Physiology of the respiratory system refers to the act of respiration, also known as breathing or pulmonary ventilation. Pulmonary ventilation involves repetitive cycles of inhalation and exhalation, in which movement of oxygen happens from the outside environment to the cells within tissues, followed by the removal of carbon dioxide in the opposite direction. A pressure difference between air pressure within the lungs and the air outside of the body causes air to flow in and out of the lungs.
Common Respiratory Terms:
- Eupnoea: respiratory rate at rest
- Bradypnoea: decreased respiratory rate
- Tachypnoea: increased respiratory rate
- Apnoea: temporary cessation of breathing
- Dyspnoea: difficulty breathing
- Orthopnoea: difficulty breathing when laying down
- Respiratory Arrest: permanent breathing cessation
- Hyperventilation: fast breathing rate in which Carbon Dioxide is expelled faster than it is produced, lowering the Carbon Dioxide level in the blood leading to an increase in the blood pH
- Hypoventilation: slow breathing rate in which Carbon Dioxide in the blood is increased due to not expelling it at the same rate as it is produced
Normal Respiratory Rates:
- Adults: 12-20 breaths per minute
- Children: 18-30 breaths per minute
- Infants (up to 1 year): 30-60 breaths per minute
Respiratory muscles used in quiet inspiration:
- Diaphragm – lowers to increase the thoracic cavity depth
- External Intercostal Muscles – elevate the ribs leading to widening of the thoracic cavity
Respiratory muscles used in forced inspiration:
- Sternocleidomastoids and Pectoralis Minor – elevate the sternum and ribs leading to the widening of the thoracic cavity
- Scalenes – elevate ribs 1 and 2 leading to the widening of the thoracic cavity
- Internal Intercostals (part of) – elevate the ribs leading to widening of the thoracic cavity
Respiratory muscles used in quiet expiration:
- Diaphragm
- Thoracic Cage
- Lung Elasticity
Respiratory muscles used in forced expiration:
- Internal Intercostals (part of)
- Rectus Abdominis
- Internal and External Obliques
- Transversus Abdominis
Neural Breathing Control
Breathing happens through repetitive brain stimuli within the medulla oblongata:
Inspiratory neurons activate during quiet and forced inspiration, firing impulses leading to the diaphragm (through the phrenic nerve) and the external intercostal muscles (through the intercostal nerves) contracting. The inspiratory muscles relax when the inspiratory neurons stop firing, causing expiration.
Expiratory neurons activate during forced expiration.
The Respiratory Rate is affected by the Respiratory Centres, namely:
- Central Chemoreceptors – found in the medulla oblongata
- Peripheral Chemoreceptors – found in major blood vessels
These respond to the changes in Oxygen, Carbon Dioxide levels and pH of the blood. For example, if there is a decrease in Oxygen level, an increase in Carbon Dioxide level and a decrease in the blood pH, the Respiratory Rate is automatically increased so as to compensate for the lack of Oxygen.
Breathing can be manipulated through the cerebral cortex, which sends impulses to the diaphragm and intercostal muscles, bypassing the medulla oblongata and pons in the process. However, an increase in Carbon Dioxide level reduces the ability to control breathing manipulation.
The Pressure Gradient
The pressure gradient is the difference between the atmospheric pressure (pressure of the outside air) and the intrapulmonary pressure (pressure within the lungs). Pressure and resistance work together in determining airflow.
During inspiration, the rib cage elevates and the diaphragm depresses and flattens, leading to an increase in the thoracic volume, causing the intrapulmonary pressure to fall when compared to atmospheric pressure. Thus, air flows into the lungs.
During exhalation, the rib cage descends and the diaphragm rises in the form of a dome. Lungs recoil to a smaller volume, which causes the intrapulmonary pressure to increase when compared to atmospheric pressure. Thus, air flows out of the lungs.
Resistance to airflow depends on:
- Thoracic Wall Compliance – if the thoracic wall tissues are non-compliant, the thoracic cavity doesn’t increase, which inhibits the lungs to increase in size during inhalation
- Bronchial Diameter – bronchoconstriction causes resistance to airflow
- Alveolar Surface Tension – alveoli walls are lined by a thin film of water that creates tension at their surface
Respiratory Volumes
Tidal Volume (TV) is the volume of air inspired or expired in a normal respiratory cycle.
Inspiratory Reserve Volume (IRV) is the maximum volume of air that can be inspired during forced respiration. This does not include the tidal volume (forced inspiration amount).
Expiratory Reserve Volume (ERV) is the maximum volume of air that can be expired during forced respiration. This does not include the tidal volume.
Residual Volume (RV) is the volume of air left in the lungs following forced expiration. RV allows gas exchange to happen between respiratory cycles, allowing the alveoli to stay inflated.
Respiratory Capacity refers to the combination of more than one volume.
Total Lung Capacity (TLC) is the combination of all lung volumes:
Tidal Volume + Expiratory Reserve Volume + Inspiratory Reserve Volume + Residual Volume = Total Lung Capacity
Vital Capacity (VC) is the amount of air an individual can move in or out of the lungs:
Tidal Volume + Expiratory Reserve Volume + Inspiratory Reserve Volume = Vital Capacity
Inspiratory Capacity (IC) is the total amount of air that can be inhaled:
Tidal Volume + Inspiratory Reserve Volume = Inspiratory Capacity
Functional Residual Capacity (FRC) is the amount of air remaining in the lung following a normal tidal expiration:
Expiratory Reserve Volume + Residual Volume = Functional Residual Capacity
A PEFR measures Forced Expiratory Volume (FEV), which is the maximum amount of air that can be forcefully exhaled in one second.
Below you can find a collection of videos that can help provide a more visual approach to the physiology of the respiratory system.
Physiology of the Respiratory System – Animation
https://www.youtube.com/watch?v=kacMYexDgHg
Physiology of the Respiratory System – Animation
Lung Anatomy & Physiology
Breathing Control
Gas Exchange
Respiratory Volumes – Spirometry
Special thanks to the creators of the featured videos on this post, specifically Youtube Channels Alila Medical Media and RegisteredNurseRN.
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