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Introduction The cardiovascular/circulatory system transports food, hormones, metabolic wastes, and gases (oxygen, carbon dioxide) to and from cells. Components of the circulatory system include:
Blood Components
Vascular System - the Blood Vessels Arteries, veins, and capillaries comprise the vascular system. Arteries and veins run parallel throughout the body with a web-like network of capillaries connecting them. Arteries use vessel size, controlled by the sympathetic nervous system, to move blood by pressure; veins use one-way valves controlled by muscle contractions.  Arteries are strong, elastic vessels adapted for carrying blood away from the heart at relatively high pumping pressure. Arteries divide into progressively thinner tubes and eventually become fine branches called arterioles. Blood in arteries is oxygen-rich, with the exception of the pulmonary artery, which carries blood to the lungs to be oxygenated. The aorta is the largest artery in the body, the main artery for systemic circulation. The major branches of the aorta (aortic arch, ascending aorta, descending aorta) supply blood to the head, abdomen, and extremities. Of special importance are the right and left coronary arteries, that supply blood to the heart itself.
Capillaries  The arterioles branch into the microscopic capillaries, or capillary beds, which lie bathed in interstitial fluid, or lymph, produced by thelymphatic system. Capillaries are the points of exchange between the blood and surrounding tissues. Materials cross in and out of the capillaries by passing through or between the cells that line the capillary. The extensive network of capillaries is estimated at between 50,000 and 60,000 miles long.1Veins Blood leaving the capillary beds flows into a series of progressively larger vessels, called venules, which in turn unite to form veins. Veins are responsible for returning blood to the heart after the blood and the body cells exchange gases, nutrients, and wastes. Pressure in veins is low, so veins depend on nearby muscular contractions to move blood along. Veins have valves that prevent back-flow of blood.Blood in veins is oxygen-poor, with the exception of the pulmonary veins, which carry oxygenated blood from the lungs back to the heart. The major veins, like their companion arteries, often take the name of the organ served. The exceptions are the superior vena cava and the inferior vena cava, which collect body from all parts of the body (except from the lungs) and channel it back to the heart. Artery/Vein Tissues
Arteries and veins have the same three tissue layers, but the proportions of these layers differ. The innermost is the intima; next comes themedia; and the outermost is the adventitia. Arteries have thick media to absorb the pressure waves created by the heart's pumping. The smooth-muscle media walls expand when pressure surges, then snap back to push the blood forward when the heart rests. Valves in the arteries prevent back-flow. As blood enters the capillaries, the pressure falls off. By the time blood reaches the veins, there is little pressure. Thus, a thick media is no longer needed. Surrounding muscles act to squeeze the blood along veins. As with arteries, valves are again used to ensure flow in the right direction. Anatomy of the Heart  Audio recording: Heart Sounds Animation: Circulation through the heart The heart is about the size of a man's fist. Located between the lungs, two-thirds of it lies left of the chest midline The heart, along with the pulmonary (to and from the lungs) and systemic (to and from the body) circuits, completely separates oxygenated from deoxygenated blood. Internally, the heart is divided into four hollow chambers, two on the left and two on the right. The upper chambers of the heart, the atria (singular: atrium), receive blood via veins. Passing through valves (atrioventricular (AV) valves), blood then enters the lower chambers, the ventricles. Ventricular contraction forces blood into the arteries.
Blood Pressure and Heart Rate The heart beats or contracts around 70 times per minute.1 The human heart will undergo over 3 billion contraction/cardiac cycles during a normal lifetime. One heartbeat, or cardiac cycle, includes atrial contraction and relaxation, ventricular contraction and relaxation, and a short pause. Atria contract while ventricles relax, and vice versa. Heart valves open and close to limit flow to a single direction. The sound of the heart contracting and the valves opening and closing produces a characteristic "lub-dub" sound. The cardiac cycle consists of two parts: systole (contraction of the heart muscle in the ventricles) and diastole (relaxation of the ventricular heart muscles). When the ventricles contract, they force the blood from their chambers into the arteries leaving the heart. The left ventricle empties into the aorta (systemic circuit) and the right ventricle into the pulmonary artery (pulmonary circuit). The increased pressure on the arteries due to the contraction of the ventricles (heart pumping) is called systolic pressure. When the ventricles relax, blood flows in from the atria. The decreased pressure due to the relaxation of the ventricles (heart resting) is called diastolic pressure. Blood pressure is measured in mm of mercury, with the systole in ratio to the diastole. Healthy young adults should have a ventricular systole of 120mm, and 80mm at ventricular diastole, or 120/80. Receptors in the arteries and atria sense systemic pressure. Nerve messages from these sensors communicate conditions to the medulla in the brain. Signals from the medulla regulate blood pressure. Electrocardiography (ECG, EKG)  An electrocardiogram measures changes in electrical potential across the heart and detects contraction pulses that pass over the surface of the heart. There are three slow, negative changes, known as P, R, and T. Positive deflections are the Q and S waves. The P wave represents atrial contraction ("the lub"), the T wave the ventricular contraction ("the dub"). The Lymphatic System The lymphatic system functions 1) to absorb excess fluid, thus preventing tissues from swelling; 2) to defend the body against microorganisms and harmful foreign particles; and 3) to facilitate the absorption of fat (in the villi of thesmall intestine).  Capillaries release excess water and plasma into intracellular spaces, where they mix with lymph, or interstitial fluid. "Lymph" is a milky body fluid that also contains proteins, fats, and a type of white blood cells, called "lymphocytes," which are the body's first-line defense in the immune system. Lymph flows from small lymph capillaries into lymph vessels that are similar to veins in having valves that prevent backflow. Contraction of skeletal muscle causes movement of the lymph fluid through valves. Lymph vessels connect to lymph nodes, lymph organs (bone marrow, liver, spleen, thymus), or to the cardiovascular system.
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