Mechanism of breathing
<p>Learn about Mechanism of breathing in this comprehensive lesson.</p>
Overview
Breathing, also known as ventilation, is the process of moving air into and out of the lungs to facilitate gas exchange. It involves two main phases: inspiration (inhalation) and expiration (exhalation). These processes are driven by changes in pressure within the thoracic cavity, which are brought about by the coordinated action of the diaphragm and intercostal muscles. The mechanism of breathing is a physical process that relies on the principle of pressure gradients. When the volume of the thoracic cavity increases, the pressure inside the lungs decreases, causing air to rush in. Conversely, when the volume decreases, the pressure increases, forcing air out. This continuous cycle ensures a fresh supply of oxygen to the alveoli and the removal of carbon dioxide from the body. Understanding the specific muscle actions and the resulting pressure changes during both inspiration and expiration is crucial for IGCSE Biology. This topic often involves recalling anatomical structures and their physiological roles in maintaining efficient gas exchange.
Key Concepts
- Breathing (Ventilation): The mechanical process of moving air into and out of the lungs.
- Inspiration (Inhalation): The active process of taking air into the lungs.
- Expiration (Exhalation): The passive or active process of expelling air from the lungs.
- Diaphragm: A dome-shaped muscle that contracts and flattens during inspiration.
- External Intercostal Muscles: Muscles that contract to pull the rib cage upwards and outwards during inspiration.
- Internal Intercostal Muscles: Muscles that contract during forced expiration to pull the rib cage downwards and inwards.
- Thoracic Cavity: The chest cavity containing the lungs and heart.
- Pressure Gradient: The difference in pressure between two areas, causing air to move.
- Elastic Recoil: The natural tendency of the lungs and chest wall to return to their original size after being stretched.
- Atmospheric Pressure: The pressure exerted by the air surrounding the body.
- Intrapulmonary Pressure: The pressure within the lungs (alveoli).
Introduction to Breathing (Ventilation)
Breathing, or ventilation, is the mechanical process of moving air into and out of the lungs. It is essential for gas exchange, where oxygen is taken into the bloodstream and carbon dioxide is expelled.
There are two main phases of breathing:
- Inspiration (Inhalation): The process of taking air into the lungs.
- Expiration (Exhalation): The process of expelling air from the lungs.
These processes are involuntary, controlled by the respiratory centre in the brain, but can be consciously influenced to some extent (e.g., holding your breath).
Muscles Involved in Breathing
The primary muscles responsible for breathing are:
- Diaphragm: A large, dome-shaped sheet of muscle located at the base of the thoracic cavity, separating it from the abdominal cavity.
- Intercostal Muscles: Muscles located between the ribs.
- External Intercostal Muscles: Contract during inspiration, pulling the rib cage upwards and outwards.
- Internal Intercostal Muscles: Contract during forced expiration, pulling the rib cage downwards and inwards.
Mechanism of Inspiration (Inhalation)
Inspiration is an active process that requires muscle contraction.
- Diaphragm contracts: It flattens and moves downwards.
- External intercostal muscles contract: They pull the rib cage upwards and outwards.
- Thoracic cavity volume increases: Both the vertical and horizontal dimensions of the chest cavity increase.
- Lung volume increases: The lungs, being elastic, expand to fill the increased volume of the thoracic cavity.
- Intrapulmonary pressure decreases: As lung volume increases, the pressure inside the lungs becomes lower than the atmospheric pressure.
- Air rushes in: Air flows from the higher atmospheric pressure into the lower pressure in the lungs until the pressures equalize.
Mechanism of Expiration (Exhalation)
Expiration is typically a passive process at rest, but can be active during forced exhalation.
At Rest (Passive Expiration):
- Diaphragm relaxes: It returns to its dome shape and moves upwards.
- External intercostal muscles relax: The rib cage moves downwards and inwards due to gravity and the elastic recoil of the chest wall.
- Thoracic cavity volume decreases: Both the vertical and horizontal dimensions of the chest cavity decrease.
- Lung volume decreases: The elastic recoil of the lungs causes them to contract.
- Intrapulmonary pressure increases: As lung volume decreases, the pressure inside the lungs becomes higher than the atmospheric pressure.
- Air rushes out: Air flows from the higher pressure in the lungs to the lower atmospheric pressure until the pressures equalize.
Forced Expiration (Active Expiration):
- Internal intercostal muscles contract: They pull the rib cage further downwards and inwards.
- Abdominal muscles contract: They push the diaphragm further upwards.
- These actions further decrease the thoracic and lung volume, creating a larger pressure gradient and forcing more air out.
Pressure Changes During Breathing
The mechanism of breathing is entirely dependent on pressure gradients. Air always moves from an area of higher pressure to an area of lower pressure.
| Phase | Diaphragm Action | Intercostal Muscles Action | Rib Cage Movement | Thoracic Volume | Lung Volume | Intrapulmonary Pressure | Air Flow |
|---|---|---|---|---|---|---|---|
| Inspiration | Contracts, flattens | External contract | Upwards & Outwards | Increases | Increases | Decreases (below atm) | Into lungs |
| Expiration | Relaxes, domes | External relax | Downwards & Inwards | Decreases | Decreases | Increases (above atm) | Out of lungs |
Exam Tips
- •Clearly distinguish between inspiration and expiration, detailing the muscle actions and resulting volume/pressure changes for each.
- •Remember that inspiration is always an active process, while expiration is passive at rest but can be active during forced breathing.
- •Use diagrams to help visualize the movement of the diaphragm and rib cage, and how this affects thoracic volume.
- •Practice explaining the pressure changes: increased volume leads to decreased pressure, and vice-versa.
- •Be precise with terminology: 'contract' vs. 'relax', 'upwards/outwards' vs. 'downwards/inwards'.