Oxygen is used by organisms to indirectly break down simple molecules like amino acids, glucose, and fatty acids, etc., to procure energy to perform many activities in our body.

Carbon dioxide is harmful to our body which is why it is released during the break down of the simple molecules. Thus, we can say that oxygen is the need for the organism and carbon dioxide needs to be released out of the body.

This process of exchange of oxygen from the environment with carbon dioxide produced by our cells is called breathing. It is also known as respiration.

When you keep your hand on the chest, you must feel the chest going up and down. We know that it is due to the process of breathing.

But how do we breathe?

Respiratory organs

The animal kingdom is big and different animals breathe through different mechanisms depending on the habitat in which they live and the level of organization they show in their body structure and functions.

Exchange of gases in lower organisms

Lower invertebrates like flatworms, coelenterates, and sponges exchange gases by simple diffusion with the help of their body surface.

Earthworms make use of their moist cuticle for exchange.

Insects generally have tracheal tubes ( a network of tubes) for the transportation of atmospheric air in the body.

Gills or branchial respiration is specialized vascular structures used mostly by aquatic animals.

Pulmonary respiration with the help of lungs is done by most of the terrestrial animals.

Among vertebrates, reptiles, mammals, and birds respire through the lungs.

Amphibians do cutaneous respiration and respire with the help of their moist skin. These animals have gills too and can breathe both on water and land.

Human respiratory system

Nostrils – a pair of them are present on the external part of the face just above the lips. It leads to a nasal cavity through the nasal passage.

Pharynx – The nasal chamber opens into the common passage of food and air, called pharynx.

Larynx – It is a cartilaginous structure which looks like a box and helps in the production of sound.

Trachea – It is a straight tube-like structure that extends till the mid-thoracic cavity which further divides into right and left bronchi.

Bronchi – Bronchi also undergoes divisions and form the secondary and tertiary bronchi and bronchioles and are supported by incomplete cartilaginous rings.

Alveoli – The bronchioles terminate into several thin irregularly walled and bag like structures called alveoli.

Lungs – There are two lungs in our body covered by a pleural layer which envelops the pleural fluid between them.

•  The function of the fluid is to reduce the friction on the surface of the lungs. The outer pleural layer is in close contact with the lining of thorax whereas the inner pleural lining remains in close contact with the surface of the lung.
• The portion starting from the nostrils to the bronchioles makes the conducting part and the portion comprising of alveoli, and their ducts make the respiratory system.
• The function of conducting part is to transport the air to the alveoli and make the air clean. It also gives humidity to the dry air and brings it to the level of body temperature.
• The respiratory part of this system is involved in the diffusion of oxygen and carbon dioxide between blood and atmospheric air.
• The lungs are located in the air-tight thoracic chamber. The thoracic chamber is made of sternum, ribs, and diaphragm.

Steps of the process of respiration

• Pulmonary ventilation or breathing – oxygen is brought in, and carbon dioxide is released out in the atmosphere.
• Diffusion of carbon dioxide and oxygen across the membrane of alveoli.
• Transportation of gases through blood.
• Diffusion of gases between tissues and blood.
• Oxygen is utilized by cells, and carbon dioxide is released.

The mechanism involved in breathing

• There are two stages of breathing:
  • Inspiration – taking the air inside.
  • Expiration – Expulsion of air out in the atmosphere.
  • There is a pressure gradient that is built between the lungs and the atmosphere which results in the movement of air in and out of the body.
• Inspiration occurs when the pressure in the lungs is less than the pressure in the atmosphere which means there is a negative pressure in the body (lungs) with respect to pressure in the atmosphere.

• Expiration occurs when the pressure in the lungs is more than the pressure in the atmosphere.

• Diaphragm – Diaphragm with some specialized muscles, namely internal intercostals, situated between the ribs, assist in the generation of pressure gradients. The contraction of diaphragm increases the volume of the thoracic cavity and initiate inspiration. The ribs and sternum are lifted and increases the volume of the thoracic cavity. The overall increase in volume causes an increase in pulmonary volume.
• It leads to a reduction in pressure inside the lungs which forces the air from the atmosphere to move into the lungs.
• Relaxation of the diaphragm and the muscles bring the diaphragm and sternum in their original position resulting in a decrease in thoracic volume and thus the pulmonary volume.
• It leads to an increase in pressure in the lungs more than the pressure outside and causes the expulsion of air from the lungs.
• We can increase the strength of expiration and inspiration with the help of muscles in the abdomen.
• The average human breathing rate is 12-16 times per minute.
• The instrument used to estimate the volume of air during breathing is called spirometer and helps doctors to assess the functioning of the lungs.