Control of cardiac output Flashcards
What is the definition of cardiac output?
The volume of blood pumped by each ventricle per minute ( NOT the total amount of blood pumped by the heart)
What is the CO the product of in equation form?
CO= HR x SV
cardiac output = heart rate x stroke volume
What is stroke volume?
And what is the equation?
The volume of blood (ml) ejected per contraction
SV= End diastolic volume - End systolic volume
The strength of cardiac muscle contraction and accordingly SV can be graded by what?
1) varying the initial length of the cardiac mucle fibres, which in turn depend upon EDV (intrinsic control)
2) Varying the extent of sympathetic stimulation (extrinsic control)
The relationship between increased EDV resulting in increased SV - as more blood returns to the heart, the heart pumps out more, is not a simple relationship, why?
The heart does not eject all the blood it contains
What does intrinsic control depend on?
The length-tension relationship of cardiac muscle (which is similar to that of skeletal muscle)
What causes cardiac muscle fibres to vary in length before contraction?
Main dterminant is the degree of diastolic filling (preload)
What is the intrinsic relationship between EDV and SV known as?
Frank-starling law of the heart
What is the Frank-starling law of the heart stated simply?
the heart normally pumps out during systole the volume of blood returned to it during diastole; increased venous return results in increased stroke volume
What are the two advantages of the cardiac length-tension relationship?
1) Equalising output between the left and right sides of the heart
2) When a larger CO is required e.g during exercise, venous return is increased through action of the sympathetic NS. The resulting increase in EDV automatically increases SV
What are the two cellular basis of the Frank-starling mechanism?
1) Greater initial length increases the sensitivity of contractile proteins in the myofibrils to Ca2+
2) Increased initial fibre length may also increase Ca2+ release from the sarcoplasmic reticulum
Why is the aterial blood pressure called the after load?
the workload imposed on the heart after the contraction has begun
The heart may be able to compensate for a sustained increase in afterload by enlarging (hypertrophy). in a diseased or weakened heart, what can this lead to?
Heart failure
The strength of cardiac muscle contraction and, accordingly SV can be graded by what?
1) Varying the initial length of the cardiac muscle fibres, which in turn depends upon EDV (intrinsic control)
2) Varying the extent of sympathetic stimulation (extrinsic control)
