The Circulatory System

The circulatory system comprises both the blood and lymphatic vascular systems. The blood vascular system is composed of the following structures:
1- The heart, an organ whose function is to pumpthe blood.
2- The arteries, a series of efferent vessels that become smaller as they branch, and whose function is to carry the blood, with nutrients and oxygen, to the tissues.
3- The capillaries, the smallest blood vessels, constituting a complex network of thin tubules that anastomose profusely and through whose walls the interchange between blood and tissues takes place.
4- The veins, which result from the convergence of the capillaries into a system of channels. These channels become larger as they approach the heart, towardwhichtheyconvey the blood to be pumped again.
The internal surface of all components of the blood and lymphatic systems is lined by a single layer of a squamous epithelium, called endothelium.
Blood vessels are usually composed of the following layers, or tunics:
1- TUNICA INTIMA : consists of one layer of endothelial cells supported by a subendothelial layer of loose connective tissue containing smooth muscle cells. In arteries, is separated from the tunica by an internal elastic lamina.
Tunica
Intima
2- TUNICA MEDIA :Layers of smooth muscle cells with amounts of elastic fibers, retiantar and collagen fibers. In arteries, the media has a thinner external elastic lamina, which separates it from the tunica adventitia.