# Mechanism of Ventilation (Breathing) The process of breathing, or ventilation, involves the movement of air into and out of the lungs. This is not achieved by the lungs themselves but by the coordinated action of the *diaphragm* and the *intercostal muscles*, which alter the volume and pressure within the thoracic cavity. This mechanism follows Boyle's Law, where pressure and volume are inversely proportional. ### Muscles of Ventilation - **Diaphragm:** A large, dome-shaped sheet of skeletal muscle that separates the thoracic (chest) cavity from the abdominal cavity. - **Intercostal Muscles:** Two sets of muscles located between the ribs. - *External Intercostals:* Fibres run diagonally forward. - *Internal Intercostals:* Fibres run diagonally backward, opposite to the external intercostals. ### Phases of Breathing Breathing is a cyclical process consisting of two main phases: inspiration and expiration. #### Inspiration (Inhalation) This is the *active phase* of breathing, where air is drawn **into** the lungs. - **Muscle Action:** - The *diaphragm contracts* and flattens (becomes less dome-shaped). - The *external intercostal muscles contract* while the *internal intercostal muscles relax*. - **Resulting Movement:** The rib cage moves **upward and forward**. - **Volume and Pressure Change:** The actions of the muscles **increase the volume** of the thoracic cavity. This increase in volume leads to a **decrease in pressure** (intra-alveolar pressure) inside the lungs, making it lower than the atmospheric pressure outside the body. - **Air Movement:** Air flows from the area of higher atmospheric pressure to the area of lower pressure inside the lungs, causing them to inflate. #### Expiration (Exhalation) This is typically the *passive phase* of breathing during rest, where air is forced **out of** the lungs. - **Muscle Action:** - The *diaphragm relaxes* and returns to its dome shape. - The *external intercostal muscles relax*. - **Resulting Movement:** The rib cage moves **downward and backward** due to gravity and the elastic recoil of the lungs. - **Volume and Pressure Change:** The actions of the muscles **decrease the volume** of the thoracic cavity. This decrease in volume leads to an **increase in pressure** inside the lungs, making it higher than the atmospheric pressure. - **Air Movement:** Air is forced out of the lungs, from the area of higher pressure to the area of lower atmospheric pressure. **Forced Expiration:** During heavy exercise, expiration becomes an active process. The *internal intercostal muscles* and *abdominal muscles* contract to forcefully decrease thoracic volume. ### Comparison of Inspiration and Expiration | Feature | Inspiration (Inhalation) | Expiration (Exhalation) | | :--- | :--- | :--- | | **Process Type** | Active | Passive (Active during exercise) | | **Diaphragm** | Contracts and flattens | Relaxes and becomes dome-shaped | | **External Intercostals** | Contract | Relax | | **Internal Intercostals** | Relax | Contract (during forced expiration) | | **Rib Cage Movement** | Upward and forward | Downward and backward | | **Thoracic Volume** | Increases | Decreases | | **Thoracic Pressure** | Decreases (below atmospheric) | Increases (above atmospheric) | | **Air Movement** | Into the lungs | Out of the lungs | ### Role of Pleural Membranes The lungs are enclosed by a double-layered serous membrane called the pleura. The space between the layers contains pleural fluid, which reduces friction and creates surface tension, ensuring the lungs expand as the thoracic cavity expands. ### Diagram of Breathing Mechanism ### Control of Breathing (Ventilation) Breathing is regulated by both involuntary and voluntary systems. #### Involuntary Control Primarily managed by the *breathing centre* located in the **medulla** of the brain. - ***Inspiratory Centre:*** The ventral portion, which increases the rate and depth of inspiration. - ***Expiratory Centre:*** The dorsal and lateral portions, which inhibit inspiration and stimulate expiration. The rate of breathing is primarily influenced by the concentration of $CO_2$ and $H^+$ ions in the blood, detected by chemoreceptors. #### Voluntary Control The **cerebral cortex** allows for conscious control, such as intentionally changing the rate and depth of breathing or holding one's breath. - **Limitation:** This control is limited. If you hold your breath, blood $CO_2$ levels rise and $O_2$ levels drop. This triggers the involuntary system to take over, forcing a breath, which may occur after losing consciousness. #### Emotional Influence Emotions, acting through the **limbic system** of the brain, can also affect the respiratory centre, causing changes in breathing patterns (e.g., rapid breathing when scared or excited).