Each of these layers is a composite structure made up of both cells and elements of connective tissue. The cells lining the tunica intima are simple squamous epithelial cells called endothelial cells. The tunica media is the middle layer and is largely composed of smooth muscle and the tunica adventitia or tunica externa is the outermost layer which forms a tough connective tissue sheath. The thickness and the composition of these layers vary according to the function of the specialised type of blood vessel. Capillaries - Name the three types of capillaries and explain why they do not have the three layers present in arteries and veins 5.
Weaknesses can occur in blood vessels either as a congenital or age related abnormality.
Not all of the fluid that leaves the capillaries is returned via the venules. The lymph moves from the intercellular spaces into lymphatic capillaries. You should ensure that you are familiar with their anatomical structure and how these vessels differ from blood vessels. The lymph enters a series of lymph nodes which have an internal labyrinth. Age related changes occur in the cardiovascular system for example thickening of the arterial wall occurs during the progress of atherosclerosis.