Blood

Important medium for the transportation of oxygen, carbon dioxide, carbohydrates, lipids, proteins, many other nutrients, and various components of the immune system.

The erythrocytes shown (marked in red colour) are mainly responsible for the transportation of oxygen. In contrast to the two other blood cells (leucocytes), erythrocytes have lost their cell nucleus (marked in blue). Leucocytes are involved in the defense against pathogens (immune system). Apart form these two functions (transportation and defense), the blood contains a complex system for coagulation in the case of damage inflicted to the vascular system.