Types of Blood Vessels
The blood vessels are the part of the circulatory system. All blood is carried in these vessels. The blood vessels are responsible for passing nutrients, hormones, and other significant substances from and to body cells. There are five types of blood vessels: arterioles, arteries, capillaries, venules, and veins (see Fig. 6.7) .
Arteries deliver oxygen-rich blood from the heart to the tissues of the body. In addition, arteries hold a higher blood pressure than other parts of the circulatory system. Moreover, during the cardiac cycle the pressure in arteries varies. The pressure is lowest when heart relaxes and highest when the heart contracts and the variation in pressure generates a pulse. In order to keep blood pressure, the arteries can narrow passively when the heart is relaxing between beats. As the largest artery, aorta is the main high-pressure blood vessels connected to the heart’s left ventricle. Moreover, the aorta branches into numerous smaller arteries that distribute throughout the body.
After travelling from the aorta, blood travels through peripheral arteries into smaller arteries named arterioles and finally to capillaries. The larger arteries have less smooth muscle than arterioles and are more elastic. When blood enters these muscular and smaller arteries do not expand as much. Then arterioles can supply the greatest resistance to blood flow to modulate the blood pressure. Due to the muscular walls, arteries and arterioles can regulate their diameter to decrease or increase blood flow to specific part of the body.
The capillary are the starting site for venous blood back to the heart and are the ultimate destination of arterial blood from the heart. The capillary walls have very thin flattened ECs that line the inner walls of the blood vessels. The capillaries can delivery waste products from tissues into the blood and provide oxygen and nutrients into tissues from the blood.
Veins are blood vessels that carry blood from various regions of the body to the heart. Compared to the arterial system, the venous system has a low pressure that depends on muscle contractions to return blood to the heart. Due to the lower pressure in veins, veins have much thinner walls than arteries. Moreover, veins can widen as the amount of fluid in them increases. Additionally, most veins are equipped with valves to prevent blood flowing in the reverse direction.
Venules are small branches that collect the blood from the various organs. Venules are groups of capillaries that unite to form small vessels that drain the capillary beds. Venules are very porous, but with scattered smooth muscle fibers in the tunica media.