The heart is the pump of the cardiovascular system. The heart's contractions continually propel the blood through hollow blood vessels. The blood vessels transport blood to the body's tissues and back to the heart again.
LOCATION & SIZE OF THE HEART: (FIGURE 14.1)
The heart is a hollow organ that is about the size of a person's fist. The heart weighs about 300 grams (about 10 oz). The heart is about 12 cm (5 inches) long.
The heart is snugly enclosed within the region between the lungs called the MEDIASTINUM. In other words, the heart lies anterior to the vertebral column, posterior to the sternum and medial to the overlapping lungs. The heart tips slightly to the right. About 2/3's of its mass can be seen to the left of the body's midline. Its broad superior portion (called the BASE) is about 9 cm (3.5 inches) wide and is directed toward the right shoulder. Its more pointed APEX is directed inferiorly toward the left hip and rests on the DIAPHRAGM.
COVERINGS OF THE HEART: (FIG. 14.2)
The heart is enclosed within a thin double-layered sac of SEROUS MEMBRANE called the PERICARDIUM. The PARIETAL PERICARDIUM lines the internal wall of the PERICARDIAL CAVITY. At the superior part of the heart (the heart's BASE), the parietal pericardium turns downward and continues over the heart surface as the VISCERAL PERICARDIUM. The VISCERAL PERICARDIUM is the outer layer of the heart wall and often contains fat, particularly in older people.
Between the VISCERAL PERICARDIUM and the PARIETAL PERICARDIUM is a potential space, called the PERICARDIAL CAVITY, which contains a small amount of thin serous fluid, called PERICARDIAL FLUID. The pericardial fluid lubricates the serous membranes so they glide smoothly against one another during the heart beat, allowing the heart to beat in a relatively friction-free environment.
HEART WALL: (FIG. 14.2)
The heart wall is composed of 3 layers: the outermost EPICARDIUM, the middle MYOCARDIUM, and the innermost ENDOCARDIUM.
The EPICARDIUM is another name for the VISCERAL PERICARDIUM surrounding the outside of the heart.
The MYOCARDIUM is composed mainly of cardiac muscle tissue with some connective tissue. It is the thickest layer of the heart wall. This is the layer that actually contracts and beats the heart. What are the characteristics of cardiac muscle tissue?
The ENDOCARDIUM is a glistening white sheet of ENDOTHELIUM overlying a thin connective tissue layer. ENDOTHELIUM is a layer of simple squamous epithelium that lines the walls of the heart and blood vessels. The endocardium lines the heart chambers and covers the heart valves. The endocardium is continuous with the endothelial linings of the blood vessels entering and leaving the heart.
CHAMBERS OF THE HEART AND ASSOCIATED GREAT VESSELS:
The heart has 4 chambers: the superior LEFT & RIGHT ATRIA and the inferior LEFT & RIGHT VENTRICLES. See FIG. 14.3. Note that the LEFT VENTRICLE forms the APEX of the heart. The 2 atria are separated by the INTERATRIAL SEPTUM and the 2 ventricles are separated by the INTERVENTRICULAR SEPTUM.
An external groove in the heart called the CORONARY SULCUS (FIG. 14.3(a)) separates the atria from the ventricles. The ANTERIOR INTERVENTRICULAR SULCUS (FIG. 14.3(a) & (b)) is an external groove that separates the right and left ventricles. The POSTERIOR INTERVENTRICULAR SULCUS (FIG. 14.3(c)) separates the right and left ventricles on the posterior side of the heart. The sulci are grooves that contain coronary blood vessels and some fat.
See FIG. 14.4. Functionally, the right and left ATRIA are receiving chambers for blood returning to the heart from circulation. Because they need to contract only minimally to push blood into the ventricles, the atria are relatively small, thin-walled chambers. They contribute little to the propulsive pumping activity of the heart. The pouch-like outer anterior wall of the atria are called the left and right AURICLES (FIG. 14.3(a)).
FIG. 14.4. The right and left VENTRICLES are the discharging chambers or actual pumps of the heart; when they contract, blood is pushed out of the heart into arteries to be circulated to other parts of the body. The ventricle walls are more muscular, and therefore thicker, than the atrial walls, because the ventricles have to pump blood out of the heart.
PATHWAY OF BLOOD THROUGH THE HEART: (FIGS. 14.7 (a) & (b))
The heart actually consists of 2 side-by-side pumps, each serving a separate blood circuit: the pulmonary circuit and the systemic circuit. The pulmonary circuit receives low oxygen blood returning from the body and sends it to the lungs for oxygenation. The oxygenated blood then returns to the heart. The systemic circuit supplies the entire body with oxygenated blood.
The right side of the heart is the pulmonary circuit pump. Blood returning from the body, which is relatively OXYGEN-POOR and CARBON DIOXIDE-RICH, enters the RIGHT ATRIUM from the superior vena cava, inferior vena cava & coronary sinus. The SUPERIOR VENA CAVA returns blood from body regions superior to the heart. The INFERIOR VENA CAVA returns blood from body areas inferior to the heart. The CORONARY SINUS collects blood from veins in the heart wall. The blood passes from the RIGHT ATRIUM into the RIGHT VENTRICLE, which pumps the blood into the PULMONARY TRUNK. The pulmonary trunk divides into the RIGHT and LEFT PULMONARY ARTERIES, each of which carries blood to each lung. In the lungs, the pulmonary arteries divide into CAPILLARIES, that are closely associated with the AIR SACS of the lungs. Gas exchange takes place here between the capillaries and air sacs: the blood releases carbon dioxide to the air sacs and oxygen moves from the air sacs into the blood. The capillaries merge into the four PULMONARY VEINS which return the oxygenated blood to the LEFT ATRIUM.
The left side of the heart is the systemic circuit pump. Blood in the LEFT ATRIUM passes from the left atrium into the LEFT VENTRICLE. The left ventricle pumps the blood into the AORTA. The aorta branches into smaller SYSTEMIC ARTERIES that carry blood to the body tissues, where exchange of gases and nutrients occurs across the capillary walls. The capillaries empty the oxygen-depleted blood into systemic veins. The systemic veins merge together, eventually emptying into the SUPERIOR or INFERIOR VENA CAVA. The SUPERIOR & INFERIOR VENA CAVAE deliver the blood to the RIGHT ATRIUM. And now the pulmonary circuit begins again.
Notice that the left and right ventricles have unequal work loads. The pulmonary circuit is a short, low-pressure circulation, whereas the systemic circuit takes a long pathway through the entire body and encounters about 5 times as much resistance to blood flow. This functional difference is revealed in the comparative anatomy of the two ventricles. The walls of the left ventricle are at least twice as thick as the walls of the right ventricle. See FIGS. 14.4 (a) & (c). This is because there is more cardiac muscle in the walls of the left ventricle, so consequently, the left ventricle is a much more powerful pump.
HEART VALVES: (FIGS. 14.4, 14.5 & 14.6)
Blood flows through the heart in one direction: from the atria to the ventricles and out through the arteries that leave the ventricles. This one-way flow of blood is enforced by the presence of 4 heart valves. HEART VALVES are structures composed of dense irregular connective tissue and are covered with ENDOCARDIUM. They prevent backflow of blood.
The 2 ATRIOVENTRICULAR (AV) VALVES, located between the atrial and ventricular chambers on each side, prevent backflow into the atria when the ventricles are contracting. The right AV valve, the TRICUSPID VALVE, is located between the right atrium and right ventricle and has 3 flexible valve flaps or cusps. The left AV valve, called the BICUSPID VALVE, is located between the left atrium and left ventricle and it has 2 valve flaps. Attached to each of the AV valve flaps are tiny white collagen cords called CHORDAE TENDINEAE that anchor the cusps to the PAPILLARY MUSCLES protruding from the ventricular walls. Papillary muscles are small conical projections that protrude from the ventricle walls.
When blood enters an atrium, blood puts pressure on the AV valve. The papillary muscles relax and the chordae tendineae slacken, so that the AV valve flaps hang limply into the ventricular chamber. The blood flows through the open AV valve into the ventricle. When the ventricle contracts to pump blood out of the heart and into an artery, any blood pushed back toward the atrium pushes the cusps of the AV valve upward, closing the valve. When the ventricle contracts, so do the papillary muscles, causing the chordae tendineae to tighten, preventing the cusps of the valve from inverting into the atrium. If the cusps were not anchored in this way, they would be pushed upward into the atria by the blood.
The 2 SEMILUNAR VALVES prevent blood from flowing back from the aorta and pulmonary trunk into the ventricles. The AORTIC SEMILUNAR VALVE is at the junction of the aorta and left ventricle. The PULMONARY SEMILUNAR VALVE is at the junction between the pulmonary trunk and right ventricle.
Each semilunar valve consists of 3 pocket-like cusps. When the ventricles are at the peak of their contraction, the semilunar valves are forced open and their cusps flatten against the arterial walls as the blood rushes past them. When the ventricles relax, and the blood is no longer propelled forward by the pressure of the ventricular contraction, the blood begins to flow backward toward the heart. The blood fills the cusps and effectively closes the valves. This prevents backflow of blood from the arteries back into the ventricles.
The cardiac cycle can be observed in Fig. 14.10. The relaxation period between heartbeats consists of atrial and ventricular diastole. After the AV valves open, the ventricles fill with blood. When the ventricles are 75% filled with blood, the atria contract (atrial systole), pushing the remaining 25% of the blood into the ventricles. Then the ventricles contract (ventricular systole). This is followed by another relaxation period (atrial and ventricular diastole).
The sound of the heartbeat (called heart sounds) is from the blood turbulence caused by closing of the heart valves. The first heart sound, which is described as a "lubb" sound, is a result of the closing of the AV valves (bicuspid and tricuspid) during ventricular systole (contraction). The second heart sound, which is described as a "dupp" sound, is due to the closing of the semilunar valves during ventricular diastole (relaxation). For each heart beat (cardiac cycle), there is a "lubb, dupp". See FIG. 14.11 for the locations on the chest where the individual heart sounds can be heard with a stethoscope.
CONDUCTION SYSTEM: (FIG. 14.9)
The heart has its own pacemaker that allows the heart to beat on its own, without nervous stimulation. Even if all nerve connections to the heart are cut, the heart continues to beat (contract and relax) rhythmically. The SINOATRIAL NODE (SA NODE) or pacemaker is a compact mass of cells located in the right atrial wall inferior to the opening of the superior vena cava. The SA node initiates each cardiac cycle (heartbeat), so it sets the basic pace for the heart rate. The rate may be increased by nerve impulses from the AUTONOMIC NERVOUS SYSTEM or by hormones like epinephrine (also called adrenaline), but the sinoatrial node sets the resting heartrate.
The sinoatrial node is part of the heart's conduction system. The conduction system consists of specialized cardiac muscle tissue that is nerve-like and noncontractile. The conduction system generates and distributes its own electrical impulses that stimulate cardiac muscle tissue to contract. The impulse is distributed throughout the heart so that the myocardium contracts in an orderly, sequential manner from atria to ventricles.
See FIG. 10.8, page 279. Note the INTERCALATED DISCS between the cardiac muscle fibers. INTERCALATED DISCS contain GAP JUNCTIONS through which electrical impulses can travel from cardiac muscle cell to cardiac muscle cell, stimulating each cell to contract.
The SA node initiates an electrical impulse that travels from cardiac muscle fiber (cell) to cardiac muscle fiber through the GAP JUNCTIONS in the INTERCALATED DISCS. The electrical impulse spreads out through the left & right atria, stimulating them to contract. The impulse then reaches the ATRIOVENTRICULAR NODE (AV NODE), in the inferior interatrial septum. The AV node generates an electrical impulse that passes rapidly through the ATRIOVENTRICULAR BUNDLE (BUNDLE OF HIS), then the left and right BUNDLE BRANCHES in the interventricular septum, and then the PURKINJE FIBERS that penetrate into the VENTRICULAR MYOCARDIUM. The electrical impulses from the Purkinje fibers stimulate the ventricles to contract.
Keep in mind that while the electrical impulse passes through the atria, the atria contract. When the electrical impulse arrives at the AV node, the atria begin to relax. So, while the ventricles are contracting the atria are relaxed, and vice versa. Since the average heart rate is 70 beats per minute, this conduction process is repeated on average 70 times a minute.
The electric current generated and transmitted through the heart can be detected at the body's surface with an electrocardiogram (see FIG. 14.19 (b)) Atrial systole (contraction) occurs during the P wave. The ventricles contract (ventricular systole) during the QRS wave. The T wave indicates ventricular diastole (relaxation).