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Anatomy of the Respiratory System

Nose and Nasal Cavity

Air moves through the respiratory tract through the nasal cavity, the first portion of the external airway that carries air into the lungs.Anterior portion of the nasal cavity is supported and protected by cartilage, bone, muscle, and skin of the nose.A nasal cavity consists of a hollow space within the nose and skull that is lined with hairs and mucus membrane.The nasal cavity draws air into the body and warms, moistens, and filters it before it reaches the lungs.A nasal cavity lined with hairs and mucus helps trap dust, mold, pollen and other environmental pollutants before they reach the internal organs.When you exhale through the nose, you bring moisture and heat back into the nasal cavity before expelling into the environment.


Oral cavity, or the mouth, is the secondary outlet of the respiratory system.The nasal cavity is responsible for most of the normal functions of breathing, but when necessary, the oral cavity can perform those functions as well.Due to shorter pathways for air entering the body from the mouth than the nose, the mouth does not warm and moisturize the air entering the lungs as well as the nose does.In addition, the passage of air through the nose is not filtered by hairs or sticky mucus from the mouth.Breathing through the mouth has the advantage of allowing more air to enter the body quickly due to its longer distance and larger diameter.


From the posterior end of the nasal cavity, the pharynx and the larynx extend upwards to form a muscular funnel.There are three major regions of the pharynx: the nasopharynx, oropharynx, and laryngopharynx.In the nasal cavity, the nasopharynx is the region of the throat located directly above the base of the nose.Inhaled air travels to the nasopharynx and descends through the oropharynx, which lies behind the oral cavity.As air enters the oral cavity it enters the pharynx by the oropharynx.Following inhalation, air is diverted into the opening of the larynx by the epiglottis as it descends into the laryngopharynx.An epiglottis is a flap of elastic cartilage that connects the trachea and the esophagus.As the pharynx is also used for swallowing food, the epiglottis ensures that air enters the trachea by covering the opening to the esophagus.During swallowing, the epiglottis covers the trachea to ensure that food enters the esophagus and to prevent choking.


Larynx, also known as the voice box, is the section of the airway that connects the laryngopharynx with the trachea.Just inferior to the hyoid bone and superior to the trachea, the larynx is found in the anterior portion of the neck.It is composed of several cartilage structures, which give the larynx its structure.One of the laryngeal cartilage pieces, the epiglottis protects the larynx when swallowing.The thyroid cartilage, generally smaller and visible in adult males than the epiglottis, lies just below the epiglottis.This gland holds the vocal folds open at the anterior end of the larynx.The ring-shaped cricoid cartilage is found immediately below the thyroid cartilage, at the base of the larynx and supporting the back.A vocal fold, in addition to cartilage, allows the body to produce sounds for speech and singing.Vocal folds are folds of mucous membrane that produce vocal sounds.By changing the tension and vibration speed of the vocal folds, one can control the pitch of the voice.


There are five rings of hyaline cartilage in the trachea, which are lined with columnar pseudostratified ciliated epithelium.Through the trachea, air passes through the neck and into the thorax, connecting the larynx with the bronchi.Tracheal rings allow the air to constantly flow through.As a result, the open end of the cartilage rings faces the esophagus posteriorly, allowing the esophagus to expand into the space occupied by the trachea to handle large pieces of food moving through the esophagus.

In addition to providing a path for air in and out of the lungs, the trachea is also responsible for carrying waste products.The tracheal epithelium produces mucus that traps dust and other contaminants and prevents them from reaching the lungs.Cilia in the epithelial cells direct the mucus out of the pharynx to the gastrointestinal tract, where it can be swallowed and digested.

Bronchi and Bronchioles

.The left and right bronchi enter each lung before branching into smaller secondary bronchi.Primary bronchi carry air into the lobes of the lungs - two of them in the left lung and three in the right lung.In turn, the secondary bronchi divide into many smaller tertiary bronchi within each lobe.Many smaller bronchioles spread throughout the lungs.Each terminal bronchiole further divides into smaller branches less than a millimeter in diameter.Finally, tiny terminal bronchioles carry air to the lungs.

In the split between the bronchi and the bronchioles, the structures of the walls of the bronchi and bronchioles begin to change.Several cartilage rings extend from the primary bronchi, holding the airways open and giving them a cross-sectional shape similar to a circle or a letter D.bronchi produce smooth muscle and elastin protein in the walls as they branch into secondary and tertiary bronchi.There is no cartilage in the bronchioles, unlike those in the bronchi.Smaller bronchi and bronchioles are able to move and contract because of smooth muscles and elastin.

A major function of the bronchioles and trachea is to carry air into the lungs.Their smooth muscle walls control airflow into the lungs.A large volume of air is required by the body during exercise, so the smooth muscle relaxes to widen the bronchial tubes.Dilated airways provide less resistance to airflow and enable more air to pass into and out of the lungs.While at rest, smooth muscle fibers contract to prevent hyperventilation.In addition, mucus and cilia of the epithelial lining of the bronchi and bronchioles are used to trap and remove dust and other pollutants from the lungs.


Lungs are two large spongy organs that lie above and below the diaphragm in the thorax.Each lung has a pleural membrane that provides space for expansion as well as a negative pressure space relative to the body's exterior.During relaxation, the lungs fill passively with air due to the negative pressure.It is because of the heart pointing leftward that the left and right lungs are slightly different in their sizes and shapes.Left lung has two lobes while right lung has three; left lung is therefore slightly smaller than right lung.

A lung's interior is composed of spongy tissues with many capillaries and around 30 million tiny sacs called alveoli.A cup-shaped structure, the alveoli are located at the end of the terminal bronchioles and are surrounded by capillaries.Air entering the alveoli is lined with simple squamous epithelium that allows the exchange of gases with the blood passing through it.

Muscles of Respiration

The lungs are surrounded by muscles that force air into or out of the lungs. .When the diaphragm contracts, it moves inferiorly a few inches into the abdominal cavity, expanding the thoracic cavity and drawing air into the lungs.Exhalation allows air to flow back out through a relaxed diaphragm.

In between the ribs there are several small muscles that assist the diaphragm in expanding and compressing the lungs.Two groups of intercostal muscles exist: internal and external.There are a set of deep muscles that compress the ribs and force the thoracic cavity to squeeze out air from the lungs as the muscles contract.Intercostals are superficial to the internal intercostals and are responsible for elevating the ribs, thus expanding the thoracic cavity and allowing the lungs to inhale air.

Physiology of the Respiratory System

Pulmonary Ventilation

Ventilation of the lungs consists of moving air into and out of the lungs in order to facilitate air exchange.During pulmonary ventilation, the respiratory system uses both a negative pressure system and contractions of muscles.By establishing a negative pressure gradient between the alveoli and the external atmosphere, the respiratory system establishes negative pressure.Pleural membranes seal the lungs and keep the lungs at a pressure slightly beneath atmospheric pressure when the lungs are relaxed.This results in air passively filling the lungs at rest due to the pressure gradient.After filling with air, the lungs' pressure rises to match the atmospheric pressure.This allows for more air to be inhaled by the diaphragm and external intercostal muscles, causing the thorax to expand, dropping the lungs' pressure below that of the atmosphere again.

.In the process, the pressure gradient inside the lungs is reversed, resulting in the exhalation of air until there is an equal difference between the pressure inside and outside the lungs.As a result of the lungs' elastic properties, they recoil back into their resting volume, restoring the pressure gradient involved in inhalation.

External Respiration

Air fills the alveoli and blood fills the capillaries that surround the alveoli, resulting in external respiration.In comparison with blood in capillaries, oxygen in the atmosphere has a higher partial pressure while carbon dioxide has a lower partial pressure.By passive diffusion along the gradient of partial pressures from high to low, the gases diffuse through the simple squamous epithelium lining of the alveoli.Oxygen is transported from the air into the blood as a result of external respiration, while carbon dioxide is transferred from the blood into the air.Once oxygen has been transferred to the body's tissues, carbon dioxide is released into the atmosphere.

Internal Respiration

Body tissues exchange gases with blood in capillaries, in a process called internal respiration.As it passes through tissue, capillary blood has a higher partial pressure of oxygen and a lower partial pressure of carbon dioxide.Throughout the endothelium lining of capillaries, the difference in partial pressure leads to the diffusion of gases along their pressure gradient from high to low pressure.During internal respiration, oxygen diffuses into the tissues and carbon dioxide diffuses into the blood.

Transportation of Gases

Blood carries oxygen and carbon dioxide, the two major gases of respiration, throughout the body.Some dissolved oxygen and carbon dioxide are transported in the blood plasma, but most gases are bonded to molecules.A transport molecule in red blood cells, hemoglobin carries almost 99% of the oxygen in the bloodstream.As well as carrying oxygen, hemoglobin can transport carbon dioxide from the tissues back to the lungs.As a result, the vast majority of carbon dioxide is absorbed by the plasma as bicarbonates.Anhydrase catalyzes a reaction between carbon dioxide and water in the tissues when the partial pressure of carbon dioxide is high.After that, carbonic acid dissociates into hydrogen ions and bicarbonate ions.As carbon dioxide partial pressure falls, the reactions reverse, and carbon dioxide is liberated into the lungs for exhalation.

Homeostatic Control of Respiration

.The body maintains eupnea until less exertion results in a greater demand for oxygen and greater carbon dioxide production.As part of autonomic sensing, chemoreceptors in the body measure the partial pressures of oxygen and carbon dioxide in the blood and send signals to the respiratory center in the brain stem.A respiratory center regulates breathing rate and depth to return blood pressure to normal levels.

Health Issues Affecting the Respiratory System

When something impairs the exchange of carbon dioxide for oxygen, this is clearly a serious problem.Allergies, asthma, pneumonia and lung cancer are some of the health conditions that can cause respiratory problems.Infections may be caused by bacteria or viruses, pollutant exposure (cigarette smoke, for instance), genetic factors, or a combination of factors.There are times when the onset of a condition is so gradual that we don't seek medical care until it has become worse.It's not uncommon for symptoms to become apparent gradually, as with alpha-1 antitrypsin deficiency (A1AD), which is often undiagnosed or misdiagnosed.Tests for DNA health can screen for A1AD genetic risk.