Human respiratory system:

Respiration is the process by which energy is released from food in our body. The function of respiratory system is to breathe in oxygen for respiration producing energy from food, and to breathe out carbon dioxide. With the help of lungs gases are exchanged between the blood and the air. Gases exchanged are oxygen and carbon dioxide. We can live without food and water for many days but we cannot live for more than a few minutes without air, as it is necessary for breathing. So, before discussing the human respiratory system, it is necessary to know the process of breathing which is an important part of respiration.

Process of Breathing:

During breathing we take air in to our lungs through the nose, and then expel it. The taking in of air rich in oxygen in to the body during breathing is called inhalation and giving out of the air rich in carbon dioxide is called exhalation. Both the process takes place regularly during breathing. Therefore, a breath means ‘one inhalation plus one exhalation’.


Mechanism of breathing is as follows:

In this we will learn how air from outside is sucked in to our lungs during inhalation and how air from lungs is pushed out during exhalation and this happens with the help of lungs. Lungs are connected to our nostrils (holes in the nose) through nasal passage and wind pipe. When we inhale air, it enters our nostrils, passes through nasal passage and windpipe, and reaches our lungs. Our two lungs hang in an airtight space in our body called ‘chest cavity’. Around the side of the chest cavity is the rib cage with sheets of muscles between the ribs which encloses the lungs in it. At the bottom of the chest cavity is a curved sheet of muscle called diaphragm. Therefore, it forms the floor of the chest cavity. So, breathing involves the movement of the rib cage and the diaphragm.

This happens as follows:

(a) Breathing in: When we breathe in or inhale, two things happen at the same time:

(i) The muscles between the ribs contract causing the rib cage to move upward and outward.

(ii) The diaphragm contracts and moves downward.

All these movement increases the space in the chest cavity and make it larger and air is sucked in from outside in to the lungs. As a result, the lungs filled up with air and expand.

(b) Breathing out: When we exhale or breathe out two things happen simultaneously:

(i) The muscles between the ribs relax causing the rib cage to move downward and inward.

(ii) The diaphragm relaxes and moves upward.

Due to these movements the space in our chest cavity decreases and makes it smaller, which pushes air out from the lungs.

Respiratory System

Many organs take part in the process of respiration in Humans. Various organs are: Nose, Nasal passage or Nasal cavity, Trachea, Bronchi, Lungs and Diaphragm.

Respiratory system begins from the nose. Our nose has two holes known as nostrils and the passage behind the nostrils are called nasal passage or nasal cavity. The air for respiration is drawn in to our body through the nostrils present in the nose. This air goes in to the nasal passage. It is separated from the mouth cavity or buccal cavity or oral cavity by a hard, bony palate so that we can breathe in air even when we are eating food. The nasal passage is lined with fine hair and mucus. Mucus is secreted by the glands inside the nasal passage. When air passes through the nasal passage, the dust particles and other impurities present in are trapped by nasal hair and mucus so that clean air goes in to the lungs. Pharynx is the part of throat between the mouth and wind pipe. So, air enters from nasal passage to pharynx and then goes in to the wind pipe (trachea).

Now air passes through trachea, it does not collapse even when there is no air in it because it is supported by rings of soft bones called cartilage. The upper end of the trachea has a voice box called larynx. The trachea runs down the neck and divides in to two smaller tubes called ‘bronchi’ at its lower end and these are connected with the lungs. The lungs lie in the chest cavity or thoracic cavity which is separated from abdominal cavity by a muscular partition called diaphragm. The lungs are covered by two thin membranes called pleura. The lungs are also enclosed in a ‘rib cage’ made of bones called ‘ribs’.

The singular of bronchi is bronchus and they divide in the lungs to form a large number of still smaller tubes called ‘bronchioles’. They have tiny pouch like air sacs at their ends called ‘alveoli’ (singular alveolus). Walls of alveoli are very thin and surrounded by blood capillaries. From alveoli oxygen is taken in to the body and carbon dioxide is eliminated. So, in alveoli gaseous exchange takes place.

Millions of alveoli are there in lungs which provide a large area for the exchange of gases. Availability of large area maximises the exchange of gases. Diaphragm helps in ‘breathe in’ and ‘breathe out’ .The muscles of chest also help in breathing in and out. Now, we will see How Respiratory System works:

When we breathe in air, the diaphragm and muscles attached to the ribs contract due to which our chest cavity expands. This movement increases the volume inside the chest cavity. Due to this, the air pressure decreases inside the chest cavity and air from outside being at high pressure rushes in to the lungs through the nostrils, trachea and bronchi.

In this way, during the process of ‘breathing in’ the air sacs or alveoli of the lungs get filled with air containing oxygen. And alveoli are surrounded by blood capillaries due to which oxygen of air diffuses out from the alveoli walls in to the blood. The oxygen is carried by a red pigment called haemoglobin present in blood to all the parts of the body. As the blood passes through the tissues of the body, the oxygen present in it diffuses in to the cells due to its higher concentration in the blood. This oxygen combines with the digested food or glucose present in the cells to release energy. Carbon dioxide is produced as a waste and diffuses in to the blood due to its higher concentration in body tissues. Blood carries carbon dioxide back to the lungs where it diffuses in to the alveoli. When we breathe out air, the diaphragm and the muscles attached to the ribs relax due to which our chest cavity contracts and become smaller. This contraction movement of the chest pushes out carbon dioxide from the alveoli of the lungs in to the trachea, nostrils and then out of the body in to air. The process of gaseous exchange is completed in this way.

We should know that during the breathing cycle, when air is taken in or inhaled and let out or exhaled, the lungs always contain a certain residual volume of air so that there is sufficient time ‘for the oxygen to be absorbed’ in to the blood and ‘for carbon dioxide to be released’ from the blood.

Rate of Breathing

Breathing occurs involuntarily but the rate of breathing is controlled by the respiratory system of brain. The average breathing rate in an adult man at rest is about 15 to 18 times per minute. During physical exercise breathing rate increases which supplies more oxygen to body cells for producing more energy.

The deficiency of haemoglobin in the blood of a person reduces the oxygen-carrying capacity of blood resulting in breathing problems, tiredness and lack of energy. The person looks pale and loses weight. During carbon monoxide poisoning carbon monoxide binds very strongly with haemoglobin in the blood and prevents it from carrying oxygen to the brain and other parts of the body. If carbon monoxide is inhaled for a long time, then the person becomes unconscious and can even die due to oxygen starvation. The persons having breathing problems are given oxygen masks to facilitate breathing. In serious case, the patient is put on a machine called ‘ventilator’ in which a tube is inserted directly in to the trachea or wind pipe of the patient to help him in breathing comfortably.