🔹 Muscles Involved in Breathing

Two main muscle groups help in breathing:

1. Diaphragm

• A large, dome-shaped muscle.
• Located between the chest cavity (thorax) and abdominal cavity.
• Made of skeletal muscle, which means we can control it — like when we hold our breath.

2. Intercostal Muscles

• These are muscles located between the ribs.
• There are two types:
  • External intercostal muscles: Used during inhalation.
  • Internal intercostal muscles: Used during forceful exhalation (like blowing or coughing).

🔸 Inhalation (Inspiration)

Let’s understand what happens when you breathe in:

1. Diaphragm contracts:
   • It moves downward and becomes flat.
   • This increases the space inside your chest.
2. External intercostal muscles contract:
   • This raises the ribs up and out.
   • The rib cage expands — like opening an umbrella.
3. Thoracic cavity becomes larger:
   • This increases the volume of the chest.
4. Pressure inside lungs drops:
   • When chest volume increases, pressure inside decreases.
   • It becomes lower than outside air pressure.
5. Air rushes into the lungs:
   • Air always moves from high pressure to low pressure.
   • So, outside air flows into the lungs to fill the low-pressure space.

🧠 Example:
Open a sealed cold drink bottle. You hear a “psst” sound because air rushes into the low-pressure area inside the bottle — this is what happens during inhalation.

🔸 Exhalation (Expiration)

Now let’s see what happens when you breathe out:

1. Muscles relax:
   • Diaphragm goes back to dome shape.
   • Intercostal muscles relax, ribs move down and inward.
2. Thoracic cavity becomes smaller:
   • Volume decreases inside the chest.
3. Pressure inside lungs increases:
   • When volume decreases, pressure rises above atmospheric pressure.
4. Air is pushed out:
   • Because the pressure inside the lungs is now higher, air flows out naturally.

📌 Important Note: Normal exhalation is usually a passive process — muscles just relax.

🧠 Example:
Think of squeezing a sponge. When you press it (like exhalation), water (air) is pushed out. When you stop pressing, it expands again (like inhalation).