2) Heart As A Pump

Question 0

Describe the spread on an A/P in the heart

Answer 0

• produced at pacemaker in SAN
• contraction spreads across atria
• to the AVN, delayed by ~120ms
• continues down Purkynje fibres
• contraction spreads through ventricles from inner epicardium to outer
• contraction in bands from apex up

Question 1

Describe the histology of cardiac muscle

Answer 1

-striated
-branching
-central nuclei
-intercalated disks
-gap junctions (allows synchronous contraction, low resistance)
-t-tubules align with Z-bands (not A-I)
1 A/P spread easily between cells via gap junctions so 1A/P leads to 1 heart beat (~200/300ms)

Question 2

Compare systole and diastole

Answer 2

SYSTOLE
-contraction, 280ms, constant duration
DIASTOLE
-relaxation, 700ms, variable duration (shorter at ^HR)

Question 3

Describe the cardiac cycle (with reference to the cardiac graph)

Answer 3

1. EARLY DIASTOLE, IV pressure falls and A/V valves open, A pressure exceeds V pressure
2. Atria have been filling during systole so RAPID FILLING PHASE as soon as A/V valves open
3. Ventricle fill passively throughout diastole (at low HR ventricles fill til almost full) until IV pressure rises to match atrial pressure
4. ATRIAL SYSTOLE, tops up the ventricles. Delay of ~120ms
5. SYSTOLE, IV pressure exceeds atrial, A/V valves close via back flow in cusps
6. ISOVOLUMETRIC CONTRACTION until IV pressure exceeds arterial pressure
7. Semi-lunar valves open
8. RAPID EJECTION PHASE both IV and arterial pressure rise to a max
9. Towards end of systole, IV pressure falls until below arterial pressure, back flow forces semi-lunar valves shut

Question 4

What causes heart sounds?

Answer 4

Rapid movement (valves) or turbulent flow of back flow or incompetent valve
1st HS, A/V valves close
2nd HS, semi-lunar valves close
Splitting of sounds by unpaired valve closing
3rd (early systole) and 4th (atrial contraction) are normal