Have you ever thought, why does the chest move during breathing? 

During inhalation, the diaphragm, which is a sheet of dome-shaped muscle that separates the chest and abdominal cavities, contracts and moves down. It increases the space in the chest cavity. Along with this, the muscles between the ribs also contract to pull the rib cage. It results in the upward and outward movement of ribs. During exhalation, the exact opposite happens. All these results in the movement of the chest during breathing.

Breathing practice is considered effective in fighting anxiety, stress and depression by psychologists. The process of exchange of oxygen from the atmosphere with carbon dioxide produced by the cells is called breathing.

Let’s understand the processes of breathing. 

Table of Contents

• Mechanism of Breathing
• Difference Between Inspiration and Expiration
• Abdominal Breathing
• Thoracic Breathing
• Hering - Breuer Reflex
• Practice Problems of Mechanism of Breathing
• FAQs of Mechanism of Breathing

Mechanism of Breathing

Breathing or ventilation, is the process of movement of air in and out from the lungs to bring about the exchange of gases. This movement of air into and out of the lungs is brought about by creating a pressure gradient between the lungs and the atmosphere. 

Pressure Gradient

The pressure difference which creates the flow of gases from their higher pressure region to the lower pressure region is called the pressure gradient.

Respiratory movements in humans are carried out mainly with the help of intercostal muscles and diaphragm.

Phases in Breathing

Breathing involves two phases : 

• Inspiration 
• Expiration 

Inspiration

It is the process during which the atmospheric air is taken in. Inspiration in mammals is an active process. It occurs due to the following reasons: 

Contraction of the Diaphragm

There is contraction of the muscle fibres (phrenic muscles) of the diaphragm, by which it becomes flat and is lowered down. Due to this there is increase in volume of thoracic cavity in anterior-posterior axis.

Contraction of External Intercostal Muscles

The contraction of the external intercostal muscles helps in the movements of the ribs and sternum. Sternum and ribs move upwards and outwards. This causes an increase in volume of thoracic cavity in the dorso-ventral axis.

Combined effect of contraction of muscle fibres of the diaphragm and external intercostal muscles leads to increase in the volume of pulmonary cavity. This leads to a decrease in the pressure within the pulmonary cavity, than the atmospheric pressure. Due to this atmospheric air enters into lungs which is called Inhalation.

Expiration

Process by which the alveolar air from the lungs is released is called expiration. It is a passive process due to relaxation of external intercostal muscles and muscle fibres of the diaphragm. It brings the thoracic cavity to its original position.

Relaxation of Diaphragm

During relaxation of phrenic muscles, the diaphragm is pulled up and the volume of the thoracic cavity decreases in the longitudinal direction.

Relaxation of External Intercostal Muscles

During relaxation of the external intercostal muscles, the bibcage is moved inwards and downwards. Due to this, the volume of the thoracic cavity decreases in the dorsoventral direction. In forceful expiration, internal intercostal muscles and some abdominal muscles also contract.

Difference Between Inspiration and Expiration

Abdominal Breathing

Abdominal breathing involves breathing majorly with the diaphragm. When the diaphragm goes flat down during inhalation, it exerts a pressure on the abdominal cavity, making it move out. Abdominal breathing is seen during quiet breathing, relaxed times like rest and sleep etc. A person can also concentrate and breathe almost exclusively with the diaphragm while sitting quietly. This is a breathing technique that helps to destress a person and has many health benefits.

Thoracic Breathing

It is indicated by an upward and outward movement of the chest. It is seen during situations like vigorous exercise, or in emergency conditions. If a person constantly does thoracic breathing, it can make the body tense. So, it is better to practice abdominal breathing. The rate of breathing in a normal healthy person is 12 - 16 times/min.

Hering - Breuer reflex 

Stretch receptors that are located on the walls of bronchi and bronchioles. These are stimulated by overstretching of the alveoli of the lungs. The nerve impulses are directed along the vagus nerve to inhibit the respiratory area. As a result there is an initiation of the expiration. It is considered as a protective mechanism which prevents the excessive inflation of the lungs. 

Practice Problems of Mechanism of Breathing

Q1. A man suffers a puncture in his chest cavity due to an accident without any damage to the lungs. It can cause 

• reduced breathing rate
• rapid increase in breathing rate
• no change in respiration
• cessation of breathing

Solution:  Puncture in the chest cavity (thoracic cavity) leads to the elimination of the pressure gradient which is essential for breathing due to which breathing would cease. Hence, the correct option is d.

Q2. Mark the correct statement from the following with reference to normal breathing.

• Inspiration is considered as a passive process and expiration is considered as an active process
• Inspiration is considered as an active process but expiration is considered as a passive process
• Inspiration and expiration are active processes
• Inspiration and expiration are passive process

Solution: During inspiration, the muscles of the diaphragm as well as external intercostal muscles contract and use energy for contraction. Hence inspiration is called an active process. Expiration happens due to the relaxation of the diaphragm (that contracted during inspiration) and external intercostal muscles. As muscles relax during expiration, they do not require energy. Hence expiration is a passive process. So the correct option is b.

Q3. Thoracic breathing takes place during

i) Emergency situation

ii) Vigorous exercise

iii) Rest and sleep

• Only ii is correct
• Only i is correct
• ii and iii are correct
• i and ii are correct

Solution: Thoracic breathing is the breathing primarily using external intercostal muscles,

during emergency situations or vigorous exercises. Hence, the correct option is d.

Q4. During heavy breathing, which of the following muscles contract, in case of exhalation?

1. Diaphragm 
2. External intercostal muscles
3. Internal intercostal muscles
4. Sternocleidomastoid

Solution: Heavy breathing occurs during exercise or when we suffer from illness. During heavy breathing, more muscles are employed than quiet breathing. During exhalation while heavy breathing, the internal intercostal muscles contract. Hence, the correct option is c.

FAQ’s of Mechanism of Breathing

Question 1.- Name the structure involved in creating a pressure gradient between the atmosphere and the lungs in normal respiration.

Answer. Contraction of diaphragm - This makes the space in the thoracic cavity bigger in the longitudinal (upward-downward) direction along the anterior-posterior axis. 

Contraction of external intercostal muscles - The expansion of the thoracic cavity in the dorso-ventral direction is mediated by the expansion (upward and outward movement) of the ribcage. It is due to the contraction of muscles associated with ribcage, i.e., external intercostal muscles, which elevate the ribs and sternum. Relaxation of the diaphragm and external intercostal muscles during expiration increases the pressure of air inside the lungs and air is expelled out.

Question 2.- Which condition of intrapulmonary and atmospheric pressure is required for inspiration?

Answer. A pressure gradient is required to guide the air movement into and out of the lungs. The air movement that occurs between the lungs and the atmosphere is carried out by the pressure gradient created between them. Negative pressure in the lungs with respect to the atmosphere can result in the movement of atmospheric air into the lungs which is called inspiration. So, inspiration can occur only, when intrapulmonary pressure is less than that of the atmospheric pressure. 

Question 3.- During inspiration, what are the changes seen in the lume in the lungs, and the intrapulmonary pressure?

Answer. During inspiration, the diaphragm contracts because of which it gets pulled down and the thoracic cavity expands in the longitudinal direction. The contraction of the external intercostal muscles results in the expansion of the chest in the upwards and outwards direction. The expansion of the chest in the upwards and outwards direction, results in the expansion of the thoracic cavity in the dorsoventral direction. The thoracic cavity expansion increases the volume inside the lungs. Boyle’s law states that for gases at a constant temperature, the volume is inversely proportional to pressure. During inspiration, when the volume inside the lungs increases, the intrapulmonary pressure (pressure inside the lungs) decreases and falls below the atmospheric pressure. Hence, air enters into the lungs.

Question 4.- During expiration, what are the changes seen in the volume in the lungs, and the intrapulmonary pressure?

Answer. During expiration the volume of the chest cavity decreases and the intrapulmonary pressure increases more than the atmospheric pressure.

Related Topics to Mechanism of Breathing in Biology

• Lung Volumes and Capacities
• Respiration
• Breathing and Exchange of Gases

NCERT Class 11 Biology Chapters