Respiratory System Anatomy and Physiology

Oxygen: Essential for Survival

All the cells of the body require oxygen to generate energy from food in the body. Without it, they would not have survived to move, build, reproduce, and turn food into energy. This oxygen is obtained from breathing in air, which happens involuntarily. The respiratory system’s primary function is ‘to supply the blood with oxygen so that it can be distributed to all parts of the body.’ The respiratory system is responsible and carries out this work through breathing. The breathing process involves ‘taking in’ or inhaling oxygen and ‘giving out’ or exhaling carbon dioxide.

Respiration is a process that involves ‘a cycle of events resulting from the exchange of oxygen and carbon dioxide between the atmosphere and the body cells.’ The breathing process or ventilation is stimulated by the nerve impulse, during which the air moves through a series of passages into and out of the lungs. Later, the exchange of gases between the lungs and the blood takes place. This is called ‘external respiration.’ The blood transports gases to the tissues. The exchange of gases between the blood and the tissues is called ‘internal respiration.’ Finally, the cells utilize oxygen for their activities. This is called ‘cellular metabolism’ or ‘cellular respiration.’ The above-said activities collectively characterize the process of respiration.

Based on function and anatomy, the respiratory system is categorized as follows.1

Functionally, it is divided into the conducting zone and the respiratory zone.

The conducting zone comprises of:

  • Nose
  • Pharynx
  • Larynx
  • Trachea
  • Bronchi
  • Bronchioles

The respiratory zone, which is found inside the lungs comprises of:

  • Respiratory bronchioles
  • Alveolar ducts
  • Alveoli

Anatomically, the structure of the respiratory system is divided as follows:

  • Upper respiratory tract
  • Lower respiratory tract

Air Passage Through the Upper Respiratory Tract

The upper respiratory tract includes the head and neck region. It consists of:

  • Nose
  • Pharynx
  • Larynx


The nose consists of an external nose and nasal cavity. The nasal cavity consists of tiny hairs or cilia. The air enters through the nostrils to the nasal cavity. The nasal cavity cleans, warms, and dampens the air that enters through it. The cilia protect the nasal passageways and other parts of the respiratory tract by filtering out dust and other particles that enter the nose during breathing.

Nose anatomy
Nose anatomy


Nasopharynx, oropharynx, and laryngopharynx are parts of the pharynx. The pharynx is part of the digestive system and the respiratory system because it carries food and air. The end of the pharynx divides into two pathways.

  • Pathway for food (esophagus)
  • Pathway for air (trachea)

The epiglottis is a small flap of tissue that covers the air passage during swallowing, preventing food and liquid from entering the lungs. The air enters these regions before entering the larynx.


This is the passageway between the pharynx and the lower respiratory structures. It is made up of supportive cartilage, ligaments, muscle, and mucosal lining. It contains a pair of vocal cords, which can produce sound by vibration.

The function of the supportive cartilage is to prevent the food and water-entering windpipe while swallowing.

Pharynx and larynx anatomy
Pharynx and larynx anatomy

Air Passage Through the Lower Respiratory Tract

The lower respiratory tract is divided as follows:2

  • Trachea
  • Bronchial tree
  • Lungs 
Respiratory system anatomy
Respiratory system anatomy


The trachea is also called the windpipe. It is approximately 10–12 cm long and runs through the lower neck and chest. The tracheal walls are made up of hyaline cartilage.

The tracheal walls help the trachea to remain open for the air to enter.

 Bronchial Tree

The bronchial tree consists of three parts.

  • Primary bronchi
  • Secondary bronchi
  • Tertiary bronchi

The trachea splits into right and left primary bronchi. Once the primary bronchi entered the lungs, they split into secondary bronchi, leading to tertiary bronchi. There are eight bronchial segments in the left lung and ten bronchial segments in the right lung to conduct the air.

The air from larynx enters the trachea and primary bronchi before it enters the lungs.


Lungs are cone-shaped breathing organs. There is a pair of lungs, a left lung, and a right lung, which occupies most of the space in the chest cavity along with the heart. The substance of the lungs is light, porous, and spongy, and it floats in water. It is highly elastic.3 The lungs are divided into lobes or sections separated by dividers. The right lung has three lobes and the left lung has two lobes.

There are about ten pyramidal-shaped areas separated by membranes in each lung. The lungs can also be divided into smaller segments called ‘bronchopulmonary segments.’ A double-layered membrane called ‘pleurae’ covers each lung. The pleurae have two layers, namely ‘visceral pleura’ and ‘parietal pleura.’ The space between these two membranes is called ‘pleural cavity’ and is filled with serous fluid produced by the pleura.

Lung anatomy
Lung anatomy

Vascular Nature of Lungs

Lungs receive a large supply of blood and hence are vascular in nature. The artery that supplies blood to the lungs is the pulmonary artery, which comes directly from the heart.

Absorption of Oxygen into the Blood in the Lungs

After repeated branching of the tertiary bronchioles in the lungs, it finally forms thin-walled air sacs called ‘alveoli.’ Each alveolus is covered with a network of blood vessels called capillaries. These capillaries are branches of the pulmonary artery.

In the alveoli, the exchange of gases takes place.4 The air in the alveoli and the blood in the capillaries are very close to each other to facilitate the exchange of gases. The blood supplied to the lungs is low in oxygen and rich in carbon dioxide. The alveoli are rich in oxygen. The presence of a thin barrier between air and capillaries allows oxygen to move from the alveoli into the blood and allows carbon dioxide to move from the blood in the capillaries into the alveoli. After the gases are exchanged, the blood, now rich in oxygen, travels back through the pulmonary veins to the left side of the heart. From the heart, the blood is pumped to all parts of the body. The carbon dioxide, which has reached the alveoli, is then exhaled.

The Breathing Process

The lungs’ main function is ‘inhalation’ (breath in) and ‘exhalation’ (breath out). The lungs, diaphragm, ribs, intercostal muscles (muscles present between the ribs), and pleural membrane work together to perform the inhalation and exhalation process. The diaphragm is a muscle sheet extending across the chest. At rest, it is curved like a dome. During inhalation, the diaphragm contracts and flattens, expanding the chest to draw more air into the lungs.

These inhalation and exhalation processes are controlled by ‘the breathing center in the brain,’ which response to the levels of carbon dioxide and hydrogen ion (acid) in the blood. Conditions of lower or higher air pressure are created in the thoracic cavity by stimulating the diaphragm and the intercostals muscles. If the pressure in the thoracic cavity is lower than the pressure of the air in the environment, then the air rushes into the lungs, and this is called the negative pressure breathing. If the air pressure in the thoracic cavity is higher than the air pressure in the environment, air rushes out of the lungs. To equalize the pressure, air moves from areas of high pressure to the areas of low pressure. 


Respiration is essential for life, as one cannot survive without breathing even for a few minutes. Hence, it is essential to maintain healthy lungs by breathing unpolluted air as much as possible and providing the body with good nutrition.


1. AMA’s Current Procedural Terminology, Revised 1998 Edition.

2. Available at: Accessed on: 13th April 2008.

3. Gray H. Anatomy of the Human Body. Available at: Accessed on: 14th April 2008.

4. Van De Graaff, Kent M. Human Anatomy. Burr Ridge, IL: McGraw Hill Publishing,2002.