These illustrations shows structures of some tissues in the circulatory system. The vein and artery cross-sections shows three layers: the tunica intima (inner endothelial lining), tunica media (smooth muscle layer), and tunica externa (outer connective tissue), with arteries having a thicker tunica media to withstand higher pressure. The section of the vena cava indicates the same layers but a thinner muscle wall, which represents its function of low-pressure return of blood. The human blood smear reveals red blood cells for oxygen transport and other cellular elements of immunity and clotting. The lymph gland section includes macrophages and lymphocytes, critical for immune response and defense against pathogens.