Blood is the life-maintaining fluid, pumped by the heart through all the arteries, veins and capillaries. This accounts for approximately 7% of total human body weight.
Functions of blood:
Transportation:-
- Oxygen from the lungs to the cells of the body and carbon dioxide from the cells to the lungs.
- Nutrients from the gasrointestinal tract to body cells.
- Hormones form the endocrine glands to other body cells.
- Heat that regulates body temperature
- Waste products away from the cells such as excess water.
Regulate :-
- Electrolyte balance (that is a substance containing free ions, that behaves as an electrically conductive medium).
- Helps to regulate pH.
Protection :-
- Certain blood cells protect against blood loss (clotting mechanism).
- Leukocytes provide immunity against infection.
Blood Composition
Blood is composed of two main parts that is the liquid and non-liquid part. The liquid part is the plasma while the non-liquid part is composed of different types of cells such as red blood cells, white blood cells and platelets.
Seperated Blood
If a sample of blood is taken in an anti-coagulated (chemical that prevents blood clotting) test tube and centrifuge it, the cells will settle down to the bottom of the test-tube because they are heavier and the plasma will settle on top of the cells as it is lighter. On average, more than 99% of the cells are red blood cells also called erythrocytes. These occupy about 45% of the total blood volume and plasma accounts for the remaining 55%. White Blood cells or leukocytes and platelets represent less than 1% of the total blood volume. These form a thin layer betwen the plasma and erythrocytes known as the buffy coat.
Plasma is the pale yellow fluid portion of whole blood that consists of water, sugar, fat, protein, potassium and calcium salts. It also contains many chemicals such as fibrinogen, that help blood to form necessary blood clots to stop bleeding. More than 92% of plasma is water.
Cells
All living organisms are made of small units called cells. ‘Cells are small compartments that hold all of the biological equipment necessary to keep an organism alive and successful on Earth’
Blood Cells are continually dying and being replaced within hours, days or weeks. This process where blood cells are formed in the bone marrow and destroyed by the spleen is termed as Haematopoiesis (from Ancient Greek: haema meaning 'blood' and poiesis meaning 'to make').
The below diagram shows how blood cells originate from the bone marrow, develop and mature into five types of cells:-
Erythroblast form mature erythrocytes
Myeloblast form mature neutrophils, basophils and eosinophils.
Monoblast form mature monocytes.
Megakaryoblast form mature platelets.
Lymphoblast form mature lymphocytes.
Haematopoiesis
Red Blood Cells (Erythrocytes)
Red Blood Cells are circular biconcave non-nucleated discs. They contain Haemoglobin that is the most important component of red blood cells. Haemoglobin is a complex protein, consisting of globin and an iron–containing substance called haem, which binds with oxygen to form oxyhaemoglobin. In this way, red blood cells transport Oxygen from the lungs to the tissues . Erythrocytes are also involved to transport waste products and carbon dioxide from the body cells to the lungs for excretion. Haemoglobin is also responsible for the red colour in the red blood cells. Erythrocytes usually survive for 120 days before they are destroyed and new red cells are formed.
Basophils
These represent 0.5% of the white blood cells. Basophils are the smallest circulating granulocytes and they contain heparin and histamine. The number of basophils increases during infection. They accumulate at the site of infection or other inflammation, were they secrete histamine, which increase the blood flow to the infected area. Then, they assist the movement of phagocytes and protective substances such as antibodies, into the tissue spacing, were they accumulate in areas of local inflammation at the healing stage.
Eosinophils
Make up 1.5% of the total number of white cells. These cells migrate to the area of the body were they are exposed to the external environment, that is, just under the skin, in the mucous membrane of the respiratory and digestive system and in the lining of the vagina and uterus. They also possess anti-histamine properties. In fact, the number of eosinophils is increased in allergic conditions such as asthma, hay fever, food and drug sensitivity and skin conditions.
Neutrophils (Phagocytes)
These constitute 70% of the total number of white cells. There main function is to protect against any foreign materials that enter the human body and to remove waste materials. When there is an infected area in the body, they move in large quantities were they ingest and kill the microbes by a process known as ‘phagocytosis’.
The number of neutrophils is increased following severe exercise and in later stages of normal pregnancy.
Monocytes
Monocytes are large mononuclear cells that have a bean-shaped nucleus. They represent 4% of the total white blood cells. Monocytes are actively phagocytic and ingest bacteria. Monocytes are able to migrate from the blood stream to inflamed areas of the body where they develop into ‘macrophages’. If they encounter large amounts of foreign or waste material, they tend to multiply at the site and ‘wall off’ the area.
Platelets (Thrombocytes)
Platelets are transparent irregularly shaped membrane bound cell fragments, frequently aneculated and formed from large bone marrow cells called megakaryocyte. There main function is to initiate the mechanism of blood clotting. Their life span is of about 8 to 11 days in the circulation.
Lymphocytes
These are large mononuclear cells that are rounded and possess only a small quantity of cytoplasm. In the adult they represent 24% of white blood cells. Lymphocytes are associated with the protection of the body against foreign material. There are two types of lymphocytes, the T-Lymphocytes and B-Lymphocytes.
The below diagram shows the white blood cells in a more simplified way.
Agranular Leukocytes (Mononuclear leukocytes )
These cells possess no granules in their cytoplasm and they are either an oval or bean-shaped nucleus. There are 2 types:- Monocytes and lymphocytes.
Granulocytes (Polymorphonuclear leukocytes)
These cells originate in the red bone marrow. Their names represent the dyes they take up when stained in the laboratory. These are:-
Eosinophils take up red dye, eosin.
Basophils take up alkaline methylene blue
Neutrophils are purple because they take up both dyes.
Blood Circulation
These above mentioned cells form the blood that moves through the body through the blood vessels. Blood vessels form a closed system of tubes that carries blood away from the heart in arteries, transports it to different organs and tissues of the body in arterioles, capillaries and venules and then returns to the heart in veins.
Arteries carry the oxygen-rich blood that the heart pumps to the rest of the body. The heart pumps the blood out through one main artery, which branches out into smaller arteries called arterioles. As the arterioles enter a tissue, they branch into countless microscopic vessels called capillaries. Substances are exchanged between the blood and body tissues through the thin walls of the capillaries. Before leaving the tissues, groups of capillaries unite to form small veins called venules. These continuie to merge to form larger blood vessels called veins.
Veins then convey blood from the tissues back to the heart. These contain one-way valves to keep the blood flowing toward the heart, even against the pull of gravity. Because the blood in veins contains so little oxygen, it appears bluish rather than bright red. That's why the veins you can see through your skin (for example, in the underside of your wrist) are blue.
Blood Donation
This blood that runs in our veins is so special that it is priceless, yet we all have adequate amounts of it. Have a look by click
here, to see if you're eligible or not to be one of our precious blood donors.
Testing Donated Blood
Blood type
Blood is tested to determine the donor's ABO group (A, B, O, AB) and Rh type (positive or negative). This is critical in selecting compatible blood for a patient in need of a transfusion.
Antibody Detection Test
All donors are tested to determine if their plasma contains antibodies to red blood cell antigens. If these antibodies are not identified, they may cause problems in a patient.
Tests for Infectious Disease
Screening donated blood for infectious diseases that can be transmitted through blood transfusion is very important in ensuring safety. A positive screening test in any of the following tests for infectious disease is followed by a confirmatory test to eliminate the possibility of a false-positive test result. A false positive occurs when the screening test is positive but it cannot be confirmed during subsequent, more detailed testing. This means a donor was not exposed to the infectious agent being tested for but the screening test was positive.
Hepatitis B Surface Antigen (HBsAg) and Core Antibody (Anti-HBc)
The hepatitis B virus has an inner core and an outer envelope (the surface). The HBsAg test detects the outer envelope or surface of the virus. The anti-HBc tests for the presence of antibodies to the inner core of the hepatitis B virus. In the absence of any other positive test for hepatitis B, the presence of antibodies to HBc suggests the individual may have had past exposure to the hepatitis B virus and it is unlikely he or she is infectious.
Hepatitis C Virus Antibody (Anti-HCV) and Nucleic Acid Test (NAT)
Two tests are done to detect hepatitis C infection. The anti-HCV test detects antibody to the hepatitis C virus. A positive result suggests the donor has been exposed to the hepatitis C virus and may be infectious. The second test detects RNA, the genetic material of the hepatitis C virus.
Syphilis Antibody
Syphilis tests detect the presence of an antibody to the organism Treponema pallidum that causes syphilis. The risk of transmitting syphilis through a blood transfusion today is very small since the rate of infection is low in blood donors and the organism causing syphilis is very fragile and unlikely to survive blood storage.
Human Immunodeficiency Virus 2 (HIV-1/2) Combo Antibody and Nucleic Acid Amplification Testing (NAT)
HIV-1 and/or HIV-2 virus cause acquired immunodeficiency syndrome, or AIDS. HIV-1 is more common in the West while HIV-2 is prevalent in Africa. Two tests are done for detecting HIV. One screens for antibody to both HIV-1 and HIV-2 viruses. The other detects RNA, the genetic material of the HIV-1 virus.
Blood Transfusion
Blood is used for treatment in a variety of clinical settings including surgery, trauma, cancer chemotherapy, and diseases of the blood (e.g., sickle cell disease and thalassemia). While RBCs, the most commonly transfused component, are used to correct anemia (low red blood cell count), platelet and plasma components are used to treat bleeding disorders, either to halt further bleeding or to reduce risk of bleeding during surgery.
Many surgeries today can be safely performed without use of blood; hernia and gallbladder surgeries are just two examples. Often, patients undergoing such relatively "minor" surgeries do not get their blood typed, but this is left to the discretion of the surgeon. For more complex surgeries where blood loss is expected to be significant, a set amount of blood is usually reserved for the patient based on the average amount of blood used for the type of surgery performed in the hospital.
Open heart, aortic aneurysm bypass, brain tumor, and orthopedic joint replacement (i.e., hip replacement) cases are examples of surgeries that usually require blood transfusion support. Organ transplant surgery, performed in more specialized hospitals, also requires considerable support by transfusion of blood components. For some types of elective surgery such as orthopaedic joint replacement, it may be desirable for the patient to pre-donate blood (i.e., autologous blood donation).
Surgeons must also notify the hospital blood bank if any special needs are anticipated, such as transfusion of platelets or plasma, so that the blood bank can prepare these components on a timely basis. (Frozen plasma, for example, requires 30-40 minutes thawing time.) Regardless of the type of surgery, however, since there is always the potential need for blood, it is important for hospital blood banks to inform surgeons and other physicians of the status of the blood supply as it may be necessary to postpone elective surgical procedures during times of severe blood shortage.