The Lungs as Organs

The inflated lungs are conical; the upper part is the rounded apex, and the lower concave part is the base (Figure 2-1). The costal (rib) surface of the lungs is smooth and convex and adjoins the inner chest wall. The surfaces adjoining the mediastinum are concave; the mediastinum is the central portion of the chest cavity containing the heart, aorta, esophagus, great veins, trachea, and mainstem bronchi.

Figure 2-1 shows that the apices of the lung extend above the clavicles into the base of the neck, with their top borders lying at the level of the first thoracic vertebra. The anterior border of the left lung forms the cardiac notch, which accommodates protrusion of the heart into the left half of the thoracic cavity.

The lung bases rest on the diaphragm, the major muscle of ventilation, which consists of two distinct, separately innervated muscles—the left and right hemidiaphragms. The diaphragm separates the thoracic and abdominal cavities, bowing deeply upward into the thoracic cavity (see Figures 2-1 and 2-9, A). The concave lung bases fit over the domes of the diaphragm. When the body is at rest, the outer margins of the lung’s bases lie at the level of the tenth thoracic vertebra; however, the highest points of their concave diaphragmatic surfaces bow up to the level of the eighth to ninth thoracic vertebra. The left hemidiaphragm surface is slightly lower than the right hemidiaphragm surface because (1) the heart rests on the left half of the diaphragm, pushing it downward, and (2) the liver, situated in the abdominal cavity directly below the right half of the diaphragm, props up this area (see Figure 2-1).

The area on the lung’s mediastinal surfaces through which arteries, veins, and the main bronchus enter is called the hilum and can be thought of as the “root” of the lung (Figure 2-2). The pulmonary ligament just below the hilum connects the membrane that covers the lung’s surface with the diaphragm below. The hilar structures and pulmonary ligament suspend and stabilize the lungs in the chest cavity. A thin anterior portion of the upper lobe of the left lung overlaps the heart and continues downward to a narrow point, forming the lingula, the tonguelike anatomical counterpart of the middle lobe of the right lung (see Figure 2-2).

Pleural Membranes

The visceral and parietal pleurae are one continuous membrane, forming sealed envelopes surrounding each lung. The visceral pleura, attached to the lung’s surface, doubles back at the hilar region to form the parietal pleura, which is attached to the inner chest wall surface (Figure 2-3). The potential space between visceral and parietal membranes is called the pleural space. Normally, the visceral and parietal pleurae are separated only by an extremely thin layer of serous pleural fluid. Pleural fluid lubricates the membranes, allowing nearly frictionless movement as they slide over one another during breathing. The cohesive forces of pleural fluid molecules prevent separation of the membranes, similar to the way a film of water prevents two glass microscope slides from being pulled apart. The lung passively follows movements of the chest wall and diaphragm. Intrapleural pressure, or pressure between the pleural membranes, is subatmospheric during normal breathing because the chest wall and lung recoil in opposite directions, creating a vacuum in the sealed pleural space.

The lowest margin of the diaphragm meets the chest wall in an area called the costophrenic recess. The lung’s borders do not extend into this recess, but the parietal pleural membrane does (see Figure 2-1). If the pleural membranes become inflamed by disease, fluid may form in the pleural space, creating what is called a pleural effusion; in such situations, fluid settles into the costophrenic recess of the pleural space, blunting the normally sharp angles of the costophrenic junctions as seen on a chest x-ray image. This fluid can be removed with a syringe and large-gauge needle (thoracentesis) or by surgically inserting a chest tube into the pleural cavity; several liters of fluid can collect in the pleural space, which compresses the lung and restricts its expansion.

CONCEPT QUESTION 2-2

What effect would a large pleural effusion have on the pressure required to inflate the lungs?

Blood Supply to the Lungs

The metabolic requirements of the lung are met by two separate blood supplies—the pulmonary and systemic circulations. The pulmonary circulation originates from the right ventricle of the heart as the pulmonary artery and carries oxygen-poor blood to the lungs to be reoxygenated. Pulmonary arterioles subdivide many times to form extensive capillary beds that surround the alveoli like a fine net. Beyond the alveoli, capillaries converge to form venules and pulmonary veins, which carry oxygenated blood to the heart’s left atrium (Figure 2-4). The entire cardiac output flows through the pulmonary circulation and its fine capillary network; in this way, the pulmonary capillaries act as a kind of filter through which all blood flow must pass. The main function of this circulation is to bring blood into contact with alveolar gas so that oxygen and carbon dioxide exchange (respiration) can occur.

The lung’s systemic blood supply, the bronchial circulation, arises from the aorta as the bronchial arteries, which supply all of the airway walls from the major bronchi down to the respiratory bronchioles. In contrast to the pulmonary circulation, it is only a small fraction of the cardiac output. When blood flow passes through the capillary beds of the bronchial wall, oxygen diffuses out of the blood into airway wall tissues. On leaving the capillaries, this oxygen-poor blood takes at least two different courses: (1) one third to one fourth of it is channeled into the true bronchial veins into the azygos vein and then into the heart’s right atrium and (2) the remaining two thirds to three fourths of it drains directly into the pulmonary veins, mixing oxygen-poor bronchial venous blood directly with the freshly oxygenated blood of the pulmonary veins. This mixing reduces the overall oxygen content of the pulmonary venous blood that enters the left atrium, which the left ventricle eventually pumps into the systemic arterial circulation (see Figure 2-4).

These vascular connections between bronchial and pulmonary circulations are called bronchopulmonary-arterial

anastomoses, which constitute part of the normal “anatomical shunt” found in the pulmonary circulation. Shunting refers to the mixing of unoxygenated blood with oxygenated blood. In this case, unoxygenated venous blood from the bronchial circulation mixes with oxygenated blood in the pulmonary veins, which is eventually pumped into the aorta and the systemic arteries. This normal anatomical shunting means that systemic arterial blood can never have the same partial pressure of oxygen as alveolar gas; this gives rise to the normal P(A-a)O₂ (alveolar-to-arterial oxygen pressure difference). Vessels carrying blood to the heart are called veins, and vessels carrying blood away from the heart are called arteries. In the systemic circulation, arteries carry oxygenated blood, and veins carry deoxygenated blood; this is reversed in the pulmonary circulation. In common medical usage, the terms arterial blood and venous blood generally refer to oxygenated blood and deoxygenated blood.

Lymphatics of the Lung

Lymph is a clear, protein-containing fluid found in the interstitial spaces of the body (i.e., extracellular and extravascular spaces). The body’s capillaries filter about 30 L of fluid per day out of the blood into interstitial spaces but reabsorb only about 27 L back into the blood. If the extra 3 L of fluid remained in the interstitial spaces, fluid would accumulate and eventually flood the alveoli, causing pulmonary edema. The lymphatic system normally removes this interstitial fluid and returns it to the bloodstream. In addition, lymph nodes filter and clear the fluid of microorganisms and foreign materials.

The lung’s lymphatics are organized into superficial and deep vessel networks. The superficial vessels drain the lung surfaces and pleura; the deep vessels drain the main lung tissues, also known as the lung parenchyma. The flow of lymph through deep and superficial systems is directed toward the hilum, where numerous lymph nodes are located. Lymph eventually returns to the bloodstream through the right thoracic lymph duct.

Nervous Control of the Lungs and Thoracic Musculature

The voluntary skeletal muscles of the chest wall and diaphragm are innervated by the somatic nervous system, whereas the involuntary smooth airway muscle of the lung is innervated entirely by the autonomic nervous system. The sympathetic and parasympathetic divisions of the autonomic system control most of the body’s visceral functions. (The term “viscera” refers to the soft organs of the thoracic and abdominal cavities.) Somatic and autonomic nervous systems are compared in Table 2-1. The somatic system provides only motor innervation to the ventilatory muscles; the autonomic system supplies both motor (efferent) and sensory (afferent) nerves to the lung.

Somatic Innervation

The paired phrenic nerves supply motor innervation to the hemidiaphragms. Phrenic nerves originate from the right and left cervical nerve plexuses as branches of cervical spinal nerves C3 to C5 (Figure 2-5). Phrenic nerves cross in front of the scalenus anterior muscles of the neck and enter the chest, sandwiched between subclavian arteries and veins. Thoracic surgery, neck trauma, and cancerous tumors sometimes injure or compress the phrenic nerve, causing paralysis of the diaphragm. However, breathing may still be possible even with a paralyzed diaphragm if intercostal nerves and muscles are intact.

The intercostal nerves are spinal nerves that provide motor innervation to the muscles between the ribs, the intercostal muscles. They originate from the thoracic portion of the spinal column and follow the ribs anteriorly.