Cardiac Physiology Flashcards
Describe the first and second heart sound
- The vanes of the valves vibrate due to sudden pressure changes -> heart sounds
- S1 = closure of the AV valves, low pitch and longer lasting.
- S2 = semilunar valves close, rapid snap
Define the refractory periods
- Absolute refractory period - time in which a normal cardiac impulse cannot re-excite an already excited area of cardiac muscle.
- Relative refractory period - it is much more difficult to re-excite already excited muscle, but a strong impulse may overcome this.
Describe the period of ejection (phase of systole)
- As the LV pressure exceeds 80mm Hg and RV exceeds 8mm Hg, the semilunar valves open and blood rushes out of the ventricles.
- 70% of the blood ejected during systole is ejected in the first 1/3 (of systole).
- The first 1/3 of systole is called the period of rapid ejection, the last 2/3 is called the period of slow ejection.
What is the function of the papillary muscles?
- Contract when the ventricular walls contract, and they prevent the valves from bulging too far into the atria (by pulling the vanes of the valve inward)
- They do NOT help the valves close
- If a papillary muscle ruptures, the valve bulges backwards causing severe valvular incapacity
What are the different phases of systole?
1) Period of isovolumic (isometric) contraction
2) Period of ejection
3) Period of isovolumic (isometric) relaxation
What is the formula for the duration of the cardiac cycle?
1/HR
e.g. if HR is 72 the duration is 1/72 = 0.0139min/beat
How can the stroke volume be increased?
- Increasing end-diastolic volume.
- Decreasing the end-systolic volume.
Define end-systolic volume (ESV)
The volume of blood that remains in the ventricles at the end of contraction/systole.
Describe the period of isovolumic relaxation
- At the end of systole, the ventricles relax, allowing the right and left intraventricular pressures to rapidly decrease.
- The aorta and pulmonary arteries which have become distended with blood, push blood back towards the ventricles, causing the aortic and pulmonary valves to close.
- The ventricular muscles continue to relax and the volume does not change -> isovolumic relaxation.
- The intraventricular pressures rapidly decrease to diastolic levels; then the AV valves open to start the next cycle.
What are the differences between the AV and semilunar valves?
1) The AV valves have much softer closure.
2) The velocity of blood flow through the semilunar valves is much greater (openings more narrow)
3) The aortic and mitral valves are subjected to much greater mechanical abrasion
4) The AV valves are supported by chordae tendineae, unlike the semilunar valves
Explain the processes involved in excitation-contraction coupling
- As the action potential passes over the cardiac muscle membrane, it spreads to the interior of the muscle fibre along the T (transvere) tubules.
- The T tubule action potential spreads to the membranes of the longitudinal sarcoplasmic tubules, releasing calcium into the cell from the sarcoplasmic reticulum. From here, calcium diffuses to the actin and myosin filaments.
- In addition to the calcium from the SR, the action potential opens voltage-gated Ca channels allowing calcium to diffuse from the T tubules into the cell. This calcium also activates ryanodine receptor channels in the SR membrane -> Ca release into the cell sarcoplasm.
- Calcium interacts with troponin to initiate cross-bridging.
- The strength of contraction of cardiac muscle is strongly influenced by extracellular calcium concentration.
- Cross-bridge formation is then the same as for skeletal muscle contraction.
- At the end of the plateau phase, calcium ions are rapidly pumped back into the SR and T tubule extracellular fluid. The calcium ATPase pump moves Ca into the SR, and Na-Ca exchanger removes Ca from the cell.
What are the different phases of the ventricular action potential?
- Phase 0 (depolarisation): the ventricular cells are depolarised, Fast Na channels open, Na rushes in and depolarises the cell to a membrane potential of +20mV.
- Phase 1 (initial repolarisation): Fast Na channels close, small amount of K leaves the cell through open K channels (repolarising).
- Phase 2 (plateau): Ca channels open and K channels close. Brief initial repolarisation occurs, then the plateau occurs as Ca ion permeability increases and K ion permeability decreases (during phases 0 & 1 Ca channels open slowly and Ca influx occurs, and K channels close slowly and less K efflux occurs).
- Phase 3 (rapid repolarisation): Ca channels close, K channels open and K efflux occurs rapidly, returning the cell to its resting membrane potential.
- Phase 4 (resting membrane potential): RMP is -90mV.
Define stroke volume (SV)
The volume of blood ejected during systole (EDV-ESV).
The heart has 3 types of cardiac muscle…what are they?
1) Atrial muscle
2) Ventricular muscle
3) Specialised excitatory/conductive muscle
What is an intercalated disc?
Cell membranes that separate individual cardiac muscles.
Describe the aortic pressure curve
- The LV contracts, leading to a rapid rise in ventricular pressure until the aortic valve opens
- After the valve opens, the pressure in the LV rises much less rapidly (because of blood flow into the aorta)
- As the blood flows out to the systemic arteries, their walls stretch and pressure rises to 120mm Hg
- At the end of systole, the LV stops contracting, the aortic valve closes; the arteries maintain high pressure even during diastole
- An incisura (notch) in the aortic pressure occurs in the aortic pressure curve when the aortic valve closes (short period of backflow immediately before the valve closes, followed by sudden stoppage of flow)
- After the aortic valve closes, aortic pressure slowly decreases in diastole (blood stored in distended elastic arteries flows into peripheral vessels back to the veins).
- Before the LV contracts again, aortic pressure is ~80.
- Pressure curves in the RV & pulmonary artery are similar, but pressures are only 1/6 as high.
Describe the a wave, c wave and v wave
a wave: atrial contraction
c wave: occurs when the ventricles begin to contract. The c wave is caused by
(1) backflow of blood into the atria
(2) bulging of the AV valves backwards (into the atria).
v wave: occurs toward the end of ventricular contraction, and is caused by slow flow of blood into the atria from the veins while the AV valves are closed.
At each intercalated disc, the membranes fuse to form what?
- Gap junctions.
- These allow rapid diffusion of ions.
Describe isovolumic contraction
- After ventricular contraction the ventricular pressure rises immediately and abruptly, so the AV valves close (to prevent backflow into the atria).
- Ventricular pressure continues to rise until it is high enough to push the semilunar valves open against the pressure in the aorta and pumonary arteries.
- During this period, ventricular contraction is occurring but NO emptying.
Isometric - increasing muscle tension but NO shortening of muscle fibres in this phase of systole.
Where is the SA node located?
In the superior lateral wall of the RA near the opening of the SVC.
Describe the process of diastole
- Blood accumulates in the ventricles during diastole.
- Note that prior to this (in ventricular systole), the AV valves are closed, so blood fills the atria at this time. As the ventricular pressures fall towards the end of systole and the pressures in the atria exceed this, the AV valves are forced open and blood flows quickly into the ventricles.
- This is called the period of rapid filling/rapid inflow, which lasts for the first 1/3 of diastole.
- During the middle 1/3 (diastasis) a small amount of blood flows into the ventricles.
- During the last 1/3 (atrial systole) the atria contract -> 20% contribution to filling of the ventricles.
What are the functions of the specialised excitatory/contractile cardiac muscle fibres?
Either
A) automatic rhythmical action potentials
B) conduction of action potentials through the heart
Define the end-diastolic volume (EDV)
The volume of blood in the ventricles at the end of diastole (i.e. immediately prior to systole).
Define the ejection fraction (EF)
The fraction of the end-diastolic volume that is ejected (i.e. SV/EDV).
Atrial contraction contributes what percentage of ventricular filling?
Approximately 20%
