Inspiration and Expiration — Revision Notes

⚡ 30-Second Revision

Inspiration: — Active. Diaphragm contracts (flattens), External intercostals contract (ribs up/out). Thoracic volume $\uparrow$. $P_{pul}$ $\downarrow$ (below $P_{atm}$). Air in.
Expiration (Quiet): — Passive. Diaphragm relaxes (domes), External intercostals relax (ribs down/in). Elastic recoil. Thoracic volume $\downarrow$. $P_{pul}$ $\uparrow$ (above $P_{atm}$). Air out.
Forced Expiration: — Active. Internal intercostals contract, Abdominal muscles contract. Thoracic volume $\downarrow\downarrow$. $P_{pul}$ $\uparrow\uparrow$. More air out.
Boyle's Law: — $P \propto 1/V$.
Intrapleural Pressure ($P_{ip}$): — Always negative (e.g., $-4,\text{mmHg}$ at rest, $-6,\text{mmHg}$ during inspiration). Keeps lungs inflated.

2-Minute Revision

Breathing involves inspiration (air in) and expiration (air out), driven by pressure gradients created by thoracic volume changes. Inspiration is an active process: the diaphragm contracts and flattens, while external intercostal muscles contract, pulling the rib cage up and out.

This increases thoracic cavity volume, which, by Boyle's Law, decreases intrapulmonary pressure below atmospheric pressure, causing air to rush into the lungs. Quiet expiration is typically passive: inspiratory muscles relax, and the elastic recoil of the lungs and chest wall reduces thoracic volume.

This increases intrapulmonary pressure above atmospheric pressure, expelling air. Forced breathing involves accessory muscles: sternocleidomastoid and scalenes for forced inspiration, and internal intercostals and abdominal muscles for forced expiration.

Intrapleural pressure remains negative, preventing lung collapse and facilitating lung expansion.

5-Minute Revision

The mechanics of breathing are governed by Boyle's Law, stating that gas pressure is inversely proportional to its volume. Air moves from high to low pressure. Inspiration is an active process. It begins with the contraction of the diaphragm (flattens, increasing vertical thoracic dimension) and external intercostal muscles (pull ribs up and out, increasing anterior-posterior and lateral dimensions).

This combined action significantly increases thoracic cavity volume. Due to the negative intrapleural pressure, the lungs are pulled along, their volume increases, and consequently, the **intrapulmonary pressure ($P_{pul}$) decreases** below atmospheric pressure ($P_{atm}$).

This pressure gradient ($P_{atm} > P_{pul}$) drives air into the lungs. For forced inspiration, accessory muscles like sternocleidomastoid and scalenes further elevate the rib cage.

Expiration during quiet breathing is a passive process. The inspiratory muscles (diaphragm and external intercostals) relax. The diaphragm returns to its dome shape, and the rib cage moves down and in due to gravity and elastic recoil of the lungs and chest wall.

This leads to a decrease in thoracic cavity volume. The lungs compress, causing the **intrapulmonary pressure ($P_{pul}$) to increase** above atmospheric pressure ($P_{atm}$). This pressure gradient ($P_{pul} > P_{atm}$) forces air out of the lungs.

Forced expiration is an active process, involving the contraction of internal intercostal muscles (pull ribs down and in) and abdominal muscles (push diaphragm up), further reducing thoracic volume and expelling more air.

The intrapleural pressure (pressure in the pleural cavity) is always negative, typically $-4,\text{mmHg}$ at rest, becoming more negative (e.g., $-6,\text{mmHg}$) during inspiration and less negative (e.

g., $-2,\text{mmHg}$) during expiration, crucial for keeping the lungs expanded.

Prelims Revision Notes

1
Breathing Mechanics: — Air moves due to pressure gradients. Governed by Boyle's Law ($P \propto 1/V$).
2
Atmospheric Pressure ($P_{atm}$): — Pressure outside the body, approx. $760,\text{mmHg}$ at sea level.
3
Intrapulmonary Pressure ($P_{pul}$): — Pressure inside alveoli. Fluctuates.
4
Intrapleural Pressure ($P_{ip}$): — Pressure in pleural cavity. Always negative (sub-atmospheric). Approx. $-4,\text{mmHg}$ at rest. Keeps lungs inflated.

Inspiration (Inhalation):

Nature: — Active process (requires muscle contraction & ATP).
Primary Muscles:

* Diaphragm: Contracts, flattens, moves downwards. Increases vertical thoracic dimension. * External Intercostals: Contract, pull ribs upwards and outwards. Increases anterior-posterior & lateral thoracic dimensions.

Thoracic Volume: — Increases.
Lung Volume: — Increases (lungs pulled by expanding chest wall).
Intrapulmonary Pressure ($P_{pul}$): — Decreases (e.g., to $759,\text{mmHg}$ or $-1,\text{mmHg}$ relative to $P_{atm}$). $P_{pul} < P_{atm}$.
Intrapleural Pressure ($P_{ip}$): — Becomes more negative (e.g., to $-6,\text{mmHg}$). Helps lung expansion.
Airflow: — Air rushes IN from atmosphere to lungs.
Accessory Muscles (Forced Inspiration): — Sternocleidomastoid, Scalenes, Pectoralis minor. Further elevate rib cage.

Expiration (Exhalation):

Nature (Quiet): — Passive process (no active muscle contraction, relies on elastic recoil).
Nature (Forced): — Active process (requires muscle contraction & ATP).
Primary Muscles (Quiet): — Diaphragm and External Intercostals RELAX.
Thoracic Volume: — Decreases (due to muscle relaxation and elastic recoil of lungs/chest wall).
Lung Volume: — Decreases.
Intrapulmonary Pressure ($P_{pul}$): — Increases (e.g., to $761,\text{mmHg}$ or $+1,\text{mmHg}$ relative to $P_{atm}$). $P_{pul} > P_{atm}$.
Intrapleural Pressure ($P_{ip}$): — Becomes less negative (e.g., to $-2,\text{mmHg}$). Lungs recoil.
Airflow: — Air rushes OUT from lungs to atmosphere.
Accessory Muscles (Forced Expiration): — Internal Intercostals (pull ribs down & in), Abdominal Muscles (push diaphragm up).

Key Points:

Elastic recoil is crucial for passive expiration.
Surfactant reduces alveolar surface tension, preventing collapse.
Airway resistance and lung compliance affect breathing effort.

Vyyuha Quick Recall

To remember the primary muscles of quiet breathing: Don't Expect Inspiration to be Passive.

Diaphragm
External Intercostals
Inspiration
Primary (active)