Cardiac Cycle

Question 1

Cardiac cycle

Answer 1

The heart’s role is to create pressure that pushes blood around the body. The muscular walls of the 4 chambers must contract in a coordinated sequence so blood fills the heart before it gets pumped away.

Cardiac cycle: the sequence of events in one full heartbeat.

1. Atrial systole – both right and left atria contract together, decreasing volume and increasing blood pressure. The muscle in the walls is thin, so a small pressure is created by this. This helps push blood into the ventricles stretching their walls and ensuring they are full of blood.
2. Ventricular systole – Both right and left ventricles contract together (decreasing volume, increasing pressure). Contraction starts at the apex (base) of the heart, so blood is pushed up towards the arteries.
3. Diastole – The muscular walls of all 4 chambers relax, elastic recoil causes the chambers to increase in volume, allowing blood to flow into the atria from the vena cava as that has a higher blood pressure. As ventricles continue to relax their pressure drops below that in the atria, and blood flows into them.

Question 2

Atrio-ventricular valves (mitrial/bicuspid and tricuspid):

Answer 2

After the ventricular systole ventricular walls relax and recoil.
- The pressure in the ventricles rapidly drops below the pressure in the atria, the atrial blood pressure pushes atrio-ventricular valves open. Blood entering the heart flows passively straight through the atria into ventricles (during diastole).
- The pressure in the atria and ventricles rises slowly as they fill with blood. The valves remain open as the atria contract in the atrial systole, but close when the atria begin to relax.
- This closure is caused by a swirling action in the blood around the valves when the ventricle is full.
- As the ventricles begin to contract (ventricular systole) the pressure of blood in the ventricles rises.
- When the pressure rises above that in the atria, the blood starts to move upwards. This movement
fills valve pockets, keeping them closed. Tendinous cords attached to the valves prevent them from turning inside out, preventing blood flowing back into the atria.

Question 3

Semilunar valves (pulmonary and aortic):

Answer 3

• Before ventricular contraction the pressure in the major arteries (aorta and pulmonary artery) is
  higher than in the ventricles. This means the semilunar valves are closed.
• Ventricular systole raises the blood pressure in the ventricles very quickly. Once the pressure in the
  ventricles rises above the pressure in the major arteries, the semilunar valves are pushed open.
• The blood is at very high pressure, so it is forced out of the ventricles in a powerful spurt.
• Once the ventricles have finished contracting the heart muscle starts to relax (diastole). Elastic tissue
  in the walls of the ventricles recoils. This stretches the cardiac muscle out again and returns the
  ventricle to its original size, causing the pressure in the ventricles to drop quickly (vol increase).
• As pressure drops below the pressure in the major arteries, the blood starts to flow back into the
  ventricles. The semilunar valves are pushed closed by blood collecting in the pockets of the valves.
  This prevents blood returning into the ventricles.
• The pressure created when the left semilunar valve (aortic valve) closes is the ‘pulse’ we feel.

Question 4

Pressure in blood vessels:

Answer 4

Blood enters the aorta and pulmonary artery in a rapid spurt, but tissues require blood to be delivered in an even flow.
- Artery walls close to the heart have a lot of elastic tissue. When blood leaves the heart, these walls stretch.
- As blood moves on and out of the aorta, the aortic pressure starts to drop.
- The elastic recoil of the walls helps to maintain the
blood pressure in the aorta.
- The further the blood flows along the arteries, the
more pressure drops, and fluctuations become less obvious, due to increasing cross-sectional area of the arteries.
- It is important to maintain the pressure gradient between the aorta and arterioles, as this is what keeps blood flowing towards tissues.