Finally, erythrocytes are very unusual cells in that they do not have a nucleus. White blood cells are part of the body's defence against foreign invaders, bacteria etc., which can harm the system. They can be divided into two sorts depending on where they are made. Lymphocytes which are produced in lymphatic tissues and leucocytes which are produced in bone marrow. There are at least 3 different types of lymphocytes and two types of leucocytes.
Platelets are minute colourless cells.
They are also produced in the bone marrow. Platelets play a role in blood clotting in wounds, and action designed to prevent excessive blood loss from small wounds. Arteries have thicker, stronger walls than veins which helps them to withstand the pressures generated by the pumping heart. Arteries therefore carry the blood away from the heart and to the various parts of the body while veins carry blood from the body back to the heart. This gives rise to the terms arterial and venous blood for oxygenated and deoxygenated blood.