Cardiac Physiology Chapter 9

The circulatory system has three main components: The heart establishes a pressure gradient to pump the blood The blood vessels are passageways for the distribution of pumped blood throughout the body. The blood is a transport medium, serving the needs of body cells. The pulmonary circulation is a loop of blood vessels between the heart and the lungs. The systemic circulation is the circuit of blood vessels between the heart and other body systems.

The heart is a muscular organ, about the size of a closed fist. It is located in the thoracic cavity, between the sternum and vertebrae. The base of the heart lies to the right of the sternum. The apex lies to the left of the sternum. By its location the blood can be manually driven from the heart by CPR. The heart is a dual pump. The atria are its two upper chambers. They receive blood returning to the heart. They transfer blood to the ventricles. The ventricles are the two lower chambers. They pump blood from the heart. Veins are blood vessels that carry blood from the tissues to the heart. Arteries are blood vessels that carry blood from the heart to the tissues. A septum separates the low-oxygen blood on the right side of the heart from the high-oxygen blood on the left side.

The right side of the heart receives blood from the systemic circulation. The heart pumps blood into the pulmonary circulation. The venae cavae are veins returning blood to the right atrium. Oxygen has been extracted from this blood. Carbon dioxide has been added to it. This blood is pumped from the right ventricle through the pulmonary artery to the lungs. The lungs add oxygen to this blood received from the right side of the heart. Carbon dioxide is taken away from this blood. This blood flows through pulmonary veins to the left atrium of the heart. This high-oxygen is pumped from the left ventricle through the aorta, a large artery. Therefore, the left side of the heart receives blood from the pulmonary circulation. It pumps it into the systemic circulation.

The systemic circulation is a series of parallel pathways The systemic circulation is a series of parallel pathways. Parts of it are distributed to different body regions. Each region receives a fresh blood supply. Depleted blood (low oxygen, high carbon dioxide) returns to the right side of the heart. The right and left pumps can be compared. Oxygen-poor blood on the right side soon becomes the same volume of oxygen-rich blood pumped from the left side. The pulmonary circulation is low pressure and low resistance. The systemic circulation is the opposite. Pressure is the force exerted by pumped blood on a vessel wall. Resistance is the opposition to blood flow.

The action of heart valves ensure that blood flows in the proper direction. The AV valves are the tricuspid on the right and bicuspid on the left. They allow blood to flow from the atria into the ventricles. This occurs when atria pressure is greater than ventricular pressure, during ventricular filling. During ventricular emptying, when ventricular pressure exceeds atrial pressure, the AV valves close. This prevents the blood from flowing backwards. The AV valves are anchored by chordae tendineae to papillary muscles. The other pair of valves are the semilunar, pulmonary on the right and aortic on the left. They are forced open when the ventricular pressures exceed the pressures in the pulmonary arteries and aorta. The semilunar valves close when the pressure in the ventricles falls below the pressures in these vessels. This prevents the blood from flowing backwards. A fibrous skeleton separates the atria from the ventricles. See Figures 9-5 and 9-6

The heart wall consists of three layers. The endocardium is the inner layer of epithelium. The myocardium is the middle layer of cardiac muscle tissue. The epicardium is the external membrane. Cardiac muscle fibers are interconnected by intercalated discs. They form a functional syncytia.