Blood is a connective tissue consisting of cells suspended in a liquid matrix. It carries nutrients, oxygen and water to the cells of the body as well as carrying away waste products. It is important in immune and inflammatory responses as well as maintenance of the chemical composition of the fluids and tissues of the body (homeostasis), including pH, as well as haemostasis (bleeding control) and body temperature.
Blood takes part in a large number of bodily functions and its examination can indicate underlying clinical problems and therefore plays an important part in the evaluation of a patient.
The average adult has about 5 litres of blood, 55% of which is blood plasma and 45% of which is blood cells.
Blood plasma is a clear yellow watery fluid and consists mainly of water, in which are dissolved organic and inorganic molecules.
Glucose and fats provide an energy source.
Amino acids and fats are essential building blocks for cells.
Fats also carry fat soluble vitamins around the body as well as insulate and protect the body.
Oxygen is essential for cell respiration and the release of energy.
Carbon dioxide and nitrogen are waste products.
There are 3 types of blood cells which are suspended in the blood plasma;
Erythrocytes, also known as red blood cells, are the most abundant cells in the blood. They are thin, disc-shaped cells which have a depression in the middle on both sides. This biconcavity increases the surface area to allow efficient diffusion of gases. The cells are small and flexible enough to squeeze through tiny capillaries.
The function of red blood cells is to bind with oxygen in the lungs and carry it to the tissues of the body where it is exchanged for the waste product carbon dioxide. Red blood cells do not have a nucleus and are absent of most organelles, but contain large amounts of haemoglobin (Hb). Haemoglobin is a molecule containing iron that enables it to bind and carry a large amount of oxygen and carbon dioxide. Carbon dioxide is actually transported in red blood cells as bicarbonate. Carbonic anhydrase is an enzyme which works as a catalyst in the conversion of carbon dioxide to bicarbonate (HCO) and is also carried in red blood cells.
When haemoglobin combines with oxygen, it turns a characteristically red colour, but as this oxygen is lost to the tissues of the body, it becomes more purple. The function of red blood cells is to bind with oxygen in the lungs and carry it to the tissues of the body where it is exchanged for the waste product carbon dioxide.
A haematocrit value is the percentage of red blood cells that occupy the blood.
In adults, new red blood cells are produced in the bone marrow of the sternum, vertebrae, ribs, base of the skull and the proximal ends of the long bones. Within the red bone marrow are pluripotent stem cells that give rise to all the different cell types present in the blood.
Red blood cells form from large cells with organelles and a nucleus (erythroblasts), which contain very little haemoglobin. As they mature their nuclei and most of their organelles disintegrate, and the cells become smaller and take up more haemoglobin. The production of red blood cells is carried out at the same rate as the destruction of the old red blood cells. Erythropoietin is a chemical produced by the kidneys and liver and enhances the production of red blood cells. Its release is triggered by low concentrations of oxygen in the blood.
As red blood cells do not have a nucleus and are absent of most organelles, they are unable to replicate or repair themselves. Old red blood cells are removed from the blood by the spleen and the liver, usually after they have been in circulation for about 120 days. As they are pushed through the small vessels of the spleen, old cells are phagocytosed by monocytes. The useful components such as iron and protein from the haemoglobin are reclaimed and transported to the bone marrow where they are recycled to make new red blood cells. The rest is either stored in the liver or converted into bile pigments (bilirubin) by the liver and excreted into the bile.
Leukocytes, also known as white blood cells, are larger than red blod cells and less abundant, but their numbers will increase during infection. White blood cells are also produced in the bone marrow.
There are two main types of white blood cell; granulocytes and agranulocytes. Both types contain large nuclei, but granulocytes also contain enzyme digesting granules within their cytoplasm. Agranulocytes are devoid of these granules.
Kill pathogens and then engulf the debris. They die after engulfing only a few pathogens.
Play a role in inflammatory reaction.
Each has a large, round nucleus .
Engulf debris and pathogens.
Present antigens to T-cells.
White blood cells can be found throughout the body, however they aggregate in places where they are most likely to come into contact with pathogens and antigens, such as the spleen, thymus and lymph nodes, and where they can further differentiate, such as the bone marrow.
Platelets are small fragments of large cells (megakaryocytes) produced in the bone marrow, which do not have a nucleus. They are very important in blood clotting and are only in circulation for 10 days before being recycled.
Haemostatis is the process in which bleeding is stopped following an injury.
Following an injury, platelets near the wound secrete serotonin, which causes the smooth muscle in the immediate blood vessels to contract (vasoconstrict), thereby reducing the blood flow to the site. The platelets then begin to stick to the exposed collagen found at the site of the wound, creating a soft platelet plug. The next stage in haemostasis is a complex coagulation cascade of reactions resulting in prothrombin being converted to thrombin in the presence of calcium ions. Thrombin then converts fibrinogen into fibrin a mesh of fibres that catches and traps red blood cells, forming a blood clot or scab which stops the bleeding.
There are 4 different blood groups which depend on the presence or absence of two different types of agglutinogens, which are found in red blood cells.
If a person's red blood cells do not have any agglutinogens present, they are known as having blood group O, those with anti-A agglutinogens are blood group A, those with anti-B agglutinogens are blood group B, and those with both anti-A and anti-B agglutinogens present are blood group AB.
If blood from groups containing different agglutinogens are mixed together, a reaction called agglutination occurs. The different agglutinogens cause the red blood cells to become sticky, so the blood aggregates. These blockages may damage the organs, especially the kidneys, and can result in death.
There is another agglutinogen present on red blod cells called the rhesus factor. 85% of the population possess the rhesus antigens on their red blood cells and are know as Rhesus positive (Rh+). The remaining 15% of the population that do not have the Rhesus antigen are known are Rhesus negative (Rh-).
If a Rh- recipient is given blood from an Rh+ donor the agglutinogen stimulates the production of anti-D, an anti-Rh agglutinin (antibodies). This means that if Rh+ blood is given to the recipient again, the antibodies would destroy those red blood cells and cause agglutination, which can result in death.
Blood banks rely on volunteer donors. Each donor usually donates about 1 pint (1 unit) of whole blood at each sitting a process which takes between 10-20 minutes. The donors body replenishes the fluid lost in donation in about 24 hours, but it can take up to 2 months to replace the red blood cells, so donation is limited to once every 2 months.
Apheresis Blood Donation
Apheresis is where a donor donates a specific component of blood, such as the platelets, plasma, red blood cells or white blood cells rather than donating a unit of whole blood. This procedure can take up to two hours but allows more of one particular component to be donated, than would usually be attained from a single unit of whole blood. For example, 2 units of red blood cells could be donated in one sitting (red cell apheresis). However, because it takes up to 4 months for the body to replace 2 units of red blood cells, these types of donations are limited to every 4 months.
Autologous blood donation
It is increasingly becoming common practice for a patient to donate their own blood prior to a scheduled surgery. The blood is taken prior to the non-emergency surgery and stored until the procedure takes place.
The guidelines for blood donation are different depending on the country in which you live. Listed below are a selection of the people who should not donate blood. For more information please contact the healthcare guidelines provided by your country.
Who has ever used intravenous drugs (illegal IV drugs). With HIV (AIDS virus). Who has had hepatitis. Who has risk factors for Creutzfeldt-Jakob disease (CJD) or who has an immediate family member with CJD. Who has had malaria in the past. Who has been to countries where malaria is endemic in the last 12 months. Who has been in a West Nile Virus endemic area in the last 4 weeks. Who has received or thinks they may have received a blood transfusion in the British Isles, excluding the Republic of Ireland, since 1st Jan 1980. Who has been transfused elsewhere within the last 12 months. Who has an infection. Who has had a tattoo, ear or body piercing within the last 12 months. Who is pregnant.
For more information visit;
www.transfusionguidelines.org.uk www.blood.co.uk www.aabb.org