In this lesson, you will learn to identify the major structures of the heart and the specific role in the overall functioning of the heart. Specifically, this lesson will cover:
You will recall that the heart is a remarkable pump composed largely of cardiac muscle cells that are incredibly active throughout life. Like all other cells, a cardiomyocyte requires a reliable supply of oxygen and nutrients, and a way to remove wastes, so it needs a dedicated, complex, and extensive coronary circulation (the blood vessels that supply blood to and take blood from the heart). Because of the critical and nearly ceaseless activity of the heart throughout life, this need for a blood supply is even greater than for a typical cell. However, coronary circulation is not continuous; rather, it cycles, reaching a peak when the heart muscle is relaxed and nearly ceasing while it is contracting. In this lesson, you will learn about the structure, function, and circuits of the heart.
1. Location of the Heart
The human heart is located within the thoracic cavity, the internal space formed by the sternum, ribs, and vertebrae. More specifically, the heart is found between the lungs in a space known as the mediastinum. Within the mediastinum, the heart is separated from the other mediastinal structures by a tough serous membrane known as the pericardium, or pericardial sac, and sits in its own space called the pericardial cavity.
The posterior surface of the heart lies near the bodies of the vertebrae, and its anterior surface sits deep to the sternum and costal cartilages. The great veins and great arteries are attached to the superior surface of the heart, called the base. The base of the heart is located at the level of the third costal cartilage. The inferior tip of the heart, the apex, lies just to the left of the sternum between the junction of the fourth and fifth ribs near their articulation with the costal cartilages. It is important to remember the position and orientation of the heart when placing a stethoscope on the chest of a patient and listening for heart sounds.
Position of the Heart in the Thorax—The heart is located within the thoracic cavity, medially between the lungs in the mediastinum. It is about the size of a fist, is broad at the top (called the base), and tapers toward the apex.
IN CONTEXT Everyday Connection CPR
The position of the heart in the torso between the vertebrae and sternum allows individuals to apply an emergency technique known as cardiopulmonary resuscitation (CPR) if the heart of a patient should stop. By applying pressure with the flat portion of one hand on the sternum in the area between the line at T4 and T9 as shown below, it is possible to manually compress the blood within the heart enough to push some of the blood within it out into the attached blood vessels. This is particularly critical for the brain, as irreversible damage and death of neurons occur within minutes of loss of blood flow. Current standards call for compression of the chest at least 5 cm deep and at a rate of 100 compressions per minute, a rate equal to the beat in “Staying Alive,” recorded in 1977 by the Bee Gees. At this stage, the emphasis is on performing high-quality chest compressions, rather than providing artificial respiration. CPR is generally performed until the patient regains spontaneous contraction or is declared dead by an experienced healthcare professional.
When performing CPR, chest compressions can result in broken ribs or a broken sternum. It is also possible, if the hands are placed too low on the sternum, to manually drive the xiphoid process into the liver, a consequence that may prove fatal for the patient. Proper training is essential. This proven life-sustaining technique is so valuable that virtually all medical personnel as well as concerned members of the public should be certified and routinely recertified in its application. CPR courses are offered at a variety of locations, including colleges, hospitals, the American Red Cross, and some commercial companies. They normally include the practice of the compression technique on a mannequin.
CPR Technique—If the heart should stop, CPR can maintain the flow of blood until the heart resumes beating. By applying pressure to the sternum, the blood within the heart will be squeezed out of the heart and into circulation. Proper positioning of the hands on the sternum to perform CPR would be between the lines at T4 and T9.
terms to know
Mediastinum
The region of the thoracic cavity between the lungs.
Pericardium
The serous membrane that surrounds the heart.
Pericardial Cavity
The space around the heart formed by the pericardium and filled with serous fluid.
Base (of the Heart)
The superior surface of the heart.
Apex (of the Heart)
The inferior tip of the heart.
2. Shape and Size of the Heart
did you know
The shape of the heart is similar to a pine cone, rather broad at the base and tapering to the apex. A typical heart is approximately the size of your fist: 12 cm (5 in) in length, 8 cm (3.5 in) wide, and 6 cm (2.5 in) in thickness. Given the size difference between most members of the sexes, the weight of a female heart is approximately 250–300 grams (9 to 11 ounces), and the weight of a male heart is approximately 300–350 grams (11 to 12 ounces).
The heart of a well-trained athlete, especially one specializing in aerobic sports, can be considerably larger than this. Cardiac muscle responds to exercise in a manner similar to that of skeletal muscle. That is, exercise results in the addition of protein myofilaments that increase the size of the individual cells without increasing their numbers—a concept called hypertrophy. The hearts of athletes can pump blood more effectively at lower rates than those of non-athletes.
However, enlarged hearts are not always a result of exercise; they can result from pathologies, such as hypertrophic cardiomyopathy. The cause of an abnormally enlarged heart muscle is unknown, but the condition is often undiagnosed and can cause sudden death in apparently otherwise healthy young people.
term to know
Hypertrophic Cardiomyopathy
A pathological enlargement of the heart.
3. Chambers and Circulation Through the Heart
The human heart consists of four chambers: The left and right sides of the heart have one atrium and one ventricle each. The upper chambers, the right atrium (plural, atria) and the left atrium, act as receiving chambers and contract to push blood into the lower chambers, the right ventricle and the left ventricle. The ventricles serve as the primary pumping chambers of the heart, propelling blood to the lungs or to the rest of the body.
The right ventricle pumps deoxygenated blood into the pulmonary trunk, which leads toward the lungs and bifurcates into the left and right pulmonary arteries. These vessels in turn branch many times before gas exchange occurs—carbon dioxide exits the blood and oxygen enters. The pulmonary trunk arteries and their branches are the only arteries in the postnatal body that carry relatively deoxygenated blood.
Highly oxygenated blood returning from the lungs through the pulmonary veins—the only postnatal veins in the body that carry highly oxygenated blood—enters the left atrium. The left atrium pumps blood into the left ventricle, which in turn pumps oxygenated blood into the aorta and on to the rest of the body. Eventually, these vessels will exchange nutrients and waste with the tissue fluid and cells of the body. In this case, oxygen and nutrients exit the blood to be used by the cells in their metabolic processes, and carbon dioxide and waste products will enter the blood.
The blood returning to the heart from the body is lower in oxygen concentration than when it left through the aorta. The many blood vessels from all over the body combine to form ever-larger veins, eventually flowing into the two major systemic veins, the superior vena cava and the inferior vena cava (plural, venae cavae), which return blood to the right atrium. The blood in the superior and inferior venae cavae flows into the right atrium, which pumps blood into the right ventricle. This process of blood circulation continues as long as the individual remains alive.
Dual System of the Human Blood Circulation—Blood flows from the right atrium to the right ventricle, where it is pumped to the lungs. The blood from the right ventricle is low in oxygen but relatively high in carbon dioxide. Gas exchange occurs in the lungs (oxygen into the blood, carbon dioxide out), and blood high in oxygen and low in carbon dioxide is returned to the left atrium. From here, blood enters the left ventricle, which pumps it to the rest of the body. Following exchange throughout the body (oxygen and nutrients out of the blood and carbon dioxide and wastes in), blood returns to the right atrium and the cycle is repeated.
terms to know
Atrium
An upper chamber of the heart.
Ventricle
A lower chamber of the heart.
Pulmonary Trunk
A major artery receiving blood from the right ventricle.
Pulmonary Artery
Left and right branches of the pulmonary trunk that transport blood to the left and right lungs.
Pulmonary Vein
A major vein returning blood from the lungs to the left atrium.
Aorta
Major artery receiving blood from the left ventricle.
Superior Vena Cava
Major vein returning blood from the upper body to the right atrium.
Inferior Vena Cava
Major vein returning blood from the lower body to the right atrium.
4. Heart Valve Structure and Function
Heart valves ensure unidirectional blood flow through the heart.
Between the right atrium and the right ventricle is the right atrioventricular valve (AV), or tricuspid valve. It typically consists of three flaps, or leaflets, made of endocardium reinforced with additional connective tissue. The flaps are connected by chordae tendineae to the papillary muscles, which control the opening and closing of the valves.
Heart Valves—With the atria and major vessels removed, all four valves are clearly visible, although it is difficult to distinguish the three separate cusps of the tricuspid valve.
Emerging from the right ventricle at the base of the pulmonary trunk is the pulmonary semilunar valve, also known as the pulmonic valve or the right semilunar valve. The pulmonary valve consists of three small flaps of endothelium reinforced with connective tissue. When the ventricle relaxes, the pressure differential causes blood to flow back into the ventricle from the pulmonary trunk. This flow of blood fills the pocket-like flaps of the pulmonary valve, causing the valve to close and producing an audible sound. Unlike the atrioventricular valves, there are no papillary muscles or chordae tendineae associated with the pulmonary valve.
Located at the opening between the left atrium and left ventricle is the left atrioventricular valve, also called the mitral valve or bicuspid valve. Structurally, this valve consists of two cusps, compared to the three cusps of the right atrioventricular valve. In a clinical setting, the valve is referred to as the mitral valve, rather than the bicuspid valve. The two cusps of the mitral valve are attached by chordae tendineae to two papillary muscles that project from the wall of the ventricle.
At the base of the aorta is the aortic semilunar valve, or aortic valve, which prevents backflow from the aorta. It normally is composed of three flaps. When the ventricle relaxes and blood attempts to flow back into the ventricle from the aorta, blood will fill the cusps of the valve, causing it to close and producing an audible sound.
key concept
All four cardiac valves open and close based on the difference in pressure across the valve. When the direction of high to low pressure is also the direction that a valve opens, then it will open. However, when the direction of high to low pressure reverses, it causes the valve to close, restricting blood flow in the opposite direction.
watch
View the following video to learn more about the pathway of the blood through the heart.
terms to know
Atrioventricular Valve (AV)
A valve that allows blood to flow from the atrium to the ventricle while preventing backflow of blood.
Pulmonary Valve
A valve that allows blood to flow from the right ventricle to the pulmonary artery while preventing backflow.
Mitral Valve (also, Bicuspid Valve)
A valve located between the left atrium and ventricle; consists of two flaps of tissue.
Aortic Valve
A valve that allows blood to flow from the left ventricle to the aorta while preventing backflow.
summary
In this lesson, you learned about the basics of the heart and blood flow through the heart. First, you learned about the location of the heart in the chest and its relevance to CPR. Then, you learned the approximate shape and size of the heart. You then explored the chambers and great vessels of the heart and how they serve in blood circulation through the heart. Finally, you learned about heart valve structure and function and how they regulate blood flow through the heart.
