This is a short extract from Unit 9.4 Our Body from the forthcoming Grade 9 Science Outcomes book. This extract is in draft format and will look completely different in the eventual book. You can use this now with your grade 9 science class (or any class for that matter) if you want to.

All in a heartbeat!

Heart Diagram

Study the heart diagram. Note that both sides of the heart (both pumps) each have two chambers. The upper chamber is called an atrium and the lower chamber is called a ventricle. The arrow at the top without a label should have the label "blood from the head".

The atria (plural of atrium) and the ventricles have muscular walls, which contract with each heartbeat. The ventricle walls are thicker and more muscular than the atria walls and the ventricle walls are the main working part of both pumps.

Note the direction of blood flow in the diagram and read this explanation of how the heart operates. This is the description of what happens in less than a second—in just one heartbeat.

  1. As the whole heart muscle relaxes the space inside both heart pumps increases and the pressure inside the heart reduces. This has three major effects.
    • The valves out of the ventricles (at the broken line arrows in the heart diagram) close shut because of the shape of the valves and because the pressure at the other side of the valves is greater then inside the heart.
    • The valves into the ventricles (broad black arrows in the heart diagram) open.
    • Blood starts to flow into the atria and through the valves into the ventricles at both sides of the heart. Remember at this point the pressure is still greater outside the heart and because of the shape of the valves between atria and ventricles the blood has to flow into the ventricles.
  2. As the heart muscle contracts it does so in a wave. It starts with the walls of the atria and spreads a fraction of a second later to the main heart muscle of the ventricles. This very slight difference in time between atria walls and ventricle wall contractions results in a quick rush of blood from the atria into the ventricles.
  3. The ventricle muscles then contract fully and powerfully. This causes a sudden increase in pressure, which closes the valves from the atria into the ventricles (note how these valves are shaped again). It forces the blood through the other valves (dotted line arrows) out of the right ventricle into the pulmonary artery going to the lungs and from the left ventricle into the aorta for the blood going to all other parts of the body.
  4. As the whole heart muscle relaxes—the process starts again and so on over and over again—for so long as you shall live!

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