Cardiovascular and Pulmonary Anatomy

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Chapter 3

Cardiovascular and Pulmonary Anatomy

Elizabeth Dean

The heart lies in series with the lungs, constituting the cardiovascular and pulmonary unit, the central component of the oxygen transport pathway.1,2 Virtually all the blood returned to the right side of the heart passes through the lungs and is delivered to the left side of the heart for ejection into the systemic, coronary, and bronchopulmonary circulations. Because of this interrelationship, changes in lung function can exert changes in heart function and vice versa. A detailed understanding of the anatomy of the heart and lungs and how these organs work synergistically is essential to the practice of cardiovascular and pulmonary physical therapy.

This chapter presents the anatomy of the cardiovascular and pulmonary systems, including the skeletal features of the thoracic cavity; muscles of respiration; anatomy of the tracheobronchial tree; lung parenchyma; basic anatomy of the heart; and peripheral, pulmonary, and lymphatic circulations.3–11

Thorax

The bony thorax covers and protects the principal organs of respiration and circulation as well as the liver and the stomach (Fig. 3-1). The anterior surface is formed by the sternum and the costal cartilage. The lateral surfaces are formed by the ribs. The posterior surface is formed by the 12 thoracic vertebrae and the posterior part of the 12 ribs. At birth, the thorax is nearly circular, but during childhood and adolescence it becomes more elliptical. In adulthood, the transverse diameter of the chest wall is greater than the anteroposterior diameter.

Fig. 3-1 The lungs in relationship to the rib cage and other thoracic structures. *(From Beachey W:* Respiratory care anatomy and physiology: foundations for clinical practice, *ed 2, Philadelphia, 2008, Mosby.)*

Sternum

The sternum, or breastbone, is a flat bone with three parts: manubrium, body, and xiphoid process. The manubrium is the widest and thickest bone of the sternum. Its upper border is scalloped by a central jugular notch, which can be palpated, and by two clavicular notches that house the clavicles. Its lower border articulates with the upper border of the body at a slight angle, the sternal angle or angle of Louis. This angle, which is easily palpated, is a landmark located between thoracic vertebrae T4 and T5, and is on a level with the second costal cartilages. The bifurcation of the trachea into the right and left main stem bronchi also occurs at the sternal angle. The manubrium and body are joined by fibrocartilage, which may ossify in later life.

The body of the sternum is twice as long as the manubrium. It is a relatively thin bone and can be easily pierced by needles for bone marrow aspirations. The heart is located beneath and to the left of the lower third of the body of the sternum. Although it is attached by cartilage to the ribs, this portion of the sternum is flexible and can be depressed without breaking. This maneuver is used, with care, in closed cardiac massage to artificially circulate blood to the brain and extremities. The lower margin of the body is attached to the xiphoid process by fibrocartilage. This bone is the smallest of the three parts of the sternum and usually fuses with the body of the sternum in later life.

Ribs

A large portion of the bony thoracic cage is formed by 12 ribs located on either side of the sternum. The first seven ribs connect posteriorly with the vertebral column and anteriorly through costal cartilages with the sternum. These are known as the true ribs. The remaining five ribs are known as the false ribs. The first three have their cartilage attached to the cartilage of the rib above. The last two are free or floating ribs. The ribs increase in length from the first to the seventh rib and then decrease to the 12th rib. They also increase in obliquity until the ninth rib and then decrease in obliquity to the 12th rib.

Each rib has a small head and a short neck that articulate with two thoracic vertebrae. The shaft of the rib curves gently from the neck to a sudden sharp bend, the angle of the rib. Fractures often occur at this site. A costal groove is located on the lower border of the shaft of the ribs. This groove houses the intercostal nerves and vessels. Chest tubes and needles are inserted above the ribs to avoid these vessels and nerves. The ribs are separated from each other by the intercostal spaces that contain the intercostal muscles.

Movements of the Thorax

The frequency of movement of the bony thorax joints is greater than that of almost any other combination of joints in the body. Two types of movements have been described: the pump-handle movement and the bucket-handle movement (Fig. 3-2).¹² The upper ribs are limited in their ability to move. Each pair swings like a pump handle, with elevation thrusting the sternum forward. This forward movement increases the anteroposterior diameter and the depth of the thorax and is called the pump-handle movement. In the lower ribs, there is little anteroposterior movement. During inspiration, the ribs swing outward and upward, each rib helps to elevate the rib above it. This bucket-handle movement increases the transverse diameter of the thoracic cage. Thus during inspiration, the thorax increases its volume by increasing its anteroposterior and transverse diameters.

Fig. 3-2 The movements of the ribs. A, “Bucket handle,” lower rib. B, “Pump handle,” first rib.

Muscles of Respiration

Inspiration

Inspiration is an active movement involving the contraction of the diaphragm and intercostal muscles. Additional muscles may come into play during exertion in a healthy person. In disease, the role of these accessory muscles of inspiration may have an important role even at rest. The accessory muscles include the sternocleidomastoids, scalenes, serratus anterior, pectoralis major and minor, trapezius, and erector spinae. The degree to which these accessory muscles are recruited by the patient depends on the severity of cardiovascular and pulmonary distress.

Diaphragm

The diaphragm is the principal muscle of respiration. During quiet breathing, the diaphragm contributes approximately two-thirds of the tidal volume in the sitting or standing positions and approximately three-fourths of the tidal volume in the supine position. Approximately two-thirds of the vital capacity in all positions is contributed by the diaphragm.

The diaphragm is a large, dome-shaped muscle that separates the thoracic and abdominal cavities. Its upper surface supports the pericardium (with which it is partially blended), heart, pleurae, and lungs. Its lower surface is almost completely covered by the peritoneum and overlies the liver, kidneys, suprarenal glands, stomach, and spleen (Fig. 3-3). This large muscle can be divided into right and left halves. Each half is made up of three parts: sternal, lumbar, and costal. These three parts insert into the central tendon, which lies just below the heart. The sternal part arises from the back of the xiphoid process and descends to the central tendon. On each side is a small gap, the sternocostal triangle, which is located between the sternal and costal parts. It transmits the superior epigastric vessels and is often the site of diaphragmatic hernias. The costal parts form the right and left domes. They arise from the inner surfaces of the lower four ribs and the lower six costal cartilages, then interdigitate and transverse the abdomen to insert into the anterolateral part of the central tendon, the central part of the diaphragm. The lumbar part arises from the bodies of the upper lumbar vertebrae and extends upward to the central tendon. The central tendon is a thin, strong aponeurosis situated near the center of the muscle, somewhat closer to the front of the body. It resembles a trefoil leaf, with its three divisions or leaflets. The right leaflet is the largest, the middle is the next largest, and the left leaflet is the smallest.

Major vessels traverse the diaphragm through one of three openings (see Fig. 3-3). The vena caval opening is located to the right of the midline in the central tendon and contains branches of the right phrenic nerve and the inferior vena cava. The esophageal opening is located to the left of the midline and contains the esophagus, the vagal nerve trunks, and branches of the gastric vessels. The aortic opening is located in the midline and contains the aorta, the thoracic duct, and sometimes the azygos vein. The diaphragm is also pierced by branches of the left phrenic nerve, small veins, and lymph vessels.

The position of the diaphragm and its range of movement vary with posture, the degree of distention of the stomach, size of the intestines, size of the liver, and obesity. Average movement of the diaphragm in quiet respiration is 12.5 mm on the right and 12 mm on the left. This can increase to a maximum of 30 mm on the right and 28 mm on the left during increased ventilation. An individual’s posture determines the position of the diaphragm. In the supine position, the resting level of the diaphragm rises. The greatest respiratory excursions during normal breathing occur in this position; however, the lung volumes are decreased because of the elevated position of the abdominal organs within the thoracic cavity. In a sitting or upright position, the dome of the diaphragm is pulled down by the abdominal organs, allowing a larger lung volume. For this reason, individuals who are short of breath are more comfortable sitting than reclining. In a side-lying position, the dome of the diaphragm on the lower side rises farther into the thorax than the dome on the upper side (Fig. 3-4). The abdominal organs tend to be displaced forward in a side-lying position, allowing greater excursion of the dome on the lower side. In contrast, the upper side moves little with respiration. On radiograph, the position of the diaphragm can indicate whether the film was taken during inspiration or expiration and may also indicate pathology in the lungs, pleurae, or abdomen.

Fig. 3-4 When the patient is lying on his or her side, the dome of the diaphragm on the lower side rises farther in the thorax than the dome on the upper side.

Each half of the diaphragm is innervated by a separate nerve—the phrenic nerve on that side. Although the halves contract simultaneously, it is possible for half of the muscle to be paralyzed without affecting the other half. Generally, the paralyzed half remains at the normal level during rest. With deep inspiration, however, the paralyzed half is pulled up by the negative pressure in the thorax.

Contraction of the diaphragm increases the thoracic volume vertically and transversely. The central tendon is drawn down by the diaphragm as it contracts. As the dome descends, abdominal organs are pushed forward as far as the abdominal walls will allow. When the dome can descend no farther, the costal fibers of the diaphragm contract to increase the thoracic diameter of the thorax. This occurs because the fibers of the costal part of the diaphragm run vertically from their attachment at the costal margin. Thus contraction of these fibers elevates and everts the ribs (Fig. 3-5). If the diaphragm is in a low position, it will change the angle of pull of the muscle’s costal fibers. Contraction of these fibers creates a horizontal pull, which causes the lateral diameter to become smaller as the ribs are pulled in toward the central tendon.

Fig. 3-5 Contraction of the costal fibers of the diaphragm causes rib eversion and elevation.

As the diaphragm descends, it compresses the abdominal organs, increasing intraabdominal pressure. At the same time, the intrathoracic pressure decreases as the lung volume is increased by the descending diaphragm. Inspiratory airflow occurs as a result of this decrease in intrathoracic pressure (see Chapter 4). The pressure gradient between the abdominal and thoracic cavities also facilitates the return of blood to the right side of the heart.

Movement of the diaphragm can be controlled voluntarily to some extent. Vocalists spend years learning to manipulate their diaphragms so they can produce controlled sounds when singing. The diaphragm momentarily ceases movement when a person holds his or her breath. The diaphragm is involuntarily involved in parturition, bearing down in bowel movements, and laughing, crying, and vomiting. Hiccups are spasmodic, sharp contractions of the diaphragm that can indicate disease (e.g., a subphrenic abscess) if they persist.

Intercostals

The external intercostals extend down and forward from the tubercles of the ribs (above) to the costochondral junction of the ribs (below), where they become continuous with the anterior intercostal membrane (Fig. 3-6). This membrane extends the muscle forward to the sternum. There are 11 external intercostal muscles on each side of the sternum. They are thicker posteriorly than anteriorly and thicker than the internal intercostal muscles. They are innervated by the intercostal nerves, and contraction draws the lower rib up and out toward the upper rib. This action increases the volume of the thoracic cavity.

Fig. 3-6 Respiratory muscles (anterior view).

There are 11 internal intercostals on each side. These are considered primarily expiratory in function. The intercartilaginous or parasternal portion of the internal intercostals contracts with the external intercostals during inspiration to help elevate the ribs. In addition to their respiratory functions, the intercostal muscles contract to prevent the intercostal spaces from being drawn in or bulged out during respiratory activity.

Sternocleidomastoid

The sternocleidomastoid (SCM) muscles are strong neck muscles arising from two heads, one from the manubrium and one from the medial part of the clavicle (see Fig. 3-6). These two heads fuse into one muscle mass that inserts behind the ear into the mastoid process. It is innervated by the accessory nerve and the second cervical nerve. There are two of these muscles, one on each side of the neck. When one SCM contracts, it tilts the head toward the shoulder of the same side and rotates the face toward the opposite shoulder. If the two SCM muscles contract together, they pull the head forward into flexion. When the head is fixed, the muscles assist in elevating the sternum, increasing the anteroposterior diameter of the thorax.

The SCMs are the most important accessory muscles of inspiration. Their contractions can be observed in all patients during forced inspiration and in all patients who are dyspneic. These muscles become visually predominant in patients who are chronically dyspneic (see Chapter 5).

Scalenes

The anterior, medial, and posterior scalenes are three separate muscles that are considered a functional unit. They are attached superiorly to the transverse processes of the lower five cervical vertebrae and inferiorly to the upper surface of the first two ribs (see Fig. 3-6). They are innervated by their corresponding cervical spinal nerves. These muscles are primarily supportive neck muscles, but they can assist in respiration through reverse action. When their superior attachment is fixed, the scalenes act as accessory respiratory muscles and elevate the first two ribs during inspiration.

Serratus Anterior

The serratus anterior arises from the outer surfaces of the first eight or nine ribs. It curves backward, forming a sheet of muscle that inserts into the medial border of each scapula. It is innervated by the long thoracic nerve (cervical nerves C5, C6, and C7). There are two of these muscles, one on each side of the body. Normally they assist in forward pushing of the arm (as in punching). When the scapulae are fixed, the serratus anterior muscles act as accessory respiratory muscles and elevate the ribs to which they are attached.

Pectoralis Major

The pectoralis major is a large muscle arising from the clavicle, the sternum, and the cartilages of all the true ribs (see Fig. 3-6). This muscle spreads across the anterior chest and inserts into the intertubercular sulcus of the humerus. It is innervated by the lateral and medial pectoral nerves and cervical nerves C5, C6, C7, C8, and T1. There are two of these muscles, one on each side of the body. This muscle rotates the humerus medially and draws the arm across the chest. During climbing and pull-ups, it draws the arms toward the trunk. During forced inspiration when the arms are fixed, it draws the ribs toward the arms, thereby increasing thoracic diameter.

Pectoralis Minor

The pectoralis minor is a thin muscle originating from the outer surfaces of the third, fourth, and fifth ribs near their cartilages. It inserts into the coracoid process of the scapula. It is innervated by the pectoral nerves (cervical nerves C6, C7, and C8). There are two of these muscles, one on each side of the body. They contract with the serratus anterior to draw the scapulae toward the chest. During deep inspiration, they contract to elevate the ribs to which they are attached.

Trapezius

The trapezius consists of two muscles that form an extensive diamond-shaped sheet extending from the head down the back and out to both shoulders (Fig. 3-7). Its upper belly originates from the external occipital protuberance, curves around the side of the neck, and inserts into the posterior border of the clavicle. The middle part of the muscle arises from a thin diamond-shaped tendinous sheet, the supraspinous ligaments, and the spines of the upper thoracic region; it then runs horizontally and inserts into the spine of the scapula. Its lower belly arises from the supraspinous ligaments and the spines of the lower thoracic region, runs upward, and inserts into the lower border of the spine of the scapula. This large muscle is innervated by the external or spinal part of the accessory nerve and cervical nerves C3 and C4. Its main function is to rotate the scapulae during arm elevation and control gravitational descent of the arms. It also braces the scapulae and raises them, as when shrugging the shoulders. Its ability to stabilize the scapulae makes it an important accessory muscle in respiration. This stabilization enables the serratus anterior and pectoralis minor to elevate the ribs.

Fig. 3-7 Respiratory muscles (posterior view).

Erector Spinae

The erector spinae is a large muscle extending from the sacrum to the skull (see Figure 3-7). It originates from the sacrum, the iliac crest, and the spines of the lower thoracic and lumbar vertebrae. It separates into a lateral iliocostalis, an intermediate longissimus, and a medial spinalis column. This muscle mass inserts into various ribs and vertebral processes all the way up to the skull. It is innervated by the corresponding spinal nerves. These muscles extend, laterally flex, and rotate the vertebral column. They are considered accessory respiratory muscles through their extension of the vertebral column. In deep inspiration, these muscles extend the vertebral column, allowing further elevation of the ribs.

Expiration

Expiration is a passive process that occurs when the intercostals and diaphragm relax. Relaxation of the intercostals and diaphragm allows the ribs to drop to their preinspiratory position and the diaphragm to rise. These activities compress the lungs, which then raises intrathoracic pressure above atmospheric pressure and contributes to air flow out of the lungs.

Rectus Abdominis

The rectus abdominis rises from the pubic crest, extends upward, and inserts into the xiphoid process and the costal margin of the fifth, sixth, and seventh costal cartilages (see Fig. 3-6). It is innervated by corresponding spinal nerves and its action is considered within the context of the other abdominal muscles.

Obliquus Externus Abdominis

The obliquus externus abdominis arises in an oblique line from the fifth costal cartilage to the 12th rib (see Fig. 3-6). Its posterior fibers attach in an almost vertical line with the iliac crest. The other fibers extend down and forward and attach to the front of the xiphoid process, the linea alba, and the pubic symphysis. It is innervated by the lower six thoracic spinal nerves.

Obliquus Internus Abdominis

The obliquus internus abdominis originates from the lumbar fascia, the anterior two-thirds of the iliac crest, and the lateral two-thirds of the inguinal ligament (see Fig. 3-6). Its posterior fibers run almost vertically upward and insert into the lower borders of the last three ribs. The other fibers join an aponeurosis attached to the costal margin above, the linea alba in the midline, and the pubic crest below. It is innervated by the lower six thoracic nerves and the first lumbar spinal nerves.

Overview of the Process of Breathing

During quiet inspiration, the diaphragm, external intercostals, and intercartilaginous portions of the internal intercostals are the primary muscles that contract. The diaphragm contracts first and then descends, enlarging the thoracic cage vertically. When the abdominal contents prevent further descent of the diaphragm, the costal fibers of the diaphragm contract, which causes the lower ribs to swing up and out to the side (bucket-handle movement). This lateral rib movement is assisted by the external intercostals and the intercartilaginous portion of the internal intercostals. The transverse diameter of the thorax is increased by this bucket-handle movement. Finally, the upper ribs move forward and upward (pump-handle movement), also through contraction of their external intercostals and the intercartilaginous portions of the internal intercostals. This increases the anteroposterior diameter of the thorax. The epigastric area protrudes, then the ribs swing up and out laterally, and finally the upper ribs move forward and upward.

Quiet expiration is passive and involves no muscular contraction, although some electrical activity can be detected with electromyography. The inspiratory muscles relax, which raises intrathoracic pressure as the ribs and diaphragm return to their preinspiratory positions and compress the lungs. This increased pressure allows airflow from the lungs.

During forced inspiration, an additional number of accessory muscles may contract along with the muscles involved in quiet inspiration. The erector spinae contract to extend the vertebral column. This extension permits greater elevation of the ribs during inspiration. Various back muscles (e.g., erector spinae, trapezius, and rhomboids) contract to stabilize the vertebral column, head, neck, and scapulae. This enables accessory respiratory muscles to assist inspiration through reverse action. The SCM raises the sternum. The scalenes elevate the first two ribs. The serratus anterior, pectoralis major, and pectoralis minor assist bilateral elevation of the ribs. All these accessory muscles tend to elevate the ribs, thus increasing the anteroposterior diameter but not the transverse diameter of the thorax. (The transverse diameter does increase slightly as a result of the increased strength of the contraction of the normal inspiratory muscles.) The marked increase in anteroposterior diameter in relation to transverse diameter creates an impression of en bloc breathing in a patient using accessory muscles.

During forced expiration, the interosseous portion of the internal intercostals and the abdominal muscles contract to force air out of the lungs. Forced expiration can be slow and prolonged (as in patients with COPD) or rapid and expulsive (as in a cough). If the abdominal contractions are strong enough, the trunk flexes during exhalation. This flexion further compresses the lungs, forcing more air from them.

Upper Airways

Nose

Noses vary in size and shape among individuals. The nose is composed of bony and cartilaginous parts. The upper one-third is primarily bony and contains the nasal bones, the frontal processes of the maxillae, and the nasal part of the frontal bone. Its lower two-thirds are cartilaginous and contain the septal, lateral, and major and minor alar nasal cartilages. The nasal cavity is divided into right and left halves by the nasal septum. This cavity extends from the nostrils to the posterior apertures of the nose in the nasopharynx. The lateral walls of the cavity are irregular as a result of projecting superior, middle, and inferior nasal chonchae. There is a meatus located beneath or lateral to each choncha through which the sinuses drain. The chonchae increase the surface area of the nose for maximum contact with inspired air. The superior chonchae and adjacent septal wall are referred to as the olfactory region. They are covered with a thin, yellow olfactory mucous membrane consisting of bipolar nerve cells that are olfactory in function. Only a portion of inspired air reaches the olfactory region to provide a sense of smell. When people smell something specific, they sniff. This action lifts the inspired air so that more of it comes into contact with the olfactory region.

The anterior portion (vestibule) of the nasal cavity (Fig. 3-8) is lined with skin and coarse hairs (vibrissae) that entrap inhaled particles. The rest of the cavity and sinuses (with the exception of the olfactory region) are lined with respiratory mucous membrane. This membrane is composed of pseudostratified columnar ciliated epithelium (Fig. 3-9). It contains goblet cells, as well as mucous and serous glands that produce mucus and serous secretions. These secretions entrap foreign particles and bacteria. This mucus is then swept to the nasopharynx by the cilia at a rate of 5 to 15 mm/min, where it is swallowed or expectorated. The mucous membrane is vascular, with arterial blood supplied by branches of the internal and external carotid arteries. Venous drainage occurs through the anterior facial veins. The mucous membrane is thickest over the chonchae. As air is inhaled, it passes around and over the chonchae, whose vascular moist surfaces heat, humidify, and filter the inspired air. The mucous membrane may become swollen and irritated as a result of upper respiratory infections and may secrete copious amounts of mucus. Because this membrane is continuous with sinuses, auditory tubes, and lacrimal canaliculi, people with colds often complain of sinus headaches, watery eyes, earaches, and other symptoms. Secretions can be so copious that the nasal passages become completely blocked.

Fig. 3-8 Sagittal section of the head and neck. *(From Hicks GH:* Cardiopulmonary anatomy and physiology, *Philadelphia, 2000, WB Saunders.)*

Fig. 3-9 A, Pseudostratified columnar ciliated epithelium. B, Normal movements of cilia.

Pharynx

The pharynx is an oval fibromuscular sac located behind the nasal cavity, mouth, and larynx. It is approximately 12 to 14 cm long and extends from the base of the skull to the esophagus below, at the level of the cricoid cartilage opposite the sixth cervical vertebra. The pharynx has three compartments: the nasal cavity (nasopharynx), mouth (oropharynx), and larynx (laryngopharynx). The pharyngeal walls are lined with ciliated respiratory mucous membrane in the nasal portion and with stratified squamous membrane in the oral and laryngeal parts.

The nasopharynx is a continuation of the nasal cavities (see Fig. 3-8). It lies behind the nose and above the soft palate. With the exception of the soft palate, its walls are immovable, so its cavity is never obliterated as are the oropharynx and laryngopharynx. The nasopharynx communicates with the nasal cavity anteriorly through the posterior apertures of the nose. It communicates with the oropharynx and laryngopharynx through an opening, the pharyngeal isthmus, which is closed by elevations of the soft palate during swallowing.

The oropharynx extends from the soft palate to the epiglottis (see Fig. 3-8). It opens into the mouth anteriorly through the oropharyngeal isthmus. Its posterior walls lie on the bodies of the second and third cervical vertebrae. Laterally, two masses of lymphoid tissue—the palatine tonsils—may be seen. These tonsils form part of a circular band of lymphoid tissue surrounding the opening into the digestive and respiratory tracts.

The laryngopharynx lies behind the larynx and extends from the epiglottis above to the inlet of the esophagus below (see Fig. 3-8). The fourth to sixth cervical vertebrae lie behind the laryngopharynx. In front of the laryngopharynx are the epiglottis, the inlet of the larynx, and the posterior surfaces of the arytenoid and cricoid cartilages.

Larynx

The larynx is a complex structure composed of cartilages and cords moved by sensitive muscles (Fig. 3-10). It is located between the trachea and laryngopharynx, for which it forms an anterior wall. With its rapid closure it acts as a sphincteric valve, preventing food, liquids, and foreign objects from entering the airway. It controls airflow and at times closes so that thoracic pressure may be raised and the upper airways cleared by a propulsive cough when the larynx opens. Expiratory airflow vibrates as it passes over the contracting vocal chords, producing the sounds used for speech. (The larynx is not essential for speech. Humans can speak by learning to dilate the upper part of the esophagus so that air vibrates as it passes over that area; this is called esophageal speech.)

In adult men, the larynx is situated opposite the third, fourth, and fifth cervical vertebrae; it is situated somewhat higher in women and children. The larynx is essentially the same in children, but at puberty, the male larynx increases in size considerably until its anteroposterior diameter has almost doubled. All the cartilages enlarge, and the thyroid cartilage becomes prominent anteriorly.

Vocal cord adductor contraction results in approximation of the vocal cords and narrowing of the glottis. The adductors of the cords are important in protecting the lower airways. Their contraction prevents fluids, food, and other substances from being aspirated. All the intrinsic laryngeal muscles are innervated by the recurrent laryngeal nerve (a branch of the vagus nerve), with the exception of the cricothyroid muscle, which is supplied by the external branch of the superior laryngeal nerve (also a branch of the vagus nerve).

Lower Airways

Trachea

The trachea is a semirigid cartilaginous tube approximately 10 to 11 cm long and 2.5 cm wide. It lies in front of the esophagus and descends with a slight inclination to the right from the level of the cricoid cartilage (Fig. 3-11; see also Fig. 3-10). It travels behind the sternum into the thorax to the sternal angle (opposite the fifth thoracic vertebra), where it divides to form the right and left main stem bronchi. The tracheal wall is strengthened by 16 to 20 horseshoe-shaped cartilaginous rings. The open parts of the tracheal rings are completed by fibrous and elastic tissue and unstriated transverse muscle. This highly flexible part of the ring is positioned posteriorly. It indents or curves inward during coughing, which increases the velocity of expelled air. The cartilaginous rings lie horizontally one above the other, separated by narrow bands of connective tissue. The trachea is lengthened during hyperextension of the head; during swallowing, which raises the trachea; and during inspiration, when the lungs expand and pull the trachea downward. Its cross-sectional area becomes smaller with contraction of the unstriated transverse muscle fibers that complete the tracheal rings.

Fig. 3-11 Tracheobronchial tree (a three-quarter view, rotated toward the right side).

The mucous membrane of the trachea contains columnar ciliated epithelium and goblet cells. Each ciliated epithelial cell contains approximately 275 cilia. These structures beat rapidly in a coordinated and unidirectional manner, propelling a sheet of mucus toward the head, from the lower respiratory tract to the pharynx, where it is swallowed or expectorated. The cilia beat in this layer of mucus with a forceful forward stroke followed by an ineffective backward stroke that returns the cilia to their starting position. Mucociliary escalation propelling of mucus by the cilia toward the upper trachea for clearance, is essential. When cilia are paralyzed by smoking, alcohol, dehydration, anesthesia, starvation, or hypoxia, mucus begins to accumulate in distal, gravity-dependent airways, causing infiltrates and eventually localized areas of lung collapse referred to as atelectasis.

The number of mucus-containing goblet cells is approximately equal to the number of ciliated epithelial cells. Reserve cells lie beneath the ciliated and goblet cells. These reserve cells can differentiate into either goblet cells or ciliated cells. Beneath the reserve cells lie the gland cells. There are approximately 40 times more gland cells than goblet cells. Mucus is composed of 95% water, 2% glycoprotein, 1% carbohydrate, trace amounts of lipid, deoxyribonucleic acid (DNA), dead tissue cells, phagocytes, leukocytes, erythrocytes, and entrapped foreign particles. Mucus lines the airways from the trachea to the alveoli. Two separate layers have been observed: the sol layer, which lies on the mucosal surface and contains high concentrations of water, and the gel layer, which is more superficial and viscous because of its lower concentration of water.

The right main stem bronchus is an extension of the trachea and is wider, shorter, and more vertical than the left main stem bronchus. Greater width and more vertical course cause a majority of aspirated foreign material to pass through the right main stem bronchus. The azygos vein arches over the right main stem bronchus; the right pulmonary artery lies beneath it. The right main stem bronchus divides to form the right upper lobe bronchus, the right middle lobe bronchus, and the right lower lobe bronchus. The right upper lobe divides into three segmental bronchi: apical, posterior, and anterior. The apical bronchus runs almost vertically toward the apex of the lung. The posterior bronchus is directed posteriorly in a horizontal direction, and the anterior bronchus is directed anteriorly in an almost horizontal direction. The right middle lobe bronchus divides about 10 mm below the right upper lobe bronchus and descends anterolaterally. The right lower lobe bronchus divides into five segmental bronchi. The apical or superior bronchus runs almost horizontally, posteriorly. The medial or cardiac bronchus descends medially toward the heart. The anterior basal bronchus descends anteriorly. The lateral basal bronchus descends laterally, and the posterior bronchus descends posteriorly. Each segment describes its anatomic position.

The left main stem bronchus is narrower and runs more horizontally than the right main stem bronchus. The aortic arch passes over it, and the esophagus, descending aorta, and thoracic duct lie behind it. The left pulmonary artery lies anteriorly and above the left main stem bronchus. The left main stem bronchus has two major divisions: the left upper lobe bronchus and the left lower lobe bronchus. The left upper lobe bronchus has three major segmental bronchi. The anterior bronchus ascends at approximately a 45-degree angle. The apical-posterior bronchus has two branches: one runs vertically and the other posteriorly toward the apex of the left lung. The lingular bronchus descends anterolaterally, much the same as does the right middle lobe bronchus of the right lung. The right lower lobe bronchus divides into four segmental bronchi. The superior or apical bronchus runs posteriorly in a horizontal direction. The anterior bronchus descends anteriorly. The lateral bronchus descends laterally, and the posterior bronchus descends posteriorly. The segments describe their anatomic position.

The bronchi of the airways continue to divide until there are approximately 23 generations (Table 3-1). The main, lobar, and segmental bronchi are made up of the first four generations. The walls contain U-shaped cartilage in the main bronchi. This cartilage becomes less well defined and more irregularly shaped as the bronchi continue to divide. In the segmental bronchi, the walls are formed by irregularly shaped helical plates with bands of bronchial muscle. The mucous membrane in these airways is essentially the same as that in the trachea, but the cells become more cuboidal in the lower divisions.

Table 3-1

Structural Characteristics of the Air Passages

	Generation (mean)	Number	Mean Diameter (mm)	Area Supplied	Cartilage	Muscle	Nutrition	Placement	Epithelium
Trachea	0	1	18	Both lungs	U-shaped		From inferior thyroid, thoracic, and branches of bronchial arteries
Main bronchi	1	2	13	Individual lungs				Within connective tissue sheath alongside arterial vessels
Lobar bronchi	2	4	7	Lobes	Irregular shape and helical plates	Helical bands	From the bronchial circulation		Columnar, ciliated
	↓	↓	↓
	3	8	5
Segmental bronchi	4	16	4	Segments
Small bronchi	5	32	3	Secondary lobules
	↓	↓	↓
	11	2000	1
Bronchioles and terminal bronchioles	12	4000	1			Strong helical muscle bands		Embedded directly in the lung parenchyma	Cuboidal
	↓	↓	↓
	16	65,000	0.5
Respiratory bronchioles	17	130,000	0.5	Primary lobes		Muscle band between alveolar thin bands in alveolar septa	From the pulmonary circulation		Cuboidal to flat between the alveoli
	↓	↓
	19	500,000
Alveolar ducts	20	1,000,000	0.3	Alveoli				Lung parenchyma	Alveolar epithelium
	↓	↓
	22	4,000,000
Alveolar sacs	23	8,000,000	0.3					Lung parenchyma	Alveolar epithelium

Adapted from Weibel ER: Morphometry of the human lung. New York, 1963, Springer.

The subsegmental bronchi extend from the fifth to the seventh generation. Although the diameter of these airways becomes progressively smaller, the total cross-sectional area increases because of the increased number of divisions. The mucous membrane is essentially the same, and helical cartilaginous plates and cilia become sparser. These changes continue throughout the 8th to 11th generations, which are referred to as bronchioles.

The terminal bronchioles extend from the 12th to the 16th generation. The diameter of these airways is approximately 1 mm. Cartilage is no longer present to provide structural rigidity. The airways are embedded directly in the lung parenchyma, and it is the elastic properties of this parenchyma that keep these lower airways open. Strong helical muscle bands are present and their contraction forms longitudinal folds in the mucosa that sharply decrease the diameter of the airways. The epithelium of the terminal bronchioles is cuboidal and no longer ciliated. The cross-sectional area of the airways increases sharply at this level. All the airways to this level (generations 1 to 16) are considered conducting airways because their purpose is to transport gas to the respiratory bronchioles and alveoli, where gas exchange occurs. The conducting airways receive their arterial blood from the bronchial circulation (branches of the descending aorta). Airways below this point receive their arterial blood from the pulmonary arteries.

The respiratory bronchioles extend from the 17th to the 19th generation. They are considered a transitional zone between bronchioles and alveoli. Their walls contain cuboidal epithelium interspersed with some alveoli. The number of alveoli increases with each generation. The walls of the bronchioles are also buried in the lung parenchyma. The airways depend on traction of this parenchyma to maintain their lumen. Muscle bands are also present between alveoli.

Alveolar ducts extend from the 20th to the 22nd generation. Their walls are composed entirely of alveoli, which are separated from one another by their septae. Septae contain smooth muscle, elastic and collagen fibers, nerves, and capillaries.

Alveolar sacs make up the 23rd generation of air passages. They are essentially the same as alveolar ducts, except that they end as blind pouches. Communication occurs between blind pouches in the form of the pores of Kohn, which are channels in alveolar walls, and the Lambert canals, which are communications between bronchioles and alveoli. These communications are thought to be responsible for the rapid spread of lung infection. They also provide collateral ventilation to alveoli whose bronchi are obstructed. Although this ventilation does little to arterialize blood, it does help prevent collapse of these alveoli. Each alveolar sac contains approximately 17 alveoli. There are about 300 million alveoli in an adult man, 85% to 95% of which are covered with pulmonary capillaries. Alveolar epithelium is composed of two cell types. Type I cells, squamous pneumocytes, have broad thin extensions that cover about 95% of the alveolar surface. Type II cells, the granular pneumocytes, are more numerous than type I cells but occupy less than 5% of the alveolar surface. This is because of their small, cuboidal shape. These cells are responsible for the production of surfactant, a phospholipid that lines the alveoli. Surfactant keeps alveoli expanded by lowering their surface tension. Type II cells have been shown to be the primary cells involved in repair of the alveolar epithelium. Type III cells, alveolar brush cells, are rare and found only occasionally in humans.

An additional type of cell, the alveolar macrophage, is found within the alveoli. These cells are thought to originate from stem cell precursors in the bone marrow and reach the lung through the blood stream. They are large, mononuclear, ameboid cells that roam in the alveoli, alveolar ducts, and alveolar sacs. Macrophages contain lysosomes, which are capable of killing engulfed bacteria. They are especially effective in neutralizing inhaled gram-positive organisms. They also engulf foreign matter and are transported to the lymphatic system or migrate to the terminal bronchioles, where they attach themselves to the mucus. Macrophages are carried by the mucus to larger airways and eventually to the pharynx. Because cilia are not present below the 11th generation of air passages, clearance of foreign matter and bacteria from these areas is largely dependent on macrophages.

Other cells located in the distal airways that are important in the defense of the lung are the lymphocytes and polymorphonuclear leukocytes. Immunoglobulins (IgA, IgG, and IgM) in the blood serum enhance the engulfing activity of the macrophages. Two types of lymphocytes are found in the lung: the B lymphocyte and the T lymphocyte. The B lymphocytes produce gamma globulin antibodies to fight lung infections, whereas the T lymphocytes release a substance that attracts macrophages to the site of the infection. The polymorphonuclear leukocytes are important in engulfing and killing blood-borne gram-negative organisms.

Lungs

Two lungs (each covered with its pleurae—the visceral pleura and the parietal pleura), lie within the thoracic cavity. Each lung is attached to the heart and the trachea by its root and the pulmonary ligament. The lungs are otherwise free in the thoracic cavity. The lungs are light, soft, spongy organs whose color darkens with age as they become impregnated with inhaled air pollutants. They are covered with the visceral pleura, a thin, glistening serous membrane that covers all surfaces of the lungs. The visceral pleura extends to the mediastinum and inner thoracic wall, where it becomes known as the parietal pleura. The space between the two pleurae maintains a negative pressure at all times and is therefore termed a potential space. This negative pressure maintains lung inflation. A small amount of pleural fluid lubricates the two pleurae as they slide over each other during breathing. In disease, fluid, tumor cells, or air can invade the pleural space and collapse the underlying lung.

Each lung has an apex, a base, and three surfaces (costal, medial, and diaphragmatic). There are also three borders (anterior, inferior, and posterior). Each lung is divided by fissures into separate lobes. In the right lung, the oblique fissure separates the lower lobe from the middle, whereas the horizontal fissure separates the upper lobe from the middle. The right lung is heavier and wider than the left lung. It is also shorter because of the location of the right lobe of the liver. The left lung is divided into upper and lower lobes by the oblique fissure. It is longer and thinner than the right lung because the heart and pericardium are located in the left thorax. Numerous structures enter the lung at the hilus, or root of the lung, including the main stem bronchus, the pulmonary artery, pulmonary veins, bronchial arteries and veins, nerves, and lymph vessels. The root, or hilus, of the lungs lies opposite the bodies of the fifth, sixth, and seventh thoracic vertebrae. The lungs are connected to the upper airways by the trachea and main stem bronchi.

Surface Markings

Surface markings of the lungs can be outlined on the chest with a basic knowledge of bony landmarks and of the gross anatomy of each lung (Table 3-2 and Fig. 3-12). The apices of both lungs extend 2 or 3 cm above the clavicles at the medial ends. The anteromedial border of the right lung runs from the sternoclavicular joint to the sternal angle and downward to the xiphisternum. The inferior border runs from the xiphisternum laterally to the sixth rib in the midclavicular line, the eighth rib in the midaxillary line, and the 10th rib in the midscapular line. The midscapular line runs downward from the inferior angle of the scapula with the arm at rest. The inferior border joins the posterior medial border of the lung 2 cm lateral to the tenth thoracic vertebra. The posterior medial border runs 2 cm lateral to the vertebral column from the seventh cervical vertebra to the tenth thoracic vertebra.

Table 3-2

Anatomic Arrangement of the Bronchopulmonary Segments

Lobe	Right Lung: Bronchopulmonary Segments	Lobe	Left Lung: Bronchopulmonary Segments
Upper	Apical: Extends above the clavicle anteriorly; smaller area posteriorly Anterior: Occupies area between the clavicle and horizontal fissure Posterior: Remainder of upper lobe on the posterior aspect down to the oblique fissure

Upper

Apical posterior: Extends above the clavicle anteriorly; occupies area comparable to that of the apical and posterior segments of the right lung

Anterior: Occupies area between the clavicle and the border of the lingula (line comparable to the horizontal fissure of the right lung)

Middle

Lateral: Extends medially from junction of the two fissures at the third intercostal space to occupy one-third of the anterior surface of the lobe

Medial: Occupies the remaining anterior surface of the lobe

(Lingula)^‡

Superior: Occupies upper half of the lingula

Inferior: Occupies lower half of the lingula

Lower (Base)

Anterior: Occupies basal area beneath the oblique fissure anteriorly

Superior: Occupies half the area from the oblique fissure downward * on the posterior aspect

Lateral: Extends from the junction of the middle lobe over the midaxillary area to occupy one-third the area inferior to the superior segment on the posterior aspect

Posterior: Occupies two-thirds of the area posteriorly beneath the superior segment

Medial: Occupies a space on the inner aspect of the right base ^†

Lower (Base)

Anterior: Occupies area inferior to the oblique fissure anteriorly

Superior: Occupies one-third of the basal area posteriorly from the oblique fissure downward

Lateral: Occupies the lateral half of the remaining two-thirds of the left lower lobe beneath the superior segment on the posterior aspect

Posterior: Occupies the medial portion of the remaining two-thirds of the left lower lobe beneath the superior segment on the posterior aspect

^*This segment is best drained when the patient lies prone. Superior segments also called apical segments.

^†Medial basal segment has no direct exposure to the chest wall; therefore it cannot be directly auscultated. This segment is best drained when the patient is positioned for the left lateral basal segment because of the comparable angle of its bronchus.

^‡Lingula is not an area that is anatomically distinct from the right middle lobe; rather, it is anatomically part of the left upper lobe.

Fig. 3-12 Surface markings of the lungs (anterior **[A],** posterior **[B],** and lateral **[C]** aspects). The underlying bronchopulmonary segments are also shown. *(Modified from Cherniak RM, Cherniack L*: Respiration in health and disease, *ed 3, Philadelphia, 1983, WB Saunders.)*

The left lung is generally smaller than the right and accommodates the position of the heart. The medial border on the anterior aspect runs from the sternoclavicular joint to the middle of the sternal angle, down the midline of the sternum to the fourth costal cartilage. A lateral indentation of about 2 to 3 cm forms the cardiac notch at the level of the fifth and sixth costal cartilages. The courses of the inferior and medial borders on the posterior aspect are similar in the left and right lungs. In the left lung, however, the inferior border crosses at the level of the 10th thoracic vertebra, not the 12th, as is observed in the right lung.

The position of the fissures of the lungs can be outlined over the chest wall. In both lungs, the oblique fissure begins between the second to fourth thoracic vertebrae. This can be roughly estimated by following a line continuous with the medial border of the abducted scapula around the midaxillary line at the fifth rib and terminating at the sixth costal cartilage anteriorly. The horizontal fissure of the right lung originates from the oblique fissure at the level of about the fourth intercostal space in the midaxillary line and courses medially and slightly upward over the fourth rib anteriorly. The left lung has no horizontal fissure.

Bronchopulmonary Segments

The bronchopulmonary segments lie within the three lobes of the right lung and the two lobes of the left lung. There are 10 bronchopulmonary segments on the right and 8 on the left. Brief anatomic descriptions of the position of each lobe are provided in Table 3-2. Figure 3-12, A, illustrates the surface markings on the anterior view of the lungs and the position of the various bronchopulmonary segments within the major anatomic divisions provided by the fissures. Figure 3-12, B, shows some of these features from the lateral views. Figure 3-12, C, illustrates the surface markings and bronchopulmonary segments of the posterior aspect of the lungs.

Heart

The heart is a conical, hollow muscular pump enclosed in a fibroserous sac, the pericardium. Its size is closely related to body size and corresponds remarkably to the size of an individual’s clenched fist. It is positioned in the center of the chest behind the lower half of the sternum. The largest portion of the heart lies to the left of the midsternal line; the apex is found approximately 9 cm to the left, in the fifth intercostal space.

The surface markings of the heart can be traced by joining four points over the anterior chest wall. On the right, the heart extends from the third to the sixth costal cartilage at a distance of about 10 to 15 mm from the sternum. On the left, the heart extends from the second costal cartilage to the fifth intercostal space, 12 to 15 mm and 9 cm from the left sternal border, respectively. Joining the two points on the left side outlines the left atrium and ventricle. The heart is rotated to the left in the chest, so the right side of the heart is foremost. Joining the two uppermost points outlines the level of the atria, and joining the two lower points represents the margin of the right ventricle.

The heart as a whole is freely movable within the pericardial cavity and changes position during both contraction and respiration. During contraction, the apex moves forward, strikes the chest and imparts the chest-and-apex beat, which may be felt and seen. Abnormal positioning of the apex beat can indicate cardiac enlargement or displacement. During breathing, the movements of the diaphragm determine the position of the heart. This is because of the attachment of the central tendon of the diaphragm to the pericardium. Changes in position during quiet breathing are hardly noticeable, but with deep inspirations, the downward excursion of the diaphragm causes the heart to descend and rotate to the right. The opposite occurs during expiration. Pathology of the lungs can also change the position of the heart. Atelectasis shifts the heart to the same side. In tension pneumothorax, where air enters the chest (usually through an opening in the chest wall) and cannot escape, the positive pressure shifts the heart away from the side of the pathology.

The heart is enclosed by the pericardium, whose two surfaces can be visualized by visualizing the heart as a fist plunged into a large balloon. The outer surface, a tough fibrous membrane, is called the fibrous pericardium. It encases the heart as well as the organs and terminations of the great vessels. This membrane is so unyielding that when fluid accumulates rapidly in the pericardial cavity, it can compress the heart and impede venous return. When this occurs often, a window is cut in the pericardium, allowing the fluid to escape. The inner surface, the serous pericardium, is a serous membrane that lines the fibrous pericardium. Between 10 and 20 mL of clear pericardial fluid separates and moistens the two pericardial surfaces. The pericardium, with its fluid, minimizes friction during contraction. It also holds the heart in position and prevents dilation. The serous pericardium consists of an outer layer (the parietal layer) and an inner layer (the visceral layer or epicardium).

The heart is divided into right and left halves by an obliquely placed longitudinal septum (Fig. 3-13, A). Each half has two chambers: the atrium, which receives blood from veins, and the ventricle, which ejects blood into the arteries. The superior vena cava, inferior vena cava, and intrinsic veins of the heart deposit venous blood into the right atrium. Blood then passes through the tricuspid valve to the right ventricle. The right ventricle ejects the blood through the pulmonary valve into the pulmonary arteries, which are the only arteries in the body that contain deoxygenated blood. Pulmonary veins return the blood to the left atrium, and from there it passes through the mitral valve to the left ventricle. From the left ventricle it is ejected through the aortic valve into the main artery of the body, the aorta.

Fig. 3-13 Blood flow through the heart. A, The three major coronary arteries that supply blood to the heart. B, Blood flow through internal structures. *(A, from Hall J:* Guyton and Hall textbook of medical physiology, ed 12, Philadelphia, 2011, Saunders. B, from Goldberger: Clinical electrocardiography: a simplified approach, ed 7, Philadelphia, 2009, Mosby.)

The heart is divided into three layers: the epicardium, myocardium, and endocardium. The outermost layer, the epicardium, is visceral pericardium and is often infiltrated with fat. The coronary blood vessels that nourish the heart run in this layer before entering the myocardium. The myocardium consists of cardiac muscle fibers. The thickness of the layers of cardiac muscle fibers is directly proportional to the amount of work they perform. The ventricles do more work than the atria; hence their walls are thicker. The pressure in the aorta is higher than that in the pulmonary trunk. This requires greater work from the left ventricle, so its walls are twice as thick as those of the right ventricle. The innermost layer, the endocardium, is the smooth endothelial lining of the interior of the heart.

Heart Valves

The four valves of the heart, although delicate in appearance, are designed to withstand repetitive closures against high pressures. Normally, they operate properly for more than 80 years without need of repair or replacement. The tricuspid and mitral valves function differently from the other valves of the heart. Being located between the atria and ventricles, they must effect a precise closure within a contracting cavity.

During diastole, the two leaflets or cusps of the mitral valve and the three cusps of the tricuspid valve relax into the cavities of the ventricles, allowing blood to flow between the two chambers. As the ventricular chambers fill with blood, the cusps of the valves are forced up into a closed position. Fibrous cords, the chordae tendinae, are located on the ventricular surfaces of these cusps. These cords connect the cusps of the valve with the papillary muscles of the ventricular walls. As pressure builds in the ventricular chambers, contraction of these muscles prevents the cusps from being forced up into the atria. Dysfunction or rupture of the chordae tendinae or the papillary muscles may undermine the support of one or more valve cusps, producing regurgitation from the ventricles to the atria.

The pulmonic and aortic valves are similar in appearance, but the aortic cusps are slightly thicker than the pulmonic cusps. Each valve has three fibrous cusps, the bases of which are firmly attached to the root of the aorta or the pulmonary artery. The free edges of these valves project into the lumen of the vessels. At the end of systole, blood in the aorta and pulmonary artery forces the cusps of the valves shut. These valves are attached in such a manner that they cannot be everted into the ventricles by increased pressure in the vessels. During diastole, the cusps support the column of blood filling the ventricles. Contraction of the ventricles during systole increases pressure within the ventricular chambers, forcing the cusps to open and allow blood flow into the vessels.

The arterial supply of the heart muscle is derived from the right and left coronary arteries, which arise from the aortic sinuses (Fig. 3-13, B). The left coronary artery (LCA) divides into the anterior descending artery and the left circumflex artery. These arteries supply most of the left ventricle, the left atrium, most of the ventricular septum, and in 45% of people, the sinoatrial (SA) node. The right coronary artery (RCA) supplies most of the right ventricle, the atrioventricular (AV) node, and in 55% of people, the SA node. Infarction of these arteries or their branches can cause interruption or cessation of the conduction system and death of the myocardial muscle in the area supplied by the artery. The severity of the infarction is dependent on the size of the artery and the importance of the area it supplies.

The heart is drained by a number of veins. Most of the veins of the heart enter the coronary sinus, which then empties into the right atrium. A small number of veins, the thebesian veins, empty directly into the right and left ventricles.

Innervation

Innervation of the heart involves a complex balance between its intrinsic automaticity and extrinsic nerves (Fig. 3-14). The SA and AV nodes provide the heart with an inherent ability for spontaneous rhythmic initiation of the cardiac impulse. The rate of this impulse formation is regulated by the autonomic nervous system (ANS), which also influences other phases of the cardiac cycle. It controls the rate of spread of the excitation impulse and the contractility of both atria and ventricles.