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Humans have between 4 and 7 l of blood. Fifty five percent of this volume is plasma
which is an aqueous solution of proteins, blood clotting factors, dissolved ions,
wastes, dissolved gases, glucose and cholesterol. Blood also contains red blood
cells (erythrocytes), platelets and five types of white blood cells (leucocytes). They
are neutrophils, lymphocytes, monocytes, eosinophils and basophils. In addition
blood has three buffer systems: the phosphate, the carbonic acid/bicarbonate system
and the protein buffer which maintains blood pH between 7.35 and 7.45.
Blood transports O2 from the lungs to the body’s cells by binding O2 to haemoglobin, CO2 from cells to the lungs, nitrogenous waste from cells to kidneys and
sweat glands, absorbed nutrients from the small intestine to the liver and to cells and
hormones from endocrine system to target cells. Blood also transfers heat around
the body. Blood is able to coagulate to minimise blood loss when a vessel is damaged and contains antibodies and complement proteins to defend against bacteria.
Blood cells are made in the active (red) bone marrow from stem cells called haemocytoblasts by the process known as haemopoiesis. The sequence of events that
lead to blood clotting is known as haemostasis, while bleeding is known as a haemorrhage. Oxygen is transported attached to haemoglobin within the RBC – do you
see a pattern yet in the words that refer to blood?
Blood coagulation occurs when a clot of insoluble fibrin is formed from fibrinogen. The transformation of fibrinogen is produced by the enzyme thrombin. However
thrombin is present in blood as the inactive prothrombin (factor II). Prothrombin is
converted to thrombin by prothrombinase. However this enzyme is not present in
blood either until activated by factor X. Factor X is unable to activate prothrombin
until it unites with molecules that are produced either by the “intrinsic pathway” or
the “extrinsic pathway” following damage to a blood vessel. This complicated
arrangement ensures that all of the factors required to coagulate blood are present in
plasma as inactive forms but are ready to go – they don’t start the coagulation
process until a blood vessel has been damaged.
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