Lecture 6

Question 1

Why is the heart two pumps in series? Which side is which?

Answer 1

The heart must pump blood to the lungs (right side) followed by the rest of the body (left side).

Question 2

Which parts of the heart contract simultaneously, which parts don’t? Which stage is each known as

Answer 2

the left and right sides and the same chamber type will pump at the same time, the different chambers pump in different stages.
In systole the left and right ventricles are contracting, in diastole it is more likely for the atria to be contracting.

Question 3

How does cardiac contraction occur?

Answer 3

Cystolic Ca2+ ion concentration increases due to release from the sarcoplasmic reticulum, this binds with troponin to reveal binding sites for myosin on actin, the myosin then binds with actin to form a cross bridge and the sarcomeres shorten (generating a force).
Note that every myocyte is activated each heart beat.

Question 4

How is the force of cardiac cell contraction increased? Why can recruiting more cells not be done?

Answer 4

By increasing the cystolic Ca2+ level (releasing more calcium from the sarcoplasmic reticulum) more cross bridges can be formed, increasing the force.
More cells can’t be recruited as every cardiomyocyte is already recruited in every heat beat.

Question 5

What positive feedback cycle can occur in a failing heart?

Answer 5

The heart is failing so it has to pump harder, hence it releases more calcium, leading to an overflow of the system and possibly arrythmias due to to many cells trying to beat at random times.

Question 6

How do cardiac cells relax?

Answer 6

ATP binds to myosin and the calcium ions return to the sarcoplasmic reticulum, this breaks the cross bridges, causing a reduction in force (all cardiac myocytes relax each beat).

Question 7

How are cardiac muscle fibers arranged? Why?

Answer 7

In a helical pattern around the heart, the heart twists and contorts as it contracts, this transfers the force more effectively to the blood and hence creates more pressure for less work.

Question 8

What are the 5 main phases of the cardiac cycle in detail? Which stage are they in? Which stage lasts longer?

Answer 8

1. Atrial systole: contraction of the two atria, increasing pressure of the atria and ventricles, the atrioventricular valves open and the semilunar valves are closed.
2. isovolumetric ventricular contraction (AV valves and semilunar valves are closed but the ventricles contract, hence volume of blood stays the same but the ventricle volume changes and hence the pressure of the ventricles rapidly spikes).
3. Ejection (semilunar valves open and the high pressure pushes the blood through the arteries).

These first three are associated with systole

4. isovolumetric ventricular relaxation (atrioventricular and semilunar valves are closed and the ventricles relax, 40% of the blood from 3. is still in the ventricles)
5. Passive ventricular filling: Atrioventricular valves open up and allow blood to flow into the atria and ventricles, filling the heart with blood.

These last two are associated with diastole, the longer lasting of the two stages.