The cardiac cycle Flashcards
What is the function of the atria as a pump?
Acts as a priming pump to ensure maximum ventricular volume, increases efficiency of heartbeat
Why does the left ventricle have a thicker muscular wall?
Systemic circulation has 6x higher resistance, so to keep pulmonary and systemic flow the same, must generate 6x higher pressure.
Where does the heart tap on the chest wall during systole?
5th intercostal space
Why does the heart undergo a twisting motion when it contracts?
Alternating orientation of the subendocardial and subepicardial myocytes
What is the role of papillary muscles?
Attached to cusps of AV valves via chordae tendinae and contract to prevent inversion of these valves following ventricular systole
What causes valves to open/close?
Pressure gradient
Describe the state of both valves in isovolumetric phase, why?
Both closed
Ventricular pressure sufficient to close inlet but not enough to open outlet valve so avoid backflow and secure stroke volume.
What happens if you have incompetent valves?
Valves don’t close fully so backflow can cancel some forward flow (heart failure)
What happens in stenosis of valves?
Excessive calcification/atherosclerotic degranulation
Valves can’t fully open so more pressure overcome narrowing, heart grows and potentially heart failure
What causes 1st and 2nd heart sound?
S1 lub = closure of AV valve
S2 dub = closure of aortic and pulmonary valves
Describe course of opening/closing of AV and SL valves
AV valve open for ventricular filling, for isovolumetric contraction they SL and AV both closed.
Pressure rises then SL valves open.
Isovolumetric relaxation, AV and SL both closed.
What happens to volume and pressure in isovolumetric phase?
Volume constant but changes in pressure due to contracting/relaxing ventricle
What happens in ventricular diastole?
1) Chamber isovolumetric
AP has ended and pressure is falling
Pressure: artery>ventricle>atria (no flow into ventricles from atria)
2) Full recoil of ventricles as they’re relaxed
Pressure: Artery>atria>ventricles so blood flow into ventricles via AV valves
What happens in atrial systole?
Wave of depolarisation emitted from SAN causes atrial contraction forcing remaining blood into ventricles via AV valves
Why do atrial cells contract simultaneously?
Have many GJs which allow depolarisation to pass form one cell to another(cells arranged as syncytium) chemical coupling
Why do atria contract before ventricles, why is this important?
Annulus fibrosus insulates atria from ventricle electrically
Ventricles filled before ejecting blood
What happens to depolarisation conduction wave from AVN?
Conducted via bundle of His and Purkinje fibres to apex of ventricles where it sweeps from bottom to top to initiate V systole
What happens in ventricular systole?
1) Isovolumetric contraction as ventricles not pressurised enough to open SL valves (both SL and AV closed)
Increase in ventricular pressure however
Pressure: Artery>ventricle>atria
2)Pressure in ventricle exceeds pressure in aorta (80mmHg), flow out of SL valves down pressure gradient, ventricular volume falls
What follows wave of contraction, why is it important?
Relaxation phase, allows chambers to fill properly before another heart beat
How does most blood pass from atria to ventricles?
Passively
What causes dicrotic notch?
Closure of aortic valve
What is CVP?
Pressure that fills the heart
What 3 times during the cardiac cycle does central venous pressure increase?
Atrium contracts
Atrium collects blood
AV valve shuts
What is the jugular venous pressure, why is it important?
Reflects right atrial pressure
best estimated from right internal jugular vein
has the most direct channel into the atrium
How does Jugular venous pressure change?
Atrial systole, JVP and thus CVP increase (A wave)
Then as AV valves close (ventricular systole) blood bulges into atrium so short increase in JVP (C wave)
Atrium relaxes so JVP decreases (X descent)
Then atrial fill gently (JVP increases) ( V wave) until AV valve opens (Y descent)
What does area of pressure volume loop represent?
Work done
Why can’t you measure cardiac output from pressure volume loop?
Time is not shown, don’t know heart rate
What does heart do for most of cardiac cycle?
Diastole/filling
Why is synchronisaiton of cardiac myocytes important?
Produces stronger response, coordinated and regulated electrical excitation = efficient pumping
What is RMP of SAN cells?
-55mV, though unstable (from around -50 to -70 to threshold of -40 to -55)
How long is excitation delayed for as it reaches AVN?
0.1 secs
Describe delay from SAN to atria, apex and ventricles
- 04-0.06
- 16
- 20-0.22
At resting heart rate of 60bpm how much time is spent in systole and diastole, how does this change with increasing heart rate?
Systole: 1/3
Diastole: 2/3
More time spent in systole
What are the two other heart sounds that can be heard?
S3 - mitral valve open blood rushes into ventricles
S4 - after atria contract (pathological)
How does diastole and systole change with increased heart rate?
Both decrease, systole less so
How does length of cardiac cycle change with slower heart beat?
Longer
When does preload refer to?
Stretching load just before systole at end of diastole
What is the best estimate for preload, EDV or EDP?
EDV
Does the afterload include the preload?
Yes
How does very high heart rate affect stroke volume?
Reduces it (less filling time)
How does cardiac output change after standing up?
Decreases by 25% then compensated by baroreceptor reflex
What increases venous return to right heart?
Arterioles dilating Venules dilating (constrition would cause blood to stay in microcirculation)
True or false, cardiac output is a measure of both ventricles together
False, output of each ventricle per minute
What is intrinsic discharge of SAN?
100bpm
At what rate does depol spread from SAN?
1m/sec
What do calcium blockers do to AVN AP?
Decrease amplitude and increase duration
Most Ca2+ for excitation contraction is from ECF, yes or no
No
What is the difference in Ca2+ conc in cytoplasm and ECF at rest?
10000 more in myoplasm
Where does depolarisation spread the fastest?
Purkinje fibres
What is the Fick principle?
VO2 = CO(Ca-Cv)
VO2 = rate of O2 uptake
CO = cardiac output
Ca = Concentration of O2 in arterial blood
Cv = Concentration of O2 in venous blood leaving the organ
When is coronory flow maximum?
Isovolumetric relaxation (pressure in aorta high, ventricle wall not contracting)
When do heart sounds occur in cardiac cycle?
In isovolumetric filling phase and isovolumetric relaxation (at the start)
