8. Blood
Contents of the chapter
8.1 Blood
Blood is a complex liquid tissue. Its function is to supply oxygen and nutrients to the body's cells. In addition, blood transfers heat to different parts of the body, keeping our body temperature even.
The body of an average adult contains approximately five litres of blood. This blood consists of two distinct parts: a liquid part (= blood plasma, 55% of blood's total volume) and a cellular part (45% of blood's total volume).
90% of blood plasma is water, in which all other substances are dissolved. There are three types of blood cells: red blood cells, white blood cells and platelets. Red blood cells carry oxygen, white blood cells control pathogens, and platelets are involved in blood clotting.
Each person belongs to one of four blood types: O, A, B or AB. The blood type of a person must be known if a blood transfusion is needed during surgery. The wrong blood type would lead to the immune system attempting to destroy any foreign proteins on the red blood cells of the wrong blood type.
Changes in different parts of the blood can give clues about various diseases - for example, in anemia, the number of red blood cells is low.
The composition of blood.
8.2 Red blood cells
Red blood cells (RBC), also known as erythrocytes, are the smallest cells in our bodies. They are only eight thousandths of a millimetre (0.008 mm) in diameter. If all of the red blood cells of the human body were assembled into a chain, it would be approximately 200,000 kilometers long. Red blood cells have a biconcave shape which resembles a flattened disc. They are also adaptable, kind of like bean bags. Due to their structure, red blood cells are able to pass through even the narrowest capillaries.
Due to their flattened disc shape, the area of a red blood cell is one-third larger than that of a circular cell of the same size. Their lifespan is about four months. When they are no longer able to maintain their biconcave shape, the liver and the spleen remove them from the bloodstream. Two to three million new red blood cells are generated in bone marrow every second.
8.3 Red blood cells contain hemoglobin
The primary function of red blood cells is to transport oxygen to the cells. Red blood cells do not contain a nucleus or other cell organelles. Therefore, red blood cells consist only of the cell membrane, enzymes and hemoglobin. Hemoglobin is an iron-containing protein that is specialized in binding oxygen. Hemoglobin gives the blood its red color. It is also capable of binding carbon dioxide, but most of the carbon dioxide released during cell respiration is transported in the blood itself.
Thanks to the biconcave shape of red blood cells, their combined oxygen absorption surface area is approximately 3,000 square meters, or the size of 15 large detached houses. When a red blood cell dies, its hemoglobin is transferred into new red blood cells or stored in the liver.
Anemia is a condition caused by either a decrease in production of red blood cells or hemoglobin. Its symptoms include tiredness and loss of appetite. Anemia can be caused by a deficiency of important nutrients or vitamins, such as iron, zinc or vitamins A and C.
Smoking increases the carbon monoxide (CO) content of your blood. CO can become attached to hemoglobin, which prevents the blood from carrying oxygen. Almost ten percent of the red blood cells of a regular smoker actually carry carbon monoxide instead of oxygen. This causes the smoker to be in a weaker physical condition than a non-smoker.
| Blood Count | abbreviation | Reference value (that is, readings between values are considered normal) |
|---|---|---|
| Hemoglobin | B-Hb | men 130–165 g/l; women 115–150 g/l |
| Hematocrit | B-Ht | men 0,40–0,54; women 0,36–0,47 |
| Red blood cell count: Erythrocytes | RBC | men 4,2–5,6 * 1012/l; women 3,8–5,1 * 1012/l |
| Leukocytes | Leu | 3–8 * 109/l |
| Thrombocytes | B-Trom | 150–350 * 109/l |
8.4 White blood cells
White blood cells (WBC) or leukocytes are large, nucleated cells. They are much fewer in number than red blood cells. They are formed in the bone marrow and the lymph nodes of the armpits. White blood cells are an essential part of our body's immune system.
White blood cells are diverse and differ in size, number, nucleus structure and lifespan. Their lifespan varies from a few hours to several years.
Some white blood cells destroy microbes by engulfing them in, and others help by marking the pathogens so that they can be better recognised. White blood cells are also able to escape from the bloodstream into tissues.
Some white blood cells are converted into immunological memory cells. They quickly recognize and combat pathogens, and thus prevent diseases from re-emerging. This phenomenon is called immunity.
Immunity can also be obtained artificially by vaccination. In vaccination, antigens, certain molecules from the pathogen, are introduced into the body with an injection. This triggers an immune response.
8.5 Platelets
Blood platelets, also known as thrombocytes, are even smaller than red blood cells. They are cytoplasm fragments protected by a cell membrane.
Like other blood cells, platelets, are formed in the bone marrow. They play a very important role in the complex process of blood coagulation.
Blood coagulation helps to stop wounds from bleeding. When platelets are damaged, they release an enzyme into the blood. This enzyme converts a protein called fibrinogen into fibrous fibrin. Vitamin K is also required to produce this enzyme. The fibrin strands adhere to the edges of the wound. The blood cells that become attached to the fibrin strands gradually accumulate near the opening, blocking the wound.
Finally, the strands retract and squeeze out all excess fluid. A scab is created, which is like a patch on our body and should not be plucked. The scab comes off when the skin has completely repaired the wound below it.
8.6 Blood clots
Platelets can also repair damaged blood vessel walls. If the walls of your veins are uneven, for example because of too much cholesterol, the platelets may break against them and cause a blood clot to form. A blood clot that blocks blood flow near the heart or the brain can have fatal consequences.
Blood clots form in stages when cholesterol is accumulated in the artery walls. As the arterial diameter decreases, the blood pressure in the vessel rises. The blood clot forms when platelets accumulate in the narrow space. Eventually, the artery becomes blocked. 
8.7 Blood plasma
Blood plasma is the liquid part of the blood. Blood plasma makes up about 55% of the body’s total blood volume. The blood plasma itself is 90% water. The remaining 10% is made up of substances such as oxygen, carbon dioxide, hormones, waste products and plasma proteins. Such proteins include fibrinogens. Blood proteins play a role in various processess, such as blood coagulation and the transport of substances.
Blood plasma carries nutrients from the digestive system to the rest of the body. Blood plasma also transports carbon dioxide produced during cell respiration from tissues to the lungs. Blood warms up as it flows through the liver and the muscles, and this heat is transmitted with the help of blood plasma to the outermost parts of our body.
Although our body has blood vessels that are almost 100,000 kilometers long in total, not all cells are located close to this network. The more distant cells need food and oxygen just like all other cells. The thinnest blood vessels constantly leak fluid into the intercellular space and thus receive the substances they need. This tissue fluid returns to the bloodstream either through capillary veins or through lymphatic vessels.
8.8 Blood groups and transfusion
All people belong to one of the main blood groups (types of blood): A, B, AB or O. Your blood group is identified by the antibodies and antigens contained in your blood.
This means, for example, that a person with the blood type O has no antigens, but both anti-A and anti-B antibodies in their blood plasma.
Similarly, a person with the blood type AB has both A and B antigens, but no antibodies in their the blood. Because the antigen and its antibody must not be mixed, blood transfusion can be safely performed according to a specific chart.
If the wrong type of blood is given to a person, the antibodies in their blood plasma react with the new red blood cells and cause them to stick together. This was the most common cause of death in blood transfusions before blood groups were discovered.
In addition to the ABO system, red blood cells sometimes have another antigen, a protein known as the RhD antigen.
Only RhD negative blood can be given to a Rhesus negative person. A Rhesus positive blood can be given to both blood types, as long as ABO portability is considered.
The blood that is given during a transfusion is tested to make sure it matches the blood of the recipient. Blood banks, where blood is obtained from voluntary donors and supplemented with anticoagulants, enable these transfusions. There is no harm in donating blood to a blood bank, as the body's blood volume will be restored within a couple of weeks. The donated blood is tested for diseases such as HIV. Blood is rarely transfused as such. Instead, red blood cells, platelets or blood plasma are often separated from the donated blood and given to people who need these specific parts of blood.
8.9 Blood donation charts
Blood groups.
Blood group compatibility: O-blood can be given to anyone, as O-blood group red blood cells do not have antigens. A blood type AB can receive blood from all blood types. An O blood group can only receive O blood because they have antibodies against both A and B.