Blood is a special type of connective tissue related, among other functions, with the transport of nutrients and respiratory gases and the defense of the body.

Blood is a connective tissue with special properties.
Blood is a special type of connective tissue that stands out as a red, viscous fluid . It is characterized by having a liquid matrix (plasma), in which the cellular elements of the blood ( red blood cells , leukocytes and platelets ) are suspended.

In humans, blood flows within our cardiovascular system , which is closed off. This means that our blood is found only inside the heart and our blood vessels .

Generally, a person has a total blood volume that corresponds to about 7% of their body weight . With that, we have that an individual of approximately 70 kilos, must present about five liters of blood.


Blood has several functions in the body, ensuring, for example:

– Transport of nutrients;

– Transport of respiratory gases;

– Transport of metabolic waste ;

– Defense and immunity through the action of leukocytes;

– Blood clotting through the action of platelets.

→ Blood components

Blood is composed of plasma and cellular elements , which include blood cells and cell fragments. Plasma, which is the liquid matrix of blood, corresponds to 55% of the blood volume, while cellular elements occupy approximately 45% of this volume.

Here’s a little more about each of these components:


Plasma is the liquid part of the blood, it is largely composed of water and various dissolved substances. In plasma we find ions such as sodium, potassium, calcium and magnesium; proteins , such as albumins and antibodies ; and various substances that are carried by the blood, such as glucose, vitamins , hormones , respiratory gases, and waste products from metabolism. This portion of the blood is yellowish in color .

  • figurative elements

The formed elements of blood are the cellular components of this tissue. In the blood we have two types of cells (the red blood cells and the leukocytes) and the cellular fragments known as platelets. Below are the main features of each of these components:

  • RBCs, erythrocytes, or red blood cells

Red blood cells are blood cells that stand out for their shape of a small biconcave disk containing a large amount of hemoglobin , the pigment responsible for transporting oxygen .

These cells are numerous and the most found in our blood. Due to the large amount of red blood cells and the presence of hemoglobin pigment inside them, the blood has a reddish appearance .

Red blood cells have a biconcave disk shape and are rich in hemoglobin.

  • Leukocytes or white blood cells

Leukocytes are colorless cells whose main function is to defend our body. The two main forms of defense by these cells are phagocytosis and antibody production .

Phagocytosis is a process in which cells engulf and digest the foreign particle , while antibodies are defense proteins that act, for example, by signaling a cell so that it can be phagocytosed, or by neutralizing an antigen .

An interesting feature of leukocytes is their ability to cross blood vessels , which are therefore capable of acting on injured tissues.

Leukocytes act in the defense of the body.

It is worth noting that there is not only one type of leukocyte, and it is possible to identify five different types: lymphocytes, monocytes, eosinophils, basophils and neutrophils . These five types are divided into two groups: granulocytes and agranulocytes.

Granulocytes are distinguished by the presence of specific granules and an irregular nucleus, while agranulocytes do not have specific granules and their nucleus is more regular. Neutrophils, eosinophils, and basophils are granulocytes, while lymphocytes and monocytes are agranulocytes.

  • Platelets or thrombocytes

Platelets, contrary to what many people think, are not cells themselves . They are fragments of bone marrow cells called megakaryocytes. These structures are anucleate and have the shape of small disks. Its function is to ensure blood clotting and also help in repairing damage to blood vessels.

Cellular components of blood and their functions
cell component  Occupation
Red Cells They mainly transport oxygen.
leukocytes They work to defend the body.
platelets They work on blood clotting.

→ Normal amounts of cellular components

Each cellular component of blood has an amount considered normal in the body. Below are the expected amounts of each of these components:

Cellular components of blood and their normal amounts
cell component Amount per microliter of blood
Red Cells 5,000,000 to 6,000,000
leukocytes 5,000 to 10,000
platelets 250,000 to 400,000

Variations in these values ​​can indicate some problems.

  • lower than normal number of red blood cells may indicate, for example, a case of anemia .
  • The increase in the number of leukocytes , in turn, may indicate an infection, since in this type of process the defense cells multiply to control an infectious process.
  • The drop in platelets is related to several causes, such as dengue , for example, and is responsible for an increased risk of bleeding.

→ Where is blood produced?

Blood is produced in the so-called bone marrow , which is located in the medullary canal of long bones and in the cavities of spongy bones. Bone marrow can be divided into two types, red and yellow .

It is in the red bone marrow that blood cells are produced. In adults, red bone marrow is present in the vertebrae, ribs, sternum and in the spongy tissue layer (diploe) of the bones of the skull. The process that leads to the production of blood cells is called hemocytopoiesis.

→ Blood types

We know that in the human species we have four different blood types : type A, type B, type AB and type O.

People with blood type A have agglutinogen A in the red blood cells, while people with blood type B have agglutinogen B. People with blood type AB have agglutinogen A. and B, while in people with blood type O, no agglutinogen is observed in red blood cells.

Human blood can be classified into four different types.

In addition to the presence of agglutinogen in the red blood cells, there are agglutinins (antibodies) in the plasma. In type A blood, there is anti-B agglutinin, while in type B blood, there is anti-A agglutinin. In AB blood, there is no presence of agglutinin, while in O blood there is the presence of anti-A and anti-B.

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