Blood Vessels and Circulation - Regulation of both blood vessels and the heart ensure that homeostatic blood pressure is maintained. Blood vessels outside the heart are divided into 2 classes: 1. Pulmonary Vessels- transport blood from the right ventricle through the lungs and back to the left atrium. 2. Systemic vessels – transport blood from the right ventricle, through all parts of the body, and back to the right atrium. Pulmonary and systemic constitute the circulatory system. 5 functions: 1. 2. 3. 4. 5. Carries blood Exchange nutrients, waste products, and gases with tissues Transports substances Helps regulate blood pressure Directs blood flow to tissues Structural Features: Blood vessels – are hollow tubes that conduct blood through the tissues of the body. 3 main types of blood vessels: arteries, capillaries and veins. Blood leaving the heart first passes through the arteries, then flows through the smallest blood vessels, the capillaries, and moves through veins as it once again flows into the heart. Arteries – carry blood away from the heart; the blood is oxygenated. - Blood is pumped from the ventricles of the heart into large, elastic arteries, which branch repeatedly to form progressively smaller arteries. Classification: 1. - Elastic arteries The largest-diameter arteries and have the thickest wall. A greater portion of their wall composed of elastic tissue and smaller portion in smooth muscle. Stretch when the ventricles of the heart pumps blood into them. Aorta and Pulmonary truck are examples of elastic arteries. Th elastic recoil of these arteries prevents blood pressure from falling rapidly and maintains blood flow while the ventricles are relaxed. 2. Muscular arteries - Include medium-sized and small arteries. - Most of the walls thickness results from smooth muscles cells of the tunica media. o Medium sized arteries - walls are relatively thick compared to their diameter. - frequently called distributing arteries because the smooth muscle tissue enables these vessels to control blood flow to different body regions. - supply blood to small arteries. Small arteries have about the same structure as the medium-sized arteries, except for a smaller diameter and thinner walls. The smallest of the small arteries have only 3 or 4 layers of smooth muscle in their walls. Contractions of smooth muscles in blood vessels, called vasoconstriction, decreases blood vessel diameter and blood flow. Relaxation of the smooth muscle in blood vessels, called vasodilation, increases blood vessel diameter and blood flow. 3. Arterioles - Transport blood from small arteries to capillaries. - Smallest arteries in which the 3 tunics can be identified. The tunica media, consist of only 1 or 2 layers of circular smooth muscle cells. Small arteries and arterioles are adapted for vasodilation and vasoconstriction. o Blood flows from arterioles into capillaries. Capillaries: - - Exchange of substances such as oxygen, carbon dioxide, nutrients, and other waste products happens in the capillaries. Have thinner walls than arteries. Blood flows through the capillaries more slowly and there are far more of them than any other blood vessel type. Blood flow through capillary networks is regulated by smooth muscle cells called precapillary sphincters. o Precapillary sphincters – are located at the origin of the branches of the capillaries and by contracting and relaxing, regulate the amount of blood flow through the various sections of the network. Walls are consisted of endothelium, which is layer of simple squamous epithelium surrounded by delicate loose connective tissue. The thin wall of the capillaries facilitate diffusion between the capillaries and surrounding cells. Each capillary is 0.5 -1 mm long Capillaries branch without changing their diameter, which is approx. the same as the diameter of the RBC (7.5 micrometer) Capillary network are more numerous and more extensive in the lungs and in highly metabolic tissues, such as the liver, kidneys, skeletal muscles and cardia muscles than in other tissue types. o From the capillaries, blood flows into veins Veins: - Carry blood towards the heart; the blood is deoxygenated. Compared to arteries, the walls of veins are thinner and contain less elastic tissue and fewer smooth muscles. They increase in diameter and decrease in number as they progress towards the heart, ad their walls increase in thickness Classification: venules, small veins, medium-sized veins or large veins. Blood vessel wall (except capillaries and venules) - Consist of 3 layers or tunics. (From the inner to the outer tunics) 1. Tunica intima (innermost layer) - Consists of an endothelium composed of simple squamous epithelial cells, a basement membrane and a small amount of connective tissue. In muscular arteries – tunica intima contains a layer of thin elastic connective tissue. 2. Tunica media (middle layer) - consist of smooth muscle cells arranged circularly around the blood vessel. - contains variable amounts of elastic and collagen fibers, depending on the size and type of the vessels In muscular arteries – a layer of elastic connective tissue forms the outer margin of the tunica media. 3. tunica adventitia - composed of dense connective tissue adjacent to the tunica media. - the tissue becomes loose connective tissue toward the outer portion of the blood vessel wall.
