Blood plays an important role in the transport of oxygen from the lungs to other parts of the body. It also transports absorbed food materials from the digestive tract to body tissues. Beside these, it transports waste products, such as carbon dioxide from body tissues to the lungs and urea to the kidneys for excretion. It also transports heat, hormones and water.
Transport of Oxygen
Oxygen diffuses from the lungs, where the partial pressure of oxygen is higher, into the surrounding capillaries. Oxygen combines with haemoglobin in the erythrocytes to form oxyhaemoglobin. The erythrocytes are carried by circulating blood to other parts of the body where the partial pressure of oxygen is lower. Oxyhaemoglobin dissociates into haemoglobin and oxygen, and oxygen is thereby supplied for cellular respiration. Carbon monoxide binds more rapidly with haemoglobin to form carboxyhaemoglobin which does not dissociate readily. As a result there is less haemoglobin to bind with oxygen. People can suffer from carbon monoxide poisoning if they inhale gas fumes or smoke from the exhaust of a vehicle.
Transport of Carbon dioxide
Cellular respiration releases carbon dioxide. The carbon dioxide diffuses into the surrounding blood capillaries. It then combines with water to form carbonic acid. This reaction is catalysed by an enzyme in the erythrocytes. The carbonic acid then ionises to form hydrogen ions and hydrogen carbonate ions. Hydrogen carbonate ions then leave the erythrocytes and remain in the plasma.
CO2 (carbon dioxide) + H20 (water) H2CO3 (carbonic acid) H+ (hydrogen ion) + HCO3-(hydrogen carbonate ion)
About 70% of carbon dioxide is transported in the form of hydrogen carbonate ions. Another 23% combines with the amino group in haemoglobin and is transported as carbaminohaemoglobin. A small amount of 7% dissolves directly in the blood plasma.
When the blood reaches the lungs, the carbon dioxide is released and diffuses out of the blood into the alveoli. The carbon dioxide is then breathed out. When the blood reaches the lungs, the carbon dioxide is released and diffuses out of the blood into the alveoli. The carbon dioxide is then breathed out.
Transport of absorbed food material
Soluble digested food material such as simple sugars and amino acids, water soluble vitamin B and C, and mineral salts are absorbed into the capillaries of the villi in the small intestine. They are transported by the hepatic portal vein to the liver and then to the heart for general blood circulation. The fatty acids, glycerol and vitamin A, D, E and K are transported by the lymph into the blood circulatory system via the left subclavian vein.
Transport of excretory waste products
Deamination of excess amino acids occurs in the liver. The amino group is removed from the amino acid and is converted to urea. Urea is transported by blood to the kidneys to be excreted.
Transport of heat
Blood helps to regulate body temperature by distributing heat from heat-producing sites such as the skeletal muscles to areas of heat loss such as the skin.
Transport of hormones
Blood transport hormones such as insulin and glucagon produced by the endocrine glands to the target organs where they exert their effects. For example, insulin and glucagon produced by the pancreas are carried by blood to the liver.
Transport of water to tissues
Water is an important component of protoplasm and is transported by blood to provide a medium for biochemical reactions.
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