The blood is the liquid light that runs through all the veins and arteries (also called blood vessels) of our circulatory system. Its healthy appearance is deep red, which in thinner layers shows an orange coloration. It is responsible for carrying oxygen and other nutrients to all cells of the body, and performs regeneration and defense functions .Blood represents approximately 8% of body weight and its volume is 5L in a healthy adult. It is a mixture of several components: plasma , which is the predominant fluid, erythrocytes or red blood cells , leukocytes or white blood cells, and thrombocytes or platelets . Each of them fulfills a specific function.

What is blood for?

Blood is a form of connective tissue that acts by exerting several specific functions thanks to plasma and the cells that make it up.

In general, blood is used to:

  • Keep the composition of body fluids constant
  • Carry hormones and other biologically active substances
  • Regulate the function of organs such as the liver, bone marrow, and endocrine glands
  • Carry incoming oxygen (O 2 ) through respiration
  • Transport the carbon dioxide (CO 2 ) that comes out through the breath
  • Defend the body from pathogens
  • Prevent bleeding through clotting
  • Maintain the integrity of the vascular endothelium (prevents blood vessels from tearing, regenerates their internal tissue)

To better understand the functions of blood, a description of its components is needed.

Components of blood

Blood is the mixture of a series of well-differentiated components that are essential for its purposes. Among them are:

  1. Plasma
  2. Red blood cells
  3. White blood cells: Agranular (lymphocytes, monocytes), Granular (neutrophils, eosinophils and basophils)
  4. Platelets

1. Plasma

Plasma is the fluid of the blood, and comprises 55% of its volume. Its chemical composition is:

  • 90% water
  • 7% protein (fibrinogen, albumin and globulins)
  • 3% inorganic salts

In the plasma are the nutritional substances that come from the digestive system, the waste substances produced by the tissues and hormones. When blood is in contact with air or circulation is interrupted, fibrinogen precipitates in the form of a network or fibrin , leading to clotting .

When this phenomenon occurs, a clear, yellowish liquid called blood serum is obtained from the coagulated plasma .

2. Red blood cells

Red blood cells, also called erythrocytes or red blood cells, are cells that have lost all their organs during their maturation. Their appearance is greenish-yellow but in dense masses they acquire a red color , due to the high concentration of hemoglobin they carry. This pigment is prepared by the blood cells themselves, in a process called hemolysis .

In mammals, erythrocytes appear in the form of biconcave discs , and in profile they appear as elongated bodies with rounded ends. The size in the fresh state is 6 to 8 microns and in the smears it decreases to 7 microns due to the dehydration they suffer.

Erythrocytes have the physical property that they tend to adhere to each other, forming columns like stacks of coins, known as rouleaux . This adhesion is associated with the surface tension of their membranes.

Erythrocytes can vary in size, shape, and content . It is called anisocytosis when the red blood cells in a blood smear have different diameters. The poikilocytosis is the variation in the shape of erythrocytes which can be sickle, spherical or flattened.

The variation in its content refers to changes in the concentration of hemoglobin. Hypochromic red blood cells (with little color) are poor in hemoglobin and hyperchromic (with a lot of color) are rich in hemoglobin.

The red blood cell membrane is semipermeable and active transport of some substances takes place through it. These carry oxygen to the tissues and CO 2 to the lungs . They have a half-life of 120 days and are destroyed in the spleen, liver and bone marrow, by macrophages and not in the blood. The formation of new red blood cells is called erythropoiesis .

3. White blood cells

White blood cells or leukocytes are cells with a nucleus that are found in much smaller numbers than erythrocytes. The average number of leukocytes in healthy blood is 5,000 to 10,000 per cubic millimeter (mm 3 ). In disease states, these numbers are usually higher.

There are two main types of leukocytes: agranular leukocytes and granular leukocytes . This division is due to the presence of specific granules in its cytoplasm, and is used in most academic books.

3.1 Agranular leukocytes

Among the agranular leukocytes are lymphocytes , which are spherical cells that in human blood reach a diameter of 6 to 8 micrometers and represent 26% to 40% of all blood leukocytes. These are the small lymphocytes, and there are also medium and large lymphocytes, which are 10 to 12 microns in diameter.

The lymphocytes can be divided according to their function in different subcategories, each of which serves a different purpose in the immunological mechanisms:

  • T lymphocytes , which mature in the thymus, are responsible for searching for antigens.
  • B lymphocytes , which are programmed to recognize a single antigen. When stimulated by antigen, they transform into plasmablasts that subsequently divide into antibody-producing plasma cells.

Other agranular leukocytes are monocytes , which are cells that measure between 9 and 12 microns in diameter, although they can reach 20 microns in dry smears. They comprise only 2 to 8% of the leukocytes in the blood. Due to their phagocytic capacity , monocytes occupy a place among the cells that take part in the defense of the organism.

3.2 Granular leukocytes

Unlike lymphocytes and monocytes, granulocytes contain specific granules in their cytoplasm that characterize them, as well as a polymorphic nucleus, for which reason they are sometimes called polymorphonuclear leukocytes. These include neutrophils , eosinophils, and basophils .

The neutrophils are the most abundant leukocyte, ranging between 55% and 65% of the total leukocytes, and its diameter varies between 10 and 15 microns. They have bactericidal and phagocytic capacity . Although other leukocytes also present polymorphism in the nucleus, neutrophils are called polymorphonuclear cells , as they have multiple lobulations in their nucleus.

The eosinophils are named for their affinity with eosin. When fresh, they are 9 to 10 micrometers in diameter. These cells comprise 1% to 3% of the total blood leukocytes. Although they do not possess phagocytic activity, they are known to be capable of phagocytizing antigen-antibody complexes and to participate in defense mechanisms.

The basophils are the most difficult to observe cells. They comprise 0% to 1% of leukocytes and their size is between 10 and 12 microns. Their function is not well defined, although they are known to release heparin and histamine into the circulating blood, which is why they are considered to be related to mast cells in connective tissue.

4. Platelets

Platelets are corpuscles without a nucleus in the form of biconvex, round or oval discs, whose diameter is between 1.5 and 3 micrometers. Seen in profile, they are shaped like a cane. They intervene in hemostasis, through the substances they release to stimulate the contraction of injured vessels and prevent blood loss.

Also by means of agglutination at the point of injury of the endothelium, in a way that favors a solution of continuity. They participate in the formation of thromboplastin , one of the fundamental steps in the initiation of coagulation. For this reason they are also called thrombocytes , because they help heal wounds and clot blood.

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