The dense capillary branching and t

The dense capillary branching and the cumulative diameters of the capillaries provide a lower resistance pressure while effectively facilitating gas exchange. The pulmonary vasculature system is effective during times of exercise as it can increase blood-carrying capacity by as much as 5 times with little or no increase in pulmonary pressure. In a healthy individual the arterioles are thin walled, distensible vessels that are able to dilate in times of increased metabolic need. The arterioles are composed of 3 layers: intima, media, and adventia. The intima is the innermost layer and is responsible for tightening and relaxing the blood vessel, blood clotting, forming new vessels, inflammation, and immune responses. The media is the middle layer that regulates the diameter and volume of the vessels, thereby distributing blood to areas of the body in need. The adventitia is the outermost layer and comprises collagen, which gives the vessel stability. Chemical messengers, which include tumor necrosis factor and interleukin 1, serve as the mechanism of action that promotes the responsiveness of these arterial layers to increased metabolic needs.