Blood is the circulatory fluid present in animals that have a blood vascular system. The vascular system provides rapid transport of substances from one part of the body to another by mass flow. Blood is a liquid tissue as it is composed of cells embedded in a fluid matrix, plasma. Blood serves many functions in the body, some of which are carried out by the blood cells and others by the plasma. The main functions of blood are transport of substances, maintenance of homeostasis, defense against diseases and blood clotting.
Respiratory gases such as oxygen and carbon dioxide are transported in the blood, between the lungs and other parts of the body. Oxygen from the lungs is carried by the pigment hemoglobin present in red blood cells, to the heart where it is distributed to all parts of the body by the pumping action of the heart. In a similar manner, carbon dioxide from different areas of the body is carried in the blood to the gaseous exchange surface of the lungs where it is eliminated from the body.
Blood transports digested food around the body. When digestion completes, food substances are in the form of simple, soluble molecules which are carried from the digestive system to other parts of the body. Digested food including glucose, amino acids, fatty acids, and glycerol etc, are carried from the small intestine to the liver and parts of the body where they are stored or used up. Blood is the medium through which stored food in the body is carried to different parts of the body where they are needed.
The body produces waste products that have to be excreted from the body. These excretory products include carbon dioxide, urea, uric acid, which are transported in the blood to organs where they are excreted. Kidneys appear to be the most important excretory organ, but there are other organs such as the lungs and skin that are also primarily involved in excretion. Waste products dissolved in the blood are carried to these organs where they are gotten rid of.
Hormones are chemical messengers produced by endocrine glands. They are involved in the maintenance of homeostasis within the body. Most of the time, they are far away from where their action is needed. Therefore hormones need to reach their target organs before they can bring about a particular change to achieve homeostasis. Blood is the vessel that carries these chemicals to their required location. The transport of hormones in the blood is a type of communication in the body.
Blood plasma carries plasma proteins and hemoglobin (also a protein) which both show acidic and basic features due to the presence of acidic and basic amino acids. Acceptance or release of Hydrogen ions by these amino acids brings about pH balance to blood. These proteins are therefore referred to as buffers, which prevent large variations in the pH of blood, helping the maintenance of a constant blood pH.
The role that blood plays in the regulation of body temperature is remarkable. Blood distributes excess heat from deeper and warmer tissues to the cooler surface by its circulatory action. Viscosity of blood is related to the constriction or dilation of the blood vessels which effects the radiation of the heat away from the body. The contents in the blood alter the viscosity of the blood which leads to either constriction or dilation of blood vessels. Thus blood is greatly involved in maintaining a constant body temperature.
Blood is also involved in other homeostatic mechanisms such as the regulation of blood pressure and ionic and water content of the blood. The amount of water in the body changes the viscosity and volume of blood which in turn alters blood pressure. Blood circulates substances within the blood stream, tissue fluid and cells. Tissue fluid, which is formed by the seeping of water and other substances out of blood capillaries, must have an optimum amount of water. Blood helps in the maintenance of the required amount of water. To maintain a constant environment within the body, the above factors (blood pressure, pH, water content, body temperature etc) should be maintained within a narrow range.
Blood provides defense against infectious diseases. This is achieved by three main processes; blood clotting, phagocytosis and immunity in the form of antibodies. Blood contains different types of cells including red and white blood cells and cell fragments called platelets. There are different types of white blood cells. Certain white blood cells and platelets are involved in providing defense against invading pathogenic micro-organisms.
Neutrophils, monocytes and macrophages are types of white blood cells that are phagocytic, i.e. they engulf microbes. These phagocitic cells find their way into body tissues, in the blood stream, where they carry out phagocytosis. Lymphocytes are another type of white blood cell that secrete substances called antibodies. Micro-organisms such as bacteria and viruses carry specific proteins called antigens on their surface. Antibodies produce a specific response against specific antigens. They act against antigens destroying pathogenic micro-organisms.
When the skin is wounded, the blood that escapes out mixes with substances called thromboplastin. Thromboplastin, along with other substances such as clotting factors VII and X present in the plasma, initiates the clotting process. Fibrinogen is a soluble plasma protein that gets converted to fibrin in the clotting process. Blood clotting stops excess blood loss from the body. The opening in the wound that exposes tissue is sealed by the clotted blood, preventing the entry of pathogenic micro-organisms into the body.
