The following diagrams show the heart during the cardiac cycle. The arrows represent the flow of blood. Study the diagrams and answer the questions that follow:
- Identify the structures labelled A and B respectively.
- Name and explain what happens in each of the phases of the cardiac cycle represented in:
- Diagram I
- Diagram II
- Diagram III
- A: semi-lunar valve; B: Bicuspid or mitral valve
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- Diagram I: General diastole: Blood enters the atria from the venae cava (right atrium) and from the pulmonary veins (left atrium). The entire heart is relaxed.
- Diagram II: Atrial systole: Both atria contract and blood is pumped from the atria through the bi-/tricuspid valves to the ventricles. The bi - /tricuspid valves open easily downward to allow blood through.
- Diagram III: Ventricular systole:Both ventricles contract and pump blood upwards into the pulmonary artery (right ventricle) and aorta (left ventricle). The bi - / tricuspid valves close (because of chordae tendinae preventing them from opening backwards, so blood does not return to the atria. The semi-lunar vales open at the base of the major arteries to allow blood into them.