heart as a pump Flashcards
why does systemic circulation require high pressure?
efficient distribution to the whole body
why are lungs perfused at a lower pressure?
pulmonary vascular resistance is lower than systemic vascular resistance
describe the structures of the systemic arteries and pulmonary arteries
systemic - moderate size and thick muscular walls
pulmonary - large diameter thin elastic walls
what does starling’s law state?
increased preload increases end diastolic volume
explain starling’s law
more blood to the ventricles –> expand to a greater diameter –> contract more strongly –> higher EDV –> higher SV –> higher CO
what is preload and what determines it?
preload is the degree of stretching of the ventricles during diastole
determined by the volume of blood brought to the heart by the veins
what is the stroke volume of a normal healthy adult male?
70ml
what is residual volume?
the blood that is always left in the ventricles at the end of systole
how is the stroke volume calculated?
finding the difference between end diastolic volume and end systolic volume
how does preload affect the myocardial muscle fibre length?
increases the fibre length
what limits starling’s law?
if ventricles stretch beyond aa certain point then the mechanism doesn’t work
how does heart failure occur?
ventricles stretch too far
ventricles enlarge with no increase in wall thickness
contracts more weakly
larger EDV produces smaller stroke volume
explain the mechanism underlying starling’s law
actin and myosin filaments have excess overlap at rest
stretching increases the overlap and causes an increased force of contraction
define afterload
the effective flow impedance (resistance) of the aorta and the large arteries
what determines afterload
elasticity of the tissues and diameter of blood vessels
what is compliance?
reciprocal of impedance
how does a higher compliance affect after load and the work o the heart?
high compliance = lower after load = less work for the heart
explain how after load increases in old people
elasticity of arteries is lost
elastic tissue replaced by collagen
what effect does smoking have on afterload?
increases afterload
explain how a greater afterload leads to a larger residual volume
greater afterload
longer contraction of ventricles before aortic valve opens
smaller stroke volume
larger residual volume
what is inotropy?
force of contraction of ventricular muscle
how does an increase in inotropy affect stroke volume?
increases stroke volume
name factors which increase contractility
increased blood calcium levels
beta adrenergic agonists (like adrenaline)
drugs which stimulate calcium entry into myocardium
cardiac glycosides
insulin
glucagon
what do chordae tendineae do?
keep heart valves in position when closed
