Welcome to Breathing and Gas Exchange!

Have you ever wondered why you huff and puff after running a race, or why your breath can fog up a cold window? In this chapter, we explore the mechanics of how we get air in and out of our bodies, how the air changes once it's inside us, and how our brain keeps track of it all during exercise. Don't worry if it seems like a lot to take in—we'll break it down step-by-step!

Note: This chapter focuses on the process of breathing and air composition. For details on the structure of the lungs and alveoli, see the "Gas exchange surfaces and the breathing system" chapter.

1. Inspired and Expired Air

The air we breathe in (inspired air) is different from the air we breathe out (expired air). Our bodies take what they need and dump what they don't!

Comparing the Composition

Here is how the air changes:

  • Oxygen: Higher in inspired air (approx. \(21\%\)) and lower in expired air (approx. \(16\%\)) because it is used for respiration.
  • Carbon Dioxide: Very low in inspired air (approx. \(0.04\%\)) and much higher in expired air (approx. \(4\%\)) because it is a waste product of respiration.
  • Water Vapour: Variable in inspired air, but always saturated (very high) in expired air because water evaporates from the moist linings of the lungs.
  • Nitrogen: Remains roughly the same (approx. \(78\%\)) because our bodies do not use it from the air.

Testing for Carbon Dioxide

We use limewater to prove that expired air contains more carbon dioxide than inspired air.
1. When you bubble inspired air through limewater, it stays clear for a long time.
2. When you blow expired air through limewater, it turns cloudy (milky) very quickly.
Quick Tip: If the limewater turns cloudy, \(CO_{2}\) is definitely present!

2. Physical Activity and Breathing

When you exercise, your breathing changes in two ways: it gets faster (increased rate) and deeper (increased depth).

Why does this happen?

During exercise, your muscles work harder and need more energy. To get this energy, they perform more aerobic respiration. This creates two needs:
1. The muscles need more oxygen delivered to them.
2. The muscles produce more carbon dioxide, which must be removed.

Quick Review: Exercise \(\implies\) More Respiration \(\implies\) More \(CO_{2}\) \(\implies\) Faster/Deeper Breathing.

3. How We Breathe (Supplement Only)

Breathing involves changing the volume and pressure inside the thorax (the chest cavity). Air always moves from high pressure to low pressure.

Inhalation (Breathing In)

1. The external intercostal muscles contract, pulling the ribs up and out.
2. The diaphragm contracts and flattens.
3. The volume of the thorax increases.
4. This causes the pressure inside the thorax to decrease.
5. Air is pushed into the lungs from the outside.

Exhalation (Breathing Out)

1. The internal intercostal muscles contract (during forced breathing like exercise), and the external ones relax, moving the ribs down and in.
2. The diaphragm relaxes and moves up into a dome shape.
3. The volume of the thorax decreases.
4. This causes the pressure inside the thorax to increase.
5. Air is forced out of the lungs.

The Trachea and Cartilage

The trachea (windpipe) has rings of cartilage. These rings are vital because they keep the airway open, preventing it from collapsing when the pressure changes during breathing. Think of them like the plastic rings on a vacuum cleaner hose!

4. Control of Breathing (Supplement Only)

How does your body "know" to breathe faster when you run? It’s all about carbon dioxide detection.

As you exercise, the concentration of \(CO_{2}\) in your blood increases. The brain detects this high level of \(CO_{2}\). It then sends electrical impulses to the diaphragm and intercostal muscles to increase the rate and depth of breathing. This helps to exhaust the excess \(CO_{2}\) and bring in more oxygen.

5. Keeping the Lungs Clean (Supplement Only)

The air we breathe isn't always clean. Our respiratory system has a "cleaning crew" to protect us:

  • Goblet Cells: These cells secrete mucus. Mucus is sticky and traps dust, dirt, and bacteria before they can reach the lungs.
  • Ciliated Cells: These cells have tiny hair-like structures called cilia. Cilia beat in a rhythmic wave to sweep the mucus (and the trapped "junk") up and away from the lungs toward the throat, where it can be swallowed or coughed out.

Analogy: Think of mucus as a sticky flypaper trap and the cilia as a broom sweeping the floor.

Summary: Key Takeaways

Core Students:
- Inspired air has more \(O_{2}\); expired air has more \(CO_{2}\) and water vapour.
- Limewater turns cloudy when \(CO_{2}\) is bubbled through it.
- Exercise increases the rate and depth of breathing to support respiration.

Supplement Students:
- Volume and pressure are inversely related (Volume goes up, Pressure goes down).
- Inhalation: External intercostals and diaphragm contract.
- Brain Control: The brain detects \(CO_{2}\) levels, not oxygen levels, to control breathing.
- Protection: Goblet cells make mucus; Cilia sweep it away.