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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Breathing Mechanisms
Breathing is something we do without thinking, about 20,000 times a day! But how exactly does air move in and out of our lungs? In this section, we'll explore the amazing mechanisms that allow us to breathe and how our body controls this vital process.
Key Definitions:
Before we dive into how breathing works, let's look at the key parts of the breathing system:
The respiratory tract is divided into two main sections:
Air travels through these structures during breathing, getting warmed, moistened and filtered along the way.
The lungs are the main organs of respiration. They are:
Breathing involves two phases: inhalation (breathing in) and exhalation (breathing out). Both processes rely on pressure changes in the thoracic cavity.
Inhalation is an active process that requires muscular effort. Here's what happens:
The diaphragm contracts and flattens, moving downward. The external intercostal muscles contract, pulling the ribcage up and out.
These muscle movements increase the volume of the thoracic cavity, creating more space for the lungs to expand into.
As volume increases, the pressure inside the lungs decreases (below atmospheric pressure), causing air to rush in from outside.
Exhalation is usually a passive process during normal breathing. Here's what happens:
The diaphragm relaxes and moves upward (dome shape). The external intercostal muscles relax, allowing the ribcage to move down and in.
These movements decrease the volume of the thoracic cavity, reducing the space available for the lungs.
As volume decreases, the pressure inside the lungs increases (above atmospheric pressure), forcing air out.
During exercise or when we need more oxygen, we use forced breathing. In forced exhalation, the internal intercostal muscles and abdominal muscles actively contract to push more air out of the lungs. This makes exhalation an active process rather than passive.
The lungs are surrounded by two thin membranes called pleura:
Between these membranes is a thin space called the pleural cavity, which contains a small amount of pleural fluid. This fluid:
While we can consciously control our breathing to some extent, most of the time it's regulated automatically by the brain.
Breathing is controlled by the respiratory centre in the medulla oblongata of the brain. This centre:
Several factors can change how fast and deep we breathe:
Asthma is a common condition that affects the breathing mechanisms we've discussed. During an asthma attack:
These changes narrow the airways, making it harder to breathe out (exhalation becomes difficult). People with asthma often use inhalers containing bronchodilators that relax the muscles around the airways, making breathing easier.
Scientists and doctors measure different aspects of breathing using a device called a spirometer. Here are some important measurements:
Key measurements include:
Normal breathing rates vary by age:
During exercise, breathing rate can increase to 40-60 breaths per minute to meet the body's increased oxygen demand.
Breathing is a complex but beautifully coordinated process that involves:
Understanding these mechanisms helps us appreciate how our body maintains the constant gas exchange needed for survival and how problems with these mechanisms can lead to respiratory disorders.
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