Blood is composed of plasma, red blood cells, white blood cells, and platelets. Blood cells are made by bone marrow.
Plasma is a straw colored substance that is approximately 90% water. It suspends the cells.
There are more red blood cells (erythrocytes) than any other type of cell. Red blood cells never divide. About 3 million red blood cells die and are scavenged by the liver each second. Red blood cells are responsible for transporting oxygen and carbon dioxide.
White blood cells are called leukocytes. There are a lot less white blood cells than red blood cells (1:700). They play a major role in fighting infection. There are many different types of white blood cells that each perform a very specific function. These cells are called monocytes, neutrophils, eosinophils, and basophils.
Platelets are controlled by a homeostatic mechanism. They play a major role in clotting. When coagulation occurs (blood clot) it forms a plug of platelets. Platelets are cell fragments formed by megakaryocytes.
The volume of blood consists in cells, 50% (red blood cells called erythrocytes, leukocytes or white blood cells, thrombocytes or platelets) and the plasma:
- Red blood cells contain mainly hemoglobin, the pigment whose primary role is to carry oxygen from lungs to tissues
- Leukocytes include different types of cells: polymorphonuclear neutrophils and monocytes, which play an essential role in nonspecific defense against infections, lymphocytes, which are bases cells of specific immunity polymorphonuclear eosinophils, whose multiplication proves the presence of an allergy or parasitosis; polymorphonuclear basophils, whose role is still unexplained.
- Platelets play an essential role, together with coagulation factors, in blood clot formation and, consequently, in hemostasis (stopping of bleeding).
- Plasma - is a yellowish liquid, composed of 95% slightly salty water and many other elements in varying amounts, among of them being nutrients, proteins and waste.