cardiac function

Question 1

cardiac output

Answer 1

cardiac output (litres/min) = stroke volume (litres/beat) X heart rate (beat/min)

Question 2

factors affecting heart rate

Answer 2

intrinsic rate: pacemaker SAN, conduction AVN.
influenced by:
1) sympathetic nervous system- activation of b-adrenoceptors- causes increase in heart rate
2) parasympathetic nervous system- activation of muscarinic receptors causes a decrease in heart rate
3) hormones e.g. adrenaline acting on b-adrenoceptors causes increase in heart rate
4) extra/intracellular ions- alterations in membrane potential e.g. potassium

Question 3

factors affecting Stroke volume

Answer 3

SV- the vol of blood ejected by each ventricle in a single heartbeat.
SV= difference between end diastolic volume (EDV) and end systolic volume (ESV).
EDV is dependent on end-diastolic pressure; pre-load. influenced primarily by venous return. also filling time (increased HR reduces filling time- limit EDV), atrial contractility.
also depends on low large the aortic pressure it has to work against- large afterload- reduce SV

Question 4

frank-starling curve

Answer 4

the force of contraction of the cardiac muscle fibres is proportional to the degree of stretch- increased stretch (EDV)- increased force of contraction and therefore increased contractility

Question 5

cardiac contractility

Answer 5

the amount of force generated by cardiac muscle fiber can vary independently from the degree of stretch i.e. ventricular contractility can vary at any given end-diastolic volume depending upon other influences e.g. sympathetic nerve activation