Blood is connective tissue, flowing through capillaries, veins, and arteries of all vertebrates. The red color is characteristic due to the presence of hemoglobin in the red blood cells.

It is a type of connective tissue specialized with a matrix colloidal liquid and a complex constitution. It is a solid phase (formed elements), which includes the erythrocytes (or red blood cells), the leukocytes (white blood cells) and platelets, and a liquid phase, represented by blood plasma. 

These phases are also called blood components, which are divided into the serum components (liquid phase) and cellular component (solid phase).

Its main function is the logistics of distribution and systemic integration, whose containment in the blood vessels (vascular space) supports its distribution (blood circulation) to almost the entire body.

Blood composition

Functions of Blood:

  • Like all body tissues, blood serves multiple functions necessary for life as a defense against infection, gas exchange and distribution of nutrients. To fulfill all these functions has different types of cells suspended in plasma.
  • All cells that comprise the blood are produced in the bone marrow. This is in the spongy tissue of the flat bones (skull, vertebrae, sternum, iliac crests) and in the medullar canals of long bones (femur, humerus).
  • The blood is a renewable tissue of the human body; this means that the bone marrow is making, throughout life, blood cells because they have a limited life span.
  • This health “factory” in certain situations can increase production depending on the needs. For example, before bleeding increases up to seven times the production of red blood cells to infection increases the production of white blood cells.

Blood composition(or) Blood Components

Like any tissue, blood consists of cells and extracellular components (the extracellular matrix). These two tissue fractions (or) blood components are represented by:

  • Cellular fractions: The formed elements, also called figurative elements are semisolid elements (i.e., liquids and solids half) and particulates (corpuscles) represented by cells and cell-derived components.
  • Blood plasma fraction: a translucent yellowish liquid fluid representing the extracellular matrix in which the cellular components are suspended. This represents an average isotonic to blood cells, which survive in an environment that is 0.9% concentration, such as saline, to provide an example.

The formed elements constitute about 45% of the blood. Such percentage magnitude is known of hematocrit (cell fraction), ascribable almost entirely to the red cell mass. The other 55% is represented by the blood plasma (acellular fraction).

Cellular Fraction:

Formed elements of the blood are varied in size, structure, and function, and are grouped into:

  • The blood cells, which are white blood cells or leukocytes, cells that are “pass” through the blood to fulfill its function in other tissues;
  • The cellular derivatives that are not strictly cells but cell fragments; they are represented by erythrocytes and platelets; They are the only blood components that perform their duties strictly within the vascular space.

Red Blood Cells (RBC):

Red blood cells, red blood cells or red blood cells constitute about 96% of figurative elements. Its normal value (count) average is around 4,800,000 in women, and about 5,400,000 in the male, red blood cells per mm ³ (or microliter).

These corpuscles lack nucleus and organelles (only in mammals). Their cytoplasm is composed almost entirely by hemoglobin a protein responsible for transporting oxygen and also contains some enzymes. The carbon dioxide is transported in the blood (free dissolved 8%, and 27% carbamino compounds, and as bicarbonate, the latter regulating the pH in the blood). In the plasma membrane of the erythrocytes are glycoprotein (CDs) that define the different blood groups and other cell identifiers.

Erythrocytes have biconcave disc shape depressed in the center. This particular shape increases the effective membrane surface. Mature red blood cells lack a nucleus because it expelled in the bone marrow before entering the bloodstream (this does not occur in birds, amphibians and certain other animals). Adult human erythrocytes are formed in the bone marrow.


The hemoglobin -contained exclusively in the red- blood cells is a pigment a protein conjugate containing the group “heme”. It also transports carbon dioxide, most of which is dissolved in the erythrocytes and to a lesser extent in the plasma.

Normal hemoglobin levels are between 12 and 18 g / dl of blood, and this amount is proportional to the quantity and quality of red blood cells (red cell mass). Hemoglobin makes up 90% of erythrocytes and, as a pigment, gives its characteristic red color, although this only occurs when the red blood cell is loaded with oxygen.

After a half-life of 120 days, erythrocytes are destroyed and removed from the blood by the spleen, the liver and the bone marrow, where hemoglobin is degraded in bilirubin and iron is recycled to form new hemoglobin.

White Blood Cells (WBC):

The white blood cells are part of cellular players’ immune system and are cells with migratory capacity using the blood as a vehicle to access different parts of the body. Leukocytes are responsible for destroying infectious agents and infected cells and secrete protective substances such as antibodies, which fight infections.

Normal leukocyte count is within a range of 4,500 to 11,500 cells permm³ (or microliter) blood, varies according to the physiological conditions (pregnancy, stress, sport, age, etc.) and pathological (infection, cancer, immune suppression, aplasia, etc.). The percentage counts of different types of leukocytes are called “differential count” (see CBC, below).

According to the microscopic characteristics of their cytoplasm (staining) and core (morphology), they are divided into:

  • Granulocytes or polymorphonuclear cells: are neutrophils, basophils, and eosinophils; polymorph possesses a core and numerous granules in their cytoplasm, as differential staining with cell type.
  • Agranulocitos or monomorphonuclear cells: are lymphocytes and monocytes; lack of granules in the cytoplasm and have a round nucleus.

Granulocytes or Polymorphonuclear cells:

  • Neutrophils present in blood between 2,500 and 7,500 cells per mm³. They are most numerous, occupying between 55% and 70% of the leukocytes. Hence its name palely stained, they are responsible for engulfing foreign substances (bacteria, external agents, etc.) that enter the body. In situations of infection or swelling their numbers increase in the blood. Its distinctive core has of 3-5 lobes separated by thin strands of chromatin, thus before they are called “polymorphonuclear” or a simple “polynuclear” misnomer.
  • Basophiles: blood present between 0.1 and 1.5 cells per mm³, (0.2-1.2% of WBC). They have basophilic staining, which defines them. They secrete substances such as heparin, the anticoagulant properties, and histamine which contribute to the process of inflammation. Often they have a core covered with secretory granules.
  • Eosinophils : present in the blood between 50 and 500 cells per mm³ (1-4% of leukocytes). Increase in diseases caused by parasites, allergies, and asthma. Its core, characteristic, has two lobes joined by a thin strand of chromatin, and therefore is also called the “mask shaped cells”.

Agranulocytes or Monomorphonuclear cells:

  • Monocytes: average count between 150 and 900 cells per mm³ (2% to 8% of the total white blood cells). This figure rises almost always caused by viruses or parasites infections. Also in some tumors or leukemia. They are defined nucleus cells and kidney-shaped. In tissues they differentiate into macrophages or histocytes.
  • Lymphocytes: average value between 1300 and 4000 mm³ (24% to 32% of white blood cells). Their number increases especially viral infections, but also in neoplastic diseases (cancer) and may decrease in immunodeficiency. Lymphocytes are the specific effectors of the immune system, exerting cellular and humoral immunity acquired. There are two types of lymphocytes, B lymphocytes, and T lymphocytes


B-lymphocytes are responsible for humoral immunity, i.e. antibody secretion (substances that recognize and bind to bacteria and allow them their phagocytoses and destruction). Granulocytes and monocytes can better recognize and destroy bacteria when antibodies are attached to them (opsonization).  Cells are also responsible for the production of some components of blood serum, called immunoglobulin.


T-lymphocytes recognize the infected cells and destroy the virus using macrophages. These cells amplify or suppress the overall immune response by regulating the other components of the immune system, and secrete many cytokines. They constitute 70% of all lymphocytes.

Both the T lymphocytes and B have the ability to “remember” previous exposure to a specific antigen, and when there is a new exposure to it, the action of the immune system will be more effective.

Platelets (or) Thrombocytes:

Platelets (thrombocytes) are small cell fragments (2-3 mm in diameter), oval and coreless. They are produced in the bone marrow from the cytoplasm fragmentation of megakaryocytes being free in the bloodstream. The normal quantitative value is between 250,000 and 450,000 platelets per mm³.

  • Platelets are used to plug the injuries that could affect the blood vessels. In the process of coagulation (haemostasis), platelets contribute to the formation of clots (thrombi) and they are responsible for the closure of vascular wounds. A drop of blood contains about 250,000 platelets.
  • Platelets are the smallest blood cells.
  • Its function is to clot the blood when a circulatory vessel platelets surrounding the wound to reduce the size and prevent bleeding breaks.
  • Fibrinogen is converted into sticky threads together with platelets form a network to catch the red blood cells, red coagulates and forms a crust so that the bleeding is prevented.

Blood plasma Fraction:

Blood plasma is the liquid portion of blood in which the formed elements are embedded. It is the major component of blood, representing 55% of total blood volume, with about 40-50 mL / kg weight. It is salty and yellowish translucent. Besides transporting blood cells, it carries nutrients and waste substances contained in the cells.

Blood plasma is essentially a solution aqueous, slightly denser than water, with 91% water, 8% protein and trace amounts of other materials. Plasma is a mixture of many vital proteins, amino acidscarbohydrateslipids, salts, hormones, enzymes, antibodies, urea, dissolved gases and inorganic substances such as sodium, potassium, calcium chloride, carbonate, and bicarbonate. Among these proteins are fibrinogen (for coagulation), globulins (regulate the water content in the cell form antibodies against infectious disease), albumin (exert osmotic pressure to deliver water between plasma and body fluids) and lipoproteins (buffer the pH changes in the blood cells and makes the blood more viscous than water).

 Other major plasma proteins act as carriers to tissues of essential nutrients such as copper, iron, other metals, and various hormones. Plasma components are formed in the liver (albumin and fibrinogen), endocrine glands (hormones), and others in the intestine.

When the blood clots and clotting factors are consumed, the remaining fluid fraction is called serum blood.

Physico-chemical characteristics:

  • Blood is a non-Newtonian fluid and pulsed with perpetual motion, flowing unidirectional contained in the vascular space (flow characteristics adapted to the architecture of the blood vessels). Hemodynamic momentum is provided by the heart in collaboration with large elastic vessels.
  • Blood usually has a pH between 7.36 to 7.44 (values ​​in arterial blood). Their variations beyond these values ​​are conditions that must be corrected soon (alkalosis, when the pH is too basic, and acidosis, when the pH is too acidic).
  • pH values ​​compatible with life that require immediate correction are: 6.8 – 8
  • An adult has about 4-5 litres of blood (7% of body weight) at a rate of about 65 to 71 mL of blood per kg body weight.

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