The heart as a pump

Question 1

Draw the basic structure of the heart, including chambers, valves and main vessels

Answer 1

See diagram.

Question 2

Describe the properties of cardiac muscle allowing the heart to function as a pump

Answer 2

Striations
Branching
Centrally positioned nuclei (1 or 2 per cell)
Intercalated discs (for electrical & mechanical coupling with adjacent cells)
Adherens-type junctions (to anchor cells and provide anchorage for actin)
Gap junctions (for electrical coupling)
In contrast with skeletal muscle, the T tubules of cardiac muscle are inline with the Z bands and not with the A-I band junction.

Question 3

Define systole and diastole

Answer 3

Systole is the period when the myocardium is contracting

Diastole is the period when the myocardium is at rest (700ms)

Question 4

Explain the wave of excitation in systole

Answer 4

1. The SA node fires an action potential, which spreads over the atria causing atrial systole. The AP reaches the AV node, where it is delayed for about 120ms.
2. From the AV node, excitation spreads down the septum between the ventricles
3. Excitation spreads from inner (endocardial) to outer (epicardial) surface
4. Ventricle contracts from the apex up, forcing blood towards the outflow valves.

Question 5

How does the organisation of muscle in the ventricle walls facilitate the pumping of blood?

Answer 5

Ventricular muscle is organised into figure of eight bands that squeeze the ventricular chamber forcefully in a way most effective for ejection through the outflow valve.
The apex of the heart contracts first and relaxes last to prevent back flow.

Question 6

Describe the main differences between the right and left side of the heart

Answer 6

The left side of the heart has a thicker myocardium, as it must generate the force to get blood around the entire body compared to the right side that must only get it around the lungs.
The right side of the heart has the pacemaker, the SA node.

Question 7

Describe the sequence of pressure and volume changes in the atria and ventricles, during one cardiac cycle

Answer 7

In diastole, ventricles relax and pressure falls below atria
Initially flow is fast - rapid filling phase
Ventricular filling continues at a steadily decreasing rate
AV closed through atrial contraction
After a 100-150 ms delay, ventricles contract
Isovolumetric phase then pressure exceeds vessels
Followed by rapid ejection period
Valves close when ventricular pressure lower than vessel

Question 8

Describe when and how each valve of the heart opens and closes

Answer 8

Aortic and Pulmonary are outflow valves which open in systole, when ventricular pressure rises above that of the pressure in the vessels.

Tricuspid and mitral valves: open in early diastole when ventricular pressure drops below atrial pressure until pressure in the ventricles causes them to close.

Question 9

Explain the origin of the first and second heart sound

Answer 9

First Sound
As the AV valves close oscillations are induced in a variety of structures, producing a mixed sound with a crescendo-descendo quality – ‘lup’

Second Sound
As the semi-lunar valves close oscillations are induced in other structures, including the column of blood in the arteries. This produces the sound of shorter duration, higher frequency and lower intensity than the first – ‘dup’