Blood is a liquid connective tissue , also called fluid tissue , that flows in the human body inside the blood vessels (arteries and veins). It is classified as tissue because it is a collection of similar specialized cells that perform specific functions and are nevertheless suspended in a liquid matrix ( plasma ), an aspect that makes blood a fluid. On average, in the human body of an adult there are 4 to 5 liters of blood and these represent approximately 7% of the total weight of the body. Moving inside the vessels, it reaches all the organs and tissues, performing many functions essential for life. In addition to providing oxygen , a fundamental element for the life of cells, blood allows the transport of nutrients and the removal of harmful ones. The cells and molecules of the immune system that defends the body from microorganisms and other harmful elements also flow in the blood. The composition of this fluid tissue is given by the union of different elements, divided mainly into two categories based on the physical state: a liquid part , made up of plasma (about 55% in men and about 60% in women), and a corpuscular part , made up of three different types of cells (red blood cells, white blood cells and platelets) which together represent 45% of the total in men and 40% in women. The percentage of plasma and that of the corpuscular part also vary according to other characteristics. The functions performed by blood within the body are many, and all essential: transport of oxygen and nutrients throughout the body, maintenance of body homeostasis (body temperature) and defense of the organism, thanks to the presence of the elements that make up the immune system.
What is hematocrit?
The percentage of blood volume occupied by red blood cells is called hematocrit . Having a hematocrit of 45% means that for every 100 mL of blood, you have a volume of red blood cells equal to 45 mL, which is considered equal to that of the entire corpuscular component of the blood due to the fact that platelets and white blood cells represent a very limited component. The hematocrit is part of the set of tests that are part of the complete blood count, together with hemoglobin , red blood cell count, white blood cells and platelets.
It is a very important value to analyze and monitor because, in addition to being an indicator of the body’s state of hydration , it can be a warning sign of some diseases. A value that is too low is detected, for example, in anemic subjects , in whom there is a reduction in circulating red blood cells, while a high percentage is present in the case of polycythemia .
Blood that is too thick , corresponding to a high hematocrit, requires more effort from the heart and can contribute to the onset of heart disease, or damage blood vessels due to the stress they are subjected to, increasing the risk of cerebral hemorrhages . Furthermore, a high hematocrit promotes the formation of clots and a consequent increase in the risk of heart attack or stroke .
If, on the other hand, the blood is too fluid , that is, due to low hematocrit values (few red blood cells), the symptoms that are found are weakness , headache , fatigue and paleness.
Transport of oxygen and substances necessary for the life of cells and tissues
Oxygen is transported in the blood , a fundamental element for keeping cells alive and functioning. Oxygen is transported through red blood cells, cells specialized for this function thanks to the presence of a protein, hemoglobin (indicated by the acronym Hb or Hgb), capable of binding this molecule.
Responsible for the red color of blood, hemoglobin has a structure formed by four protein chains, each of which, in turn, is connected to a heme group (culminating with an Fe 2+ ion). Each heme group is capable of binding an oxygen molecule, therefore each hemoglobin molecule transports four oxygen molecules.
At the level of the pulmonary alveoli , red blood cells bind the oxygen introduced with inspiration to carry it to the peripheral tissues, where it is used to perform the functions that keep cells alive. Here, hemoglobin binds to carbon dioxide, a waste product, and carries it to the lungs, where it is eliminated during expiration.
Through the walls of the intestine, digested substances are released into the blood, which are then transported throughout the body and used by cells. The blood also takes care of the transport of waste substances: after having recovered them from peripheral tissues, it conveys them to the kidneys and liver so that they can be processed and transformed. Hormones are released into the blood , substances that regulate the functionality of various organs.
Homeostasis: an organism with stable parameters
Homeostasis is the set of functions that ensure stability of vital parameters in the organism.
One of the variables that are kept constant by homeostatic systems is represented by temperature . When body temperature exceeds 37°C, special sensors in the body warn the brain of the event, which activates the vasodilation mechanism : the superficial blood vessels increase in diameter and get closer to the skin surface, helping to disperse heat. When, on the contrary, body temperature drops below 37°C, the vasoconstriction mechanism is activated : the vessels narrow in diameter and limit their contact with the body surface, reducing heat dispersion.
Body defense (immunity)
In the bloodstream, there are cells (white blood cells) and substances (antibodies and other molecules of various types) that help defend the body from foreign or harmful agents, be they viruses , bacteria, mutated cells that could lead to the development of a tumor. The immune system also includes cells and substances capable of digesting and eliminating debris that derives from this defense activity and promoting the repair of damaged tissues. Platelets participate in this latter phenomenon, thanks to their role in the formation of the clot, which allows the blocking of blood leakage from wounds and the activation of repair systems.
Venous blood and arterial blood
Blood circulates in the cardiovascular system , which consists of the heart and blood vessels (arteries, veins, and capillaries). When the heart contracts, it pushes blood into the arteries so that it reaches all the tissues of the body. During the release phase, blood enters the heart that returns from the various organs of the body, through the veins. The blood that reaches the right atrium of the heart is venous: it carries waste substances from the metabolic activity of the peripheral tissues and is characterized by a bluish-purple red color due to the low oxygen (O₂) content and the high carbon dioxide (CO₂) content. Arterial blood, which reaches the left heart, is bright red because it is rich in oxygen (O₂). Arterial and venous blood never mix inside the heart: while the first remains in the left heart, the second travels to the right heart.
Blood oxygenation occurs in the capillaries of the pulmonary alveoli, where the CO₂-O₂ gas exchange occurs. Venous blood releases carbon dioxide (which is emitted by the body with exhalation) and hemoglobin loads the oxygen present in the lungs thanks to the inhalation of air. The oxygenated blood then returns to the left heart from which it is pumped throughout the body. The
amount of oxygen in the blood, technically called oxygen saturation , is a very important value because it expresses the level of respiratory function. Oxygen saturation indicates the percentage of hemoglobin saturated with oxygen at the time of measurement, i.e. the percentage of red blood cells in the blood that are transporting oxygen throughout the body.
The most accurate method for measuring the concentration of circulating oxygen is represented by the arterial blood sample , generally taken from the radial artery of the wrist. An alternative and less invasive, but less precise, method is the use of a pulse oximeter (or pulse oximeter). This is a type of clamp that is applied to a finger and that, thanks to an infrared light sensor, is able to detect the percentage of hemoglobin saturated with oxygen and, in some cases, also the heart rate by passing through the tissues. This tool has been widely used during the pandemic to monitor the evolution of the disease at home.
In healthy subjects, normal blood oxygen saturation values are between 97 and 99% but, in some conditions, values above 95% are still considered acceptable.
When oxygen in the blood is present in a lower percentage, hypoxemia is present, which can be mild (between 94 and 91%), moderate (between 90 and 86%) or severe (less than 90%). In case of hypoxemia, it is advisable to undergo blood gas analysis (ABG) .
In patients with advanced chronic respiratory diseases, such as chronic obstructive pulmonary disease (COPD) , blood oxygen levels can be considered normal (compatible with the disease) even if around 90%. Hypoxemia can also manifest itself with symptoms such as shortness of breath, shortness of breath, increased respiratory rate, difficulty breathing, as well as confusion, headache, tachycardia and increased blood pressure.
What is blood made of?
The main components of blood are:
- Plasma : is the liquid part of the blood;
- Corpuscular component : It is composed of blood cells (red blood cells, white blood cells and platelets), which are produced in the red bone marrow, the tissue contained within the long bones and flat bones (including the femur and pelvis).
Plasma, the liquid part of the blood
Plasma is defined as the liquid component of blood because it is made up of 90-95% water . The remaining 5-10% is composed of proteins (such as albumin and globulins and other elements that contribute to coagulation), mineral salts, sugars, vitamins, antibodies and hormones. The main functions of plasma include the transport of substances and coagulation
factors , in addition to maintaining acid-base balance . Approximately 55% of the total circulating blood is made up of plasma, the concentration of which is kept constant, within certain values, through the regulation of the thirst mechanism and the control of renal excretion . The protein fraction of blood is mainly made up of:
- Albumins (4.5 grams per 100mL): produced in the liver, these proteins regulate the passage of water into the tissues and bind and transport numerous substances (hormones, drugs, bilirubin, etc.) into the plasma;
- Globulins (2.7 grams per 100 mL): their synthesis occurs in the liver and lymphoid tissue. They are divided into three fractions: α, β (which have a transport function) and γ (also called immunoglobulins because they are the antibodies that, produced by plasma cells, are part of the immune system). Transferrin, for example, belongs to the beta class and carries out the function of iron transporter in the plasma;
- Fibrinogen (2.25 grams per 100mL): is a protein synthesized in the liver and involved in the coagulation process.
Red blood cells to transport oxygen
Red blood cells are also called erythrocytes or red blood cells . They are the most numerous elements in the blood: about 4.5 million per cubic millimeter in women and 5 million in men. The different concentration of red blood cells and hemoglobin in the two sexes is due to the greater presence of testosterone in the male organism, which stimulates erythropoiesis, or the formation of new red blood cells.
Red blood cells are produced in the bone marrow through a process called erythropoiesis (the term hematopoiesis refers to the synthesis of all blood cells). They live for an average of three months and are continually renewed: just think that 25,000 billion of them are regenerated every day . The “old” red blood cells, after having traveled about 400 km, are eliminated in the spleen and liver by specialized cells of the immune system (macrophages). These are cells without a nucleus, which is lost during hematopoiesis (maturation of the corpuscular elements). The lack of a nucleus allows more space for hemoglobin, while the biconcave shape increases the surface area of the cell, given the same volume. These characteristics serve to make the diffusion of oxygen more efficient. Red blood cells are elastic and deformable to be able to pass through capillaries, the smallest blood vessels.
Inside them there is hemoglobin , a protein that contains iron, capable of binding oxygen and making the process of cellular respiration possible: red blood cells take oxygen from the lungs to carry it to the tissues and release waste carbon dioxide into the lungs.
The characteristic color of blood is given by the iron found inside hemoglobin: as proof of this, the fact that anemic subjects , or those who have lower than normal levels of iron in their blood, are generally pale. The presence of particular proteins (antigens) on the external surface of erythrocytes allows the blood group of each subject to be defined.
When these cells assume abnormal shapes, even severe diseases can occur, such as sickle cell anemia .
Hemoglobin. Normal, high and low values
This protein is a fundamental indicator of health, which is why its dosage is often prescribed when performing blood tests .
Normal hemoglobin values at birth are approximately 16 g/100 mL , decreasing slightly in the following weeks to remain around 12 g/100 mL until the age of 10 years .
After puberty, the values increase, to decrease around the age of 50. Around the age of 65, the blood hemoglobin levels return to very low levels, approximately 12 g/100 mL.
The average hemoglobin values in adults are: 13.9 g/100 mL (in women) and 15.8 g/100 mL (in men).
Other important values that are usually detected by performing a complete blood count are represented by:
- MCV: the mean corpuscular volume of red blood cells;
- MCH: mean hemoglobin content of red blood cells in the blood;
- RDW : measures the mean size distribution width of red blood cells.
Hemoglobin electrophoresis is a technique that allows you to evaluate the presence of pathologies such as Mediterranean anemia or beta-thalassemia and other forms of anemia.
Too high hemoglobin values can be associated with diarrhea , smoking or , in the most serious cases, liver cancer , burns and heart failure .
When we talk about low hemoglobin, however, we almost always refer to a form of anemia that can be caused by multiple causes, including nutritional factors, bleeding, problems in the synthesis of red blood cells and hemoglobin, radiotherapy and chemotherapy and kidney disease.
When evaluating the results of blood tests, however, it must be taken into account that hemoglobin values can be influenced by various factors , including inadequate hydration, smoking habits, age, sex, and the altitude of the place where you are. Furthermore, within certain limits, it is physiological that women of childbearing age tend to have lower hemoglobin levels than prepubertal and postmenopausal women, due to monthly bleeding.
White blood cells or leukocytes
White blood cells are also called leukocytes and are cells with a nucleus.
Produced in the bone marrow, white blood cells are larger than erythrocytes, but are present in lower concentrations (5,000 to 7,000 per cubic millimeter). The main task of leukocytes is to defend the body against pathogens, a process that includes the containment of infectious processes (through the elimination of bacteria, viruses and fungi) and the synthesis of antibodies.
There are three different types of white blood cells:
- Granulocytes (Neutrophils, Eosinophils and Basophils);
- Monocytes;
- Lymphocytes.
Leukocytes are formed from hematopoietic stem cells that differentiate into a lymphoid lineage (from which B and T lymphocytes develop) and a myeloid lineage (from which all other white blood cells, but also red blood cells and platelets develop). The maturation of leukocytes requires the intervention of organs that are part of the immune system: the lymph nodes , the tonsils , the thymus (an endocrine gland located in the retrosternal area) and the spleen .
The leukocyte formula that can be performed on a blood sample taken from a vein in the arm allows us to distinguish the concentrations of each of these cell types.
White blood cells: normal, high (leukocytosis) and low (leukopenia) values
Normal values for the concentration of leukocytes range from 4,500 to 11,000 elements per µL , with different values depending on the cell type.
A white blood cell count above the norm ( leukocytosis ) can have various causes, but generally it is observed when there is an ongoing bacterial infection, whether acute or chronic.
The excessive presence of only eosinophilic leukocytes, on the other hand, is characteristic of allergic forms, while a significant increase in leukocytes without apparent infectious symptoms can be among the signs of a blood neoplasm (leukemia) or a bone marrow disease that causes a release of still immature white blood cells.
A concentration of white blood cells below the norm is called leukopenia and can have various causes. For example, it can be determined by infections (such as HIV ), congenital and acquired diseases of the bone marrow, autoimmune diseases, neoplastic diseases, taking certain drugs (e.g. antibiotics ), chemotherapy, etc. To establish the cause of leukopenia and formulate a diagnosis, it is therefore necessary to associate this test with other laboratory and possibly instrumental investigations, which allow for the evaluation of the general clinical picture of the patient together with the age and the possible coexistence of chronic pathologies.
Leukocytes in urine
In general, below certain limits, the presence of leukocytes in the urine is physiological.
However, if the concentration exceeds 10 units per milliliter of urine , it could be a sign of a urinary tract infection ( UTI ), such as cystitis , or other clinical conditions such as diabetes or polycystic kidney disease.
Platelets or thrombocytes and coagulation
Platelets, also called thrombocytes, are present in the blood with a concentration of about 250,000 elements per cubic millimeter. They are not true cells, but fragments of cytoplasm of larger cells ( megakaryocytes ) located in the red marrow. Their average life is 10 days (compared to 120 for red blood cells), after which they are phagocytized or destroyed by macrophages, especially in the liver and spleen, organs in which about a third of the total platelet mass is present. Every day, 30,000 to 40,000 platelets are produced per cubic millimeter; if necessary, this synthesis can increase up to 8 times.
Thrombocytes participate in the mechanism of hemostasis, that is , the blood coagulation process that occurs physiologically in the event of an injury: by gathering at the wound, they form a sort of plug that prevents the blood from flowing out. At a later stage, they begin the actual healing process.
Platelets: Normal, High and Low Values
In one milliliter of blood there are normally 150,000 to 400,000 platelets . The reference interval of the test, however, can change depending on age, sex and equipment used in the analysis laboratory. For this reason, the limit values reported on the report must be considered. Furthermore, the results of the tests must be evaluated as a whole by the doctor.
Thrombocytopenia or thrombocytopenia
A low platelet count (thrombocytopenia or thrombocytopenia) increases the risk of spontaneous bleeding when the platelet count falls below 10,000-20,000, but even values below 50,000 can put you at risk from cuts and other trauma.
Thrombocytopenia can be due to reduced cell production due to a bone marrow problem or increased destruction due to liver or spleen problems .
The most common causes of low platelet count include infections, anemia, vitamin B12 deficiency , and more serious conditions such as bone marrow tumors (leukemia and lymphoma). An increased rate of destruction can be attributed to autoimmune diseases, serious bacterial infections, and drug intake. Chronic bleeding problems ( for example caused by gastric ulcers or intestinal polyps and less frequently by recurrent bleeding from the anus due to the presence of hemorrhoids or from the nose and gums during pregnancy and for various other reasons) can also cause a reduction in the number of circulating platelets.
Thrombocytosis or thrombocythemia
A high concentration of platelets ( thrombocytosis or thrombocytosis or thrombocythemia) increases cardiovascular risk because it promotes the formation of clots that can block blood flow in the smallest vessels, a phenomenon that can cause heart attacks and strokes.
This condition can occur due to a rare disease, essential (or primary) thrombocythemia , which involves an excessive production of platelets by the bone marrow. Or due to inflammatory pathologies (for example rheumatoid arthritis ), infectious (tuberculosis), tumors or in the case of iron deficiency anemia (due to iron deficiency), surgical removal of the spleen, adverse reaction to drugs (such as the contraceptive pill ).
A temporary increase in platelets can be due to trauma or major surgery, intense physical effort or excessive alcohol consumption .
Blood and blood type
An individual’s blood group is determined by the presence of antigens (or agglutinogens or erythrocyte antigens) inside the red blood cells, thanks to which the different blood groups are distinguished.
The types of agglutinogens that exist are A and B:
- Subjects characterized by the presence of the A antigen belong to the A blood group;
- Those who possess the B antigen belong to group B;
- Individuals who have both are of blood type AB;
- Those who do not present any antigen belong to group 0.
In plasma there are antibodies, called agglutinins, which have the task of agglutinating germs and cells foreign to the organism, including red blood cells belonging to a blood group different from that of the individual, in order to make them unable to perform their functions. To prevent the risk of hemagglutination, it is therefore necessary that blood transfusions
only occur between individuals of compatible blood groups. In contrast to antigens , agglutinins can be anti-A and anti-B:
- Individuals belonging to group A present anti-B agglutinin in their plasma;
- Those belonging to group B have anti-A agglutinin;
- Individuals with blood group 0 possess both types of agglutinin;
- Those in group AB don’t even have one.
For the reason just described, people with blood group 0 are universal blood donors , as their antigen-free red blood cells cannot be attacked by any of the antibodies present in the recipient’s plasma. On the contrary, people with blood group AB are universal recipients: having no agglutinin in their plasma, they can accept red blood cells characterized by any type of antigen. People with blood group A can donate to people of groups A and AB and receive transfusions from groups 0 and A; similarly, people belonging to group B can donate to people of groups B and AB and receive transfusions from groups 0 and B.
However, this subdivision does not provide a complete picture of the compatibility between blood groups, as there is an additional agglutinogen on the red blood cells (the Rhesus factor , Rh) discovered in 1940. We speak of a Rh+ subject when this antigen is present (about 85% of people ) and of a Rh- subject when it is not present ( 15% of individuals).
In order to provide a complete overview of group compatibility , it is useful to analyze them one by one.
- An individual with group AB Rh+ , having all the antigens and no agglutinin, can receive blood from any other individual, but can donate it only to another person with group AB Rh+;
- An individual with group AB Rh- , due to the absence of Rh agglutinogen, can donate to both AB Rh+ and AB Rh- subjects and can receive transfusions from any type of group, as long as they are Rh-;
- Group A Rh+ allows transfusions to groups A and AB with Rh+ and can receive blood from groups A and 0, both with Rh positive and negative;
- A subject with group A Rh- can donate to groups A and AB both Rh+ and Rh-, but can only receive from groups 0 and A Rh-;
- Group B Rh+ allows transfusions to groups B and AB with Rh+ and can receive blood from groups B and 0, both with Rh positive and negative. Group B Rh-, on the other hand, can donate to groups B and AB both Rh+ and Rh-, but can only receive from groups 0 and B Rh negative;
- Individuals with blood group 0 Rh+ can donate to any individual, as long as they are Rh+, but can only receive blood transfusions from groups 0 Rh both positive and negative;
- As regards group 0 Rh- , donation is possible towards any other group, as there is no antigen present, but blood can only be received from individuals with group 0 Rh-.
Blood donation and plasma donation (plasmapheresis)
Currently, it is not yet possible to produce blood artificially : it is therefore important that citizens are informed and made aware of the importance of donating . Blood transfusions are essential for the treatment of various conditions: from trauma to burns to some forms of anemia.
Every day in Italy, approximately 1,800 transfusions are performed , for an annual requirement of over 2.4 million units of whole blood and more than 800,000 liters of plasma. In our country, blood collection is carried out on the basis of voluntary, unpaid donations. To become a donor , it is sufficient to be in good health, be at least 18 years old and under 70 and weigh at least 50 kilos.
Those who have risky behaviors (drug use, alcoholism, sexual intercourse with a risk of transmitting infectious diseases) or are diagnosed with infections (AIDS, hepatitis B or C, etc.) or are undergoing treatment with steroids or other anabolic hormones are excluded from donating. Each bag of blood collected is subjected to a series of tests and processes to eliminate any possible risk of contamination and to prevent the patient from being transfused with infected blood or blood containing potentially harmful substances.
The following can be donated in local centers :
- Whole blood;
- Plasma (plasmapheresis);
- Platelets (plateletpheresis);
- More blood components.
Blood and its components represent an indispensable therapy for treating some forms of anemia, tumors, hemorrhage (platelets), burns. In the case of a deficiency of individual elements (coagulation factors, factor VIII and IX for hemophilia A and B, immunoglobulins, albumin), it is possible to transfuse plasma or plasma derivatives.
Plasma as a cure for Covid-19
The administration of hyperimmune plasma has been tested in the treatment of patients affected by Sars-Cov-2 coronavirus infection . This is plasma donated by convalescent patients who, having recovered from Covid, have plasma rich in neutralizing antibodies, that is, capable of eliminating the virus. The use of plasma for therapeutic purposes is not at all new.
Unfortunately, the results of these clinical studies have not been exciting and the idea of using hyperimmune plasma transfusions with this indication has been abandoned.
BIBLIOGRAPHICAL REFERENCES
- Kohsuke Hagisawa et al. Artificial blood transfusion: A new chapter in an old story. The Physyiological Society
- National Blood Center
- Kashif Tousif et al. The relationship between knowledge, attitude, and practice of plasma donation with age and gender as moderators: a cross-sectional survey. Annals of Medicine and Surgery. (2023)
Questions and Answers
What is blood and what is it made up of?
Blood is a liquid connective tissue that flows in the human body inside the blood vessels (arteries and veins). On average, in the human body of an adult there are 4-5 liters of blood, which represents about 7% of the total body weight. Blood is carried to all organs and tissues of the body, to which it brings oxygen and nutrients and from which it removes metabolic waste products. The components of the immune system also flow in the blood. This tissue is made up of a liquid part (plasma) and a corpuscular part (composed of red blood cells, white blood cells and platelets).
What functions does blood perform?
Blood performs functions in transporting oxygen and nutrients throughout the body, maintaining homeostasis, and providing defense.
How can you increase red blood cells, a sign of anemia?
If caused by nutritional deficiencies, they can be treated by administering the deficient elements (iron, vitamin B12, folic acid), by increasing the intake of foods that contain them in the diet (meat, fish, eggs, milk, legumes, spinach) or by prescribing specific supplements.
What is the difference between plasma donation and blood donation?
Plasma donation is called plasmapheresis and consists of the infusion of a blood bag from which the corpuscular part has been removed through a centrifugation procedure. The collection of plasma from a donor takes longer than that of whole blood, because it involves passing through the centrifuge and the subsequent reinfusion of the corpuscular component into the donor. The volume of plasma that is collected is equal to 500 mL in males and 300 mL in females. This is a quantity that can be quickly reintegrated by drinking liquids before and after the collection: this makes donating blood or plasma a safe procedure for the health of those who undergo it.
Why is it important to donate plasma?
From plasma, numerous drugs known as plasma-derived drugs can be obtained, including coagulation factors, antibodies and various types of proteins.
