135 The Lungs and Pleurae

Lungs

The lungs are an essential organ which is necessary for the exchange of oxygen and carbon dioxide from the body.

The lung is the essential organ of the respiratory system. In mammals and more complex life forms, the two lungs are located near the backbone on either side of the heart.

Their principal function is to transport oxygen from the atmosphere into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere. Air travels through the mouth or nose into the pharynx, larynx, trachea, and bronchi and bronchioles in order to reach the lungs.

The exchange of gases is performed by the alveoli, the functional units of the lungs.

Anatomy of the Lungs

The human lungs flank the heart and great vessels in the chest cavity: The human lungs flank the heart and great vessels in the chest cavity.

The lungs are found in the thoracic cavity, and extend laterally into the right and left halves around the heart. The rib cage is a structure of bones that surrounds and protects the thoracic cavity, with 12 ribs protecting each of the two lungs.

The vertebral column is posteriorly attached to the ribcage and the lungs. The lungs are cone-shaped—the apex refers to the top of the lung, while the base refers to the bottom of the lung. The base is curved inward to allow it to rest on the diaphragm.

The diaphragm, a sheet of skeletal muscle that lies beneath the lungs, plays an important role in inhalation. The right lung is larger than the left lung, and the left lung contains the cardiac notch, a concave impression that the heart lies against.

The outer layer of the lungs are the pleura, a type of mesothelium (membrane tissue) that surrounds the lung and attaches it to the thoracic cavity. The pleura contain two layers, the outer parietal pleura (attached to the thoracic cavity), and the inner visceral pleura (covers the lungs).

The pleural cavity is the fluid-filled space between the parietal and visceral pleura, and provides room for the lung to expand during inhalation. The fluid inside the pleural cavity protects against irritation during inhalation as well.

Interior of the thoracic cavity: This is a transverse view of the thoracic cavity, showing the pleural cavity and the major structures around the lungs.

The interior of the lung contains the alveoli, where gas exchange occurs. The alveoli branch off from the bronchioles and bronchi that connect to the trachea and allow air to pass into the lungs.

The lungs are divided into lobes by fissures on the outer surface of the lung, and divide further into segments and finally into hexagonal lobules, the smallest divisions of the lungs.

Physiology of the Lungs

Breathing is largely driven by the muscular diaphragm at the bottom of the thorax. Contraction of the diaphragm pulls the bottom of the cavity in which the lung is enclosed downward, increasing volume and thus decreasing pressure, causing air to flow into the airways and into the lungs.

During normal breathing, expiration is passive— no muscles are contracted—and the diaphragm relaxes from its contracted state. The elastic recoil of the lungs automatically pulls the lungs inward during exhalation. The rib cage itself is also able to expand and contract to some degree during breathing, through the action of other respiratory and accessory respiratory muscles. As a result, air is transported into or expelled out of the lungs.

The major function of the lungs is gas exchange, which occurs in the alveoli of the lung. Oxygen dissolves through the extracellular matrix of the alveoli that lets the gas diffuse into the capillaries based on the relative partial pressures of the gasses (gasses flow from areas of high pressure to low pressure). Oxygen passively diffuses into the deoxygenated blood of the capillaries while carbon dioxide passively diffuses out of the deoxygenated blood and into the airways.

Only a relatively small proportion of alveoli in the lungs are perfused with blood and actually take part in gas exchange. The ratio of ventilation in the lungs and perfusion of the lungs (the air and blood supply of the alveoli respectively) is called the V/Q ratio (with Q being perfusion); it is an important indicator of efficiency in the lungs.

The V/Q in a healthy individual is variable but balanced, usually around.8, but can be higher or lower depending on the region of the lung sampled. An imbalance in V/Q ratio is called a mismatch, and indicates a severe problem.

Too low perfusion (and a higher ratio) indicates alveolar dead space, while too low ventilation (and a lower ratio) indicates a shunt, which is a lack of air supply relative to perfusion. V/Q mismatches occur during many different kinds of lung failure, and have many different causes.

Lobes, Fissures, and Lobules

The lungs are located on either side of the heart and are separated by fissures into lobes, three in the right and two lobes in the left.

The lungs are located in two chambers of the thoracic cavity on either side of the heart. Though similar in appearance, the two lungs are not identical, nor wholly symmetrical.

Fissures are double folds of pleura that divide the lung into lobes. There are three lobes in the right lung and two in the left lung. The lobes are further divided into segments and then into lobules, which are hexagonal divisions of the lungs that are the smallest visible subdivision.

The lobes are further divided into segments and then into lobules, hexagonal divisions of the lungs that are the smallest subdivision visible to the naked eye.

The Right Lung

The lobes of the lungs: The right lung has three lobes and the left lung has two.

The right lung is five centimeters shorter than the left lung to accommodate the diaphragm, which rises higher on the right side over the liver; it is also broader. The volume, the total capacity, and the weight of the right lung is greater than that of the left. The right lung is divided into three lobes.

The Upper Lobe (Right Lung)

The upper lobe is the largest lobe of the right lung. It extends from the apex of the lung down to the horizontal and oblique fissures. It bears apical, anterior, and posterior bronchopulmonary segments.

The Middle Lobe (Right Lung)

The middle lobe is the smallest lobe of the right lung, located between the horizontal and oblique fissures. It bears medial and lateral bronchopulmonary segments.

The Lower Lobe (Right Lung)

The lower lobe is the bottom lobe of the right lung. It lies beneath the oblique fissure. It bears medial, lateral, superior, anterior, and posterior bronchopulmonary segments.

The Left Lung

The Left Lung: This has a concave depression that accommodates the shape of the heart, called the cardiac notch.

The human left lung is smaller and narrower that the right lung, and is divided into two lobes, an upper and a lower, by the oblique fissure. The left lung has only two formal lobes because of the space taken up in the left side of the chest cavity by the heart, though it does have the lingula, which is similar to a lobe.

The left lung has a depression on the medial side of its surface called the cardiac notch, a concave impression molded to accommodate the shape of the heart.

The Upper Lobe (Left Lung)

The upper lobe of the left lung contains anterior and apicoposterior bronchopulmonary segments. It is above the oblique fissure.

The Lower Lobe (Left Lung)

The lower lobe of the left lung contains superior, anterior, posterior, medial, and lateral bronchopulmonary segments.

The Lingula

The lingula is not formally considered to be a lobe. It is a small, tongue-like projection of the left lung that is analogous to the middle lobe of the right lung. It contains superior and inferior bronchopulmonary segments.

The Hilium

Above and behind the cardiac impression is a triangular depression named the hilum. The hilum is the root of the lung where that contains structures that supply the lungs with blood, lymph fluid, and innervation, such as the pulmonary vein, pulmonary artery, pulmonary nerves, and lymphatic vessels.

These structures are enclosed by pleura. There is a hilium for each of the lungs found in the mediastinum (backside) of the lungs. The hilium is thinner in the left lung compared to the right lung because it lies between the cardiac notch and the groove for the aorta.

The hilium is important because it is the primary way in which the respiratory system links with the cardiovascular and nervous systems.

Blood Supply to the Lungs

Pulmonary circulation transports oxygen-depleted blood away from the heart to the lungs and returns oxygenated blood back to the heart.

There are two primary types of circulation in the human.

1. Pulmonary circulation refers to blood supply to the lungs for the purpose of gas exchange.
2. Systemic circulation refers to blood supply to the rest of the body, for the purpose of supplying oxygen to the tissues.

Bronchial circulation (by the bronchial arteries) supplies blood to the tissues of the bronchi and the pleura, and is considered part of systemic circulation.

Pulmonary Circulation

The right side of the heart deals with pulmonary circulation. At the end of systemic circulation, the veins take blood back to the heart through the vena cava.

The vena cava fills the right atrium with blood, which then ejects blood into the right ventricle by passing through the tricuspid valve. After blood fills in the right ventricle, it contracts and pumps the blood through the pulmonary valve, and into the pulmonary arteries.

There are two pulmonary arteries (one for each lung) that bring the deoxygenated blood to the lungs through the hilium. The arteries branch into the capillaries of the alveoli. Capillaries are the thinnest and smallest type of blood vessel, and they supply oxygen to individual tissues everywhere in the human body.

Gas exchange occurs by passive diffusion in the alveoli, so that dissolved oxygen enters the capillaries, while carbon dioxide leaves pulmonary circulation. The oxygenated blood then leaves the lungs through pulmonary veins (also contained in the hilium), which return the blood to the left side of the heart, completing the cycle of pulmonary circulation.

This blood then enters and fills inside the left atrium, which pumps it through the mitral valve (also called bicuspid) into the left ventricle. The blood fills inside the left ventricle and is then pumped through the aortic valve into the aorta, which marks the beginning of systemic circulation.

Systemic circulation and pulmonary circulation form the overall cycle of the circulatory system: transporting oxygen throughout the body.

Pulmonary circuit: Diagram of pulmonary circulation. Oxygen-rich blood is shown in red; oxygen-depleted blood in blue.

Problems in Pulmonary Circulation

While the cycle of pulmonary and systemic circulation is a well designed and effective system, it is not immune to certain problems. The most serious issue in pulmonary circulation is a pulmonary embolism, which is where a blood clot travels to the lung and causes an infarction of the lung (tissue death from lack of oxygen).

These blood clots typically originate in the deep veins of the legs (part of systemic circulation) as a result of blood pooling from injury or immobilization. As the veins of the leg are on their way to the right side of the heart, the clots are less likely to break up before they reach pulmonary circulation.

When the clot reaches the pulmonary artery, it obstructs the flow of blood into the lung, which causes the alveoli in the effected lung to die as a result. This results in an increase in aveolar dead space and decreased perfusion, (leading to shortness of breath and chest pain) and can be fatal if not treated in time by fibrinolytics (medications that dissolve the clot).