Human Respiratory System: Mechanism, Diagram, and Function

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What is Human Respiratory System?

o Lungs, sac like organ where the air moves in and out to the response in difference of pressure.

o When air pressure is reduced in the alveolar space, the inspiration takes place that is we breathe in air. And vice versa when the pressure exceeds in the lungs, expiration takes place.

o Pressure difference also results in flow of air to be rapid or slow.

o Air flow remains constant but can be understood by the level of how much atmospheric pressure is below or above the requirement.

o These pressure fluctuation in the alveolar space occurs due to tension and relaxation of chest muscles and abdomen resulting in expansion and contraction of lungs.

o Little addition in the expansion increases the space in the lung enclosing air. This results in the falling of per unit volume of air in the lungs so does the pressure falls.

o To equilibrise the air pressure of lungs with respect to lungs the air flow is continued and to restore the balance.

o Similarly, when the muscles are relaxed from the inspiration, air is compressed in the lung, pressure rises above the atmospheric pressure and so the air is moved out of lungs to achieve the equilibrium in the original lung volume.

o This cycle with this sequence of events occurs during normal respiratory mechanism.

o The lung volume changes this leads to either flow of air out of lung or to flow of air into the lung.

Respiratory System Diagram

Respiratory System, Respiratory System Function, Respiratory System Diagram, Respiratory System Organs, the Respiratory System 1

Respiratory System Mechanism and Lung System

o Volume of chest and lungs system changes due to the muscle contraction along with the elastic properties.

o Lungs resists the stretch to a limit to prevent the tendency of collapse by the pressure difference compared to outside and inside.

o Pressure in the lung equals the atmospheric pressure when the pressure measured in pleural space is below atmospheric pressure.

o This pressure below atmospheric pressure is a measure that helps to keep lungs expanded.

o When this pleural pressure reduces more the lungs are stretched for increasing the volume by air through inspiration.

o When air is inhaled the force also increases rapidly and similarly decreases when air is expelled.

o Therefore, the pleural pressure shows the force that is needed to keep the lung distended against elastic recoil and force that is needed for airflow.

o The force is provided by the chest and diaphragm that are stretched inwards to pull the lung for maintaining the inflation of lung and for airflow.

o Lung and chest system are like coiled spring opposing each other.

Role of Muscles in Respiration

o Respiratory muscle changes the direction of lung and chest by displacing the elastic forces by the muscle contraction.

o During inspiration, muscles contract that gives outward force to the chest that increases the additional stretch to the lungs.

o During expiration the muscles relax, lungs return to the original position by the additional refraction.

o Abdominal muscles are responsible for the equilibrium displacement.

o During contraction the abdominal muscles retracts the lungs by increasing the abdominal pressure that results in the rising the diaphragm which causes forceful expiration.

o When lung volume is restored, the muscular force is removed and relaxes the abdominal force.

Lung Volume

o Residual Volume (RV): Total amount of air left after the total expiration from lungs.

o Tidal Volume (TV): Total amount of air in a normal respiratory cycle that can be inhaled or exhaled. This helps in the mechanism of respiratory muscles, lungs and chest walls.

o Inspiratory Reserve Volume (IRV): Amount of air that can inhaled forcibly even after tidal volume. This happens during deep breathing. Value is between 1900 – 3300 mL for a normal adult.

o Expiratory Reserve Volume (ERV): Amount of air that can be exhaled forcibly even after tidal volume. Value between 700 – 1200 mL for a normal adult. ERV reduces in obesity or upper abdominal surgery.

Lung Capacity

o Inspiratory Capacity (IC): The maximum amount of air inhaled after resting capacity.


o Total Lung Capacity (TLC): The volume of air after total inspiration.


o Vital Capacity (VC): The amount of air that can be exhaled after total inhalation.


o Functional Residual Capacity (FRC): Total volume of air present in lungs after normal/passive expiration. This is also balancing the chest expansion and are equal by representing when lungs tissue would recoil.


Breathing Mechanism

o Lung and chest system is stretched by the energy used in breathing that result in airflow.

o Respiratory pump can increase the pressure with the lungs by Valsalva manoeuvre (forceful contraction between the abdominal muscles and chest with no space in the vocal cord).

o The brief Valsalva manoeuvre can suddenly open the larynx during cough.

o The high speed of air an effective way for clearing the foreign particles.

o Cilia i.e., hairline projections help in maintaining steady flow of the secretion from nose when the cough is unable to keep the speed with the rate of secretions produced.

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Human Respiratory System Citations
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