Cardiovascular System PDF

Pharmacy Department…………….……………………….………………………………………………… Histology First stage M.s.c. Atyaf ali The cardiovascular system The cardiovascular system is a transport system that carries blood to and from the tissues of the body...

Pharmacy Department…………….……………………….………………………………………………… Histology First stage M.s.c. Atyaf ali The cardiovascular system The cardiovascular system is a transport system that carries blood to and from the tissues of the body. The constitutive elements of this fluid include cells, nutrients, waste products, hormones, and antibodies. The cardiovascular system consists of a pump represented by the heart and blood vessels, which provide the route by which blood circulates to and from all parts of the body. The blood vessels are arranged so that blood delivered from the heart quickly reaches a network of narrow thin walled vessels the blood capillaries within the tissues in every part of the body. In the capillaries, a two directional exchange of fluid occurs between the blood and tissues. The fluid, called blood, carries oxygen and metabolites and passes through the capillary wall. In the tissues, these molecules are exchanged for carbon dioxide and waste products. Most of the fluid reenters the venous end of the blood capillaries. The remaining fluid enters lymphatic capillaries as lymph and is ultimately returned to the bloodstream through a system of lymphatic vessels. The heart The heart lies obliquely, on the left side of the thoracic cavity. It is surrounded by a serous fibrous sac, the pericardium which also contains the beginnings and ends of the great vessels entering and leaving the heart, there is a potential space containing a minimal amount (15–50 ml) of serous fluid between the visceral and parietal layers of the serous pericardium. This space is known as the pericardial cavity. The heart is strongly attached to the diaphragm and neighboring organs that lie in the thoracic cavity by the pericardium. The heart contains four chambers the right and left atria and right and left ventricles through which blood is pumped. Valves guard the exits of the chambers, preventing backflow of blood. The wall of the heart is composed of three layers. From the outside to the inside, they are as follow:- * The epicardium, it consists of a single layer of mesothelial cells and underlying connective and adipose tissue. The blood vessels and nerves that supply the heart lie in the epicardium and are surrounded by adipose tissue. * The myocardium, consisting of cardiac muscle, is the principal component of the heart. The myocardium of the atria is thinner than that of the ventricles. The atria receive blood from the large veins and deliver it to adjacent ventricles, a process that requires relatively low pressure. The myocardium of the ventricles is substantially thicker because of the higher pressure required to pump the blood through the pulmonary and systemic circulations. * The endocardium, consists of an inner layer of endothelium and subendothelial connective tissue. Layers of Vascular Wall The walls of arteries and veins are composed of three layers called tunics. The three layers of the vascular wall, from the lumen to outward are the following:- 1- The tunica intima, the innermost layer of the vessel, consists of three components: (a) A single layer of squamous epithelial cells, the endothelium. (b) The basal lamina of the endothelial cells. (c) The subendothelial layer, consisting of loose connective tissue. Occasional smooth muscle cells are found in the loose connective tissue. 2- The tunica media, or middle layer, consists primarily of smooth muscle cells. In arteries, this layer is relatively thick. Variable amounts of elastin, and reticular fibers, are interposed between the smooth muscle cells of the tunica media.. 3- The tunica adventitia, or outermost connective tissue layer, is composed primarily of longitudinally arranged collagenous tissue and a few elastic fibers. The tunica adventitia of large arteries and veins contains a system of vessels called the vasa vasorum that supplies blood to the vascular walls themselves, as well as a network of nerves called nervi vascularis that control contraction of the smooth muscle in the vessel walls. Arteries Arteries are classified into three types on the basis of size and the characteristics of the tunica media. 1- Large Arteries (Elastic Arteries), such as the aorta and pulmonary arteries convey blood from the heart to the systemic and pulmonary circulations. Elastic arteries have multiple sheets of elastic lamellae in their walls. * The tunica intima of elastic arteries consists of the following:- 1- Endothelium , endothelial cells provide a barrier between the circulating blood and the sub endothelial tissues. 2- The sub endothelial layer of connective tissue, elastic arteries consists of connective tissue with both collagen and elastic fibers. The main cell type in this layer is the smooth muscle cell. It is contractile and secretes extracellular ground substance as well as collagen and elastic fibers. * The tunica media, of elastic arteries consists of multiple layers of smooth muscle cells separated by elastic lamellae. * The tunica adventitia, in the elastic artery is a relatively thin connective tissue layer. It consists of collagen fibers and elastic fibers form a loose network. 2- Medium Arteries (Muscular Arteries) Muscular arteries have more smooth muscle and less elastin in the tunica media than do elastic arteries. Generally, in the region of transition between elastic arteries and muscular arteries, the amount of elastic material decreases, and smooth muscle cells become the predominant of the tunica media * The tunica intima is relatively thinner in muscular arteries than in elastic arteries and consists of an endothelial lining with its basal lamina, a sparse sub endothelial layer of connective tissue. * The tunica media, is composed of smooth muscle cells and relatively little elastic material. The smooth muscle cells are arranged in a spiral fashion in the arterial wall. Their contraction helps maintain blood pressure. * The tunica adventitia, is relatively thick and is consists of fibroblasts, collagen fibers, and elastic fibers. 3- Small Arteries and Arterioles Small arteries and arterioles are distinguished from one another by the number of smooth muscle cell layers in the tunica media, arterioles have only one or two layers, and a small artery may have as many as eight layers of smooth muscle in the tunica media. Arterioles, control blood flow to capillary networks by contraction of the smooth muscle cells. Contraction of the smooth muscle in the wall of an arteriole reduces the blood going to the capillaries. Capillaries, are the smallest diameter blood vessels. Capillaries form blood vascular networks that allow fluids containing gases, metabolites, and waste products to move through their thin walls. Each consists of a single layer of endothelial cells and their basal lamina. Veins Veins are divided into four types on the basis of size. 1- Venules, are further sub classified as postcapillary and muscular venules. They receive blood from capillaries and have a diameter as small as (0.1 mm). 2- Small veins, are less than (1 mm) in diameter and are continuous with muscular venules. 3- Medium veins, have a diameter of as much as (10 mm). 4- Large veins, usually have a diameter greater than (10 mm). Examples of such veins include the superior and inferior vena cava. Although large and medium veins have three layers also designated tunica intima, tunica media, and tunica adventitia these layers are not as distinct as they are in arteries. Typically, veins have thinner walls than their accompanying arteries, and the lumen of the vein is larger than that of the artery. Many veins, especially those that convey blood against gravity, such as those of the limbs, contain valves that allow blood to flow in only one direction, back toward the heart. Venules * Postcapillary venules, collect blood from the capillary network and it possess an endothelial lining with its basal lamina and pericytes. * Muscular venules, are distinguished from postcapillary venules by the presence of a tunica media. Whereas postcapillary venules have no true tunica media, the muscular venules have one or two layers of smooth muscle that constitute a tunica media. These vessels also have a thin tunica adventitia. Usually, pericytes are not found in muscular venules. Medium Veins The three tunics of the venous wall are most evident in medium-sized veins. 1- The tunica intima, consists of an endothelium with its basal lamina, a thin subendothelial layer with occasional smooth muscle cells scattered in the connective tissue elements. 2- The tunica media, it contains several layers of circularly arranged smooth muscle cells with interspersed collagen and elastic fibers. 3- The tunica adventitia, is typically thicker than the tunica media and consists of collagen fibers and networks of elastic fibers. Large Veins 1- The tunica intima, of these veins consists of an endothelial lining with its basal lamina, a small amount of subendothelial connective tissue, and some smooth muscle cells. 2- The tunica media, is relatively thin and contains circumferentially arranged smooth muscle cells, collagen fibers, and some fibroblasts. 3- The tunica adventitia, of large veins (e.g., the subclavian veins, and the vena cava) is the thickest layer of the vessel wall. Along with the usual collagen and elastic fibers and fibroblasts, the tunica adventitia also contains longitudinally disposed smooth muscle cells.