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Blood is the red fluid that circulates in the body?s veins, arteries, and capillaries. Blood is pumped by the heart via the arteries to the lungs and all other tissues and then is returned to the heart in veins. Blood contains plasma (a watery, straw-colored fluid liquid, which contains dissolved proteins, sugars, fats, salts, and minerals) and blood cells: red blood cells (erythrocytes), white blood cells (leukocytes) and platelets (thrombocytes). Red cells and platelets are unique among body cells as they have no nucleus. Blood cells are so small that one cubic millimeter of blood (the size of a pin head) contains about five million red cells, 7.000 white cells and 250.000 platelets.
Erythrocytes are red blood cells of which 4.5-5.0 million are found in each cubic millimeter. These cells are made in the bone marrow and are important in transporting oxygen from the lungs through the blood stream to the cells all over the body. The red blood cells contain a pigment called hemoglobin, which gives the blood its red color.
Leucocytes are white blood cells from 4.000 to 10.000 per cubic millimeter existing in several types: granulocytes and agranulocytes, which are also subdivided into different types.
Granulocytes are cells with granules in their cytoplasm formed in the bone marrow. There are three types of granulocytes: eosinophils, basophils, neutrophils.
Agranulocytes are produced in lymph nodes and spleen. There are two types of agranulocytes: lymphocytes and monocytes.
Thrombocytes or platelets are tiny cells formed in the bone marrow. They are necessary for blood clotting. Their number is 400.000 per cubic millimeter. The plasma is the fluid portion before clotting has occurred. The serum is the fluid portion of blood remaining after the coagulation process is completed.