week 5: cardiac physiology: cardiac output, blood pressure and flow Flashcards
cardiac output
volume of blood pumped out by each ventricle per unit time
most common unit: litres per min
cardiac output (CO)=
heart rate (HR) x stroke volume (SV)
stroke volume depends on
body size
cardiac index
normalises SV to body surface area
normal resting cardiac index
3.2 Lmin-1 m-2
SA node innervated by both
sympathetic and parasympathetic branches of the autonomic nervous system
noradrenaline
transmitter released by postganglionic S fibres
what transmitters do P fibers release
acetylcholine (ACh)
noradrenaline and acetylcholine action
act to change and regulate heart rate
sympathetic effects are mediated via
B1 adrenoceptors
Parasympathetic effects are mediated via
muscarine receptors
increase in sympathetic nerve firing
increase in heart rate
tachycardia
increase in parasympathetic nerve firing
decrease in heart rate
bradycardia
dominant tone in resting state
parasympathetic
intrinsic firing rate on SA node and therefore intrinsic frequency of a de-innervated heart
100 bpm
what is heart rate determined by
pacemaker potential of the SA node cells
stroke volume
vol of blood ejected out each ventricle per heartbeat
vol of blood in ventricle at end of diastole - volume og blood remaining at end of systole
end-diastolic vol - end systolic vol
ejection fraction
fraction of the EDV ejected during the subsequent ventricular contraction
EF= SV/ EDV
(end-diastolic vol)
referred to as the pre-load
end-systolic volume determined by
contractility of the ventricular muscle and the diastolic aortic blood pressure
diastolic aortic blood pressure termed
afterload- resistance to blood ejection
at the cellular level, strength of contraction depends on
initial sarcomere length and overlap of actin and myosin filaments
The Frank Sterling law
strength of contraction depends on the initial degree of stretch: within the physiological range, stretching ventricular muscle leads to an increased force in contraction
what does initial stretch depend on
end-diastolic volume (pre-load)
determinants of end-diastolic volume
venous return
what does venous return depend on
pressure in the large veins returning blood to the heart, the central venous pressure (CVP)
central venous pressure can be influenced by
- blood volume increased blood volume= increased CVP)
- postural changes
- respiratory and skeletal muscle pumps ( aid venous return and increase CVP)
- vasoconstriction ( via increased sympathetic activity)
extrinsic regulation of stroke volume comes from
autonomic nervous system
what control is autonomic regulation of SV under
sympathetic:
little parasympathetic innervation of the ventricular myocardium
during action potential, B1 adrenoreceptors
increase level of cytosolic Ca2+ in ventricular myocytes
(EC coupling)
increasing levels of Ca2+ in ventricular myocytes
increases contractility of myocardium at any given length
(starling curve shifted up)
resulting in greater stroke volume from any given end diastolic volume
blood flows down
pressure gradient
pressure results
when flow is opposed by resistance
steepest drop in pressure
occurs across arterioles, where the greatest resistance to blood flow occurs
dicrotic notch (after phase 1)
closure of the aortic semilunar valve
phase 1
diastole
no blood enters aorta from left ventricle
aortic vol and pressure decline to minimum
typical systolic blood pressure (left)
120mmHg
highest arterial pressure corresponds to the systolic phase of the cardiac cycle
typical diastolic pressure (left)
80 mmHg
lowest arterial pressure, corresponds to the diastolic phase of the cardiac cycle
pulse pressure
40 mmHg
systolic pressure- diastolic pressure
mean arterial pressure
diastolic + (pulse pressure/3)
typical 93mmHg
Darcys law
in the steady state, fluid flow between 2 points is equal to the difference in pressure between the 2 points divided by resistance to flow
flow=(p1-p2)/R
analogous to OHms law for electrical current I=V/R
Relating systemic circulation to Darcys law
flow= left ventricular output (ie CO)
P1-P2= aortic pressure- right atrial pressure (effectively the MAP)
R= The total resistance to flow imposed by all the blood vessels in the systemic circulation ( the total peripheral resistance, TPR, or systemic vascular resistance, SVR)
hence CO=MAP/TPR can be rearranged
what determines total peripheral resistance
resistance to a steady flow along a straight cylindrical tube is proportional to tube length and fluid viscosity and inversely proportional to tube radius ^4
the smaller the radius, the greater the resistance
poiseuille’s Law
combing P definition of resistance with Darcy’s law of flow
derive expression for flow throughout blood vessel
flow=(P1-P2) X pi xr^4/ 8n xL
arteriolar walls contain
high proportion of circularly arranged smooth muscle
arteriolar radius under the influence of
sympathetic nervous system
metabolic and myogenic autoregulatory influences
what does metabolic autoregulation during exercise lead to
vasodilation in vascular beds of active skeletal muscle and heart
leads to large increase in blood flow to skeletal muscle
what is vasoconstriction mediated by
sympathetic nervous system
what does vasoconstriction lead to
decrease in blood flow to the splanchnic and renal vasular beds
active hyperaemia
diversion of blood to active muscles
baroreceptors
receptors that are sensitive to changes in pressure
what do arterial baroreceptors detect
degree of stretch in blood vessel wall
mechano-receptor
arterial baroreceptors are
sprays of non-encapsulated nerve endings in the adventitial layer of the arterial walls in the carotid sinus and the aortic arch
baroreceptor afferents nerve fibers project to the
medulla oblongata
main CV control centre
increase in baroreceptor discharge firing rate leads to
increase in parasympathetic signalling
decrease in sympathetic stimulation
of heart
decreased sympathetic stimulation of systemic arterioles and veins
