Blood is a tissue that circulates through capillaries, veins and arteries. It consists of a liquid phase, the plasma, made up of proteins, and a cellular part in which white blood cells or leukocytes, red blood cells or erythrocytes and platelets are immersed. The function of the blood is vital, since it defends against infections and intervenes in the exchange of gases and the distribution of nutrients to all the organs of the body.
Blood: what it is
Blood (from Latin: sanguis, ĭnis) is a liquid connective tissue that circulates in capillaries, veins and arteries of all vertebrates. Its red color is caused by the presence of the hemoglobin pigment contained in red blood cells.
Blood is constituted by a liquid phase, represented by the blood plasma, where the formed elements or cells are immersed, which include leukocytes (or white blood cells) and cellular derivatives, formed by erythrocytes (or red blood cells) and platelets.
Its main function is the logistics of distribution and systemic integration, whose containment in the blood vessels allows its distribution (blood circulation) to practically the whole organism.
What is blood used for and where does it originate?
Blood, as hematology experts explain, fulfills multiple functions necessary for life, such as defense against infections, gas exchanges (oxygen-carbon dioxide) and the distribution of nutrients.
To fulfill all these functions it has different types of cells suspended in the plasma. It should be noted that all the cells that make up the blood are manufactured in the bone marrow. This is found in the spongy tissue of the flat bones (skull, vertebrae, sternum, iliac crests) and in the medullary canals of the long bones (femur, humerus).
Blood is a tissue in continuous renewal in the human organism, which means that the bone marrow is manufacturing, throughout life, blood cells, since they have a limited life span. This “factory”, in certain situations of need, can increase its production. For example, in the event of a hemorrhage, the production of red blood cells increases up to seven times, and in the event of an infection, the production of white blood cells increases.
What blood is made up of
Like all tissue, blood is composed of cells and extracellular components. These two tissue fractions are represented by:
– The formative elements: represented by cells and cell-derived components.
– Blood plasma: a translucent, yellowish fluid representing the liquid extracellular matrix in which the formate elements are suspended.
The formative elements constitute about 45% of blood. This percentage is known as the hematocrit (“cellular” fraction), which is almost entirely attributable to the erythrocyte mass. The other 55% is represented by the blood plasma (acellular fraction).
The formed elements of the blood are varied in size, structure and function, and are grouped as follows:
– Blood cells, which are the white blood cells or leukocytes, are cells that are “passing through” the blood to perform their function in other tissues.
– Cell derivatives, which are not strictly cells but cell fragments, are erythrocytes and platelets. They are the only blood components that perform their functions strictly within the vascular space.
1) White blood cells (leukocytes). They are part of the cells of the immune system. They are cells with migratory capacity that use the blood as a vehicle to gain access to different parts of the body. They are responsible for destroying infectious agents and infected cells. In addition, they secrete protective substances such as antibodies, which fight infection. The normal leukocyte count is between 4,500 and 11,500 cells per mm³ (microliter) of blood, which varies according to physiological (pregnancy, stress, sport, age…) and pathological conditions (infection, cancer, immunosuppression, aplasia, etc.). According to the microscopic characteristics of their cytoplasm (tinctorial) and nucleus (morphology) they are divided into:
1.1. agranulocytes or monomorphonuclear cells: these are the lymphocytes and monocytes. They have no granules in the cytoplasm and have rounded nuclei:
(a) Lymphocytes. The normal value is between 1,300 and 4,000 per mm³ (from 24 to 32% of the total white blood cells). Their number increases in viral infections and cancers and may decrease in immunodeficiencies. They are responsible for doing the work of the immune system. There are two types:
– B lymphocytes: They are in charge of the secretion of antibodies (substances that recognize bacteria, binding to them to destroy them). They are also the cells in charge of producing the components of blood serum, called immunoglobulins.
– T lymphocytes: They are in charge of recognizing virus-infected cells and destroying them, with the help of macrophages. These lymphocytes amplify or suppress the overall immune response, regulating the other components of the immune system. They constitute 70% of lymphocytes.
b) Monocytes. The normal count is between 150 and 900 cells per mm³ (2 to 8% of the total white blood cells), a figure that is elevated by infections caused by viruses or parasites, but also in some tumors and leukemias. They are cells with a defined nucleus.
1.2. Granulocytes or polymorphonuclear cells: these are the neutrophils, basophils and eosinophils. They are cells with polymorphous nucleus and many granules in the cytoplasm.
a) Neutrophils. They are present in the blood between 2,500 and 7,500 cells per mm³. They are the most numerous, occupying between 55 and 70% of leukocytes. They are responsible for capturing and digesting foreign substances (bacteria or external agents) that enter the organism. When there is an infection or inflammation their number increases in the blood.
b) Basophils. They are present in the blood between 0.1 and 1.5 cells per mm³ (0.2- 1.2% of leukocytes). They secrete substances such as heparin (with anticoagulant properties) and histamine (which contributes to the inflammation process).
c) Eosinophils. They are present in the blood between 50 and 500 cells per mm³ (1-4% of leukocytes). They are increased in diseases caused by parasites, in allergies and in asthma.
1) Red blood cells (erythrocytes). They are in the blood and transport oxygen to the rest of the cells. In humans they are formed in the bone marrow. They constitute, approximately, 96% of the elements figured. Their normal value is around 4,800,000 in women and approximately 5,400,000 in men, red blood cells per mm³. They are not cells as such, since they lack a nucleus (they expel it in the bone marrow before entering the bloodstream). Their cytoplasm consists almost entirely of hemoglobin, a protein responsible for transporting oxygen, and also contains enzymes.
2) Platelets (thrombocytes). They are small, oval cell fragments without nucleus. They are produced in the bone marrow from the fragmentation of the cytoplasm of megakaryocytes, and remain free in the blood circulation. Their normal value ranges from 250,000 to 450,000 platelets per mm³. Platelets serve to plug lesions that could affect the blood vessels, clotting the blood. Thus, when a circulatory vessel is ruptured, platelets envelop the wound to reduce its size and prevent bleeding. In the coagulation process, platelets form clots (thrombi), which are responsible for the closure of vascular wounds (thrombosis). To get an idea, a drop of blood contains about 250,000 platelets.
On the other hand, there is the blood plasma, which is the liquid portion of the blood in which the aforementioned formed elements are immersed, composed of 91% water, 8% proteins and traces of other materials. It is the major component of blood and represents 55% of the total blood volume. In addition to transporting the blood cells, it carries the nutrients and waste substances it collects from the cells.
Among the 8% of proteins it contains are: amino acids, carbohydrates, lipids, salts, hormones, enzymes, antibodies, urea, gases in solution and inorganic substances, such as sodium, potassium, calcium chloride, carbonate and bicarbonate. Also, among these proteins are:
– Fibrinogen (for coagulation).
– Globulins (regulate water content in the cell and form antibodies against infectious diseases).
– Albumins (exert osmotic pressure to distribute water between plasma and body fluids)
– Lipoproteins (buffer blood and cell pH changes, and make blood more viscous than water)