human respiratory system parts and functions

how does the respiratory system work?

The human respiratory system consists of lungs and air passages. These air passages carry fresh air to the respiratory sites.respiratory system of man (human respiratory system)

Human Respiratory System (Air passageways)

Air passageways or Human Respiratory System consists of:

  • Pharynx and Larynx
  • Trachea
  • Bronchi and Bronchioles
  • Alveolar Ducts and Alveolar Sacs

1.      Nasal Cavities

Nasal cavities are lined with mucous membranes. This mucous membrane is composed of ciliated epithelium. The projection of bones from the wall of the internal nose subdivides each nasal cavity into three passageways. Air enters the nasal cavity through the nostrils.

The hair and mucous of nostrils trap the large dust particles. The mucous membrane of the nasal cavity makes the air moist, and warm and filters the small foreign particles. The nasal cavities open into the throat or pharynx by two openings.

2. Throat or Pharynx

The pharynx is a muscular passage. It is also lined with a mucous membrane. The air is transferred from the pharynx into the larynx.

3. Larynx or Voice Box

The larynx is a complex cartilaginous structure. It surrounds the upper end of the trachea. One of the cartilages of the larynx is the epiglottis. The epiglottis is a muscularly controlled structure. It shows hinge-like action. It acts as a lid.

Epiglottis automatically closes the opening of the larynx during the act of swallowing. So, the epiglottis prevents the entry of food or liquid into the larynx. The opening of the larynx is called the glottis.

The glottis is lined by a mucous membrane. The mucous membrane of the glottis forms two thin-edged fibrous bands called vocal cords. These vocal cords are stretched across the glottis. The vocal cords vibrate by air and produce a voice.

4. Trachea or Wind Pipe

A trace is a tubular structure. It lies ventral to the esophagus. The trachea or Wind Pipe extends into the chest cavity or thorax. It divides into the right and left bronchi in the thorax. There are a series of C-shaped cartilage rings in the wall of the trachea. The cartilaginous ring prevents the trachea from collapsing. So, they keep the trachea open.

5. Bronchi and Bronchioles

Each bronchus enters the lung. Each bronchus in the lungs divides and sub-divides into small bronchi. The bronchi with a diameter of less than 1mm are called bronchioles. The bronchi have a similar cartilaginous ring to the trachea. But, these rings are progressively replaced by cartilaginous plates.

These cartilaginous plates are irregularly disturbed. The cartilages are totally absent in bronchioles. Bronchioles are made up of mainly circular smooth muscles.

6. Air Sacs and Alveoli

The bronchioles continue to divide and sub-divide deep into the lungs. These bronchioles are finally opened into a large number of air sacs. Air sacs are the functional unit of the lung. Each air sac consists of alveoli. The alveoli are microscopic single-layered structures.

There is a rich network of blood capillaries overlying the alveoli. These capillaries produce an excellent site for the exchange of gases.

7. Lungs

Lungs are very important in the Human Respiratory System. The lungs are closed sacs. They are connected to the outside through the trachea and nostril or mouth. The lungs have millions of alveoli. So, they are spongy. Lungs are placed in the chest cavity. The sides of the chest cavity are bounded by ribs and skeletal muscles.

The floor of the chest is called the diaphragm. The diaphragm is a sheet of skeletal muscles. It separates the chest cavity from the abdominal cavity. Lungs are covered with pleura. The pleura are double layered thin membranous sacs.

Mechanism of the Human Respiratory System

The pumping of oxygen-containing fresh air into the lungs and the pumping of carbon dioxide-containing air out of the lungs is called breathing (Exchange of Gases). So, breathing is a mechanical process. It consists of two phases, inspiration, and expiration.

Breathing occurs rhythmically. The frequency of breathing in a man at rest is 15 to 20 times per minute. There are three properties of lungs and their associated parts help in breathing:

  1. Lungs are spongy in nature. The lungs themselves cannot pull air in or push it out. The massive expansion of elastic lungs takes place during inspiration. Similarly, passive contraction of the lungs takes place during expiration.
  2. The floor of the chest cavity is a diaphragm. It is a muscular sheet. When these muscles relax, the shape of the diaphragm is more domes. On the other hand, when the muscles of the diaphragm contract, its shape becomes less dome-like.
  3. The walls of the chest cavity are composed of ribs and intercostal muscles. When intercostal muscles between the ribs contract, the ribs are elevated. When these intercostal muscles are relaxed, the ribs settle down.


The moving of fresh air in the lungs is called inspiration. The space inside the chest cavity is increased during inspiration. This space is increased in two ways:

  • The muscles of the ribs contract and elevate the ribs upward and forward.
  • The muscles of the diaphragm also contact and the diaphragm become less domelike.

These two movements increase the volume of the chest cavity. It reduces pressure in the chest cavity. When the pressure in the lungs is decreased, the lungs expand. The expansion of the lungs creates a vacuum in the lungs. There is higher atmospheric pressure outside. So, air rushes into the lungs from the outside. This is called inspiration.


The removal of the consumed air from the lungs is called expiration. The space in the chest cavity is reduced in two ways:

  • The intercostal muscles of the ribs are relaxed during expiration. As a result, the ribs move downward and inward. So, the space in the sides of the chest cavity is reduced.
  • At the same time, the muscles of the diaphragm are relaxed and the diaphragm becomes more domelike. So, the chest cavity is also reduced from the floor.

This reduction in the space of the chest cavity exerts pressure on the lungs. This pressure presses the lungs and the air inside the lungs moves out the lungs. This is called expiration.

Respiratory Distress Syndrome

It is common in premature infants, especially with a gestation age of fewer than 7 months. This occurs because enough surfactant is not produced in these infants. The surfactant is a mixture of lipoprotein molecules.

It is produced by the secretory cells of the alveolar epithelium. Surfactant forms a layer over the surface of the fluid within the alveoli. It reduces the surface tension of the fluid of the alveoli. So, surfactant reduces the tendency of the lungs to collapse. The lungs may collapse without enough surfactant.

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