Definition of Blood

Blood – the cell-filled fluid that carries vital chemicals and nutrients via the cardiovascular system to tissues and cells throughout the body.

HematopoiesisThe heart pumps the blood, sending it under pressure through a closed network of arteries and veins.

The blood provides volume within the cardiovascular system, establishing both blood pressure and osmotic pressure (the pressure that keeps fluid within the blood vessels).

The blood carries oxygen and nourishment to and collects metabolic wastes from the cells – also serves as the body’s primary immune response mechanism, transporting antibodies and specialized cells that defend the body from infection as well as aid in healing wounds.

The blood’s basic composition is about 55 percent plasma (liquid) and 45 percent cells. The adult human body contains about five liters, or five and a half quarts, of blood accounting for 8 percent of total body weight.

Plasma Definition

Plasma is 90 percent water. It contains a mix of proteins, electrolytes, hormones, antibodies, minerals, glucose, and other dissolved substances, forming a solution in which the blood’s cells float.

The constant churning and movement of the blood as the heart pumps it through the blood vessels keeps the cells and the plasma well mixed. However, in a collected blood sample the cells quickly settle to the bottom, leaving the plasma at the top. The primary proteins in plasma are albuminimmunoglobulin, and clotting factors.

Plasma has a higher concentration of electrolytes (salts) than fluid in the tissues, giving the blood a higher osmotic pressure that draws fluid into the blood rather than allows it to seep from the blood in the capillary beds. plasma is also essential for coagulation (clotting) as it carries both clotting factors and the enzymes that activate them.

Blood Cells Definition

The blood contains three kinds of cells:

  • erythrocytes, or red blood cells, which carry oxygen from the lungs to every other cell in the body
  • leukocytes, or white blood cells, which fight infection and take one of three forms: monocyte, lymphocyte, or granulocyte
  • platelets, also called thrombocytes, which cause blood to coagulate (clot)

Erythrocytes make up nearly the entire volume of blood cells, while leukocytes and platelets combined make up less than 1 percent. The red bone marrow synthesizes (produces) the vast majority of blood cells, a process called hematopoiesis.

Other structures, such as the spleen, can produce limited numbers of blood cells when the body is in crisis. The liver and the spleen cleanse damaged, old, and deteriorating blood cells from the blood. The liver breaks erythrocytes into their chemical components, which the body then recycles to synthesize new erythrocytes in the bone marrow.

For further discussion of blood and lymph structure and function please see the overview section “The Blood and Lymph.”


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