Control of cardiac output Flashcards
What is cardiac output?
Volume of blood pumped by each ventricle per minute
Stroke volume x Heart rate (measured in L/ min)
What is preload?
Initial stretching of the cardiac myocytes prior to contraction related to the filling of the ventricles before contraction.
[direct measure = sarcomere length but normally measured using end diastolic volume - the higher the EDV, the higher the preload]
What is afterload?
Force/ load against which the heart has to contract to eject blood.
[Measured in terms of arterial pressure]
What is stroke volume?
Volume of blood ejected from each ventricle during each ventricular contraction
Stroke Volume = End Diastolic Volume - End Systolic Volume
What is total peripheral resistance?
Resistance to flow provided by systemic circulation (e.g. due to diameter of arterioles/ viscosity)
What nerve provides parasympathetic input to the heart?
Vagus (CN X)
What is Starling’s Law?
Stroke volume of the heart increases in response to an increase in the volume of blood in the ventricles before contraction (when all other factors remain constant)
What increases as a result of increased cardiac preload?
Atrial contractility Aortic pressure Thoracic venous blood volume Central venous pressure Ventricular compliance
What is End Systolic Volume (ESV)?
The volume of blood in a ventricle at the end of a contraction (systole) and at the beginning of filling (diastole).
Lowest volume of blood in the ventricle at any point during the cardiac cycle.
What is End Diastolic Volume (EDV)?
The volume of blood in a ventricle at the end of filling (diastole).
What factors affect heart rate?
Hormones
Atrial/ Bainbridge reflex (caused by increased venous pressure) - reflex alters ANS innervation
What factors affect the end diastolic volume (EDV)?
Preload (influenced by venous return and filling time)
What factors affect end systolic volume (ESV)?
Contractility (influenced by ANS innervation and hormones)
Afterload (influenced by vasodilation or vasoconstriction)
What is the baroreceptor reflex?
Response to increased blood pressure
Increased firing of vagal nerve and glossopharyngeal nerve
Decreased firing of sympathetic nerves
What nerves provide sympathetic input to the heart?
Sympathetic cardiac nerves
What is the chemoreceptor reflex?
Response to low pO2 / high pCO2
Increased firing of sympathetic nerves
Decreased firing of vagal nerve
How do inotropic agents affect contractility?
Positive inotropic agents increase contractility.
Negative inotropic agents decrease contractility.
What decreases as a result of increased cardiac preload?
Venous compliance
Heart rate
What factors affect stroke volume?
Preload (determined by venous return and filling time)
Contractility (regulated by hormones and ANS)
Afterload (determined by vasoconstriction/ dilation)
How does an increased heart rate impact cardiac output?
An increased HR reduces cardiac output as it reduces filling time during ventricular diastole
[Exacerbated by mitral stenosis or atrial fibrillation due to loss of atrial contractility]
How does the ANS impact heart rate?
Sympathetic stimulation increases heart rate by reducing repolarisation time and increasing speed of depolarisation and also increases contractility
Parasympathetic stimulation decreases heart rate by slowing depolarisation. APs also hyperpolarise
How does the baroreceptor reflex respond to increased BP?
Baroreceptors in the carotid sinus and aortic arch detect raise in BP and increase firing of glossopharyngeal and vagal nerves to the cardiac and vasomotor centres in the medulla. This increases firing of efferent vagal fibres (to decrease HR) and decreases firing of sympathetic nerves to the heart (negative inotropy and chronotropy) and blood vessels (vasodilation).
[+ causes release of noradrenaline and inhibition of RAAS]
What is the Bainbridge (atrial) reflex?
Increased venous pressure detected by stretch receptors at the venoatrial junctions of both atria which increases firing of vagal afferents to the cardiac and vasomotor centres in the medulla. This decreases parasympathetic efferent firing and increases sympathetic efferent firing thus increasing heart rate.
[Prevents pooling of blood in venous system]
What is the chemoreceptor reflex?
Chemoreceptors in the carotid sinus and aortic arch detect changes in arterial gas concentrations. Increased pCO2 +/- decreased pO2 causes increased firing of afferent glossopharyngeal and vagal fibres to chemoreceptors in the medulla. This then causes cardiac and vasomotor centres to decrease efferent vagal stimulation and increases sympathetic stimulation to the heart and blood vessels
What is contractility?
Force of contraction of the myocardium
[directly controls stroke volume - the greater the contractility, the greater the stroke volume]
How do inotropic agents alter contractility?
Positive inotropic agents increase contractility
Negative inotropic agents decrease contractility
