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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Intercostal Muscles and Diaphragm
Breathing is something we do without thinking, but it's actually a complex process involving several muscle groups. The two main muscle groups that power our breathing are the intercostal muscles and the diaphragm. These muscles work together to change the volume of our thoracic (chest) cavity, which creates pressure differences that move air in and out of our lungs.
Key Definitions:
Breathing involves two main muscle groups:
These muscles work together to change the volume of your chest cavity, creating pressure differences that move air in and out of your lungs.
Every cell in your body needs oxygen to release energy from glucose through respiration. Breathing also helps remove carbon dioxide, a waste product of respiration. Without efficient gas exchange, cells would quickly run out of energy and accumulate toxic waste products.
The intercostal muscles are layers of muscle tissue that run between adjacent ribs. There are two main types:
Location: Outer layer between ribs
Direction: Fibres run downward and forward
Function: Contract during inhalation to pull the ribs upward and outward, increasing the volume of the thoracic cavity
Location: Inner layer between ribs
Direction: Fibres run downward and backward
Function: Contract during forced exhalation to pull the ribs downward and inward, decreasing the volume of the thoracic cavity
The diaphragm is a dome-shaped sheet of muscle that forms the floor of the thoracic cavity. It's the most important muscle for breathing.
The diaphragm consists of:
When relaxed, the diaphragm forms a dome that extends up into the thoracic cavity. When it contracts, it flattens and moves downward.
The diaphragm is responsible for about 75% of the air movement during normal, quiet breathing. The intercostal muscles contribute the remaining 25%. However, during heavy exercise, the intercostal muscles play a much larger role.
Inhalation is an active process that requires muscle contraction:
During normal, quiet breathing, exhalation is largely a passive process:
During forced exhalation (like when exercising or blowing up a balloon), the internal intercostal muscles and abdominal muscles contract to force more air out.
Inhalation:
• Diaphragm contracts
• External intercostals contract
Exhalation:
• Diaphragm relaxes
• External intercostals relax
• Internal intercostals contract (forced)
Inhalation:
• Thoracic cavity volume increases
• Lungs expand
Exhalation:
• Thoracic cavity volume decreases
• Lungs recoil
Inhalation:
• Pressure in lungs decreases
• Air flows in
Exhalation:
• Pressure in lungs increases
• Air flows out
Scientists and doctors measure various aspects of breathing to assess lung function:
Normal resting rate: 12-20 breaths per minute
During exercise: Can increase to 40-60 breaths per minute
Breathing rate increases during exercise to supply more oxygen to working muscles and remove more carbon dioxide.
Definition: The amount of air moved in and out during normal breathing
Average value: About 500 ml in adults
Tidal volume increases during exercise as the breathing muscles work harder to move more air with each breath.
Asthma: During an asthma attack, the airways narrow and breathing becomes difficult. People with asthma often use their accessory breathing muscles (in the neck and shoulders) to help force air in and out. The intercostal muscles may visibly retract between the ribs during an attack as they work harder to pull air in.
Chronic Obstructive Pulmonary Disease (COPD): In conditions like emphysema, the lungs lose their elasticity. This means patients must rely more heavily on their intercostal muscles and diaphragm to force air out, rather than letting the natural elastic recoil do the work. Over time, the diaphragm can become flattened and less effective.
Regular exercise leads to adaptations in the breathing muscles:
Olympic swimmers often have exceptional control over their breathing muscles. They can expand their lungs to about 12 litres of air - nearly twice the capacity of an average person! This helps them stay underwater longer and maintain efficient breathing patterns during races.
To summarize the role of the intercostal muscles and diaphragm in breathing:
Understanding how these muscles work together helps explain how we breathe and how our bodies adapt to different situations like exercise, high altitude, or respiratory diseases.
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