Biology Of Blood

What Is Blood

The fluid that circulates in the blood vessels of the body. Consists of plasma and cells floating within it. The cells are derived from extravascular sites and then enter the circulatory system.

They frequently leave the blood to enter the extravascular spaces.

The circulating blood carries nutrients and oxygen to the body cells, and is thus an important means of maintaining the homeostasis of the body. It carries hormones from their sites of origin throughout the body, and is thus the transmitter of the chemicals of the body.

The blood carry cytokines; any of a group of soluble proteins that are released by a cell to send messages which are delivered to the same cell (autocrine), an adjacent cell (paracrine), or a distant cell (endocrine).

The cytokine binds to a specific receptor and causes a change in function or in development (differentiation) of the target cell. Cytokines are involved in reproduction, growth and development, normal homeostatic regulation, response to injury and repair, blood clotting, and host resistance (immunity and tolerance).

Cytokines may be divided into six groups: interleukins, colony-stimulating factors, interferons, tumor necrosis factors, and chemokines.

Blood plasma also circulates immune bodies and contains several of the components essential for the formation of blood clots.

Finally, blood transports waste products to excretory organs for elimination from the body.

Blood Biology

A 70 kg person has 5 liters of blood. Blood composed of 52-62% liquid (plasma) and 38-48% cells.

The plasma is mostly water (91.5%) and is a solvent for transporting other materials. Blood is slightly alkaline (pH = 7.4).

Where Blood Cells Made

All blood cells are manufactured by stem cells, which live mainly in the bone marrow, via a process called hematopoiesis. The stem cells produce hemocytoblasts that differentiate into the precursors for all the different types of blood cells. Hemocytoblasts mature into three types of blood cells:

- erythrocytes = red blood cells
- leukocytes = white blood cells
- thrombocytes = platelets

WBCs are further subdivided into granulocytes (containing large granules in the cytoplasm) and agranulocytes (without granules).

The granulocytes consist of:

- neutrophils (55-70%)
- eosinophils (1-3%)
- basophils (0.5-1.0%)

The agranulocytes are lymphocytes (consisting of B cells and T cells) and monocytes. Lymphocytes circulate in the blood and lymph systems, and make their home in the lymphoid organs.


The plasma is the river in which the blood cells travel. It carries also nutrients, waste products (CO2, lactic acid, urea, etc.), antibodies, clotting proteins (called clotting factors), chemical messengers such as hormones, and proteins that help maintain the body's fluid balance.

When you spin blood in a centrifuge, the red blood cells go to the bottom of the container, and the white blood cells and platelets to the middle, leaving the yellowish plasma at the top.

No comments:

Post a Comment