Normal CV physiology Flashcards
What surface of the heart do the coronary arteries run on?
Epicardial surface (outside of heart)
What imaging can show the coronary arteries?
coronary angiogram (or cardiac catheter)
When does coronary blood flow occur?
during diastole (heart muscle needs to be relaxed to allow blood to flow into the coronary arteries)
What type of arteries are coronary arteries?
functional end arteries (no anastomoses between them so they all perfuse their own areas without much overlap)
How is the myocardial capillary density compared to capillary density in skeletal muscle?
very high myocardial capillary density
What is the main way of increasing oxygen supply to the heart during exercise?
metabolic hyperaemia (increased blood supply to heart)
What is autoregulation and why is it important?
capacity of the heart to maintain steady myocardial perfusion across a range of perfusion pressures
blood flow to the heart is maintained even if the blood pressure is low
Define cardiac output (CO)
the volume of blood ejected by 1 ventricle in 1 minute
Define stroke volume (SV)
the volume of blood ejected from the ventricle in systole
Cardiac output equation and normal values at rest
cardiac output (5L/min) = heart rate (70bpm) x stroke volume (75ml)
How much can cardiac output increase on exercise?
4-5 fold
Where is 2/3 of the blood situated and why is this important?
large veins, small veins and venules (systemic veins + venules)
acts as reservoir - can ‘top up’ the heart and arteries after a haemorrhage
What 3 factors can affect stroke volume?
preload
contractility
afterload
Define preload
the stretch of the ventricular fibres just before contraction
[cannot be measured directly]
How can preload be measured?
cannot be measured directly
blood stretches the heart - it has a volume and puts pressure on the walls of the heart - therefore end diastolic volume or end diastolic pressure (RVEDP) can be used as a marker of preload
RVEDP = right ventricle end diastolic pressure
What is CVP?
central venous pressure
pressure in the vena cava at the entrance to the right atrium
What is the CVP a good estimate of in a healthy heart?
right atrial pressure
When is the right atrial pressure the same as the right ventricular pressure?
during diastole
How can the CVP be an estimate of the RV end diastolic pressure (ie the preload)?
CVP is a good estimate of right atrial pressure
right atrial pressure is the same as right ventricular pressure during diastole
therefore CVP can be used as an estimate for RVEDP and therefore the preload
CVP determines right ventricular preload
What is the Frank-Starling mechanism?
the greater the preload
the greater the force of contraction
the greater the stroke volume
[also called the law of the heart]
What factors influence CVP?
volume of blood in circulation
distribution of blood between central and peripheral veins
gravity
skeletal muscle pump
sympathetic nerves
respiratory pump
pumping ability of the heart
How do guards faint when stood to attention for a long time?
decreased cardiac output to brain = faint
reduced stroke volume due to standing (decreased CVP)
gravity
standing still (skeletal muscles inactive)
hot (vasodilation, decreased volume of blood in large veins = decreased CVP)
Define contractility
intrinsic ability of cardiac muscle tissue to contract
force of contraction caused by neurohumoral factors
a change in force of contraction that is independent of fibre length (not dependent on Starling’s Law)
What can cause reduced contractility
heart failure
(reduced stroke volume)
What effect does noradrenaline have on contractility?
increases contractility
(increases stroke volume)
Define afterload
the resistance the heart must overcome to eject its contents
[ie the left ventricle has to generate the same pressure as the aorta before the aortic valve can open)
[the more pressure the LV has to generate to reach the aortic pressure, the less energy is available for contraction therefore the stroke volume falls]
How does high aortic arterial pressure affect the output?
‘pump function curve’
high arterial pressure impairs output
the more pressure the LV has to generate to reach the aortic pressure, the less energy is available for contraction therefore the stroke volume falls
How does an increase in sympathetic activity affect heart rate?
tachycardia
How does an increase in parasympathetic activity affect heart rate?
bradycardia
How does heart rate increase with activity?
decrease in parasympathetic activity
increase in sympathetic activity
How does stroke volume increase in activity?
increased preload:
- skeletal muscle pump
- peripheral venoconstriction
increased contractility:
- increased sympathetic activity
How is maximum heart rate calculated?
220 - age in years
Describe metabolic hyperaemia in active muscles during exercise
vasodilation in active muscles (metabolic hyperaemia)
active muscles release metabolites that cause dilatation of the local resistance vessels in the muscle thereby increasing the blood flow
How can hypotension occur during exercise?
vasodilation of active muscles causing a reduction in systemic vascular resistance
BP roughly equal to COxSVR (systemic vascular resistance)
compensatory vasoconstriction in non-active tissues reduces the fall in SVR
Systemic vascular resistance definition
resistance to blood flow offered by all of the systemic vasculature (however the main resistance vessels are the terminal arteries and arterioles)
What is most of the increase in cardiac output in swimming due to?
increase in heart rate
(less dependent on stroke volume in horizontal exercise)
How does static (isometric) exercise and resistance exercise increase BP?
eg. sustained handgrip or weightlifting
total peripheral resistance (ame as SVR) increases (partly due to compression of intramuscular blood vessels)
What would happen to the stroke volume and heart rate in an athletic heart?
increased stroke volume
decreased resting HR
