Types of Blood Vessel
Arteries carry blood away form the heart via the pulmonary artery and aorta.
As major arteries begin to branch , they become smaller and capable of distkntion of constriction because of their muscle layer. These arterioles control the amount- of blood flow that will be delivered to the capillary bed. The precapilliary sphincters that lie immediately proximal to the capillary bed control the capillary filling and shunting. They can increase central blood volume by constriction or increase the peripheral blood supply by dilatation.
These are very small vessels providing almost 60,000 miles of vascular tubules and composed almost exclusively of endothelial cells, thereby facilitating the passage of nutrients, plasma, oxygen and waste products across their membrane. They are extensive in organs with high metabolic demands.
Blood form capillaries travels through small veins called venules ( which become larger as they approach the heart, derminating in the two largest veins-the superior and the inferior vena cavae. Veins have a thicker adventitia but a much thinner middle muscle layer. Veins contain valves that promote blood flow to the heart against the forces of body and gravity.