Blood is essential to life. We’re all born with it and will die with it. And unlike a hip or a heart or a lung, it can be replenished but never replaced.
The volume of blood in our bodies depends on a person’s body weight and their gender, says Hong Kong haematologist Dr Herman Liu Sung-yu. Blood accounts for about 7 per cent of a person’s total body weight. During pregnancy a woman’s blood increases by about 50 per cent. This means a woman who weighs 55kg has, give or take, 3.8 litres coursing around her body. An average man will have more, usually between 4.2 and 5.7 litres.
Blood serves many life-giving functions. It is composed of platelets which control clotting to stop the body bleeding out; white blood cells which form our defence system, fighting infection; and red blood cells which transport food and oxygen to every part of our bodies while helping cells clear waste.
It also contains plasma, the pale yellow liquid which accounts for 55 per cent of blood volume. Plasma is 90 per cent water and is the vehicle that everything else rides in as blood courses around the 161,000km of blood vessels that form the adult human circulatory system.
Keeping well hydrated is important for keeping our blood freely moving. Dehydration coupled with immobility – on a plane, for example, or post-surgery – could give rise to dangerous clots forming as blood thickens.
There are four main blood groups – A, B, O, AB – with each coming in either positive or negative forms, meaning you can be any one of eight (A positive, B negative and so on). Red blood cells sometimes contain a protein known as the RhD antigen. If this is present, your blood group is RhD positive. If it is absent, your blood group is RhD negative.
AB negative is the rarest blood group. O positive is the most common, but O negative is the most safely donated; it can be given to anybody irrespective of the recipient’s blood type, so it is valuable in emergencies.
Types of blood diseases
Although blood is the life force of the body, it is also susceptible to disorders. Hong Kong haematologist Liu says the most common disease that affects blood is iron deficiency anaemia, which affects up to one-third of the world’s population. The most common genetic blood disorder is thalassaemia, which affects the production of haemoglobin, resulting in severe anaemia.
Thalassaemia is prevalent in Mediterranean areas and southeast China. In Hong Kong, 12 per cent of the population is a thalassaemia carrier, though the majority show no symptoms. Due to its high prevalence, it is recommended for all pregnant women to have a prenatal screening for the condition.
Blood can also develop cancers which affect the production and function of blood cells. Most begin in the blood cell factory, the bone marrow. The three main types are leukaemia, which is caused by the rapid production of abnormal white blood cells; lymphoma, which develops from another type of white blood cell, lymphocytes; and myeloma, a cancer of the plasma cells.
Another inherited blood disorder is haemophilia, which causes blood to not clot properly. The disorder mostly affects males; Queen Victoria’s male descendants suffered from it, as did the Russian royal family, the Romanovs!