The circulatory apparatus consists of the heart, covered by the pericardium, the arteries, veins, and capillaries, and circulating fluid — the blood. The pericardium is a serofibrous membrane which invests the heart and the great vessels at their origin for about two inches. The pericardium consists of a fibrous and serous layer, between which is a small amount of serous fluid, preventing friction, as at each rhythmic contraction of the heart these layers rub against each other. The serous layer (epicardium) invests the heart muscle and is reflected to the fibrous layer. The heart is a hollow muscular organ situated in the thorax between the right and left lobes of the lungs, enclosed by the pericardial sac. It lies obliquely, the base being directed upward, backward, and toward the right, its position corresponding to the surface of the chest wall extends from the fifth to the eighth thoracic vertebra. The apex looks downward, forward, and to the left, its impulse against the chest wall being felt in the fifth interspace on the left side, about 3| inches from the middle of the sternum. The posterior or postero-inferior surface is flat, formed chiefly by the wall of the left ventricle, and rests on the diaphragm; its anterior or anterosuperior surface is convex, formed chiefly by the wall of the right ventricle, but also partly by the left, and is overlapped by the lungs, except in the central part. The borders of the heart are right and left, the former is long and thin, the left shorter and thicker. The length of the heart is about 5 inches, the greatest breadth 3| inches; its thickness about 2J inches. Its weight is approximately 10 to 12 ounces in the male, 8 to 10 ounces in the female, and these increase with age. Externally its surface presents a deep transverse groove, auriculoventricular, which marks an upper auricular and lower ventricular portion; this latter part presents a longitudinal furrow on the front and the back, the former being somewhat to the left, the latter to the right, marking off the right and left ventricles (interventricular groove). The heart contains four chambers. They are divided by a longitudinal partition or septum into a right and left part, and these in turn are subdivided into an auricle and a ventricle by a transverse partition, which is perforated on each side by an opening, called the auriculoventricular, connecting the auricle and ventricle. The cavities of the heart are lined throughout by a thin layer of membrane called the endocardium. It is continuous with the lining membrane of the great blood vessels opening into the heart cavities, and helps to form the various valves by a process whereby the membrane is thrown into folds. The endocardium consists of a flattened layer of endothelial cells resting on a membrane formed of subendothelial (fibro-elastic) tissue. On opening the heart the following structures can be seen within its cavity: The right auricle, larger than the left, with walls about one line in thickness and capacity two ounces. Within the auricle are the orifice of the superior vena cava, the orifice of the inferior vena cava, a projection called the tubercle of Lower, the opening of the coronary sinus, small openings (foramina Thebesii) of the small veins of the heart, the auriculoventricular opening, and the Eustachian valve. The right ventricle is pyramidal, extending nearly to the apex of the heart. It has a capacity of three ounces. On opening the ventricle are the auriculoventricular orifice, the tricuspid valve, the opening of the pulmonary artery, and the semilunar valves. The left auricle is smaller and thicker walled than the right, with features including the orifices of the pulmonary veins, the auriculoventricular orifice, and musculi pectinati. The left ventricle forms the apex of the heart, three times as thick-walled as the right. Within it are the auriculoventricular orifice guarded by the mitral valve, and the aortic opening guarded by the semilunar valves. The muscle fibers of the heart are attached to cartilaginous rings which surround the auriculoventricular and arterial orifices. Nerves — (See pages 121, 383.) The circulation of the blood is propelled by the rhythmic contraction of the heart muscle, aided by the elasticity of arteries, veins, and capillaries throughout the body. Without this rhythmical contraction, life cannot be maintained. Blood courses through the cavities of the heart as follows: Allowing that the heart has emptied its chambers, venous blood is returned to the right auricle from the superior and inferior vena cavae, passes to the right ventricle through the auriculoventricular opening, then into the pulmonary artery and branches to the lungs. In the lungs, carbon dioxide is given up and oxygen absorbed, changing color to bluish red or scarlet. This new blood is carried back by the pulmonary veins to the left auricle of the heart, flows through the left auriculoventricular opening into the left ventricle and thence through the aorta to small arteries, where it gives up its oxygen to tissues and absorbs carbon dioxide, changing color again. The cycle repeats with venous return to the right auricle. Disease or injury to the bundle of His causes heart-block, which is fatal. When any portion of the heart contracts, it is called systole; relaxation, diastole. The heart having two cavities on each side, when their walls contract and relax in succession we speak of an auricular systole and diastole, and a ventricular systole and diastole. The contraction systole starts as a wave in the great veins and passes to the base of the heart in both auricles, then extends rapidly over the ventricles to the apex; during this period the auricles and ventricles empty their cavities into the pulmonary and arterial systems from the right and left sides of the heart respectively. Following this contraction wave is a pause or relaxation of the muscle — diastole — during which time blood flows into the auricles and ventricles, and at the end of the relaxation there is a period of rest when the auricles and ventricles fill with more blood. The part played by the heart muscle and valves during the course of the blood through the chambers of the heart involves the contraction or systole of the ventricle forcing blood into the aorta and pulmonary artery, closing the semilunar valves; while the ventricle contracts, the auricles are filling from the venae cavae and pulmonary veins. The cardiac cycle consists of an auricular contraction, ventricular contraction, and period of repose during which time the auricles and ventricles are at rest. There are 72 cycles per minute made by a healthy adult heart, with each cycle lasting about eight-tenths of a second divided as follows: Auricular systole, Auricular diastole, Ventricular systole, Ventricular diastole, Common pause. The heart beats 72 times per minute in a healthy adult; during the first year of life it decreases to 128 per minute; from the eighth to the fourteenth year, 84 per minute. It is more rapid in females, averaging 8 to 10 more beats per minute. The pulse of a person lying down is a few beats less than when sitting or standing. Exercise and digestion temporarily increase the number of beats. The heart sounds are two resembling the pronouncing of the syllables lubb-dupp, lubb-dupp, occurring with each pulsation of the heart. They are called the first and second sounds; the former is dull and long, due to contraction of the muscular walls of the ventricles, gradual closure and vibrations of the mitral and tricuspid valves, and sudden pressure of the apex against the chest wall. The second sound is short and clear, due to sudden closure and vibrations of the semilunar valves in the aorta and pulmonary artery, following ventricular systole or contraction; also added by the whirling of the column of blood against those closed valves at the beginning of diastole or relaxation of the ventricles. The fetal circulation differs from that observed in adults: Blood from the inferior cava passes into the right auricle and then is directed to the left auricle through the foramen ovale, where it mixes with a small quantity of blood returned from the lungs by the pulmonary veins before passing into the left ventricle and aorta.
Key Takeaways
- The heart is a hollow muscular organ with four chambers, divided by septa into right and left parts, each part further subdivided into auricle and ventricle.
- The heart's rhythmic contraction is essential for maintaining life; disruptions can be fatal.
- Understanding the cardiac cycle helps in diagnosing and treating heart conditions.
Practical Tips
- Learn to identify the location of the heartbeat on a patient’s chest, which can help in assessing their condition during an emergency.
- Recognize the importance of maintaining proper hydration and nutrition to support heart health.
- Understand that stress and physical exertion can affect heart rate; monitor these factors when assessing a patient.
Warnings & Risks
- Be cautious with manual chest compressions, as improper technique can cause injury.
- Avoid administering medications without proper training, as incorrect dosages can be harmful.
- Do not attempt to diagnose or treat complex cardiac issues without professional medical knowledge.
Modern Application
While the historical techniques in this chapter provide foundational understanding of heart anatomy and physiology, modern survival preparedness has advanced with tools like portable ECG machines and CPR training. However, the core principles of recognizing and responding to heart-related emergencies remain crucial.
Frequently Asked Questions
Q: What are the main components of the circulatory apparatus described in this chapter?
The circulatory apparatus consists of the heart, covered by the pericardium, arteries, veins, capillaries, and circulating fluid — the blood. The heart itself is divided into four chambers: two auricles (right and left) and two ventricles (right and left), each with specific functions.
Q: How does the fetal circulatory system differ from that of an adult?
In fetuses, blood from the inferior cava passes into the right auricle and then is directed to the left auricle through the foramen ovale. This mixed with a small quantity of blood returned from the lungs by the pulmonary veins before passing into the left ventricle and aorta.
Q: What are the heart sounds, and how do they occur?
The heart sounds consist of two parts: the first sound (lubb) occurs with cardiac systole or contraction due to the contraction of the muscular walls of the ventricles, gradual closure and vibrations of the mitral and tricuspid valves, and sudden pressure of the apex against the chest wall. The second sound (dupp) is short and clear, occurring at the commencement of diastole or relaxation of the heart muscle due to the sudden closure and vibrations of the semilunar valves in the aorta and pulmonary artery.