
- systolic blood pressure
- diastolic blood pressure
systemic arterial blood pressure =
the pressure exerted by the blood on the walls of the aorta and systemic arteries when the heart contracts
systemic arterial blood pressure under resting conditions
should not normally reach or exceed 90mmHg under resting conditions
values on graph of arterial pressure against time when blood pressure
- systolic pressure (SBP) is the peak of the curve (120mmHg)
- diastolic pressure (DBP) is the dip in the curve (80mmHg)
- mean pressure= approx 93mmHg at rest (DBP + 1/3 pulse pressure)
- pulse pressure= 120-80= 40mmHg
pulse pressure
difference between systolic and diastolic blood pressure
mean arterial blood pressure (MAP)
average blood pressure during a single cardiac cycle, which involves contraction and relaxation of the heart
why is average arterial blood pressure (MAP) not obtained by averaging the systolic and diastolic pressures
because during a normal cardiac cycle, the relaxation (diastolic) portion of the cardiac cycle is about twice as long as the contraction (systolic) portion of the cardiac cycle
how is MAP estimated
-MAP= [(2xsystolic) + diastolic] divided by 3
or
-can be estimated by adding DBP + 1/3 pulse pressure (diff between SBP and DBP)
normal values of arterial blood pressure (SBP and DBP)
-
normal range of MAP
about 70-105mmHg
minimum value of MAP that is ESSENTIAL to perfuse coronary arteries, brain and kidneys
- at least 60mmHg
- if MAP drops below 60mmHg, organs wont be receiving enough oxygen and nutrients
blood supply to the heart
via coronary arteries
why does MAP need to be regulated within a narrow range
- to ensure pressure is high enough to perfuse internal organs including the brain, heart and kidneys
- to ensure pressure is not too high to damage the blood vessels or place an extra strain on the heart
relationship between MAP, CO and TPR
MAP= CO x TPR
= (SV x HR) x TPR
MAP
mean arterial blood pressure
TPR
- total peripheral resistance
- sum of resistance of all peripheral vasculature in the systemic circulation
cardiac output (CO)
volume of blood pumped by each ventricle of the heart per minute
CO=
SV (stroke volume) x HR (heart rate)
stroke volume (SV)
volume of blood pumped by each ventricle of the heart per heart beat
what are the major resistance vessels involved in systemic circulation of the heart
- arterioles
- mean arterial pressure drops significantly through arterioles
systemic circulation
1.the main part of the blood circulation, as distinct from the pulmonary circulation
how is TPR (total peripheral resistance) regulated/controlled
by vascular smooth muscles:
- contraction of vascular smooth muscles causes vasoconstriction and increases TPR and MAP (ie.pressure upstream) (increases blood pressure)
- relaxation of vascular smooth muscle causes vasodilatation and decreases TPR and MAP (decreases blood pressure)
main site of TPR (total peripheral resistance)
arterioles (make up 50% of TPR) as they are the major resistance vessel
components of total peripheral resistance (TPR)
- arteries make up 20%
- arterioles make up 50%
- capillaries make up 20%
- veins make up 10%
vascular smooth muscles (nerve supply and neurotransmitter)
- supplied by sympathetic nerve fibres
- neurotransmitter = noradrenaline which acts on alpha receptors to cause vasoconstriction
vasomotor tone
- vascular smooth muscles are partially constricted at rest, this is called the vasomotor tone (causing increase in TPR and MAP)
- vasomotor tone is caused by tonic(continuous) discharge of sympathetic nerves- continuous release of noradrenalin (which acts on alpha receptors to cause vasoconstriction)
- vasomotor tone takes a role in regulation of HR (as increased vasomotor tone causes increased vasoconstriction and increase HR)
effect of contraction of vascular smooth muscles on TPR and MAP
causes vasoconstriction which increases both values
effect of relaxation on TPR and MAP
causes vasodilation which decreases both values
how is the total peripheral resistance (TPR) modified
- by the autonomic nervous system (in particular sympathetic):
- increased sympathetic discharge/release of noradrenalin which acts on alpha receptors to cause vasoconstriction, will increase the vasomotor tone resulting in vasoconstriction (causing increased TPR and MAP- pressure upstream)
- decreased sympathetic discharge/noradrenalin release (which acts on alpha receptors to cause vasoconstriction) will decrease vasomotor tone resulting in vasodilation (which causes a decrease in TPR and MAP)
why do the parasympathetics not play a significant role in modifying TPR
- because there is no significant parasympathetic innervation of arterial smooth muscle- exceptions are penis and clitoris
- parasympathetics only effect HR
how is TPR increased by sympathetic discharge
increased sympathetic discharge/release of noradrenalin which acts on alpha receptors to cause vasoconstriction, will increase the vasomotor tone resulting in vasoconstriction (causing increased TPR and MAP- pressure upstream)
how is TPR decreased by sympathetic discharge
decreased sympathetic discharge/noradrenalin release (which acts on alpha receptors to cause vasoconstriction) will decrease vasomotor tone resulting in vasodilation (which causes a decrease in TPR and MAP)
baroreceptors reflex
- important in moment-moment regulation of arterial blood pressure including prevention of postural changes(as a result of body position)
- short term regulation of mean arterial blood pressure (pressure sensors/baroreceptors, either aortic or carotid, detect change in BP, signals are sent to the control centre in the brain aka medulla, and effectors are released involving the heart (HR and SV) and involving the blood vessels (TPR) to regulate the change)
- by increasing these things you increase the BP if it drops below a certain level (and vise versa)
baroreceptor
receptor sensitive to changes in pressure (pressure sensors)
location of baroreceptors
- two groups of baroreceptors:
- one in aortic arch (where blood is passed into aorta)
- one in carotid sinus (where blood is going into the brain)
how do signals from aortic baroreceptors reach the medulla
- medulla = control centre in brain
- via the vagus nerve (CNX)
how do signals from carotid baroreceptors reach the medulla
- medulla= control centre in brain
- via the herrings nerve which is a branch of the GLOSSOPHARYNGEAL nerve (CNIX)
baroreceptors reflex in response to decreased blood pressure
- > decreased ABP causes a decreased baroreceptor discharge (signals to the brain)
- > this is detected by the cardiovascular integrating centre/control centre/medulla
- > vagal activity is decreased to cause an increase in HR, causing an increase in CO which increases the arterial blood pressure
- > cardiac sympathetic activity is increased, causing an increase in SV which also contributes to the inceased CO and furthermore increased ABP (arterial blood pressure)
- > sympathetic constrictor tone/vasomotor tone is increased which causes both venoconstriction and vasoconstriction
- venoconstriction increases SV which increases CO, increasing ABP
- vasoconstrictionincreases TPR which increases ABP
vasoconstriction
narrowing of the lumen of blood vessels especially as a result of vasomotor action
vasoconstriction
constriction of a vein
baroreceptors reflex in response to increased blood pressure
- > increased ABP causes an increase in baroreceptor discharge (signals to the brain)
- > this is detected by the cardiovascular integrating centre/control centre/medulla
- > vagal activity is increased to cause a decrease in HR, causing a decrease in CO which decreases the arterial blood pressure
- > cardiac sympathetic activity is decreased, causing a decrease in SV which also contributes to the decreased CO and furthermore decreased ABP (arterial blood pressure)
- > sympathetic constrictor tone/vasomotor tone is decreased which causes both venodilation and vasodilation
- venodilation decreases SV which decreases CO, decreasing ABP
- vasodilation decreases TPR which decreases ABP
why will baroreceptor firing decrease if high BP is sustained
- because baroreceptors only respond to acute changes in blood pressure
- therefore the baroreceptors will not help to regulate this change in BP
- baroreceptors will only reset/fire again once if there is an acute change in MAP above the new higher steady state level
- therefore baroreceptors cannot supply information about steady state blood pressure at a particular time
how is MAP controlled in long term
largely controlled by the control of blood volume
