Pulmonary Breathing: Anatomy, Mechanics of Breathing, Defenses Against Infection and Respiratory Disorders

The respiratory system is a biological system that consists of specific organs and structures used to exchange gases in animals and plants.

The anatomy and physiology that make this happen vary greatly, depending on the organism’s size, the environment in which it lives, and its evolutionary history. In terrestrial animals, the respiratory surface is internalized as the lining of the lungs.

Gaseous exchange in the lungs occurs in millions of tiny air sacs called alveoli in mammals and reptiles but atria in birds. These microscopic air sacs have a wealthy blood supply, which causes the air to contact the blood.

Breathing (or ventilating) is moving air in and out of the lungs to facilitate the exchange of gases with the internal environment, mainly by bringing oxygen and removing carbon dioxide.

All aerobic creatures need oxygen for cellular respiration, which uses oxygen to break down food into energy and produces carbon dioxide as a waste product.

Breathing, or “external breathing,” brings air to the lungs, where the exchange of gases occurs in the alveoli through diffusion. The body’s circulatory system transports these gases to and from the cells, where ” cellular respiration ” takes place.

The breathing of all vertebrates with lungs consists of repetitive cycles of inhalation and exhalation through a highly branched system of tubes or airways that lead from the nose to the alveoli.


The number of respiratory cycles per minute is the respiratory rate, and it is one of the four main vital signs of life.

Under normal conditions, the depth and speed of respiration are controlled automatically and unconsciously by various homeostatic mechanisms that maintain constant partial carbon dioxide and oxygen pressures in the arterial blood.

Maintaining the partial pressure of carbon dioxide in the arterial blood without changes under various physiological circumstances contributes significantly to the strict control of the pH of extracellular fluids (FEC).

Excessive breathing ( hyperventilation ) and insufficient respiration (hypoventilation), which decrease and increase the partial blood pressure of carbon dioxide, respectively, cause an increase in the pH of extracellular fluids in the first case and a decrease in pH in the second. Both cause distressing symptoms.

Breathing has other essential functions. It provides a mechanism for speech, laughter, and similar expressions of emotions. It is also used for reflexes, such as yawning, coughing, and sneezing.

Animals that can not be thermoregulated by perspiration because they lack sufficient sweat glands can lose heat by evaporation through panting.


In humans, the respiratory tract is the part of the anatomy of the respiratory system that participates in the process of respiration. Air is breathed through the nose or mouth.

In the nasal cavity, a layer of mucous membrane acts as a filter and traps pollutants and other harmful substances found in the air.

Then, the air moves towards the pharynx, a passage that contains the intersection between the esophagus and the larynx.

The opening of the larynx has a unique flap of cartilage, the epiglottis, which opens to allow air to pass, but closes to prevent food from moving into the airways.

From the larynx, the air moves to the trachea and to the intersection that branches to form the right and left primary bronchi.

Each of these bronchi branches into secondary bronchi (labors) that branch off into tertiary (segmental) bronchi that branch off into smaller airways called bronchioles that eventually connect with small specialized structures called alveoli that function in gas exchange.

The lungs found in the thoracic cavity are protected from physical damage by the rib cage. At the base of the lungs, there is a sheet of skeletal muscle called the diaphragm.

The diaphragm separates the lungs from the stomach and intestines. The diaphragm is also the primary breathing muscle and is controlled by the sympathetic nervous system.

The lungs are encased in a serous membrane that folds over itself to form the pleura: a two-layer protective barrier.

The internal visceral pleura covers the surface of the lungs, and the outer parietal pleura is attached to the inner surface of the thoracic cavity. The pleura encloses a cavity called a pleural cavity that contains pleural fluid.

This fluid decreases the amount of friction the lungs experience during breathing.

Mechanics of respiration

The lungs cannot inflate and will expand only when there is an increase in the volume of the thoracic cavity.

In humans, as in other mammals, this is mainly achieved by contraction of the diaphragm but also by contraction of the intercostal muscles that pull the rib cage.

During forced inhalation, the inhalation muscles that connect the ribs and the sternum to the cervical vertebrae and the base of the skull, in many cases through an intermediate union to the clavicles, exaggerate the movements of the pump handle and the bucket, causing a more significant change in the volume of the thoracic cavity.

During the exhalation, at rest, all the muscles of the inhalation relax, returning the thorax and the abdomen to a position called “resting position,” which is determined by its anatomical elasticity.

At this point, the lungs contain the functional residual capacity of air, which, in the adult human being, has a volume of approximately 2.5-3.0 liters.

During heavy breathing (hyperpnea), for example, during exercise, the exhalation is caused by the relaxation of all the muscles of inhalation (in the same way as at rest).

But, in addition, the abdominal muscles, instead of being passive, now contract strongly, causing the rib cage to be pulled down (front and side).

This not only decreases the size of the rib cage but also pushes the abdominal organs up against the diaphragm and thus swells deeply in the thorax.

The final exhalation lung volume is now less air than the ” functional residual capacity ” at rest.

However, in a typical mammal, the lungs can not be emptied. There is always at least a liter of residual air in the lungs in an adult human being after maximum exhalation.

Diaphragmatic breathing causes the abdomen to swell and regress rhythmically. Therefore, it is often called “abdominal breathing.” These terms are often used interchangeably because they describe the same action.

When the accessory muscles of inhalation are activated, especially during difficult breathing, the clavicles are lifted upward, as explained above.

This external manifestation of the use of the accessory muscles of inhalation is sometimes called clavicular breathing, which is observed mainly during asthma attacks and in people with chronic obstructive pulmonary disease.

Air passage

Usually, the air is inhaled and exhaled through the nose.

The nasal cavities (between the nostrils and the pharynx) are pretty narrow, first divided in two by the nasal septum and, secondly, by lateral walls that have several longitudinal folds or shelves, called nasal conchae.

Thus exposing a large area of ​​nasal mucous membrane to the air as it is inhaled (and exhaled).

This causes the inhaled air to absorb moisture from the wet mucus and heat from the underlying blood vessels. The atmosphere is almost saturated with water vapor and is almost at body temperature when it reaches the larynx.

Some of this moisture and heat is recaptured as the exhaled air moves over the partially dried mucus and cools in the nostrils during expiration. The sticky mucus also traps much of the particulate matter breathed in, preventing it from reaching the lungs.

Lower respiratory tract

The anatomy of a typical mammalian respiratory system, below the structures, usually listed among the “upper airways” (nasal cavities, pharynx, and larynx), is often described as a respiratory tree or a tracheobronchial tree.

The larger airways give rise to slightly narrow branches but more numerous branches than the “trunk” airway that gives rise to the components.

The human respiratory tree may consist, on average, of 23 such branches in progressively smaller airways, while the mouse respiratory tree has up to 13 such units.

The proximal divisions (those closest to the upper part of the tree, such as the trachea and bronchi) work mainly to transmit air to the lower respiratory tract.

Subsequent divisions, such as respiratory bronchioles, alveolar ducts, and alveoli, are specialized in exchanging gases.

The trachea and the first portions of the main bronchi are outside the lungs. The rest of the branches of the “tree” inside the lungs finally extends to each part of the lungs.

The alveoli are the blind terminals of the “tree,” which means that any air that enters must leave by the same route that used to enter the alveoli.

A system like this creates a dead space, a volume of air that fills the airways (quiet area) at the end of inhalation and exhales, unchanged, during the next exhalation, never having reached the alveoli.

Similarly, the dead space is filled with alveolar air at the end of exhalation. It is the first air that breathes back into the alveoli before the fresh air reaches the alveoli during inhalation. A typical adult human’s volume of dead space is approximately 150 ml.

The exchange of gases

The primary purpose of breathing is to bring the atmospheric air (in small doses) to the alveoli, where the gas exchange with the gases in the blood occurs. The equilibrium of the partial pressures of the gases in the alveolar blood and the alveolar air occurs by diffusion.

At the end of each exhalation, the adult human lungs contain 2,500-3,000 ml of air, their residual functional capacity or FRC. With each breath (inhalation), only about 350 ml of the warm and humid atmosphere are added, mixing well with the available residual capacity.

Consequently, the gas composition of the functional residual capacity changes very little during the breathing cycle.

Since the pulmonary capillary blood is balanced by this virtually invariable mixture of air in the lungs (which has a composition substantially different from that of the ambient air), the partial pressures of arterial blood gases also do not change with each breath.

Therefore, the tissues are not exposed to oxygen oscillations and carbon dioxide stresses in the blood during the breathing cycle. The peripheral and central chemoreceptors do not need to “choose” the point in the respiration cycle at which They must measure the blood gases and their response.

Therefore, homeostatic control of the respiratory rate depends on the partial pressures of oxygen and carbon dioxide in the arterial blood. This also maintains the constancy of the pH of the blood.


The frequency and depth of respiration are controlled automatically by the respiratory centers receiving information from the peripheral and central chemoreceptors.

These chemoreceptors continuously monitor the partial pressures of carbon dioxide and oxygen in the arterial blood.

In the first place, the sensors are the central chemoreceptors on the surface of the medulla oblongata, which are particularly sensitive to pH and the partial pressure of carbon dioxide in the cerebrospinal blood fluid.

The second group of sensors measures the partial pressure of oxygen in the arterial blood. The latter is known as the peripheral chemoreceptors found in the aortic and carotid bodies.

The information of all these chemoreceptors is transmitted to the respiratory centers of the pons and the medulla oblongata, which respond to deviations in the partial pressures of carbon dioxide and oxygen in the arterial blood from ordinary.

By adjusting the frequency and depth of breathing in such a way as to restore the partial pressure of carbon dioxide to 5.3 kPa (40 mm Hg), the pH to 7.4, and, to a lesser extent, the partial pressure of oxygen to 13 kPa (100 mm Hg).

For example, exercise increases the production of carbon dioxide by active muscles. This carbon dioxide diffuses into the venous blood and ultimately improves the partial pressure of carbon dioxide in the arterial blood.

This is detected immediately by the carbon dioxide chemoreceptors in the brainstem.

Respiratory centers respond to this information by causing the frequency and depth of breathing to increase so that the partial pressures of carbon dioxide and oxygen in the arterial blood return almost immediately to the same levels as at rest.

The respiratory centers communicate with the muscles of respiration through the motor nerves, of which the phrenic nerves, which innervate the diaphragm, are probably the most important.

Automatic breathing can be canceled to a limited extent by simple choice or to facilitate swimming, speaking, to sing, or other vocal training.

It is impossible to suppress the need to breathe to the point of hypoxia, but training can increase the ability to maintain breathing; For example, in February 2016, a Spanish professional freediver broke the world record of keeping his breath underwater in just over 24 minutes.

There are also other automatic reflexes of breath control.

Immersion, particularly in the face, in cold water, triggers a diving reflex response. This, in the first place, has the result of closing the airways against the influx of water. The metabolic rate slows down.

This is combined with intense vasoconstriction of the arteries in the extremities and abdominal viscera. This reserves the oxygen in the blood and lungs at the beginning of the dive, almost exclusively for the heart and the brain.

The diving reflex is a response often used in animals that routinely need to dive, such as penguins, seals, and whales. It is also more effective in babies and young children than in adults.

Defenses against infection

The epithelial lining of the upper respiratory tract is interspersed with goblet cells that secrete protective mucus. This helps filter the waste, which is eventually swallowed in the highly acidic stomach environment or expelled by spitting.

The epithelium that lines the respiratory tract is covered with tiny hairs called cilia. These beat rhythmically from the lungs, moving the foreign particles secreted from mucus into the laryngopharynx up and out in the mucociliary escalator.

In addition to keeping the lower respiratory tract sterile, they prevent the accumulation of mucus in the lungs. The macrophages in the alveoli are part of the immune system that engulfs and digests the harmful inhaled agents.

The hair in the nostrils plays a protective role, trapping particles like dust. The cough reflex expels all the irritants inside the mucous membrane to the outside.

The airways of the lungs contain muscle rings. When the ducts are irritated by an allergen, these muscles can contract.

Respiratory disorders

Abnormal breathing patterns include Kussmaul breathing, Biot breathing, and Cheyne-Stokes breathing.

Other breathing disorders include shortness of breath ( dyspnea ), stridor, apnea, sleep apnea ( obstructive sleep apnea ), mouth breathing, and snoring.

Many conditions are associated with obstructed airways. Hypopnea refers to breathing too shallow; hyperpnea refers to rapid and deep breathing caused by the demand for more oxygen, such as exercise.

Hypoventilation and hyperventilation also refer to shallow, rapid, and deep breathing, respectively, but in inappropriate circumstances or diseases.

However, this distinction (for example, hyperpnea and hyperventilation) is not always fulfilled, so these terms are often used interchangeably.

A variety of breath tests can diagnose diseases such as dietary intolerances. A rhinomanometer uses acoustic technology to examine airflow through the nasal passages.

Breathing and humor

Specific breathing patterns tend to occur with certain moods. Due to this relationship, practitioners of various disciplines feel that they can encourage the emergence of a particular perspective by adopting a specific breathing pattern.

For example, the most common recommendation is that deep breathing that uses the diaphragm and abdomen more can encourage a more relaxed and confident mood.

Practitioners from different disciplines often interpret the importance of regulating breathing and its perceived influence on the mood in different ways.

Buddhists may consider that it helps to precipitate a sense of inner peace, holistic healers that fosters a general state of health, and business advisers that provide relief from work-based stress.