Vessels of circulatory system are the aorta, arteries, arterioles, capillaries, venules, veins and venae cavae. There are structural differences between different blood vessels.
1- Arterial System:
Arterial system comprises the aorta, arteries and arterioles. Walls of the aorta and arteries are formed by three layers:
- Outer Tunica Adventitia: It is made up of connective tissue layer. It is the continuation of fibrous layer of parietal pericardium.
- Middle Tunica Media: It is formed by smooth muscles.
- Inner Tunica Intima: It is made up of endothelium. It is the continuation of endocardium.
Aorta, arteries and arterioles have two laminae of elastic tissues:
- External Elastic Lamina: It is present between tunica adventitia and tunica media.
- Internal Elastic Lamina: It is found between tunica media and tunica intimae.
Aorta and arteries have more elastic tissues and the arterioles have more smooth muscles. Arterial branches become narrower and their walls become thinner while reaching the periphery. Aorta has got the maximum diameter of about 25 mm. Diameter of the arteries is gradually decreased and at the end arteries. It is about 4 mm. It further decreases to 30 u in the arterioles and ends up with 10 u in the terminal arterioles. Resistance (peripheral resistance) is offered to blood flow in the arterioles and so these vessels are called resistant vessels.
Arterioles are continued as capillaries, which are small, thin walled vessels having a diameter of about 5 to 8 u. Capillaries are functionally very important because the exchange of materials between the blood and the tissues occurs through these vessels.
2- Venous System:
From the capillaries, venous system starts and it includes venules. veins and venae cavae. Capillaries end in venules, which are the smaller vessels with thin muscular wall than the arterioles. Diameter of the venules is about 20 u. At a time, a large quantity of blood is held in venules and hence the venules are called capacitance vessels. Venules are continued as veins, which have the diameter of 5 mm. Veins form superior and inferior venae cavae, which have a diameter of about 30 mm.
Walls of the veins and venae cavae are made up of inner endothelium, elastic tissues, smooth muscles and outer connective tissue layers In the veins and venae cavae, the elastic tissue is less but the smooth muscle fibers are more.
Complications In Blood Vessels:
- Aorta and Arteries: Arterial blood vessels are highly susceptible for arteriosclerosis and atherosclerosis. Arteriosclerosis is the disease of the arteries, associated with hardening, thickening and loss of elasticity in the wall of the vessels. Atherosclerosis is the disease marked by the narrowing of lumen of arterial vessel due to deposition of cholesterol.
- Arterioles: When the tone of the smooth muscles in the arterioles increases, hypertension occurs.
- Capillaries: Permeability of the capillary membrane may increase resulting in shock or edema due to leakage of fluid, proteins and other substances from blood.
- Veins: Inflammation of the wall of veins leads to the formation of intravascular clot called thrombosis. The clot gets dislodged, as thrombus. The thrombus travels through blood and causes embolism. Embolism obstructs the blood flow to vital organs such as brain, heart and lungs, leading to many complications.