Respiratory System


Cells in the body require oxygen to survive. Vital functions of the body are carried out as the body is continuously supplied with oxygen. Without the respiratory system exchange of gases in the alveoli will not be made possible and systemic distribution of oxygen will not be made possible. The transportation of oxygen in the different parts of the body is accomplished by the blood of the cardiovascular system. However, it is the respiratory system that carries in oxygen to the body and transports oxygen from the tissue cells to the blood. Thus, cardiovascular system and respiratory system works hand in hand with each other. A problem in the cardiovascular system would affect the other and vice versa.

Respiratory System

Functional Anatomy of the Respiratory System

The nose is the only external part of the respiratory system and is the part where the air passes through. During inhalation and exhalation, air enters the nose by passing through the external nares or nostrils. Nasal cavity is found inside the nose and is divided by a nasal septum. The receptors for the sense of smell, olfactory receptors are found in the mucosa of the slit-like superior part of the nasal cavity which is located beneath the ethmoid bone. Respiratory mucosa lines the rest of the nasal cavity and rests on a rich network of thin-walled veins that warms the air passing by.

Important information about nose is the presence of the sticky mucus that is produced by the mucosa’s gland. This important characteristic moistens the air and traps the incoming bacteria and other foreign debris passing through the nasal cavity. Cells of the nasal mucosa are ciliated and it creates a gentle current that moves the contaminated mucus posteriorly towards the throat, where it is swallowed and digested by stomach juices.

In cases where the temperature of the environment is cold, the cilia become sluggish. Thus, more mucus are allowed to accumulate in the nasal cavity and to dribble outward through the nostrils. This is the main reason why a “runny nose” is noted during a cold day.

Conchae – these are three mucosa-covered projections or lobes that greatly increase the surface area of the mucosa exposed to the air.  Aside from that, conchae increase the air turbulence in the nasal cavity.

Palate – a partition that separates the nasal cavity from the oral cavity.  Anteriorly, the palate that is supported by a bone called the hard palate and the one which is unsupported is the soft palate.

Paranasal Sinuses – these are structures surrounding the casal cavity and are located in the frontal, sphenoid, ethmoid and maxillary bones.


The pharynx is a 13 cm long muscular tube that is commonly called the throat. This muscular passageway serves as a common food and air pathway. This structure is continuous with the nasal cavity anteriorly via the internal nares.

Parts of pharynx:

  1. Nasopharynx – the superior portion of the pharynx. The pharyngotympanic tubes that drain the middle ear open in this area. This is the main reason why children who have otitis media may follow a sore throat or other tyoes of pharyngeal infections since the two mucosae of these regions are continuous.
  2. Oropharynx – middle part
  3. Laryngopharynx – part of pharynx that enters the larynx.

When food enters the oral cavity, it travels to the oropharynx and laryngopharynx. However, instead of entering the larynx, the food is directed into the esophagus and not to the larynx.

Tonsils – clusters of lymphatic tissues found in the pharynx.

Types of Tonsils:

  1. Palatine tonsils – tonsils found at the end of the soft palate.
  2. Pharyngeal tonsils – lymphatic tissues located high in the nasopharynx. This is also called adenoid.
  3. Lingual tonsils – located at the base of the tongue.

The larynx is the one that routes the air and food into their proper channels. Also termed as the voice box, it plays an important role in speech. This structure is located inferior to the pharynx and is formed by:

  1. Eight rigid hyaline cartilages
  2. Spoon-shaped flap of elastic cartilage, which is called the epiglottis.

Thyroid cartilage – this is the largest hyaline cartilage that protrudes anteriorly in males and is referred to as the Adam’s apple.

Epiglottis – this is a flap of tissue that serves as a guardian of the airways as it protects the superior portion of the larynx. The epiglottis does not restrict passage of air into the lower respiratory passages when a person is not swallowing. However, when a person swallows food, the epiglottis tips and forms a lid or blocks the opening of the larynx so that food will not be directed to the lower respiratory passages. The food will be then routed to the esophagus and in cases where it enters the larynx, a cough reflex is triggered to expel the substance and prevent it from continuing into the lungs. This protective reflex does not work when a person is unconscious that is why it is not allowed to offer or administer fluids to an unconscious client.

Vocal folds – a pair of folds which is also called the true vocal cords that vibrate when air is expelled.

Glottis – the slit-like passageway between the vocal folds.


Also called the windpipe, the trachea is about 10 to 12 cm long or about 4 incheas and travels dwon from the larynx to the fifth thoracic vertebra. This structure is reinforced with C-shaped rings of hyaline cartilage and these rings are very important for the following purposes:

  1. The open parts of the rings abut the esophagus that allows the structure to expand anteriorly when a person swallows a large size of food.
  2. The solid portions of the C-rings are supporting the walls of the trachea to keep it patent or open even though pressure changes during breathing.

The trachea is lined with ciliated mucosa that primarily serves for this purpose: To propel mucus loaded with dust particles and other debris away from the lungs towards the throat where it can either be swallowed or spat out.

Main Bronchi

The main bronchi, both the right and the left, are both formed by tracheal divisions. There is a slight difference between the right and left main bronchi. The right one is wider, shorter and straighter than the left. This is the most common site for an inhaled foreign object to become lodged. When air reaches the bronchi, it is already warmed, cleansed of most impurities and well humidified.


The lungs are fairly large organs that occupy the most of the thoracic cavity. The most central part of the thoracic cavity, the mediastinum, is not occupied by the lungs as this area houses the heart.

Apex – the narrow superior portion of each lung and is located just below the clavicle

Base – the resting area of the lung. This is a broad lung area that rests on the diaphragm.

Divisions of the Lungs

The lungs are divided into lobes by the presence of fissures. The left lung has two lobes while the right lung has three.

Pleural Layers

Visceral pleura – also termed as the pulmonary pleura and covers each surface of the lings.

Parietal pleura – covers the walls of the thoracic cavity.

Pleural fluid – a slippery serous secretion that allows the lungs to slide along over the thorax wall during breathing movements and causes the two pleural layers to cling together.

Bronchioles – smallest air-conducting passageways.

Bronchial tree or respiratory tree – a network formed due to the branching and rebranching of the respiratory passageways within the lungs.

Alveoli – air sacs. This is the only area where exchange of gases takes place. Millions of clustered alveoli resembles bunches of grapes and these structures make up the bulk of the lungs.

Respiratory Zone – this part includes the respiratory bronchioles, alveolar ducts, alveolar sacs, alveoli.

Physiology of Respiration

The respiratory primarily supplies oxygen to the body and disposes of carbon dioxide through exhalation. Four events chronologically occur, for respiration to take place.

  1. Pulmonary ventilation – this process is commonly termed as breathing. With pulmonary ventilation, air must move out into and out of the lungs so that the alveoli of the lungs are continuously drained and filled with air.
  2. External respiration – this is the exchange of gases or the loading of oxygen and the unloading of carbon dioxide between the pulmonary blood and alveoli.
  3. Respiratory gas transport – this is the process where the oxygen and carbon dioxide is transported to the and from the lungs and tissue cells of the body through the bloodstream.
  4. Internal respiration – in internal respiration the exchange of gases is taking place between the blood and tissue cells.
Mechanics of Breathing

Breathing, also called pulmonary ventilation is a mechanical process that completely depends on the volume changes occurring in the thoracic cavity. Thus, a when volume changes pressure also changes, and this would lead to the flow of gases equalizing with the pressure.

Inspiration – also called inhalation. This is the act of allowing air to enter the body. Air is flowing into the lungs with this process. Inspiratory muscles are involved with inspiration which includes:

  1. The diaphragm
  2. External intercostals

These muscles contract when air is flowing in and thoracic cavity increases. When the diaphragm contracts it slides inferiorly and is depressed. As a result the thoracic cavity increases. The contraction of the external intercostal muscles lifts the rib cage and thrusts the sternum forward. This increases the anteroposterior and lateral dimensions of the thorax.

Expiration – also called expiration. It the process of breathing out air as it leaves the lungs. This process causes the gases to flow out to equalize the pressure inside and outside the lungs. Under normal circumstances, the process of expiration is effortless.