The Cardiac Cycle Flashcards
What is the cardiovascular system formed of?
The heart, blood vessels and blood
Why is the cardiovascular system described as a closed loop?
Blood is pumped out of the heart by arteries and returned to the heart by veins
What are the functions of the cardiovascular system?
- Transport of oxygen and nutrients to meet the metabolic demands of the body
- Transport of metabolic waste products (e.g. CO2) for excretion
- Transport of hormones
- Maintains constant body temperature and transfers heat
- Aids response to infection and injury
- Assists regulation of fluid and pH in the body
What are the individual cells of the heart called?
Cardiomyoytes
What type of muscle tissue is the myocardium?
Specialised striated muscle tissue
What does the structure of the myocardium allow?
Co-ordinated pumping actions so that the metabolic demands of the tissue organs can be met
How are cardiomyocytes connected to one another?
Gap junctions, intercalated discs and desmosomes
Which ventricle has a thicker wall?
Left ventricle - responsible for pumping blood through systemic circulation. Needs to overcome aortic resistance so blood can enter systemic circulation
Which ventricle has a larger stroke volume?
Generally both ventricles pump the same volume of blood - same stroke volume
What are the 2 AV valves called?
Tricuspid valve (right AV valve) Bicuspid/mitral valve (left AV valve)
What are the 2 semilunar valves?
Pulmonary valve
Aortic valve
What does the pulmonary valve permit?
Permits blood to flow from right ventricle and pulmonary artery
What does the aortic valve permit?
Permits blood flow from left ventricle to the aorta
What causes the heart valves to open?
Pressure differences across the valves (opening and closing of heart valves is a passive process)
What happens to the AV valves when pressure in the atrium is greater than pressure in the corresponding ventricle?
Valve is forced open and blood flows from atrium into ventricle
What happens when the contracting ventricle achieves an internal pressure that is greater than the corresponding atrium?
The AV valve is forced closed
What happens during systole (ventricular contraction)?
Blood flows from ventricles into aorta (left) and pulmonary artery (right)
What do the semilunar valves prevent?
They prevent the back flow of blood from the atria into the ventricles during ventricular relaxation
What are the AV valves fastened to?
Papillary muscles
What are papillary muscles?
Muscular projections of the ventricular wall
What connects papillary muscles to valve cusps?
Chordae tendinae
What is the function of papillary muscles?
Prevent the back flow of blood by limiting the valve’s movements
What do the “lub” and “dub” sounds correspond to?
"lub" = closure of the AV valves "dub" = closure of the semilunar valves
When may an additional heart sound (other than the “lub” “dub” sounds) be heard?
Due to oscillation of blood flow into ventricle or various disease states e.g. heart valve defect
What is valve regurgitation?
Blood leaks back into chambers, occurs by a valve not closing tightly
What is valve stenosis?
Thickening/stiffening of valve cusps
Prevents heart valve from opening fully; not enough blood can flow through
What is the end diastolic volume?
The volume of blood in the ventricle prior to contraction
What is the end systolic volume?
The volume of blood in the ventricle after each ejection
Ventricles not completely emptied during systole
How do you calculate the stroke volume?
End diastolic volume - end systolic volume = stroke volume
EDV - ESV = Stroke volume
What is the stroke volume (SV)?
Volume of blood ejected per heart beat
What is heart rate (HR) determined by?
HR is determined by the rate which the cardiac pacemaker (sino-atrial node) fires action potentials
Which range do “normal” resting heart rate values lie within?
60-100 bpm
What determines the rate of action potential firing by the sino-atrial node?
Activity of the autonomic nervous system
What do sympathetic nerves release (principally)?
Noradrenaline
Which receptor does noradrenaline bind to on SA node?
Beta-1-adrenoceptors
What happens when noradrenaline binds to beta-1-adrenoceptors on the SA node?
Activation of a G-protein and production of cyclic AMP
Increases heart rate - positive chronotrophy
What do parasympathetic nerves release (principally)?
Acetylcholine (ACh)
Which receptor does Ach bind to on the SA node?
M2 muscarinic receptors on SA node
What happens when acetylcholine binds to M2 muscarinic receptors on the SA node?
Activates inhibitory G protein, blocking cAMP pathway and allows K+ efflux from cell
Decreases heart rate - negative chronotrophy
Which type of adrenoceptor does the heart primarily contain on nodal tissues, the cardiac conduction system and the myocardium?
Beta-1
Which hormones bind to the beta-1 adrenoceptor?
Noradrenaline
Adrenaline
Activation of the beta-1 adrenoceptor results in…
- Positive inotrophy (increases strength of myocardial contraction)
- Positive chromotrophy (increases heart rate)
- Positive lusitrophy (increases rate of myocardial relaxation)
- Positive dromotrophy (increases conduction speed in AV node)
What is positive inotrophy?
Increased strength of myocardial contraction
What is positive chronotrophy?
Increased heart rate
What is positive lusitrophy?
Increased rate of myocardial relaxation
What is positive dromotrophy?
Increased conduction speed in AV node
Describe the spread of depolarisation throughout the heart
- Depolarisation spreads from the SA node through myocytes of right atrium and left atrium
- Spread of depolarisation through right atrium to AV node
- AP propagates from AV node along interventricular septum via the Bundle of His
- Bundle of His separates into left and right bundle branches which innervate the walls of the myocardium
- Rapid propagation of AP along purkinje fibres initiates coordinated contraction of the cardiac myocytes of the left and right ventricles
Why is propagation of the AP through the AV node relatively slow compared to propagation from the SA node?
To allow for completion of atrial contraction prior to initiation of ventricular excitation and contraction
What is the SA node?
The autonomic pacemaker of the heart
How is the SA node action potential initiated?
Due to influx and efflux of ions across the membrane
Describe the movement of ions during the SA action potential
- Na+ slowly enters cell through leaky Na+ channels
- AP is fired once threshold is reached
- Voltage-gated Ca2+ channels open - Ca2+ enters cell
- Voltage-gated K+ channels open - K+ leave the cells. Ca2+ channels close. Depolarisation occurs.
- Voltage-gated K+ channels close
How is ventricular systole initiated by the movement of ions into myocytes?
Opening of Na+ channels -> influx of Na+ causes rapid depolarisation of cardiac muscle cells and this initiates systole
What does excitation-contraction coupling refer to?
Couples the electric excitation brought by the action potential to the mechanical contraction of the cardiac myocytes
Describe the cardiomyocyte contractile cycle
- Calcium binds to troponin C, leading to a conformational change that displaces tropomyosin from the actin binding sites
- Cross-bridge formation occurs with ATP hydrolyzation into ADP and Pi.
- Power stroke moves actin filament toward the centre of the sarcomere. ADP and Pi are released from the myosin heads
- Actin released with ATP binding to myosin. Myosin heads cocked back into firing position, ready to make cross bridges further downstream.
What is an ECG?
Electrocardiogram
- Measures electrical activity of the heart
- Detects phasic changes in potential difference between 2 electrodes
What is an ECG useful for?
Checks how heart is fucntioning
Useful in diagnosis of arrhythmias, post myocardial infarction damage etc.
Describe the PQRST wave
P wave = atrial depolarisation QRS complex = ventricular depolarisation T wave = ventricular repolarisation P-R interval = delay through AV node S-T interval = plateau phase of ventricular action potential
What is long QT syndrome?
Increased time between Q and T waves
Heart muscle takes longer to recharge between beats
What is bradycardia?
Slowing heart rate
What is tachycardia?
Quickening of heart rate
What is atrial fibrillation?
Atrium flutter - they don’t contract properly
One of the most common types of cardiac arrhythmia
What is cardiac output (CO)?
Volume of blood pumped by the heart per minute
How do you calculate CO?
CO = SV x HR
What does Starling’s Law state?
The force of muscle contraction increases as the muscle is stretched in response to increased filling of the heart’s chambers.
-> The magnitude of the stretch predicts the strength of the contraction
What is Cardiac Preload?
Initial stretching of cardiac myocytes prior to contraction
Indicated by end-diastolic volume (pressure)
What is Cardiac Afterload?
The pressure the heart must eject blood against
Left ventricle after load is related to aortic pressure
