Function of the blood
The blood is one of the two fluid tissues found within the body. The bloods main function is to transport substances to different parts of the body, for example it transports food and oxygen to respiring tissues; it takes carbon dioxide and waste products away from respiring cells to the various organs that remove them and it carries hormones that are secreted by the endocrine glands to the target organs. The blood has many other functions such as, it helps to maintain constant body temperature, it provides pressure that many organs depend on to enable them to carry out their functions, it acts as a buffer by keeping the pH constant and it defends the body against infection.
The four main components that make up the blood are the plasma, erythrocytes (red blood cells), leucocytes (white blood cells) and thrombocytes (platelets). All of these components perform different functions which allow bodily processes to take place i.e. respiration, digestion, and immunisation.
The plasma is a pale, straw-coloured liquid, of which 90% of the composition is water and the other 10% is composed of dissolved substances. The plasma makes up 55% of the blood and the dissolved substances which are present in plasma include protein, glucose, amino acids, vitamins, hormones, oxygen, carbon dioxide, antibodies, anti-toxins and other nutrients.
Carbon dioxide is a by-product which is produced through respiration and is dissolved in the plasma after the cells have respired aerobically. It is then transported to the lungs where it is exhaled, forming the 4% of exhaled carbon dioxide in exhalation. Urea is another waste product that circulates in the plasma until it reaches the excretory organ which will remove it.
The plasma is also responsible for the transportation of hormones, which have been secreted by the endocrine glands, to the target organs. Hormones like erythropoietin, which are responsible for stimulating the production of erythrocytes, are transported in plasma. Other hormones like testosterone, oestrogen, ADH, thyroxin, adrenaline, insulin, FSH and LH are also transported in the plasma.
The red blood cells are produced in a complex process called erythropoiesis. This is where the red blood cells are formed as stem cells, which are synthesised in the bone marrow, and then develop into erythrocytes. This process is stimulated by the hormone erythropoietin, which is usually produced by the kidneys.
The primary function of erythrocytes is to deliver oxygen around the body to aerobically respiring cells. Red blood cells are suited to this function as they contain pigmentations called haemoglobin, which bind to oxygen to form oxyhaemoglobin. This allows for oxygen to be transported around the body. In addition to this is the biconcave shape of the red blood cells, which allows for maximum oxygen absorption. Also erythrocytes are characterised by the absence of a nucleus, which also allows for maximum oxygen absorption.
Haemoglobin in the blood is comprised of globular proteins with the addition of a prosthetic group (iron), which binds to oxygen in the process of diffusion. In correlation to this, is the erythrocytes ability to take on carbon dioxide in deoxygenated blood to the lungs for exhalation. This is a shared ability in which the plasma shares this function with red blood cells.
The main role of white blood cells is to defend against disease. They destroy micro-organisms and produce anti-bodies to fight bacteria. Anti-toxins are produced to neutralise the toxins produced by bacteria. In order to assist this function there are five different types of white blood cells, each leucocyte interlinks with another to enable a compatible eradication of pathogens. The five different types include: neutrophils, basophils, eosinphils, lymphocytes and monocytes.
Neutrophils destroy invading pathogens by releasing highly toxic substances and carry out phagocytosis. Basophils are mainly responsible for the inflammation which acts to remove injurious stimuli. Lymphocytes contain receptors that stimulate the production of antibodies and also perform phagocytosis. Monocytes circulate in the blood where they mature into macrophages in bodily tissue and share the same ability as Neutrophils and lymphocytes and eosinophils combat infections caused by parasites
Platelets or thrombocytes are small fragments of cells, that have no nucleus, which are used to clot the blood when a wound or cut is present to prevent severe blood loss. Together with leucocytes they form one percent of the total composition of blood. The thrombocytes don't actually transport any substances around the body.
The primary function of the platelets is to assist in blood clotting. The formation of blood clots is the started by the process of haemostasis. This intrinsic process involves vasoconstriction, which is the narrowing of the blood vessels resulting from the contracting of the muscular wall of the vessels, to slow and eventually stop the bleeding.
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