HEART PHYSIOLOGY

Blood Pressure:
Blood pressure is the force exerted by blood against the wall of a blood vessel. It is commonly measured in a large artery with an inflatable cuff (Fig. 9-4) known as a blood pressure cuff or blood pressure apparatus, but technically called a sphygmomanometer. Both systolic and diastolic pressures are measured and reported as systolic then diastolic separated by a slash, such as 120/80. Pressure is expressed as millimeters of mercury (mm Hg), that is, the height to which the pressure can push a column of mercury in a tube. Blood pressure is a valuable diagnostic measurement that is easily obtained.

Cardiovascular System:
Blood circulates throughout the body in the cardiovascular system, which consists of the heart and the blood vessels (Fig. 9-1). This system forms a continuous circuit that delivers oxygen and nutrients to all cells and carries away waste products. Also functioning in circulation is the lymphatic system, which drains fluid and proteins from the tissues and returns them to the bloodstream.
The Heart:
The heart is located between the lungs, with its point or apex directed toward the left (Fig. 9-2). The thick muscle layer of the heart wall is the myocardium. This is lined on the inside with a thin endocardium and is covered on the outside with a thin epicardium. The heart is contained within a fibrous sac, the pericardium.Each of the upper receiving chambers of the heart is an atrium (plural, atria). Each of the lower pumping chambers is a ventricle (plural, ventricles). The chambers of the heart are divided by walls, each of which is called a septum. The interventricular septum separates the two ventricles; the interatrial septum divides the two atria. There is also a septum between the atrium and ventricle on each side.The heart pumps blood through two circuits. The right side pumps blood to the lungs to be oxygenated through the pulmonary circuit. The left side pumps to the remainder of the body through the systemic circuit.

Blood Flow Through the Heart:
The pathway of blood through the heart is shown by the arrows in Figure 9-2. The right atrium receives blood low in oxygen from all body tissues through the superior vena cava and the inferior vena cava. The blood then enters the right ventricle and is pumped to the lungs through the pulmonary artery. Blood returns from the lungs high in oxygen and enters the left atrium through the pulmonary veins. From here it enters the left ventricle and is forcefully pumped into the aorta to be distributed to all tissues.Blood is kept moving in a forward direction by one-way valves. The valve in the septum between the right atrium and ventricle is the tricuspid valve (meaning three cusps or flaps); the valve in the septum between the left atrium and ventricle is the bicuspid valve (having two cusps), usually called the mitral valve (so named because it resembles a bishop’s miter). The valves leading into the pulmonary artery and the aorta have three cusps. Each cusp is shaped like a half-moon, so these valves are described as semilunar valves. The valve at the entrance to the pulmonary artery is specifically named the pulmonic valve; the valve at the entrance to the aorta is the aortic valve.Heart sounds are produced as the heart functions. The loudest of these, the familiar lubb and dupp that can be heard through the chest wall, are produced by alternate closing of the valves. The first heart sound (S1) is heard when the valves between the chambers close. The second heart sound (S2) is produced when the valves leading into the aorta and pulmonary artery close. Any sound made as the heart functions normally is termed a functional murmur. (The word murmur used alone with regard to the heart describes an abnormal sound.)

The Heartbeat:
Each contraction of the heart, termed systole (SIS-to_-le _), is followed by a relaxation phase, diastole (di_-ASto_-le_), during which the chambers fill. Each time the heart beats, both atria contract and immediately thereafter both ventricles contract. The wave of increased pressure produced in the vessels each time the ventricles contract is the pulse. Contractions are stimulated by a built-in system that regularly transmits electrical impulses through the heart. The components of this conduction system are shown in Figure 9-3. They include the sinoatrial (SA) node, called the pacemaker because it sets the rate of the heartbeat, the atrioventricular (AV) node, the AV bundle (bundle of His), the left and right bundle branches, and Purkinje ( pur-KIN-je_) fibers. Although the heart itself generates the heartbeat, factors such as nervous system stimulation, hormones, and drugs can influence the rate and the force of heart contractions.

The Vascular System:
The vascular system consists of:
1. Arteries that carry blood away from the heart (Fig. 9-5). Arterioles are small arteries that lead into the capillaries.
2. Capillaries, the smallest vessels, through which exchanges take place between the blood and the tissues.
3. Veins that carry blood back to the heart (Fig. 9-6). The small veins that receive blood from the capillaries and drain into the veins are venules.
All arteries, except the pulmonary artery (and the umbilical artery in the fetus), carry blood high in oxygen. They are thick-walled, elastic vessels that carry blood under high pressure. All veins, except the pulmonary vein (and the umbilical vein in the fetus), carry blood low in oxygen. Veins have thinner, less elastic walls and tend to give way under pressure. Like the heart, veins have one-way valves that keep blood flowing forward.
Nervous system stimulation can cause the diameter of a vessel to increase (vasodilation) or decrease (vasoconstriction).These changes alter blood flow to the tissues and affect blood pressure.