Form, Size, and Location of the Heart

Knowledge of the heart’s position in the thoracic cavity is important in hearing heart sounds, obtaining electrocardiograms (ECGs), and performing cardiopulmonary resuscitation (CPR). The heart, illustrated in Figure 12-1, is located in the thoracic cavity between the two lungs. It is posterior to the sternum and anterior to the vertebral column, and it rests on the diaphragm. About two thirds of the heart mass is to the left of the body’s midline, and one third is to the right. The apex, or pointed end of the heart, extends downward to the level of the fifth intercostal space. The opposite end, the base, is larger and less pointed than the apex and has several large vessels attached to it. Its most superior portion is at the level of the second rib. The size of the heart varies with the size of the individual. On average, it is about 9 cm wide and 12 cm long, which is about the size of a closed fist.

Coverings of the Heart

The heart is enclosed by a loose-fitting, double-layered sac called the pericardium (pair-ih-KAR-dee-um) or pericardial sac. The outer layer of the pericardium consists of tough, white fibrous connective tissue and is called the fibrous pericardium. The fibrous pericardium is lined with a serous membrane called the parietal pericardium. Where the pericardium is attached to the vessels at the base of the heart, the parietal pericardium reflects onto the surface of the heart to form the visceral pericardium, or epicardium. The small space between the parietal and visceral layers of the pericardium is the pericardial cavity. It contains a thin layer of serous fluid that reduces friction between the membranes as they rub against each other during heart contractions.

Layers of the Heart Wall

The heart wall is formed by three layers of tissue: an outer epicardium, a middle myocardium, and an inner endocardium. The epicardium (eh-pih-KAR-dee-um) (which is the same as the visceral pericardium) consists of a serous membrane. It is a thin protective layer that is firmly anchored to the underlying muscle. Blood vessels that nourish the heart wall are located in the epicardium.

The thick middle layer is the myocardium (my-oh-KAR-dee-um). It forms the bulk of the heart wall and is composed of cardiac muscle tissue. Refer to Chapter 5 for a review of the different types of muscle tissue. Contractions of the myocardium provide the force that ejects blood from the heart and moves it through the vessels.

The smooth inner lining of the heart wall is the endocardium (en-doh-KAR-dee-um). Its smooth surface permits blood to move easily through the heart. The endocardium also forms the valves of the heart and is continuous with the lining of the blood vessels. Figure 12-2 illustrates the layers of the heart wall.

Chambers of the Heart

The internal cavity of the heart is divided into four chambers (Figure 12-3):

The two atria are thin-walled chambers that receive blood from the veins. The two ventricles are thick-walled chambers that forcefully pump blood out of the heart. Differences in thickness of the heart chamber walls are caused by variations in the amount of myocardium present, which reflects the amount of force each chamber is required to generate.

The right atrium (AY-tree-um) receives deoxygenated blood from the superior vena cava and the inferior vena cava. The superior vena cava returns blood to the heart from the head, neck, and upper extremities. The inferior vena cava returns blood to the heart from the thorax, abdomen, pelvis, and lower extremities. The left atrium receives oxygenated blood from the lungs through four pulmonary veins, two on the right and two on the left. Because the atria are “receiving” chambers rather than “pumping” chambers, their myocardium is relatively thin. The right and left atria are separated by a partition called the interatrial septum. A thin region, the fossa ovalis, is found in the interatrial septum. This represents an opening, the foramen ovale, that is present between the atria in the fetal heart.

The right ventricle (VEN-trih-kull) receives blood from the right atrium and pumps it out to the lungs, where it picks up a new supply of oxygen. The left ventricle receives blood from the left atrium and pumps it out to the tissues of the whole body. The ventricles are “pumping” chambers, and this is reflected by a thick myocardium. Because the left ventricle pumps blood to the whole body and the right ventricle sends blood only to the lungs, the left ventricle has to generate a lot more pumping force than the right ventricle. This is reflected in the fact that the left ventricular wall has a thicker myocardium than the right. Both ventricles hold about the same volume of blood. The thick, muscular partition between the right and left ventricles is the interventricular septum.

Highlight on the Circulatory System

Blood doping: Blood doping is a practice reportedly used by some athletes to improve their endurance for aerobic activities such as running, swimming, and cycling. A few weeks before a competition, blood is drawn from the athlete and the red blood cells (RBCs) are separated and frozen. Normal hematopoiesis replaces the lost RBCs and brings the blood cell count back to normal. Then, just before the competition, the frozen RBCs are thawed and injected into the athlete. This creates an artificial polycythemia. The idea is that the additional RBCs are able to deliver more oxygen to the muscles and improve aerobic endurance. Whether this occurs is questionable, and the practice is not without danger. All blood transfusions carry some risk. Furthermore, the additional cells increase the viscosity or thickness of the blood and put a strain on the heart.

Pericarditis: Pericarditis is an inflammation of the pericardium. This may interfere with production of the serous fluid that lubricates the surfaces of the parietal and visceral layers. Painful adhesions may form and interfere with contraction of the heart.

Damaged heart valves: Sometimes disease processes damage the heart valves so that they are unable to function properly. Incompetent valves permit a “backflow” of blood, and the heart has to pump the same blood over and over to get it into the vessels. In valvular stenosis, the valves are stiff and have narrow openings. The heart has to work harder to pump blood out through the small opening. Defective heart valves may result in abnormal heart sounds. For example, if the atrioventricular valves are faulty, a hissing sound may be heard between the first and second heart sounds.

Angina pectoris: Angina pectoris is chest pain that results when the heart muscle’s demand for oxygen exceeds the oxygen supply. Nitroglycerin is sometimes used in the treatment of angina pectoris because it dilates blood vessels, so the patient is less likely to develop a myocardial oxygen deficit.

Coronary artery blockage: If a branch of a coronary artery becomes blocked, blood supply to that region of the heart is cut off and the muscle cells in that area die because of lack of oxygen. This is a myocardial infarction (MI), also called a coronary or a heart attack. The extent of the damage and chances of recovery depend on the location of the blockage and the length of time that elapses before medical intervention occurs.

Heart enzymes: When heart muscle is damaged, the dying cells release enzymes into the bloodstream. These enzymes can be measured and are useful in confirming an MI. The enzymes assayed are creatine kinase (CK) and lactate dehydrogenase (LDH).

Dysrhythmias: Variations in normal contraction patterns are called dysrhythmias (arrhythmias). One type of dysrhythmia occurs when a conduction myofiber or heart muscle cell independently depolarizes to threshold and triggers a premature heart contraction. The cell responsible for the premature contraction is called an ectopic focus. Sometimes ectopic foci form feedback loops within the conduction system, causing myocardial contractions to occur at a rapid rate. If not treated properly, this often leads to ventricular tachycardia, fibrillation, and death.

Red bone marrow: By the age of 25 years, a person’s red bone marrow for hematopoiesis is limited to the flat bones of the skull, iliac crests, ribs, sternum, vertebrae, and proximal ends of the humerus and femur.

Smoking: About 20% of a cigarette smoker’s hemoglobin is nonfunctional for transporting oxygen because it is bound to carbon monoxide from the cigarette smoke.

Reticulocytes: The reticulocyte count in circulating blood gives information about the rate of hematopoiesis. Normally 0.5% to 1.5% of the RBCs in normal blood are reticulocytes. A number below 0.5% indicates a slowdown in production. Values above 1.5% indicate a greater-than-normal rate of RBC formation.

Vitamin K: The K in vitamin K is for koagulation, the German word for clotting. In other words, vitamin K is the “koagulation” vitamin. Although this vitamin is necessary in the diet, it is also produced by bacteria in the large intestine and absorbed into the blood.

Aneurysm: An aneurysm is a bulge, or bubble, that develops at a weakened region in the wall of an artery. This is especially dangerous if it is in the aorta or arteries of the brain. If diagnosed soon enough, the aneurysm sometimes may be removed and the vessel surgically repaired. Because the wall is weakened, an aneurysm is subject to rupture. Little can be done when this happens because the massive bleeding usually leads to death before medical care can be obtained.

Blood vessels: It is estimated that if all the capillaries in the body were placed end to end, they would encircle the earth at the equator two and one-half times! The difference in blood pressure between arteries and veins is obvious when the vessels are cut. Blood flows smoothly and freely from a vein, but it spurts forcefully from an artery.

Varicose veins: Varicose veins are veins that are twisted and dilated with accumulated blood. These frequently occur in the legs. Conditions that hinder venous return, such as pregnancy, obesity, and standing for long periods of time, allow blood to accumulate in the veins of the extremities. This stretches the veins, so the valve flaps no longer overlap and they permit the backflow of blood. Superficial veins are more susceptible because they receive less support from surrounding tissue.

Sunbathing: When you are in the sun for an extended period, the cutaneous blood vessels dilate to bring more blood to the skin’s surface, which helps keep the body cool. This action decreases the amount of blood in other parts of the body and may diminish the blood supply to the brain. If you are sunbathing and stand up abruptly, you may feel dizzy. This is because the blood momentarily remains in the dilated cutaneous vessels instead of returning to the heart. This causes a decrease in blood pressure. The dizziness is a signal that the brain is not receiving enough oxygen.

Rising rapidly and dizziness: Sometimes there is a feeling of dizziness when rising rapidly from lying down to a standing position. This is because the body has not had time to respond to the decrease in blood pressure caused by the downward pull of gravity on the blood. The dizziness is a signal that the brain is not receiving enough blood.

Hole in the heart: When the foramen ovale between the two atria fails to close after birth, the result is an interatrial septal defect. Because pressure in the right atrium is lower than in the left, blood flows from the left atrium back into the right atrium without going through the systemic circulation. This defect overloads the pulmonary circulation and puts a strain on the heart as it attempts to pump enough blood to maintain adequate supplies to the body tissues. 

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