3. Organisms Exchange Substances - The Cardiac Cycle

Question 1

What is systole?

Answer 1

Contraction of the heart.

Question 2

What is diastole?

Answer 2

Relaxation of the heart.

Question 3

What is the first phase of the cardiac cycle?

Answer 3

Diastole (relaxation of the heart)

Question 4

What happens during diastole?

Answer 4

• Blood returns to the atria of the heart through the pulmonary vein and vena cava
• As the atria fill, the pressure in them rises
• When this pressure exceeds that in the ventricles, the atrioventricular valves open allowing the blood to pass into the ventricles
• Muscular walls of both the atria and ventricles are relaxed at this stage
• Semilunar valves close

Question 6

What causes the semilunar valves to be closed during diastole?

Answer 6

The relaxation of the ventricle walls causes them to recoil and reduces the pressure within the ventricles. This causes pressure to be lower than that in the aorta and the pulmonary artery, so semilunar valves in aorta and pulmonary artery close.

Question 7

What is the second stage of the cardiac cycle?

Answer 7

Atrial systole (contraction of the atria)

Question 8

What happens during atrial systole?

Answer 8

• The atrial walls contract, and along with the recoil of the relaxed ventricle walls, forces the remaining blood into the ventricles from the atria
• Throughout this stage, the muscle of the ventricles walls remains relaxed
• During this, AV remain open and semilunar shut

Question 9

What happens during the third stage of the cardiac cycle?

Answer 9

Ventricular systole (contraction of the ventricles)

Question 10

What happens during ventricular systole?

Answer 10

• After a short delay to allow the ventricles to fill with blood, their walls contract simultaneously
• This increases the blood pressure within them, forcing shut the atrioventricular valves and preventing back flow of blood into the atria
• With the AV valves shut, the pressure in the ventricles rises further
• Once it exceeds that in the aorta and pulmonary artery, blood is forced from these ventricles into these vessels