Breathing and Respiration in Humans

We can live without food for several days but we cannot live without breathing even for a short while. Breathing provides oxygen to the cells of our body for oxidation of food in order to generate energy for various activities.

Respiration may be divided into two steps.

  1. Breathing involves inhalation of air containing oxygen and exhalation of carbon dioxide.

  2. Cellular respiration is responsible for release of energy by oxidation of food (glucose), and its conversion into ATP (adenosine triphosphate) - The energy module.

Respiration is different from breathing.

Breathing is the physical process of respiratory gaseous exchange between the organism and the environment by diffusion. It takes place in the lungs. On the other hand, respiration involves oxidation of food and release of energy which takes place in the cells along with respiratory gaseous exchange.

Respiratory System

Respiratory system of human beings has the following parts:

  • External nares or nostrils.
  • Nasal cavities inside the nose.
  • Internal nostrils opening into pharynx.
  • Pharynx that leads into the wind pipe or trachea.
  • Trachea divides into two bronchi (one bronchus) which lead into the two lungs.

The opening of the pharynx into the trachea is called glottis. Trachea is thin walled but its walls do not collapse even when there is not enough air in it as it is supported by rings of cartilage. Trachea bifurcates into bronchi.

Lungs enclose within them branches of bronchi called bronchioles which branch further and end in very thin walled sac-like structures called air sacs or alveoli.

The voice box or larynx is present on the trachea.

Mechanism of Breathing or Ventilation of Lungs

Lungs are located in the chest cavity or the thoracic cavity. Below the chest cavity is the abdominal cavity. These two cavities are separated from each other by a dome-shaped (upwardly arched) muscular sheet called diaphragm. The movement of diaphragm helps in breathing. Breathing, also called ventilation involves two processes - inhalation and exhalation.

1. Inhalation

Inhalation (drawing the air inwards) is the result of increase in the volume of the thoracic cavity. This increase is caused by the changes that take place in the position of diaphragm and ribs.

  • Diaphragm straightens out due to contraction of its muscles.

  • Ribs are raised upward and outward and volume of chest cavity enlarges by contraction of rib muscles.. As volume of chest increases pressure of air in it decreases.

  • Atmosphereic air rushes in and reaches the alveoli. It brings in oxygen which diffuses into the capillaries from the alveoli.

2. Exhalation

Exhalation is the result of decrease in the volume of the thoracic cavity. This decrease in the volume is caused when:

  • Diaphragm relaxes and resumes its dome shape, arching upwards.

  • Ribs are lowered downwards and inwards.

  • Thoracic cavity is compressed and the pressure inside the lungs is increased.

  • The alveolar carbon dioxide diffuses out and is pushed out through the trachea and nose.

  • This breathing out of carbon dioxide laden air is called exhalation.

Breathing Rate

When at rest, an adult human breathes about 16 to 18 times per minute. Breathing rate increases during physical exercise, disease, fever, pain and under stress.

Exchange of gases between blood and tissues

Inhalation fills in the alveoli of lungs with oxygenated air. This oxygen has to reach the various tissues of the body. Thus as the first step, blood capillaries on alveoli pick up oxygen from alveoli and carbon dioxide brought by the capillaries from the tissues is exchanged for oxygen. Oxygen diffuses into alveoli.

In the tissues, oxygen gets used up and carbon dioxide is accumulated which is now exchanged for oxygen in blood. The carbon dioxide picked up by blood from tissues is carried to the heart by veins.

Cellular Respiration

Once inside the tissues, oxygen acts upon the digested food (glucose) which has reached the cells of the tissues. As a result energy and carbon dioxide are released. This occurs in the mitochondria of the cells and is called cellular respiration.

Why mountaineers and sea divers carry oxygen cylinders and wear oxygen masks?

As we climb higher and higher altitudes, the air pressure becomes lower and lower. Reduced oxygen supply causes breathing troubles and oxygen masks facilitate breathing. People living in hilly areas have evolved adaptation such as increased number of red blood corpuscles and large thoracic cavity. Divers carry oxygen masks because we derive our respiratory oxygen from air and not water.