The composition of blood

As a general rule, blood makes up about 7% of our body mass. This means that on average, a normal and healthy adult male has about 5 to 6 litres of blood within his body, while a female may have slightly less than this quantity [1]. In its natural state, whole blood consists of millions of microscopic cells suspended within a fluid known as plasma. The cells, also known as cellular elements, constitute approximately 40% of the total volume of blood [2], and are all manufactured in the bone marrow as stem cells, where, depending upon the specific needs of the body at a particular period of time, they are then converted into one of three major cellular groups. Together with the plasma, there are thus four main blood components, each of which plays a particular role in ensuring the orderly and healthy functioning of the body. These components, with their main roles, are:

i) Red corpuscles (erythrocytes): The main function of these cells is to transport oxygen from the lungs to tissue all over the body. When red cells are still young or freshly loaded with oxygen, they give blood its characteristic red colour, but when they carry carbon dioxide or are near the end of their 120-day life spans, they acquire a violet tinge (hence the appearance of blue veins);

ii) White corpuscles (leucocytes): These cells, of which there are many variants, form an integral part of the immune system, by acting as a second line of defence should any foreign micro-organism, such as a virus, penetrate the primary protective layers of the body (i.e. the skin and mucus membranes). In the event of an infection, the number of white corpuscles in the body increases rapidly, with some being tasked with identifying the unknown intruder, while others are responsible for breaking it down and then eliminating it. Depending on the particular variant, a white corpuscle can live anywhere from only a few hours to over six months;

iii) Platelets (thrombocytes): In the case of a tissue injury where blood vessels are cut or severed, platelets perform the function of ensuring that any loss of blood that may occur is staunched. This is done via a process known as coagulation, whereby the platelets stick to the walls of the affected blood vessel in order to former a platelet plug, or clog, at the location of the injury, which helps stop blood loss and heal the wound. On average, the lifespan of a platelet varies from 7 to 10 days.

iv) Plasma: This pale yellow fluid, which is made up mainly of water (92%) and proteins (7%), is used to transport the corpuscles and platelets around the body to the locations where they are required [3]. In addition, plasma also transports the hormones and nutrients that are essential for maintaining tissue strength and growth, as well as simultaneously moving waste products to their appropriate filtering locations.

As was previously noted, one of the early obstacles preventing blood transfusion from being undertaken on a wider scale was the belief that blood was a homogenous fluid, which could therefore be easily transfused between people without complication. It was only after Landsteiner discovered the ABO groups that it was realised that the blood of different individuals had special characteristics that could affect their degree of compatibility with blood from other people. The particular blood group to which a person belongs can be determined by looking at whether a set of chemical markers, known as the A and B antigens, are present on the surface of that persons red corpuscles. An individual whose red corpuscles contain only antigen A belongs to blood group A, but if only antigen B appears, then the blood is of type B status. Should a blood sample contain both antigens, then the person will be a member of the AB group, but should neither antigen be present, then the blood belongs to the O group [4].

These antigens can be identified by the manner in which they react to two antibodies, anti-A and anti-B, which may be present in an individuals plasma sample. Should the antibodies of a blood recipient encounter red corpuscles from transfused blood containing antigens different to those that they are familiar with, they will react to this foreign presence by engaging in a process known as agglutination, whereby they clump together with these foreign red cells and destroy them. By contrast, a recipient’s antibodies will usually not react with transfused blood that contains a set of antigens compatible with those of the host. By definition, type A blood contains anti-B, but no anti-A, antibodies, while type B blood has anti-A, but no anti-B antibodies; since AB blood contains both antigens, it will obviously contain no antibodies, while type O blood will carry both anti-A and anti-B antibodies (as it has neither antigen).

Due to the adverse reactions that occasionally arise due to agglutination, the standard medical procedure is to ensure that patients receive only blood that will not provoke a response from the antibodies in their plasma, which means that they should only accept blood from their own group. The general exception to this rule relates to type O blood, which is considered to be an universal substitute, since, in an emergency situation, it can be transfused into any person should the blood group of the recipient be unknown, or should blood of the correct group be unavailable at the required moment [5]. This feat is possible since type O blood has no markers on its red cells with which to provoke a response from a recipient’s own antibodies, enabling it to stealthily penetrate that person’s blood stream. The basic characteristics of the different blood groups just covered are summarized in Table1.

Table 4.1 Blood group compatibility [6]

Blood group	Antigen on red cells	Antibodies in serum	Can receive blood from:
A	A	Anti-B	A, O
B	B	Anti-A	B, O
AB	A and B	None	A, B, AB, O
O	None	Anti-A and Anti-B	O

 

The operation of the transfusion process is complicated by the presence of the rhesus (Rh) factor, which is carried, like the A and B antigens, on the surface of the red blood corpuscles [7]. If a Rh negative person receives a transfusion of Rh positive blood, there is a risk that a process known as sensitisation may occur, where the body manufactures antibodies to destroy the transfused blood that has been received. Although a Rh-negative person may not suffer significant side effects from sensitisation following an initial exposure to Rh-positive blood, should a subsequent exposure occur, then the outcome may be more severe. The implications of such a phenomenon are most widely felt in the case of pregnancy, when a Rh-negative mother bears a Rh-positive child to a Rh-positive father. In a small percentage of cases, the mother may become sensitised when red blood cells from the foetus cross the placenta and enter her blood stream. The antibodies that she develops may then cross back over the placenta, where they will destroy the still developing red cells of the foetus. The outcome is that the baby will usually suffer from anaemia or jaundice, although if sensitisation occurred in a previous pregnancy, there is a possibility that the baby may even die.

In Table 2, the relative distribution of the overall South African population into the main blood groups and respective rhesus types is shown. It should be noted that these are merely guidelines, as the distribution varies along racial and ethnic lines, with blood from the rare groups usually being more common among certain ethnic groups, especially those of black African descent.

 Table 4.2 Blood group distribution of the South African population [8]

ABO classification	Rhesus positive	Rhesus negative	Population distribution
A	35.7%	6.3%	42.0%
B	8.5%	1.5%	10.0%
AB	3.4%	0.6%	4.0%
O	37.4%	6.6%	44.0%

 

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