Respiratory System

The respiratory system is a biological system consisting of organs and processes that makes gas exchange between living things and the external environment possible.

The respiratory system is dependent on the anatomy and physiology of the organism and the environment in which it lives. For example, most land animals, except for birds, make use of lungs, and air sacs known as alveoli, while birds use the atria.

The air sacs have a constant supply of blood, which brings enables it to pass oxygen into the blood. The air sacs engage with the external environment through a system of airways or hollow tubes, the largest being the trachea, which branches in the middle of the chest to form a bronchus.

The bronchi enter the lungs, where they divide into smaller branches called bronchioles. These bronchioles are known as parabranchioles in birds. Air from the external environment is pumped into the air sacs or atria by the rhythmic movement of the rib cage, a process known as breathing.

The respiratory system in most fish and other aquatic animals consists of gills. These gills are either partial or complete external organs, and they are constantly bathed with water.

The water flows over the gills by active means such as when the animal is in motion, or by passive means like when the animal is at rest. The exchange of gases occurs in the gills, which are made up of thin, flat filaments and lamellae, which has a very large surface area for gas exchange.

Animals like insects, arthropods, and molluscs have respiratory systems consisting of simple anatomical features such as spiracles and lung books. The skin of amphibians plays an essential role in the exchange of gases.

Plants utilize tiny holes in their leaves for the exchange of gases through a process called diffusion.

The Human Respiratory System

The human respiratory system consists of the air passages, the lungs, pulmonary vessels, and breathing muscles, helps the body to exchange gases between the air and blood, and between the blood and the body’s plethora of cells.

Most of the organs that make up the respiratory system help in the redistribution of air, but only the tiny alveoli (air sacs) and the alveolar ducts are responsible for actual gas exchange.

The respiratory system is divided into two parts:

Upper respiratory tract

This includes the nose, mouth, and pharynx. The nose is a protruding part of the face with a hollow internal area known as the nasal cavity. It is divided into two equal parts by a bony wall called the nasal septum.

Both cavities open into the pharynx. The nose enables oxygen from the external environment to pass into the lungs. The pharynx is a cavity that connects the nose and mouth with the larynx and esophagus.

It is also known as the throat. The pharynx has both digestive and respiratory purposes. As part of the respiratory system, the pharynx enables the movement of air from the nasal and oral cavities to the larynx in the process of breathing.

Lower respiratory tract

This consists of the trachea, the bronchi, bronchioles, and the lungs.

The organs are located in the chest cavity. They are defined and protected by the ribcage, the sternum (chest bone), and the muscles between the ribs and the diaphragm.

The trachea, also known as the windpipe, is a 4 inch long and an inch wide tube, beginning from under the voice box (larynx) and running down the breastbone. The trachea connects the throat to the bronchi.

The trachea divides into two bronchi, each into leading into the left and right lung, respectively. Each bronchus divides into smaller bronchi as they get closer to the lung tissue and become bronchioles. The bronchioles evolve into the tiny air sacs called alveoli, which is the site of gaseous exchange.

In the process of exchanging gases, oxygen is absorbed from the air in the air sacs into the blood capillaries. The pumping action of the heart circulates the oxygenated blood through all the tissues in the body.

At the same time, carbon dioxide, a waste product of respiration, is transported from the blood capillaries into the air sacs. This is then expelled through the bronchi and then the upper respiratory tract.

The lungs provide a surface area for the exchange of gases to take place. It consists of the bronchial tree, which further divides into smaller air tubes, each branch ending in a pulmonary alveolus.

How do we breathe?

Breathing occurs in two stages; inhalation and exhalation. Inhalation is the intake of air into the lungs via expansion of the chest volume, while exhalation is the discharge of air from the lungs through the contraction of the chest volume.

The muscles involved in inhalation and exhalation are the rib muscles (muscles between the ribs in the chest) and the diaphragm muscle. The constant relaxing and contracting of these muscles (occurring approximately 16 times a minute) controls the breathing process.

The following occurs during inhalation;

  • Contraction of the diaphragm muscles which causes it to flatten, thus enlarging the chest cavity.
  • Contraction of the rib muscles, which makes the ribs rise, thus increasing chest volume.
  • Expansion of the chest which reduces air pressure and causes air to be drawn into the lungs.

The following actions take place during exhalation;

  • Relaxation of the muscles which results in a decrease in chest volume.
  • Contracts of the chest cavity, thus increasing air pressure and causing the air in the lungs to be released through the upper respiratory tract.

Inhalation and exhalation are involuntary process and require control to function.

Diseases of the Respiratory system

Common diseases that affect the respiratory system include:


This is a condition that causes the airways to swell, become narrow, and produce too much mucus. It makes it difficult to breathe and triggers intense coughing fits, wheezing, and shortness of breath.


It is a contagious disease typically caused by the bacterium Mycobacterium tuberculosis. It usually affects the lungs but can also affect other parts of the body, such as the brain, kidney, and spine.


This causes permanent thickening and enlargement of the airways in the lungs. Symptoms of bronchiectasis include shortness of breath, chronic cough with mucus production, chest pain, and coughing up blood.

Lung cancer

This condition is also known as lung carcinoma. Lung cancer is a malignant tumor resulting in uncontrolled cell growth of the tissues of the lung. It can be triggered by smoking or other heavy metals from the environment.


This is an inflammation of the air sacs in one or both lungs. It causes the air sacs to be filled with fluid or cough. Pneumonia can be caused by a variety of organisms, such as bacteria, viruses, and fungi. Symptoms include fever, coughing with phlegm, chills, and difficulty breathing.

Chronic obstructive pulmonary disease

This long-term condition that exacerbates over time. It includes bronchitis and emphysema.

Cystic fibrosis

This is a genetic condition that affects the lungs and digestive system. It makes the body to produce thick, sticky mucus that can clog the lungs and obstructs the pancreas. The condition can be fatal, and people with the condition tend to have a shorter life span than average.

Idiopathic pulmonary fibrosis

This is a chronic lung disease characterized by scarring of the lungs. It causes an irreversible decline in lung function. The symptoms of idiopathic pulmonary fibrosis include a dry cough and shortness of breath.


This is an abnormal collection of inflammatory cells that form lumps called granulomas. The condition begins in the lungs, skin, or lymph nodes. Symptoms of the disease are dependent on the organ it affects.

Pleural effusion

This occurs when excess fluid builds up between the layers of the pleura outside the lungs. Pleura are thin membranes that line the lungs and the inside of the chest cavity. They help to lubricate and facilitate breathing. Symptoms include sharp pain in the chest and shortness of breath.