ANATOMY AND PHYSIOLOGY OF THE RESPIRATORY SYSTEM

The respiratory system consists of the nose, the nasal cavity, the pharynx, the larynx,

the trachea, the bronchi, and the lungs. The upper respiratory tract refers to the nose, nasal

cavity, pharynx, and associated structures; and the lower respiratory tract includes the larynx,

trachea, bronchi, and lungs. These terms are not official anatomical terms, however, and there

are several alternative definitions. For example, one alternative places the larynx in the upper

respiratory tract. Although air frequently passes through the oral cavity, it is considered to be

part of the digestive system instead of the respiratory system.

NOSE AND NASAL CAVITY

The term nose refers to the visible structure that forms a prominent feature of the face.

Most of the nose is composed of cartilage, although the bridge of the nose consists of bone.

The bone and cartilage are covered by connective tissue and skin.

The nasal cavity extends from the nares to the choane. The nares, or nostrils, are the

external openings of the nose and the choane are the openings to the pharynx. The nasal

septum is a partition dividing the nasal cavity into the right and left sides. The hard palate forms

the floor of the nasal cavity, separating the nasal cavity from the oral cavity. Air can flow

through the nasal cavity when the mouth is closed or when the oral cavity is full of food.

Three prominent bony ridges called conchae resembling a conch shell, are present on

the lateral walls on each side of the nasal cavity. The conchae increase the surface area of the

nasal cavity.

Paranasal sinuses are air-filled spaces within the bone. The maxillary, frontal,

ethmoidal, and sphenoidal sinuses are named after the bones in which they are located. The

paranasal sinuses open into the nasal cavity and are lined with a mucous membrane. They

reduce the weight of the skull, produce mucus, and influence the quality of the voice by acting

as resonating chambers.

Mucus produced by the epithelium of the paranasal sinuses, drains through small

passageways into the nasal cavity. When the mucous membranes become swollen because of

nasal infections, sinus infections, or allergies, these passages can become blocked. The mucus

then accumulates within the sinuses, and the increasing pressure can produce a painful sinus

headache.

The nasolacrimal ducts, which carry the tears from the eyes, also open into the nasal

cavity. Sensory receptors for the sense of smell are found in the superior part of the nasal

cavity.

Air enters the nasal cavity through the nares. Just inside the nares the epithelial lining is

composed of stratified squamous epithelium containing coarse hairs. The hairs trap some of

the large particles of dust suspended in the air. The rest of the nasal cavity is lined with

pseudostratified columnar epithelial cells containing cilia and many mucus-producing goblet

cells. Mucus produced by the goblet cells also traps debris in the air. The cilia sweep the

mucus posteriorly to the pharynx, where it is swallowed. As air flows through the nasal cavities,

it is humidified by moisture from the mucus epithelium and is warmed by blood flowing through

the superficial capillary networks underlying the mucous epithelium.

PHARYNX

The pharynx is the common passageway of both the respiratory and digestive systems.

It receives air from the nasal cavity and air, food, and water from the mouth. Inferiorly, the

pharynx leads to the rest of the respiratory system through the esophagus. The pharynx can be

divided into three regions; the nasopharynx, the oropharynx, and the laryngopharynx.

The nasopharynx is the superior part of the pharynx and extends from the choane of the

nasal cavity to the level of the uvula, a soft process that extends from the posterior edge of the

soft palate. The soft palate forms the floor of the nasopharynx. The nasopharynx is lined with

pseudostratified ciliated columnar epithelium that is continous with the nasal cavity. The

auditory tubes extend from the middle ears and open into the nasopharynx. The posterior part

of the nasopharynx contains the pharyngeal tonsil, which aids in defending the body against

infections. The soft palate and the uvula are elevated during swallowing; this movement results

in the closure of the nasopharynx, which prevents food from passing from the oral cavity into the

nasopharynx.

The oropharynx extends from the uvula to the epiglottis, and the oral cavity opens into

the oropharynx. Thus food, drink, and air all pass through the oropharynx. The oropharynx is

lined with stratified squamous epithelium, which protects against abrasion. Two sets of tonsils,

the palatine tonsil and lingual tonsil, are located near the opening between the mouth and the

oropharynx. The palatine tonsils are located in the lateral walls near the border of the oral

cavity and the oropharynx. The lingual tonsil is located on the surface of the posterior of the

tonque.

The laryngopharynx passes posterior to the larynx and extends from the tip of the

epiglottis to the esophagus. The laryngopharynx is lined with stratified squamous epithelium

and ciliated columnar epithelium.

LARYNX

The larynx is located in the anterior throat, and it connects superiorly to the pharynx and

inferiorly to the trachea. The larynx consists of an outer casing of nine cartilages that are

connected to one another by muscles and ligaments. Three of the nine cartilages are unpaired,

and six of them form three pairs. The largest cartilage is the unpaired thyroid cartilage, or

Adam’s apple. The thyroid cartilage is attached superiorly to the hyoid bone. The most inferior

cartilage of the larynx is the unpaired cricoid cartilage, which forms the base of the larynx on

which the other cartilages rest. The thyroid and cricoid cartilages maintain an open

passageway for air movement.

The third unpaired cartilage is the epiglottis. It differs from the other cartilages in that it

consists of elastic cartilage rather than hyaline cartilage. Its inferior margin is attached to the

thyroid cartilage anteriorly, and the superior part of the eipiglottis projects as a free flap toward

the tonque. The epiglottis helps prevent swallowed materials from entering the larynx. As the

larynx elevates during swallowing, the epiglottis tips posteriorly to cover the opening of the

larynx.

The six paired cartilages consists of three cartilages on either side of the posterior part

of the larynx. The top cartilage on each side is the cuneiform cartilage, the middle cartilage is

the corniculate cartilage, and the bottom cartilage is the arytenoid cartilage. The arytenoids

cartilages articulate with the cricoid cartilage inferiorly. The paired cartilages form and

attachment site for the vocal folds.

Two pairs of ligaments extend from the posterior surface of the thyroid cartilage to the

paired cartilages. The superior pair forms the vestibular folds or false vocal cords, and the

inferior pair composes the vocal cords, or true vocal cords. When the vestibular folds come

together, they prevent air from leaving the lungs, such as when a person holds his breath.

Along with the epiglottis, the vestibular folds also prevent food and liquids from entering the

larynx.

The vocal folds are the primary source of voice production. Air moving past the vocal

cords causes them to vibrate, producing sound. Muscles control the length and tension of the

vocal folds. The force of air moving past the vocal folds controls the loudness, and the tension

of the vocal folds controls the pitch of the voice. An inflammation of the mucous epithelium of

the vocal folds is called laryngitis. Swelling of the vocal folds during laryngitis inhibits voice

production.

TRACHEA

The trachea or windpipe is a membraneous tube that consists of connective tissue and

smooth muscle, reinforced with 16 – 20 C-shaped pieces of cartilage. The adult trachea is

about 1.4 – 1.6 centimeters in diameter and about 10 – 11 cm. long. It begins immediately

inferior to the cricoid cartilage, which is the most inferior cartilage of the larynx. The trachea

projects through the mediastinum, and divides into the right and left primary bronchi at the level

of the fifth thoracic vertebra. The esophagus lies immediately posterior to the trachea.

C-shaped cartilages form the anterior and lateral sides of the trachea. The cartilages

protect the trachea and maintain an open passageway for air. The posterior wall of the trachea

has not cartilage and consists of a ligamentous membrane and smooth muscle. The smooth

muscle can alter the diameter of the trachea.

The trachea is lined with pseudostratified columnar epithelium, which contains numerous

cilia and goblet cells. The cilia propel mucous produced by the goblet cells, as well as foreign

particles embedded in the mucus, out of the trachea, through the larynx, and into the pharynx,

from which they are swallowed.

Constant irritation of the trachea by cigarette smoke can cause the tracheal epithelium to

change to stratified squamous epithelium. The stratified squamous epithelium has no cilia and

therefore lacks the ability to clear the airway of mucus and debris. The accumulations of mucus

provide a place for microorganisms to grow, resulting in respiratory infections. Constant

irritation and inflammation of the respiratory passages stimulate the cough reflex, resulting in

“smoker’s cough”.

BRONCHI

The trachea divides into the left and right main (primary)

bronchi, each of which connects to a lung. The left main bronchus is

more horizontal than the right main bronchus because it is displaced

by the heart. Foreign objects that enter the trachea usually lodge in

the right main bronchus, because it is more vertical than the left main

bronchus and therefore more in direct line with the trachea. The main bronchi extend from the

trachea to the lungs. Like the trachea, the main bronchi are lined with pseudostratified ciliated

columnar epithelium and are supported by C-shaped pieces of cartilage.

LUNGS

The lungs are the principal organs of respiration. Each lung is cone-shaped, with its

base resting on the diaphragm and its apex extending superiorly to a point about 2.5 cm above

the clavicle. The right lung has three lobes called the superior, middle, and inferior lobes. The

left lung has two lobes called the superior and inferior lobes. The lobes of the lungs are

separated by deep, prominent fissures on the surface of the lung. Each lobe is divided into

bronchopulmonary segments separated from one another by connective tissue septa, but these

separations are not visible as surface fissures. Individual diseased bronchopulmonary segment

can be surgically removed leaving the rest of the lung relatively intact, because major blood

vessels and bronchi do not cross the septa. There are nine bronchopulmonary segments in the

left lung and 10 in the right lung.

The main bronchi branch many times to form the tracheobronchial tree. Each main

bronchus divides into lobar bronchi as they enter their respective lungs. The lobar (secondary)

bronchi, two in the left lung and three in the right lung, conduct air to each lobe. The lobar

bronchi in turn give rise to segmental (tertiary) bronchi, which extend to the bronchopulmonary

segments of the lungs. The bronchi continue to branch many times finally giving rise to

bronchioles. The bronchioles also subdivide numerous times to give rise to terminal

bronchioles, which then subdivide into respiratory bronchioles. Each respiratory bronchiole

subdivides to form alveolar ducts, which are like long, branching hallways with many open

doorways. The doorways open into alveoli, which are small air sacs. The alveoli becomes so

numerous that the alveolar duct wall is little more than a succession of alveoli. The alveolar

ducts end as two or three alveolar sacs, which are chambers connected to two or more alveoli.

There are about 300 million alveoli in the lungs.

As the air passageways in the lungs become smaller, the structure of their walls

changes. The amount of cartilage decreases and the amount of smooth muscle increases, until

at the terminal bronchioles the walls have a prominent smooth muscle layer, but no cartilage.

Relaxation and contraction of the smooth muscle within the bronchi and bronchioles can change

the diameter of the air passageways. For example, during exercise, the diameter can increase,

thus increasing the volume of air moved. During an asthma attack, however, contraction of the

smooth muscle in the terminal bronchioles can result in greatly reduced airflow. In severe

cases, air movement can be so restricted that death results.

As the air passageways of the lungs becomes smaller, the lining of their walls also

changes. The trachea and bronchi have pseudostratified ciliated columnar epithelium, the

bronchioles have ciliated simple columnar epithelium, and the terminal bronchioles have ciliated

simple cuboidal epithelium. The ciliated epithelium of the air passageways functions as a

mucus cilia escalator, which traps debris in the air and removes it from the respiratory system.

As the air passageways beyond the terminal bronchioles become smaller, their walls

become thinner. The walls of the respiratory bronchioles are cuboidal epithelium and those of

the alveolar ducts and alveoli are simple squamous epithelium. The respiratory membrane of

the lungs is where gas exchange between the air and blood takes place. It is mainly formed by

the walls of the alveoli and the surrounding capillaries, but there’s some contribution by the

alveolar ducts and respiratory bronchioles. The respiratory membrane is very thin to facilitate

the diffusion of gases. It consists of:

1. A thin layer of fluid lining the alveolus.

2. The alveolar epithelium composed of simple squamous epithelium.

3. The basement membrane of the alveolar epithelium.

4. A thin interstitial space.

5. The basement membrane of the capillary endothelium.

6. The capillary endothelium composed of simple squamous epithelium.

The elastic fibers surrounding the alveoli allow them to expand during inspiration and recoil

during expiration. The lungs are very elastic, and when inflated, they are capable of

expelling the air and returning to their original, uninflated state. Specialized secretory cells

within the walls of the alveoli secrete a chemical called surfactant that reduces the tendency

of the alveoli to recoil.

PLEURAL CAVITIES

The lungs are contained within the thoracic

cavity. In addition, each lung is surrounded by

a separate pleural cavity. Each pleural cavity is

lined with a serous membrane called the pleura.

The pleura consists of a parietal and visceral

part. The parietal pleura, which lines the walls

of the thorax, diaphragm, and mediastinum, is

continuous with the visceral pleura, which

covers the surface of the lung.

The pleural cavity, between the parietal and

visceral pleurae, is filled with a small volume of pleural fluid produced by the pleural

membranes. The pleural fluid performs two functions: (1) it acts as a lubricant, allowing the

visceral and parietal pleurae to slide past each other as the lungs and thorax change shape

during respiration, and (2) it helps hold the pleural membranes together. The pleural fluid acts

like a thin film of water between two sheets of glass (the visceral and parietal pleurae); the glass

sheets can slide over each other easily, but it is difficult to separate them.

LYMPHATIC SUPPLY

The lungs have two lymphatic supplies. The

superficial lymphatic vessels are deep to the

visceral pleura and function to drain lymph from the

superficial lung tissue and the visceral pleura. The

deep lymphatic vessels follow the bronchi and

function to drain lymph from the bronchi and

associated connective tissues. No lymphatic

vessels are located in the walls of the alveoli. Both

the superficial and deep lymphatic vessels exit the

lungs at the main bronchi.

Phagocytic cells within the lungs phagocytize carbon particles and other debris from

inspired air and move to the lymphatic vessels. In older people, the surface of the lungs can

appear gray to black because of the accumulation of these particles, especially if the person

smoked or lived most of his life in a city with air pollution. Cancer cells from the lungs can also

spread to other parts of the body through the lymphatic vessels.