2. Cardiac Cycle

Pacemaker (Non-examinable)

Sino-atria node (pace maker) is situated at the top of the right atrium. Sino-atria node will generate nerve fibres that travel down the nerve fibres. As nerve impulse travels down the nerve fibres, it causes the cardiac muscles of the atria and ventricles to contract respectively.

Activity of the pacemaker is dependent on the nervous system and circulating hormones such as adrenaline.

Question:

What is the likely effect of adrenaline on the rate of heart beat?

CARDIAC CYCLE (EXAMINABLE)

RELAXATION OF THE ATRIA AND VENTRICLES

Deoxygenated blood enters the right atrium through the vena cava. Oxygenated blood enters the left atrium through the pulmonary vein.

CONTRACTION OF THE ATRIA

• • Muscles of the left atrium and right atrium contract simultaneously.

  • Bicuspid valves on the left side of the heart and tricuspid valves on the right side of the heart open to allow oxygenated blood to enter the left ventricle and deoxygenated blood to enter the right ventricle respectively.

CONTRACTION OF THE VENTRICLES

• • Muscles of the left ventricle and right ventricle contract simultaneously.

  • Bicuspid valves on the left side of the heart and tricuspid valves on the right side of the heart close.

  • Semi-lunar valves in the aorta and pulmonary artery open.

  • Oxygenated blood is forced out of left ventricle into the aorta while deoxygenated blood is forced out of right ventricle into the pulmonary artery.

  • The cycle repeats itself.