The average adult has about five liters of blood living inside of their body, coursing through their vessels, delivering essential elements, and removing harmful wastes. Without blood, the human body would stop working.
Blood is the fluid of life, transporting oxygen from the lungs to body tissue and carbon dioxide from body tissue to the lungs. Blood is the fluid of growth, transporting nourishment from digestion and hormones from glands throughout the body. Blood is the fluid of health, transporting disease fighting substances to the tissue and waste to the kidneys.
Because it contains living cells, blood is alive. Red blood cells and white blood cells are responsible for nourishing and cleansing the body. Since the cells are alive, they too need nourishment.

Vitamins and Minerals keep the blood healthy. The blood cells have a definite life cycle, just as all living organisms do. Approximately 55 percent of blood is plasma, a straw-colored clear liquid. The liquid plasma carries the solid cells and the platelets which help blood clot. Without blood platelets, you would bleed to death.
When the human body loses a little bit of blood through a minor wound, the platelets cause the blood to clot so that the bleeding stops. Because new blood is always being made inside of your bones, the body can replace the lost blood. When the human body loses a lot of blood through a major wound, that blood has to be replaced through a blood transfusion from other people.
But everybody's blood is not the same. There are four different blood types. Plus, your blood has Rh factors which make it even more unique. Blood received through a transfusion must match your own. Patients who are scheduled to have major surgery make auto logous blood donation (donations of their own blood) so that they have a perfect match.


Blood performs many important functions within the body including:
*Supply of oxygen to tissues (bound to hemoglobin which is carried in red cells).
*Supply of nutrients such as glucose , amino acids and fatty acids (dissolved in the blood or bound to plasma proteins).
*Removal of waste such as carbon dioxide , urea and lactic acid.
*Immunological functions, including circulation of white cells, and detection of foreign material by antibodies.
*Coagulation , which is one part of the body's self-repair mechanism.
*Messenger functions, including the transport of hormones and the signalling of tissue damage.
*Regulation of body pH (the normal pH of blood is in the range of 7.35 - 7.45).
*Regulation of core body temperature.
*Hydraulic functions.

Red Blood Cells

Red Blood Cells: Riding on the Red Road Red blood cells perform the most important blood duty. A single drop of blood contains millions of red blood cells which are constantly traveling through your body delivering oxygen and removing waste. If they weren't, your body would slowly die.
Red blood cells are red only because they contain a protein chemical called hemoglobin which is bright red in color. Hemoglobin contains the element Iron, making it an excellent vehicle for transporting oxygen and carbon dioxide. As blood passes through the lungs, oxygen molecules attach to the hemoglobin. As the blood passes through the body's tissue, the hemoglobin releases the oxygen to the cells. The empty hemoglobin molecules then bond with the tissue's carbon dioxide or other waste gases, transporting it away.
Over time, the red blood cells get worn out and eventually die. The average life cycle of a red blood cell is 120 days. Your bones are continually producing new blood cells, replenishing your supply. The blood itself, however, is re-circulated throughout your body, not being remade all of the time.
Since the human body is continually making more blood, it is safe for healthy adults to donate blood. The blood is then stored for use in emergency situations. Initially after giving blood, the donor may feel some momentary lightheadedness due to the loss of oxygen-rich red blood cells and blood sugar. The body quickly stabilizes itself.

White Blood Cells

Battling Blood Cells Whenever a germ or infection enters the body, the white blood cells snap to attention and race toward the scene of the crime. The white blood cells are continually on the lookout for signs of disease. When a germ does appear, the white blood cells have a variety of ways by which they can attack. Some will produce protective antibodies that will overpower the germ. Others willsurround and devour the bacteria.
The white blood cells have a rather short life cycle, living from a few days to a few weeks. A drop of blood can contain anywhere from 7,000 to 25,000 white blood cells at a time. If an invading infection fights back and persists, that number will significantly increase.
A consistently high number of white blood cells is a symptom of Leukemia, a cancer of the blood. A Leukemia patient may have as many as 50,000 white blood cells in a single drop of blood.


Platelets: Sticky Situations The human body does not handle excessive blood loss well. Therefore, the body has ways of protecting itself. When, for some unexpected reason, sudden blood loss occurs, the blood platelets kick into action.
Platelets are irregularly-shaped, colorless bodies that are present in blood. Their sticky surface lets them, along with other substances, form clots to stop bleeding.
When bleeding from a wound suddenly occurs, the platelets gather at the wound and attempt to block the blood flow. The mineral calcium, vitamin K, and a protein called fibrinogen help the platelets form a clot.
A clot begins to form when the blood is exposed to air. The platelets sense the presence of air and begin to break apart. They react with the fibrinogen to begin forming fibrin, which resembles tiny threads. The fibrin threads then begin to form a web-like mesh that traps the blood cells within it. This mesh of blood cells hardens as it dries, forming a clot, or "scab."

Calcium and vitamin K must be present in blood to support the formation of clots. If your blood is lacking these nutrients, it will take longer than normal for your blood to clot. If these nutrients are missing, you could bleed to death. A healthy diet provides most people with enough vitamins and minerals, but vitamin supplements are sometimes needed.
A scab is an external blood clot that we can easily see, but there are also internal blood clots. A bruise, or black-and-blue mark, is the result of a blood clot. Both scabs and bruises are clots that lead to healing. Some clots can be extremely dangerous. A blood clot that forms inside of a blood vessel can be deadly because it blocks the flow of blood, cutting off the supply of oxygen. A stroke is the result of a clot in an artery of the brain. Without a steady supply of oxygen, the brain cannot function normally. If the oxygen flow is broken, paralysis, brain damage, loss of sensory perceptions, or even death may occur.


Plasma: The Importance of Plasma It's a straw-colored, clear liquid that is 90 percent water, and it is an essential ingredient for human survival.
It might seem like plasma is less important than the blood cells it carries. But that would be like saying that the stream is less important than the fish that swims in it. You can't have one without the other.
Besides water, plasma also contains dissolved salts and minerals like calcium, sodium, magnesium, and potassium. Microbe-fighting antibodies travel to the battlefields of disease by hitching a ride in the plasma.
Without plasma, the life-giving blood cells would be left floundering without transportation. Never underestimate the importance of plasma.


Blood plasma that escapes from the blood vessels is absorbed into the surrounding tissue. This tissue fluid collects in tubes throughout the body and is known as lymph. From the lymph tubes, it returns to the blood after passing through a lymph node. If you've ever broken a blister, you've seen lymph. It's a colorless, slightly sticky liquid.
Lymph is an important part of the circulatory system. It aids the body's absorption of nutrients and helps to remove waste from the tissue. The lymph collects the body's waste and then deposits it in a lymph node as it passes through. Lymph nodes are clumps of tissue that collect the waste deposits. Your tonsils and adenoids are two examples of lymph nodes.
The lymph nodes also assist the spleen and the bones in producing new white blood cells.

Blood Types

In some ways, every person's blood is the same. But, when analyzed under a microscope, distinct differences are visible. In the early 20th century, an Austrian scientist named Karl Landsteiner classified blood according to those differences. He was awarded the Nobel Prize for his achievements.

ABO System

Landsteiner observed two distinct chemical molecules present on the surface of the red blood cells. He labeled one molecule "A" and the other molecule "B." If the red blood cell had only "A" molecules on it, that blood was called type A. If the red blood cell had only "B" molecules on it, that blood was called type B. If the red blood cell had a mixture of both molecules, that blood was called type AB. If the red blood cell had neither molecule, that blood was called type O.
If two different blood types are mixed together, the blood cells may begin to clump together in the blood vessels, causing a potentially fatal situation. Therefore, it is important that blood types be matched before blood transfusions take place. In an emergency, type O blood can be given because it is most likely to be accepted by all blood types. However, there is still a risk involved.

A person with type A blood can donate blood to a person with type A or type AB.
A person with type B blood can donate blood to a person with type B or type AB.
A person with type AB blood can donate blood to a person with type AB only.
A person with type O blood can donate to anyone.
A person with type A blood can receive blood from a person with type A or type O.
A person with type B blood can receive blood from a person with type B or type O.
A person with type AB blood can receive blood from anyone.
A person with type O blood can receive blood from a person with type O.
Because of these patterns, a person with type O blood is said to be a universal donor.
A person with type AB blood is said to be a universal receiver. In general, however, it is still best to mix blood of matching types and Rh factors.

Rh Factors

Rh Factors: Are You Positive or Negative? Scientists sometimes study Rhesus monkeys to learn more about the human anatomy because there are certain similarities between the two species. While studying Rhesus monkeys, a certain blood protein was discovered. This protein is also present in the blood of some people. Other people, however, do not have the protein. The presence of the protein, or lack of it, is referred to as the Rh (for Rhesus) factor.
If your blood does contain the protein, your blood is said to be Rh positive (Rh+). If your blood does not contain the protein, your blood is said to be Rh negative (Rh-).
This Rh factor is connected to your blood type. For example, your blood may be AB+ which means that you have type AB blood with a positive Rh factor. Or, you might have O- blood which means that you have type O blood with a negative Rh factor.
It is particularly important for expectant mothers to know their blood's Rh factor. Occasionally, a baby will inherit an Rh positive blood type from its father while the mother has an Rh negative blood type. The baby's life could be in great danger if the mother's Rh negative blood attacks the baby's Rh positive blood. If this happens, an exchange transfusion may save the baby's life. The baby's blood can be exchanged for new blood that matches the mother's.


General medical disorders

Disorders of volume

Injury can cause blood loss through bleeding . Thrombocytes are important for blood coagulation and the formation of blood clots which can stop bleeding. Trauma to the internal organs or bones can cause internal bleeding , which can sometimes be severe.

Disorders of circulation

*Shock is the ineffective perfusion of tissues.
*Atherosclerosis reduces the flow of blood through arteries, because atheroma lines arteries and narrows them. Atheroma tends to increase with age, and its progression can be compounded by many causes including smoking, high blood pressure , excess circulating lipids (hyperlipidemia ), and diabetes mellitus .
*Coagulation can form a thrombosis which can obstruct vessels.
*Problems with blood composition, the pumping action of the heart, or narrowing of blood vessels can have many consequences including hypoxia (lack of oxygen) of the tissues supplied. The term ischaemia refers to tissue which is inadequately perfused with blood, and infarction refers to tissue death (necrosis ) which can occur when the blood supply has been blocked (or is very inadequate).

Hematological disorders

*Anemia Insufficient red cell mass (anemia ) can be the result of bleeding, blood diseases like thalassemia , or nutritional deficiencies; and may require blood transfusion . Several countries have blood banks to fill the demand for transfusable blood. A person receiving a blood transfusion must have a blood type compatible with that of the donor.
*Disorders of cell proliferation
Leukemia is a group of cancers of the blood-forming tissues.
Non-cancerous overproduction of red cells (polycythemia vera ) or platelets (essential thrombocytosis ) may be premalignant .
Myelodysplastic syndromes involve ineffective production of one or more cell lines.
*Disorders of coagulation

Hemophilia is a genetic illness that causes dysfunction in one of the blood's clotting mechanisms . This can allow otherwise inconsequential wounds to be life-threatening, but more commonly results in hemarthrosis , or bleeding into joint spaces, which can be crippling.
Ineffective or insufficient platelets can also result in coagulopathy (bleeding disorders).
Hypercoagulable state (thrombophilia ) results from defects in regulation of platelet or clotting factor function, and can cause thrombosis.
*Infectious disorders of blood
Blood is an important vector of infection. HIV , the virus which causes AIDS , is transmitted through contact between blood, semen , or the bodily secretions of an infected person. Hepatitis B and Hepatitis C are transmitted primarily through blood contact. Owing to blood-borne infections , bloodstained objects are treated as a biohazard .
Bacterial infection of the blood is bacteremia or sepsis . Viral Infection is viremia. Malaria and trypanosomiasis are blood-borne parasitic infections.

Carbon monoxide poisoning

Substances other than oxygen can bind to hemoglobin; in some cases this can cause irreversible damage to the body. Carbon monoxide , for example, is extremely dangerous when carried to the blood via the lungs by inhalation, because carbon monoxide irreversibly binds to hemoblobin to form carboxyhemoglobin , so that less hemoglobin is free to bind oxygen, and less oxygen can be transported in the blood. This can cause suffocation insidiously. A fire burning in an enclosed room with poor ventilation presents a very dangerous hazard since it can create a build-up of carbon monoxide in the air. Some carbon monoxide binds to hemoglobin when smoking tobacco.

Medical treatments

Blood products

Blood for transfusion is obtained from human donors by blood donation and stored in a blood bank . There are many different blood types in humans, the ABO blood group system , and the Rhesus blood group system being the most important. Transfusion of blood of an incompatible blood group may cause severe, often fatal, complications, so crossmatching is done to ensure that a compatible blood product is transfused.
Other blood products administered intravenously are platelets, blood plasma, cryoprecipitate and specific coagulation factor concentrates.

Intravenous administration

Many forms of medication (from antibiotics to chemotherapy ) are administered intravenously, as they are not readily or adequately absorbed by the digestive tract.
After severe acute blood loss, liquid preparations, generically known as plasma expanders, can be given intravenously, either solutions of salts (NaCl, KCl, CaCl2 etc...) at physiological concentrations, or colloidal solutions, such as dextrans, human serum albumin, or fresh frozen plasma. In these emergency situations, a plasma expander is a more effective life saving procedure than a blood transfusion, because the metabolism of transfused red blood cells does not restart immediately after a transfusion.


In modern evidence-based medicine bloodletting is used in management of a few rare diseases, including haemochromatosis and polycythemia . However, bloodletting and leeching were common unvalidated interventions used until the 19th century, as many diseases were incorrectly thought to be due to an excess of blood, according to Hippocratic medicine.

Blood Pressure

Blood pressure (strictly speaking: vascular pressure) refers to the force exerted by circulating blood on the walls of blood vessels , and constitutes one of the principal vital signs . The pressure of the circulating blood decreases as blood moves through arteries , arterioles , capillaries , and veins ; the term blood pressure generally refers to arterial pressure, i.e., the pressure in the larger arteries, arteries being the blood vessels which take blood away from

the heart. Arterial pressure is most commonly measured via a sphygmomanometer , which uses the height of a column of mercury to reflect the circulating pressure (see Non-invasive measurement). Although many modern vascular pressure devices no longer use mercury, vascular pressure values are still universally reported in millimetres of mercury (mmHg).
The systolic arterial pressure is defined as the peak pressure in the arteries, which occurs near the beginning of the cardiac cycle ; the diastolic arterial pressure is the lowest pressure (at the resting phase of the cardiac cycle). The average pressure throughout the cardiac cycle is reported as mean arterial pressure ; the pulse pressure reflects the difference between the maximum and minimum pressures measured.
Typical values for a resting, healthy adult human are approximately 120 mmHg (16 kPa ) systolic and 80 mmHg (11 kPa) diastolic (written as 120/80 mmHg, and spoken as "one twenty over eighty") with large individual variations. These measures of arterial pressure are not static, but undergo natural variations from one heartbeat to another and throughout the day (in a circadian rhythm); they also change in response to stress , nutritional factors, drugs , or disease. Hypertension refers to arterial pressure being abnormally high, as opposed to hypotension , when it is abnormally low. Along with body temperature, blood pressure measurements are the most commonly measured physiological parameters.

Prof. Andre Glebovich Vasilev.

Prof. Andre Glebovich Vasilev.
Head of the Department of Pathphysiology in Saint Petersburg State Pedriatric Medical Academy.