STRUCTURE AND FUNCTIONS

Since gas exchange occurs inside the body, air from the atmosphere must be carried from the surrounding space and into the body. Air flows into the lungs by means of conducting passages that connect different parts of the respiratory system to one another and allow the air to flow in and out. These passages are called airways, and they are usually divided into the upper airways and the lower airways; the division between the two systems is designated as the point where the esophagus and the trachea branch off from one another at the top of a person’s throat. The upper airways are the parts above this point and include the nose, the mouth, the sinuses, and the pharynx, or upper part of the throat. The lower airways include the larynx (also called the voice box), the trachea, and the bronchi, which connect the trachea to the lungs.

The inner functions of the lungs are the central feature of the process of respiration. Air that is breathed in travels into the body through the nose or mouth, flows down the trachea, and enters the bronchi. There are two bronchi, one leading from the trachea to each lung. After leaving the bronchi, the air flows into the lungs, reaching smaller, narrower passages known as bronchioles. At the end of the bronchioles are small, clustered air sacs called alveoli. The alveoli are exquisitely thin, delicate sacs made of a set of cells called type 1 alveolar cells that provide the interface between the many capillaries that line the wall of the gases in the alveoli. Molecular oxygen (O₂) diffuses across these type 1 alveolar cells and into the capillaries that surround them. Once the oxygen enters the capillaries, it is carried by hemoglobin in the red blood cells throughout the body by the network of blood vessels.

Pressure changes within the lungs move air into the lungs (inspiration) or drive air from the lungs (expiration). These pressure changes result from changes in lung volume. Gas molecules exert pressure on their containers by colliding with the sides of the container. The smaller the volume of the gas, the more frequently the gas molecules strike the sides of the container, which results in greater pressure. Therefore, as the volume of a gas decreases, the pressure of that gas increases. To change the volume of the lungs, a large, parachute-shaped muscle underneath the lungs, called the diaphragm, moves down as it contracts and rises as it relaxes. During inspiration, the diaphragm lowers as it contracts, which increases the volume of the lungs and decreases the pressure within the lungs. When the pressure in the lungs decreases below atmospheric pressure, air from the atmosphere flows into the lungs in response to this pressure disparity. During expiration, the diaphragm rises as it relaxes, which decreases the volume of the lungs and increases the pressure within the lungs. When the pressure within the lungs exceeds atmospheric pressure, the gases with in the lungs move into the atmosphere in response to these pressure differences.

The lungs must expand and contract as air is inhaled and then exhaled, so they require an enclosure that allows them to maintain their position in the body while still being able to increase and decrease in volume. The lungs are also quite delicate structures, so they need protection from outside forces that might be directed against the body. The design of the thoracic cavity in which the lungs are situated accommodates these competing needs for both strength and flexibility. The ribs that make up the ribcage encase the lungs in a lattice of sturdy bone to protect them from injury. However, to keep this structure from being too rigid and potentially even damaging the lungs it is supposed to protect, the ribs are attached to the thoracic vertebrae in the spinal column by articulated facets and to the sternum by cartilage. Cartilage is firm tissue that is more flexible than bone, yet stronger than the softer tissue of which organs are composed. This means that the ribs encircling the lungs can move slightly to allow the lungs to function while keeping them safe. The lungs expand and contract within the thoracic cavity, protected by two membranes called pleurae. One pleura, called the visceral pleura, encases the lung itself; the other, the parietal pleura, coats the inside of the thoracic cavity. A small amount of fluid flows between the two pleurae to prevent the lungs from rubbing against the inside of the thoracic cavity.

The expansion and contraction of the lungs is aided by the presence of large amounts of a protein called elastin, which acts like a molecular rubber band that can stretch and snap back repeatedly. This ability of the lungs to stretch and distend is called lung compliance. However, the inner side of the alveoli are quite wet, and the cohesive properties of water would cause the contracting alveoli to collapse during expiration and not reinflate upon inspiration. To prevent their collapse during expiration, the walls of the alveoli contain a second cell type called type 2 alveolar cells that secrete a detergent-like concoction called surfactant. Surfactant decreases the cohesive properties of water and reduces the surface tension within the alveoli. Consequently, the lungs require less energy to expand and surfactant discourages alveolar collapse during expiration.

A good way of seeing the human respiratory system in action is to follow the way the body responds to strenuous exercise while the activity is occurring. During exercise, the body’s need for energy increases because it is moving more quickly and forcefully than it normally would. This means that there is a greater need for oxygen to enter the bloodstream and for carbon dioxide to be removed from the blood. To provide additional oxygen, the rate of breathing increases so that more breaths are taken per minute and each breath is deeper than it would be if the person were at rest. This increases the amount of oxygen entering the body, but this extra oxygen still needs to reach all the body’s different parts. To help accomplish this, the heart rate increases along with the increase in the rate of breathing. As the heart beats faster and stronger, it pushes the oxygenated blood out to the body’s extremities much more quickly, making sure that they receive the oxygen-rich blood they need to sustain the exercise.

Measuring the increase in breathing rate is quite straightforward, as one only needs to count the number of breaths in a specific timeframe during exercise and compare it with the number counted without exercise. Measuring how deep a breath is can be a more complicated undertaking, because it requires the use of a special apparatus called a spirometer. To use a spirometer, the person being evaluated must place their lips around a straw-like device and inhale through it. The spirometer measures the volume of air being removed from it and drawn into the lungs. These devices are used not only to monitor and measure respiration but also as a type of respiratory therapy for patients who have had surgery or who are recovering from some type of trauma affecting their respiration. A frequent consequence of these experiences is that the lungs become partially deflated and must therefore be conditioned to return to their full capacity. This can be challenging, especially with children, because breathing is often painful during a recovery period. For this reason, doctors will often use breathing devices to encourage patients to inhale as much as they can to expand their lungs. Most of the devices have a straw one must breathe through and a ball that rises along an enclosed tube as air is pulled out of its chamber. Patients are told to use this device several times per hour and to use their breath to pull hard enough to raise the ball to a designated level.

One of the major vulnerabilities of the human respiratory system is the possibility of contaminants entering the body in the same way that air does. This could be very dangerous because small particles of matter carried along by the air could damage the delicate structures inside the lungs, cause an infection, or both. The body has several systems in place to help prevent these harms from occurring. Very small hairs grow on the inside of the nose to filter out larger particles such as dust and pollen. Particles that are too small to be captured this way may instead become stuck in the layer of mucus that lines the nasal passages and other airflow conduits. There is also an organ at the back of the throat called the epiglottis, which functions to make sure that food and liquids go down the esophagus to the stomach, rather than traveling down the trachea and potentially into the lungs. Upon swallowing, the epiglottis reflexively folds over the trachea to close it off. After the solid or liquid has been swallowed, the epiglottis then pulls back from covering the trachea.

DISORDERS AND DISEASE

There are several diseases and disorders that can afflict the human respiratory system. One of the most common is pneumonia, which is essentially an infection of the alveoli caused by bacteria. The infection causes fluid to build up in the alveoli, which interferes with the ability of oxygen to pass through the walls of the alveoli and enter the bloodstream. If this process continues long enough, the person may suffer from oxygen deprivation and require supplemental oxygen from an oxygen tank, delivered through a face mask. While not generally fatal, pneumonia can cause death in patients who are very old, very young, or immunocompromised in some fashion.

One of the symptoms exhibited by a person suffering from pneumonia is coughing, which is one of the primary ways the human respiratory system has of clearing up materials or conditions that are interfering with regular breathing. Coughing occurs when a person inhales a volume of air, closes the glottis (the opening between the vocal folds in the larynx, not to be confused with the epiglottis), forces the inhaled air against the closed glottis under pressure, and then opens the glottis to allow the inhaled air to blast outward. Coughing is used to try to expel foreign matter that has entered into the respiratory system, such as when a person takes a sip of a beverage and the liquid goes down the trachea instead of the esophagus. Coughing can be initiated voluntarily or as a reflex. Voluntary coughing may occur when people wish to clear out obstructing matter from their airway. Involuntary coughing results from a situation that causes the body to attempt to expel matter in the airway to prevent choking. An example of involuntary coughing is that which occurs when a person ingests water while drowning.

Some illnesses can cause the pleurae to become inflamed, which can make it extremely painful to breathe. In fact, the lungs are so delicate that they can collapse, in much the same way that a balloon collapses when punctured. This can happen when air or fluid enters the thoracic cavity due to an injury, a condition known as pneumothorax; the air or fluid presses on the lung and prevents it from fully inflating.

Fetal lungs do not make surfactant until the last two months of fetal life. Premature babies born before their lungs make sufficient quantities of surfactant are unable to keep their alveoli inflated between breaths and suffer from infant respiratory distresssyndrome (IRDS, also called or hyaline membrane disease). A lack of sufficient airway causes the lungs to completely or partially collapse; a condition called atelectasis. Atelectasis triggers inflammation and swelling of the lung tissue (pulmonaryedema). Blood that passes through capillaries in the atelectatic portions of lung does not receive adequate oxygen and the infant suffers from oxygen deficient (hypoxemia). IRDS is treated by aerosolizing synthetic surfactant into the respiratory passages of the baby. In severe cases, mechanical respiration may be needed.

PERSPECTIVE AND PROSPECTS

Many people are unaware of the role played by the human respiratory system in the everyday activity of speech. Human beings communicate in many ways, but speech is the most common, barring conditions that prevent a person from speaking. Speech is the act of creating sounds to signify specific concepts. The respiratory system is critical to the production of vocalized speech because to produce intelligible sounds, one vibrates the vocal cords in the larynx while exhaling to cause air to pass over them. By varying the type of vibration, people can change the tone and volume of their speech, producing anything from a whisper to a shout. If the vocal cords become damaged or paralyzed, or if a person has trouble breathing, then speech can be problematic or even impossible. Some injuries and illnesses can cause the muscles that control the vibration of the vocal cords to become paralyzed, such as throat cancer, vocal-cord infections, or accidents affecting the neck and throat.