In a resting adult human the heart beats about 70 times per minute and around 2.5 million beats in an average life time.
The heart has four chambers joined together to make one large block of muscle. Like every other muscle in the body, the heart needs a constant supply of food and oxygen, and to get rid of carbon dioxide. The blood flowing through the heart cannot do this effectively enough so the muscles of the heart have their own blood supply.
Structure of Blood Vessels
All over the human body there are three main types of blood vessels: arteries, veins and capillaries.
Arteries are the vessels that carry blood away from the heart and to the rest of the body; these do not contain valves and have a relatively small inner diameter.
Veins carry blood back to the heart (except in hepatic portal vein). This is the major artery of the three as it has a relatively large inner diameter and also contains valves.
Capillaries are the smallest of the three blood vessels. They do not contain valves and have the inner diameter of about one red blood cell. Capillaries carry blood through organs and tissues.
Contents/Function of the Blood
The blood is the red liquid which gets transported all around the body and is a complex mixture of substances; all of them do different jobs.
In the blood there is a mixture of white blood cells, red blood cells, platelets and plasma. There is about 55% of plasma in the blood, 45% of red blood cells and only 1% of white cells and platelets.
White blood cells are designed to protect the body against any type of infection which may get into the body. These cells are colourless and have a nucleus, they’re also larger than the red blood cells although there are fewer of the white cells and they only stay in the blood for about 10% of their life. White blood cells are made in the bone marrow and in the lymph nodes. There are three different types of white blood cells and these are the lymphocyte, granulocyte and the monocyte.
Red blood cells are also made in the bone marrow and as they develop, the nucleus disappears therefore mature red cells have no nucleus. Red blood cells are shaped like a disc and are dished on each surface; this shape is called the biconcave disc. This shape also makes it easier for the cell to bend and fold as they pass through the smallest blood vessels. Worn out red cells are removed in the liver or spleen and replaced by new ones from the bone marrow. The life span of a red blood cell is about 100-120 days.
Platelets are much smaller than the red blood cells and vary in shape; they also help the blood to clot. They are produced in the bone marrow again and are created by ‘budding off’ from special cells.
Plasma is a clear, pale yellow liquid that is 90% water. The remaining 10% contains many substances including glucose, amino acids, fats, calcium, urea, carbon dioxide, antibodies (which destroy microbes) and also hormones which control the way the body works.