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..at rest, the heart of an elite athlete can beat as slowly as 30 beats per minute? The heart of an average adult beats between 70 and 80 times per minute, at rest.

The heart is a muscular structure, about the size of a man's fist, weighing about 500 grams. It is situated between the lungs and sternum (breast bone) and lies towards the left, protected by the sternum and ribs.

Journey of Blood Through the Heart

Atrium and ventricle: The heart can be divided into two halves, as each side has a slightly different purpose to the other. Both sides are further divided into two chambers: atrium or ventricle. Thus we have a left atrium, a right atrium, a left ventricle and a right ventricle. The main functions of each are:

  • Right atrium: Entering the right atrium is blood from the superior vena cava. This is blood from the body, and it is low in oxygen and carries the waste products of the body.
  • Right ventricle: The deoxygenated blood then passes a valve and enters the right ventricle. The walls of the right ventricle push the blood upwards, passed another valve, and into the pulmonary artery.  From here, the blood travels through the pulmonary artery to the lungs, where such waste products as carbon dioxide are expelled, and oxygen enters the bloodstream. Thus the blood that leaves the lungs is high in oxygen. This returns to the heart and passes through the pulmonary veins*.
  • Left atrium: From the pulmonary veins, blood which is high in oxygen enters the left atrium. It passes through a valve and enters the..
  • Left ventricle: From here, it gets pumped upwards into the aorta, which carries the oxygen-rich blood to  the rest of the body. Thus is the journey of blood through the heart.

*Note that the pulmonary veins are the only veins of the body to carry oxygenated blood.

Do you notice in the diagram above that the vena cava (in blue) appears slightly smaller than the aorta (in red)? This is because it is a vein, and all veins in the body have thinner walls than arteries, and the blood within them is at a much lower pressure.

Do you notice also that the wall of the right ventricle is slightly thinner than that of the left ventricle? This is because the blood being pumped from the right ventricle only travels a short distance to the lungs. Blood leaving the left ventricle, however, requires much more force, as it needs to be distributed to the rest of the entire body.

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Within the body are two main types of blood vessels: arteries and veins.

  • Arteries: These blood vessels carry blood that is rich in oxygen (from the lungs) and high in nutrients (from the intestines). This blood needs to be distributed throughout the body so that the oxygen and nutrients it carries can nourish all parts of the body.
  • Veins: These blood vessels carry blood which has already been used by the body. This blood has little or no oxygen, and it contains waste products, such as carbon dioxide and urea, from cell activity.

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