TRANSPORT IN HUMANS**
Structure and composition of blood
Plasma
Plasma is a pale yellowish liquid. Plasma is about 90% water and the rest is a complex mixture of various dissolved substances.
Red blood cells or erythrocytes
There are about five million red blood cells which is erythrocytesin each cubic millimetre of blood. It contains the pigments haemoglobin which is a special kind of iron-containing protein.
White blood cells
White blood cells are larger than red blood cells but fewer in number. It colourless and does not contain haemoglobin and also in irregular shape and contains a nucleus.
Blood platelets and thrombocytes
Blood platelets and thrombocytes are not true cells. They are membrane bound fragments of cytoplasm from certain bone marrow cells. They play a part in the clotting of blood.
Function of blood
How does blood transport oxygen to the cells of the body
The blood pass through the lungs, oxygen diffuses from the alveoli into the blood.
Clotting or coagulation of blood
Blood exposed to air will soon clot or form a solid lump. The clot seals the wound, preventing excessive loss of blood. The clot also prevents foreign particles from entering the bloodstream.
Phagocytes
The process of engulfing or ingesting foreign particles, such as bacteria, by the white blood cells.
Circulatory system
Arteries
Arteries receive blood directly from the heart. It have walls which are thick, muscular and elastic. The elastic layer is much thicker in the arteries nearest the heart. Which mean that the thick elastic walls to help to withstand the high blood pressure in the artery.
Veins
The blood pressure in veins is much lower than the arteries. Blood flows more slowly and smoothly in the veins. Veins also contain less elastic tissue. Most veins have internal valves along their length to prevent blood from flowing backwards. Valves are folds of the inner walls shaped like half moons. Hence they are called semi lunar valves.
Blood cycle
Pulmonary circulation
Blood flows from the main circulation of the body to the heart, then to the lungs and back to the heart again before it pump back into the main circulation. The circulation linking the lungs to the heart is known as the Pulmonary circulation.
- From the heart, the Pulmonary arteries carry the blood to the lungs.
- Oxygenated blood is returned to the heart by the Pulmonary veins.
Systemic circulation
The circulation of the blood around the body is called the systemic circulation.
- Oxygenated blood leaves the left side of the heart and it distributed by arteries to all parts of the body.(except the lungs)
- Veins carry the blood from all parts of the body back to the right side of the heart.
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The blood is used to transport various substance from one part of the body to another by flowing continuously around the body. In vertebrates, the blood flows through a closed system of blood vessels called the circulatory system. The blood flow is called blood circulation.