Cardiac Function and Control of Blood Pressure Flashcards
Skeletal muscle
Muscle cells made up of sarcomeres that contain thick filaments (composed of myosin) and thin filaments (composed of actin)
Shortening if sarcomere occurs via sliding filament mechanism where actin filaments slide along adjacent myosin filaments
Cardiac muscle
Similar to skeletal muscle in terms of composition and function
Different in terms of how cells interact
Cardiac muscle acts like a syncytium - a single, multi-nucleated cell formed from many fused cells
A wave of depolarisation is followed by atrial and ventricular contraction
Cardiac excitation
The heart can generate its own rhythm
1% cardiac cells with pacemaker activity
99% cardiac cells contractile function
SA node
Approx 8mm long and 2mm thick
Specialised region in the right atrial wall at the junction between the superior vena cava and the right atrium
Cells within SA node normally fire very fast to generate a heart beat
Do not have a resting potential
Cardiac excitation
Needs to be efficient and co-ordinated
Action potentials generated at SA node
Rapid excitation through both atria
Excitation reaches AV node where conduction is SLOW
Allows atria to contract and empty blood into ventricles
Excitation spreads rapidly down the bundle of His and Purkinje fibres to ensure almost simultaneous activation of ventricular cells leads or efficient co-ordinated pump
Atrial systole
AV valves open and atria empty blood into ventricles
Atrial excitation and contraction should be complete before onset of ventricular contraction
Ventricular systole
Ventricles contract so rise in pressure closes AV valve
Pressure in ventricle rises above aortic pressure so aortic valve opens and blood is ejected from the heart
Ventricular diastole
Pressure in ventricles falls below aortic pressure so aortic valve closes
Pressure in ventricles falls below atrial pressure so AV valve opens and filling begins
Stroke volume is influenced by what factors?
End diastolic volume (preload)
Aortic pressure (after load)
Contractility
End diastolic volume
Resting cardiac muscle stretched prior to contraction
Venous tone, blood volume, posture and intrathoracic pressure
Aortic pressure
Pressure against which the heart must work to eject blood during systole
Aortic stiffness, peripheral resistance, blood volume
Contractility
Strength and vigour of theheart’s contraction(myocardial fibres) during systole, at a given preload and afterload
Increased by sympathetic nerve stimulation and circulating catecholamines, calcium, positive inotropic drugs (Sympathetic to noradrenaline to β1-adrenoceptors)
Decreased by hypoxia, acidosis, infarction
Factors affecting heart rate
Sympathetic nervous system
Parasympathetic nervous system
Systolic blood pressure
Force the heart exerts on the walls of the arteries every time it beats
Determined by:
SV and aortic elasticity
Diastolic blood pressure
Pressure within the arteries during the period when the heart is not beating, the heart fills with blood
Determined by:
Peripheral resistance
Aortic elasticity
Heart Rate
