Digested food and oxygen are carried to the body cells eg. hair papilla in the blood which is enclosed in a series of tubes called blood VESSELS. Blood is circulated through the vessels by the pumping of the heart. THE BLOOD, BLOOD VESSELS AND HEART from the circulatory system.
Blood travels around the body in tubes called vessels. Vessels carrying blood away from the heart are called ARTERIES. Whilst those returning blood to the heart are called VEINS. In order to reach every cell of the body, the blood carried down finer and finer arteries called ARTERIOLES, until the tubes become so fine their walls are only cell thick. There microscopic vessels exist in almost every tissue of the body and are called CAPILLARIES.
Blood consists of a watery liquid called PLASMA which contains a large number of red cells and a smaller number of white cells and fragmented cells known as platelets. The products of digestion are thus carried in solution in the blood plasma. Oxygen is carried as Ox-haemoglobin in red blood cells.
There are four main parts of blood PLASMA the liquid part of blood, which is a thin yellowish watery liquid. The function to transport dissolved substances including digested food hormones, waste products such as carbon dioxide and urea (urea is formed in the liver from amino acids and is later excreted in the urine by the kidneys). Also carries red and white blood cells and blood platelets. RED CELLS, small bio-concave cells with no nuclei, live only three to four months. Are made in bone marrow, contain haemoglobin, a compound of protein and iron which makes red cells. These red cells Carry oxygen from the lungs to the tissues. Oxygen combines with the haemoglobin to form oxyhaemoglobin, which easily gives up oxygen in the tissues.
WHITE CELLS larger than red cells, but fewer in number (five hundred to one White), white cells fight infection. The Cells pour through the wall of the capillaries, to reach germs in tissues, they surround bacteria and digest them. PLATELETS, fragments of cells with no nuclei. These help the clarity of blood.
We have learnt that a supply of oxygen is essential for the life of every living cell, as it is there that internal respiration and the release of energy occur.
Oxygen is taken into the body through the respiratory surface, which in the human body is the inner surface of the lungs. It is one of the functions of the blood to transport oxygen from the lungs to the other living cells of the body. The oxygen carrying capacity of the blood, is increased enormously by the presence of a pigment Called haemoglobin in the red cells. Because of this substance our blood can carry about fifty times the amount of oxygen that it could without it.
In the lungs, Where oxygen is abundant, the haemoglobin combines with oxygen to form a compound known as oxyhaemoglobin. Whilst ordinary(or reduced)haemoglobin is bluish red in colour, oxyhaemoglobin is bright scarlet. It follows therefore, that the blood leaving the lungs will be brighter red than that arriving. In the tissues oxygen is in low concentration for it is constantly being consumed in cellular respiration. Here the oxyhaemoglobin breaks down releasing oxygen for the use of the cell and returning to reduced haemoglobin. The latter will pick up more oxygen when the red cell, which carried it, arrives again in the lungs. The body demand for oxygen is high, in order to meet this demand, the number of red cells is very large and one cubic millimetre of blood (about the size of a pinhead) contains about five million red cells. Since an average person has some five million cubic millimetres of blood, you can see the total number of red cells (about 250,000,000,000) is very large indeed. Red cells have a life of about 120 days and about one million are disposed of every second, mainly by the spleen. New cells are formed in red bone marrow from cells which initially possess a nucleus and have no haemoglobin and eject the nucleus. The spleen acts as a store for red cells, as well as disposing of them. During prolonged exercise, such as a long run, the spleen contracts and discharges red cells into the bloodstream. Increasing the total blood volume by as much as 25%.
Capillaries are only one cell wide so you can imagine how small these microscopic blood vessels are. We have one going to every hair on our bodies.
