Respiratory System

The human respiratory system is a series of organs responsible for taking in oxygen and expelling carbon dioxide. Breathing is the first step in respiration for almost all organisms.

Organs in respiratory system:

  1. External nostrils
  2. Pharynx
  3. Larynx
  4. Trachea
  5. Bronchi 
  6. Bronchioles
  7. Alveoli
  8. Lungs

External Nostrils/ Nose

The nose has two exterior nostrils. These are parted by a cartilaginous (made up of cartilage) structure termed as the septum. Hairs cover the interior lining of the nostrils. The nasal chambers are a wide cavity the nostrils open up into.


The nasal chambers opens up into a wide void termed as the pharynx. It is a common path for air and food. It also prevents the entry of food into the windpipe. The epiglottis is a covering over the windpipe. If one talks during swallowing, the epiglottis is forced to open so that the air can go outwards), and food enters the windpipe, triggering cough.


The larynx also known as the voice box, is a short section of the airway that connects the laryngopharynx and the trachea. The larynx is located in the anterior portion of the neck, just inferior to the hyoid bones and superior to the trachea.


The trachea, or windpipe, is a 5-inch long tube made of C-shaped hyaline cartilage rings lined with pseudo stratified ciliated columnar epithelium. The trachea connects the larynx to the bronchi and allows air to pass through the neck and into the thorax. The rings of cartilage making up the trachea allow it to remain open to air at all times. The main function of the trachea is to provide a clear airway for air to enter and exit the lungs.

Bronchi and Bronchioles

The trachea splits into two tubes termed as bronchi, which enter each lung individually. The bronchi divide additionally into secondary bronchi, tertiary bronchi, and then bronchioles. The bronchioles then divide into small air sacs termed as alveoli.


The alveoli are minute sacs of air whose walls are so thin that their walls are single celled. These provide a crossing point for the respiratory gases to get diffused in and out of the blood stream, as the air sacs that consist of respiratory gases are in touching base with the stream of blood vessels.


The lungs are a pair of conical hollow organs, the right one being larger than the left one. The lower surfaces of the lungs are concave to accommodate the diaphragm which divides the body cavity into the thoracic and abdominal cavities. The lungs are enclosed in a double layered membrane called pleura. The lungs are well protected in the body case consisting of the breast bone on the front (ventral side) and the ribs on the sides (lateral sides) and the vertebral column on the back (dorsal side).

Mechanism of Breathing

There are two main steps in breathing: Inspiration and Expiration

  1. Mechanism of Inspiration: It is the process of breathing in, by which air is brought into the lungs. During inspiration the intercostal muscles in between the ribs pull them upwards and outward. The muscular diaphragm contracts and flattens. Because of these movements of the ribs and diaphragm the chest expands and the volume of the thoracic cavity is increased. The simultaneous expansion of the pleural cavities surrounding the lungs creates a partial vacuum. The result is the rushing in of external air through the trachea into the lungs. The process of inspiration is also called inhalation.
  2. Mechanism of Expiration: The expelling of air or exhalation from the lungs takes place when the size and pressure of the thoracic cavity are reduced. During this process the ribs are lowered by the intercostal muscles and the diaphragm becomes raised to resume its original position. As a result of these movements the size of the thoracic cavity is reduced and the lungs become compressed. The pressure in the lungs is higher than the atmospheric pressure and air rushes out of the lungs through the trachea and nose. Even after maximum forced exhalation, some air always remains in the lungs. This is called residual volume. The maximum volume of inspirable air is called inspiratory capacity. ital capacity is the amount of air that can be forcibly exhaled after a maximum inspiration. Vital capacity is the amount of air that can be forcibly exhaled after a maximum inspiration. Vital capacity is taken as the measurement of pulmonary (lung) function. The volume of air normally inhaled which is equal to the volume of exhaled air, is called tidal volume.

Gaseous exchange in the Lungs

The movement of air in and out of the lungs and the distribution of air within the lungs are referred to as ventilation of lungs is referred to as ventilation of lungs. The exchange of oxygen (O2) and carbon dioxide (CO2) between alveolar air and lung capillaries, takes place by simple diffusion. The diffusion of gases is caused due to the differential partial pressure of the respiratory gases. Gases move from a high pressure area to a low partial pressure area.

Venous blood enters the lung capillaries carrying oxygen at a pressure of 40 mm (at rest) and carbon dioxide at a pressure of 46 mm. The oxygen pressure of the alveolar air is 100 mm and the carbon dioxide pressure is 40 mm. Therefore oxygen passes from the alveolar air into blood and carbon dioxide from blood to air sacs. Arterial blood leaves the lungs, carrying oxygen at a pressure of 100mm and carbon dioxide at a pressure of 40mm. The average tissue fluid pressure of oxygen is 40 mm and carbon dioxide 46 mm. Therefore, oxygen passes from the blood to tissue fluid and carbon dioxide from tissue fluid into blood.

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