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Inspiration: Definition, Process, Muscles Involved, Practice Problems and FAQs

Inspiration: Definition, Process, Muscles Involved, Practice Problems and FAQs

You might have noticed that a deep breath can often help in reducing anxiety and relieving tension. Breathing is an integral part of staying alive. Can you control your breathing? You may be able to hold your breath for a few seconds or even a minute but not beyond that. You must have noticed that you breathe unconsciously even when you are sleeping. It is an involuntary action which can be voluntarily controlled only to a certain extent. Breathing is crucial to life because it helps in taking in fresh oxygen for respiration and throwing out the waste carbon dioxide produced as a result of respiration.

Have you ever focussed on how you breathe? Take a long, deep breath in and breathe out. When you inhale deeply, your lungs are completely filled with air coming in from your nose and your chest rises along with your abdomen. Just the opposite happens when you breathe out. The process of breathing in and breathing out are termed as inspiration and expiration, respectively. Both inspiration and expiration exhibit movements of the chest and the abdomen which are assisted by the contraction and relaxation of some involuntary muscles of the body. In this topic we will learn in detail about the process of inspiration and the muscles involved in this. So let’s start.

Table of Contents

The respiratory passage

Air enters the respiratory tract via the nostrils (the external nares). It then travels through the nasal cavities and passes through the pharynx (common chamber for air and food). It is then led into a second chamber called the larynx (also called the sound-box). Food also passes via the pharynx on its way to the food pipe (oesophagus). In order to ensure that the food doesn’t accidentally enter the larynx, the cartilaginous flap called the epiglottis covers the larynx while swallowing food. Now, after the larynx, air passes to the trachea (windpipe), which eventually divides into two bronchi, each one leading to a lung. The bronchi then branch into smaller bronchi, the terminal branches of which are called bronchioles. The bronchioles are surrounded by clusters of sac-like structures called alveoli.

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Fig: Respiratory tract

Thoracic cavity

Thoracic cavity is a cavity enclosed by vertebral column, ribs, and sternum.Dorsally it is covered by the vertebral column, ventrally by the sternum, laterally by the ribs and on the lower side by the diaphragm.

Pleural space refers to the area between the inner thoracic wall and the outside surface of the lungs. Usually filled with pleural fluid, this creates a seal that uses surface tension to retain the lungs against the thoracic wall. This seal makes sure that the lungs expand or contract in proportion to the size of the thoracic cavity.

This anatomical setup of lungs in thorax is such that any change in the volume of the thoracic cavity will be reflected in the lung (pulmonary) cavity. Such an arrangement is essential for breathing, as we cannot directly alter the pulmonary volume.

Fig: Thoracic cavity

Boyle’s law

According to Boyle's law, pressure and gas volume are inversely related (when temperature is constant). Therefore:

When the volume increases the pressure decreases and when the volume decreases the pressure increases.

The muscles' contraction and relaxation during breathing alters the thoracic cavity's volume. The volume of the lungs changes as the thoracic cavity and lungs move together, which also affects the pressure inside the lungs.

Process of Inspiration

Inspiration is the process of drawing air into the lungs from outside the body. Inspiration is accomplished by establishing a pressure differential between the lungs and the atmosphere. Muscles involved in the process of inspiration are diaphragm and external intercostal muscles.

Contraction of the diaphragm

Diaphragm is a dome-shaped muscle that separates thoracic cavity and abdominal cavity. It is also known as the phrenic muscle. It gets signals via the phrenic nerve and blood supply from phrenic arteries. When diaphragm contracts and is pulled down it causes the expansion of the thoracic cavity in longitudinal (up down) direction.

Fig: Movement of diaphragm

You can think of the bottle as the chest cavity, and the rubber sheet as the diaphragm.

When we pull the rubber sheet down, the volume in the bottle is lowered. Similarly, when the diaphragm contracts it is pulled down , thoracic cavity volume rises in longitudinal direction.

Fig: Demonstration of inspiration

Contraction of external intercostal muscles

External intercostal muscles are the muscles present between the ribs. Contraction of these muscles causes the expansion of the rib cage by movement of ribs in upwards and outwards direction. This results in the expansion of the thoracic cavity in the dorso-ventral (front-back) direction.

Fig: Inspiration

When the volume in the chest cavity increases, the lungs being highly elastic get space to expand to fill the increased space in the thoracic cavity. When the volume in the lungs increases, the pressure in the lungs, also called intrapulmonary pressure, decreases. The intrapulmonary pressure decreases and goes below the atmospheric pressure. Thus, a pressure gradient created between the atmosphere and the lungs

In other words, the lungs have acquired a negative pressure compared to the atmosphere as a result air rushes into the lungs.

Fig: Movements during inspiration

Practice Problems

Q1. Inspiration occurs when the pressure within the lungs is

  1. less than the atmospheric pressure
  2. greater than the atmospheric pressure
  3. equal to the atmospheric pressure
  4. Both a and c

Solution: Inspiration is a process in which air is drawn into the lungs.

During inspiration, the diaphragm and the external intercostal muscles contract because of which the chest expands. Due to the expansion of the thoracic cavity (chest cavity), the volume inside the lungs increases. So when the volume increases, the pressure decreases and goes below the atmospheric pressure. Thus, during inspiration the pressure within the lungs is less than that of the atmospheric pressure. As a result of this the atmospheric air moves into the lungs from the atmosphere. Thus, the correct option is a.

Q2.What is the correct passage of air during inspiration?

  1. Larynx → Trachea → Bronchiole → Bronchi → Alveoli
  2. Larynx → Pharynx → Trachea → Bronchiole → Bronchi → Alveoli
  3. Pharynx → Trachea → Bronchiole → Bronchi → Alveoli
  4. Pharynx → Larynx → Trachea → Bronchi → Alveoli

Solution: When we inhale ,the air enters our body through our nose travelling down the nasal cavity and passes the pharynx then to the larynx, from larynx air moves into the trachea. From trachea air enters into two smaller airways: the left and right primary bronchi, bronchi enters into the lungs. Left lung is served by one bronchial tube, and the right lung is served by the other. Within lungs air passes from bronchi into much smaller airways the bronchioles. From bronchioles air moves into the little air sacs at the end of the bronchioles, known as alveoli, where oxygen diffuses from the inhaled air into the blood in the capillaries. Thus, the correct option is d.

Q3. During inspiration, the diaphragm ___________ and the expansion of the thoracic cavity is in the __________ direction.

  1. contracts, dorso-ventral
  2. contracts, longitudinal
  3. relaxes, dorso-ventral
  4. relaxes, longitudinal

Solution: The diaphragm is a dome-shaped muscular structure that separates the thoracic cavity and the abdominal cavity. Inspiration is a process in which the air is drawn into the lungs. During inspiration, the diaphragm contracts because of which it gets pulled down and the thoracic cavity expands in the longitudinal direction. Thus, the correct option is b.

Q4. During inspiration, the volume in the lungs _________ and the intrapulmonary pressure _________.

  1. increases, increases
  2. decreases, increases
  3. increases, decreases
  4. decreases, decreases

Solution: During inspiration, the diaphragm contracts because of which it gets pulled down and the thoracic cavity expands in the longitudinal direction.The external intercostal muscles also contract because of which the chest expands upwards and outwards. As the chest expands upwards and outwards, the thoracic cavity expands in the dorsoventral direction.

Due to the expansion of the thoracic cavity, the volume inside the lungs increases.

It is known from Boyle’s law 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. Thus, the correct option is c.


Q1. What is abdominal breathing?
Abdominal breathing, or breathing with your belly, involves breathing mostly with your diaphragm. When the diaphragm goes down during breathing in, it exerts a pressure on the abdominal cavity, and so the abdomen moves out. This is what mostly happens during quiet breathing, during relaxed times like rest and sleep.One can also concentrate and breathe almost exclusively with the diaphragm while sitting quietly. This is a breathing technique that helps to destress and has several health benefits.

Q2. What is the Hering - Breuer reflex ?
There are stretch receptors located on the walls of bronchi and bronchioles which are stimulated when the alveoli of the lungs are overstretched. These receptors send nerve impulses along the vagus nerve to the brain to inhibit further increase in the respiratory area. This phenomenon is known as the Hering - Breuer reflex. It results in the initiation of the expiration and protects the lungs from excessive inflation.

Q3. What is inspiratory capacity?
Inspiratory capacity is the volume of air that can be taken into the lungs by maximal inspiration after a normal inspiration.

Q4. When does thoracic breathing take place?
Thoracic breathing is characterised by an upward and outward movement of the chest. This type of breathing usually occurs during vigorous exercise, or emergency situations. This breathing primarily uses external intercostal muscles.I f we constantly use chest breathing, it can make the body tense, as if it is under stress.

YOUTUBE LINK: https://youtu.be/B6kdUqeAaEk (0:00 - 32:47)


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