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Cardiovascular System
The cardiovascular system is the term used to collectively describe the heart, blood, and vessels in an organism. The cardiovascular system, also refered to sometimes as the circulatory system, comes from the Greek kardia for "heart," and the Latin vasculum, for "small vessel."

Human Heart and Blood Flow
The human cardiovascular system is comprised of a heart, vessels that carry the blood from the heart to the rest of the body, and the blood itself. The human heart consists of four different chambers: two atria and two ventricles. The atrium and ventricles that are located on the right side of your body are called the right heart while the atrium and ventricles that are located on the left side of your body are called the left heart. An atrium receives blood from the body and pumps it into a more muscular chamber, the ventricle. The ventricle pumps the blood to the lungs where gas exchange occurs and to other parts of the body once oxygenated. The right heart is responsible for sending blood through the pulmonary circuit and the left heart is responsible for sending blood through the systemic circuit. Located in the heart are different valves that prevent the backflow of blood. The atrioventricular valves, located between the atria and ventricles, stop blood from flowing back into the atria when the ventricles contract. The pulmonary valve and the aortic valve are positioned between the ventricles and the major arteries to prevent blood from flowing back into the ventricles.

Deoxygenated blood starts its circulation through the heart by entering the right atrium from the superior or inferior vena cava. The superior vena cava brings blood in from the upper portion of the body while the inferior vena cava collects blood from the lower portion. At the contraction of the right atrium, blood travel through an atrioventricular valve and into the right ventricle. After the right ventricle is filled with blood it contracts, pumping the blood into the pulmonary artery, which takes the blood to the lungs. After gas exchange has occurs in the lungs, blood returns to the heart via pulmonary veins. The oxygenated blood enters at the left atrium where it collects before entering the left ventricle through an atrioventricular valve. The muscular walls of the left ventricle exert a large force on the ventricle forcing the blood from the ventricle, through the aortic valve, and into the aorta where the oxygenated blood will circulate throughout the rest of the body. Both sides of the heart contract at the same time. The cardiac cycle consist of the contraction of the two atria followed by the contraction of the two ventricles and a period of relaxation. Contraction of the ventricles is called ventricular systole, and the relazation of the ventricles is called ventricular diastole.

Blood Pressure
Blood pressure is the pressure exerted by the blood on the walls of the blood vessesl. Blood pressure is generally referred to as systemic arterial blood pressure, or the pressure in the large arteries that deliver blood to body parts not including the lungs. Blood pressure measurement is generally taken on the arm using a sphygmomanometer, an inflatable pressure cuff and a pressure gague, and a stethoscope. By inflating the cuff pressure is increased so that both the arteries and veins close in the arm. Pressure in the cuff is gradually lowered until the sound of a pulsing flow of blood through the construction in the artery is heard. When this occurs, the pressure in the cuff is just below the peak systolic presure in the artery. Pressure is further lowered in the cuff until the sound becomes continuous. At this moment, the cuff is just below the diastolic pressure in the artery. Normal values for blood pressure in an individual are 120 mm Hg during systole and 80 mm Hg during diastole, or 120/80.

Heartbeat
A normal hearthbeat begigs with an action potential in the sinoatrial node, also known as the SA node. The SA node is located at the junction of the superior vena cava and the right atrium. These pacemaker cells are cardiac mucsle cells that have the ability to initiate action potentials without stimulation from the nervous system. When one of these cells fires an action potential, they stimulate neighboring cells to contract. The action potential for the SA node results from the opening of voltage-gated calcium channels. Once the action potential starts in the sinoatrial node it spreads rapidly throughout the electrically couples cells of the atria, causing a contraction. Located at the junction of the atria and the ventricles is another nodule of modified cardiac muscle cells called the atrioventricular node, also known as the AV node. The depolarization of the atria stimulates the AV node and it generates action potentials that are conducted to the ventricles through a bundle of fibers called the bundle of His. These fibers divide into the right and left bundle branches that run to the tips of the right and left ventricles and through the ventriculr muscle mass known as Purkinje fibers. The Purkinje fibers are responsible for ensuring that the ventricles contract at the right moment so that blood passes progressively from the artia to the ventricles to the arteries in a timely manner.

Bloodflow through Body


Blood circulates throughout the body in an intricate system of arteries, capillaries, and veins. Arties have collagen and elastic fibers which allow them to withstand the high pressure from the blood that is just leaving the heart. Veins do not have this elastic layer because blood pressure is so low by the time it reaches the veins that it is not needed for protection. Blood flows throughout the body from large to small arteries before reaching the arterioles and finally the capillares where exchange of materials between blood and tissue occurs. Capillaries have thin, permeable walls, and blood flows through these vessels slowly, making the exchange easier. Materials are exchanged in the capillaries easily by means of filtration, osmosis, and diffusion. Capillaries are permeable to water and smaller molecules and ions but not to larger molecules. Blood pressure is able to squeeze water and smaller molecules out of the capillaries and into the surrounding intercellular spaces. E.H. Starling proposed the method by which how blood volume is maintained in capillary beds. Starling stated that when blood pressure is greater than colloidal osmotic pressure, water leaves the capillary and when blood pressure falls below osmotic pressure water returns to the capillary.

Blood
Blood is classified as atypical connective tissue. Blood consists of cells suspended in an extracellular matrix. The cellular portion of blood is comprised of erythrocytes, or red blood cells, leukocytes, and platelets. The plasma, or complex aqueous solution, is made of water and a solvent that is responsible for osmotic balance, pH buffering, clotting, and immune responses. It is the erythocytes that are responsible for transporting oxygen and carbon dioxide. White blood cells are responsible fore destroying foreign cells, producing antibodies. Platelets are primarily responsible for blood clotting.