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

Superior vena cava

Left pulmonary artery

Walls Heart

vein

FIGURE 9-1. The cardiovascular system. (Reprinted with permission from Cohen BJ, Wood DL. Memmler's The Human Body in Health and Disease. 9th Ed. Philadelphia: Lippincott Williams & Wilkins, 2000.)

Inferior vena cava

Left pulmonary vein

FIGURE 9-1. The cardiovascular system. (Reprinted with permission from Cohen BJ, Wood DL. Memmler's The Human Body in Health and Disease. 9th Ed. Philadelphia: Lippincott Williams & Wilkins, 2000.)

Pulmonic valve

Brachiocephalic artery

Left common carotid artery Left subclavian artery

Superior vena cava

Right pulmonary -artery (branches)

Ascending aorta

Right _

pulmonary veins

Aortic arch

Pulmonary artery

Right atrium

Tricuspid valve

Right ventricle

Inferior vena cava

Blood high in oxygen Blood low in oxygen

Endocardium Myocardium Epicardium

Pulmonic valve

Superior vena cava

Figure The Heart And Great Vessels

FIGURE 9-2. The heart and great vessels. (Reprinted with permission from Cohen BJ, Wood DL. Memmler's The Human Body in Health and Disease. 9th Ed. Philadelphia: Lippincott Williams & Wilkins, 2000.)

Left pulmonary artery (branches)

Left pulmonary veins

Left atrium Aortic valve

Mitral

(bicuspid)

valve

Left ventricle

Apex

Interventricular septum

FIGURE 9-2. The heart and great vessels. (Reprinted with permission from Cohen BJ, Wood DL. Memmler's The Human Body in Health and Disease. 9th Ed. Philadelphia: Lippincott Williams & Wilkins, 2000.)

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-AS-to-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 atrioventricu_lar (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.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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