The Cardiac Cycle Flashcards
What is preload?
Degree of stretch the ventricular wall undergoes when blood fills the ventricle
The greater the preload, the greater the resting length of the ventricualr sarcomeres
What is the Frank Starling Law?
Why does it occur
Frank Starling Law: Stroke volume increases in response to an increase in the volume of blood in the ventricles (i.e. increased preload)
WHY:
Increase in preload → increased stretch of sarcomeres → increased overlap of thin and thick filament → increases the amount of cross-bridges that can form → increasing contractility → increased SV
Remember: there is a limit to the benefits
Describe Frank-Starling’s Law in congestive heart failure
CHF patients have a right shifted and flattened curve
What is afterload?
the resistance that the ventricles must overcome to eject blood
What does afterload depend on?
- Arterial pressure
- Ease of blood flow through the pulmonary and aortic valves
Thus dueing high BP or semilunar valve stenosis there is an increased afterload
What is the End Systolic Volume (ESV)
Residual blood remaining in ventricle after it pumps
What is stroke volume?
how much blood was pumped out of the ventricle successfully during systole
What is HR
BPM
What are the 5 stages of the cardiac cycle?
- Late Diastole
- Atrial systole
- Isovolumetric contraction
- Ventricular Ejection
- Isovolumetric relaxation
- 1,2,5 are part of ventricular diastole
- 3,4 are part of ventricular systole
Describe the relative pressures (e.g. atria, ventricle, arteries) and movement of blood in Late diastole
- P(atria) > P(ventricle)
- Thus passive filling of ventricle
Describe the relative pressures (e.g. atria, ventricle, arteries) and movement of blood in atrial systole
- Atria contract
- Additional 15% of blood is pumped into the ventricle
Describe the relative pressures (e.g. atria, ventricle, arteries) and movement of blood in Isovolumetric contraction
- Ventricle contract
- Blood fills sinus of AV valves
- AV valves close
- P(ventricle) < P(arteries)
- Thus semilunar valves stay closed
- No movement of blood
Describe the relative pressures (e.g. atria, ventricle, arteries) and movement of blood in ventricular ejection
- P(ventricles) finally > P(arteries)
- blood ejected
Describe the relative pressures (e.g. atria, ventricle, arteries) and movement of blood in isovolumetric relaxation
- Ventricles relax but P(ventricles) > P(atria)
- Thus AV valves stay closed
- No movement of blood
Identify what step in the cardiac cycle occurs at the the various deflections on an ECG
Describe the events that occur at A
- Aortic valve opens
- Sudden spike in pressure as blood is initially pumped into aorta
Describe the events at B
- Dicrotic notch
- P(ventricle) < P(aorta)
- blood in aorta flows backwards towards heart
- This fills sinuses of aortic valves and closes them
- Pressure of blood pushing back against the valve causes the spike
Describe the events at C
- Ventricular systole
- P(ventricle) spikes
Describe the events at D
- P(ventricle) < P(atria)
- blood starts to flow passivle form atria to ventricle
- this slowly increases the pressure in the ventricles
What causes heart sound 1
Closing of the mitral and tricuspid valves (the atrioventricular valves)
At this point ventricular pressure is higher than atrial pressure
What causes heart sound 2
Closure of thepulmonic and aortic valves
At this point aortic/pulmonary artery pressure is higher than ventricular pressure
How is CO calculated?
CO = SV x HR
How is SV calculated?
EDV - ESV
How is the EDV increased (2)
- Increasing venous return
- Decreasing heart rate
- Increases filling time
How is ESV increased (2)
- Improving contractility
- Increased sympathetic input
- Increased circulating adrenaline
- Increase calcium within myocyte
- Ionotropic drugs
- Increase preload
- Decreasing the afterload
- Decreasing arterial blood pressure
- Repairing stenotic (narrowed) aortic or pulmonary valves (making it easier for the heart to pump blood)
How is HR modulated (2)
- Increased sympathetic tone
- Decreased parasymathetic tone
What reflex responds to a high BP?
Baroreceptor reflex
What happens when BP inceases?
- Stretching of baroreceptorsat theaortic arch and carotid sinus
- Opens the Na+ ion channels → increases the rate of AP’s it shoots off → does a number of things:
- Causes activation of the cardioinhibitory centre of the parasympathetic nervous system → CO decreased → BP decreased
- Inhibits the cardioaccelatory centre of the sympathetic nervous system → CO is decreased → BP decreased
- Inhibits the vasomotor centre of the sympathetic nervous system → causing vasodilation of the blood vessels → BP decreased
- Homeostasis restored
If blood pressure falls below normal, the opposite of the above happens (1. stimulation of sympathetic nervous system, 2. inhibition of the parasympathetic nervous system, and 3. vasoconstriction)
What reflex responds to a low BP?
Why does this work at low BP
Chemoreceptors
At a low BP (< 60mmHg) we are going to have an issues with increased levels of carbon dioxide in the systemic circulation as the blood is not going to flow fast enough to remove all the CO2 produced by the tissues
Where are the chemoreceptors located?
- Aortic bodies
- Carotid bodies
What do the chemoreceptors sense
decrease in O2, increase in CO2, decrease in pH
What is the chemoreceptors response when stimulated?
- Activates respiratory centre
- Inhibits the cardioinhibatory centre
- Activates the cardioaccelatory centre
- Activates the vasomotor centre
Increases O2, decreases CO2, increases pH
