CORE Biology (9221) | Breathing

Breathing: Fueling the Engine (CORE Biology 9221)

Hello future scientists! Welcome to the fascinating world of breathing. In this chapter, we discover how your body manages to suck in the essential ingredient needed for life: Oxygen.

Breathing is part of the larger section called Bioenergetics because the main job of breathing is to bring in the gas (Oxygen) needed to release energy from food (Respiration). Without breathing, there is no respiration, and without respiration, there is no energy! Let's get started.

What You Will Learn:

• The main structures of the human gas exchange system.
• How we breathe in and out (the mechanism of ventilation).
• How gases (Oxygen and Carbon Dioxide) move between the lungs and the blood.

Section 1: The Structure of the Breathing System

Air needs a clear pathway to get from the outside world into your lungs. Think of this system as an upside-down tree, getting smaller and smaller as it branches out.

The Pathway of Air

Air enters the nose/mouth and travels through the following key structures:

1. Trachea (Windpipe): The main tube that carries air down towards the chest. It is kept open by rings of cartilage (a tough, flexible tissue).
2. Bronchi (Singular: Bronchus): The trachea splits into two bronchi—one going to each lung.
3. Bronchioles: The bronchi repeatedly divide into thousands of smaller, narrower tubes called bronchioles.
4. Alveoli (Air Sacs): At the very end of the bronchioles are tiny clusters of air sacs. This is where the magic happens!

Did you know? If you flattened out all the surfaces of the alveoli in your lungs, they would cover a tennis court! This huge area is vital for efficient gas exchange.

Other Structures Involved in Breathing

These structures are muscles or bones needed to physically move the air in and out.

• Ribs: Bones that form the rib cage, protecting the lungs.
• Intercostal Muscles: Muscles located between the ribs. There are external and internal intercostal muscles.
• Diaphragm: A strong sheet of muscle beneath the lungs. It separates the chest cavity (thorax) from the abdominal cavity.

Section 2: The Mechanism of Breathing (Ventilation)

Breathing in and out is officially called Ventilation. You don't "suck" air in; you change the volume and pressure inside your chest cavity (the thorax) so that air moves in or out automatically.

Remember this rule: Air always moves from an area of high pressure to an area of low pressure.

1. Inhalation (Breathing In)

To get air into the lungs, we must decrease the pressure inside the chest below the external air pressure. We do this by increasing the volume of the chest.

Step-by-Step Inhalation:

1. The Diaphragm contracts (pulls down and flattens).
2. The External Intercostal Muscles contract, pulling the ribs up and outwards.
3. These movements dramatically increase the volume of the thorax.
4. Because the volume increases, the pressure inside the lungs decreases (becomes lower than the atmosphere).
5. Air rushes in through the nose and mouth to balance the pressure.

Memory Aid: INhalation = INcrease Volume.

2. Exhalation (Breathing Out)

To push air out, we must decrease the volume of the chest, increasing the pressure inside the lungs above the external air pressure.

Step-by-Step Exhalation (Usually Passive):

1. The Diaphragm relaxes (moves up into a dome shape).
2. The External Intercostal Muscles relax, allowing the ribs to move down and inwards.
3. These movements decrease the volume of the thorax.
4. The pressure inside the lungs increases (becomes higher than the atmosphere).
5. Air rushes out of the lungs.

Section 3: Gas Exchange in the Alveoli

Breathing moves air, but the essential process of swapping gases between the air and the blood happens only at the Alveoli.

The Process: Diffusion

Gases move using a process called Diffusion.

• Diffusion is the net movement of particles from an area of high concentration to an area of low concentration.

Think of it like opening a bottle of perfume in one corner of a room. Eventually, the smell spreads everywhere until it's evenly distributed. Gases behave the same way!

Efficiency of the Alveoli

The alveoli are perfectly adapted for rapid gas exchange:

1. Large Surface Area: There are millions of alveoli, providing a massive area for gas exchange.
2. Thin Walls: The walls of the alveoli (and the surrounding capillaries) are only one cell thick. This creates a very short distance for gases to diffuse across.
3. Good Blood Supply: Each alveolus is wrapped in a dense network of tiny blood vessels called Capillaries. This ensures that blood low in oxygen is always arriving, and blood high in oxygen is always carried away.

The Exchange Action

When fresh air arrives in the alveoli, the following exchanges happen simultaneously due to the concentration differences:

1. Oxygen (\(O_2\)) Exchange:
   • Concentration of \(O_2\) is High in the Alveoli.
   • Concentration of \(O_2\) is Low in the blood arriving from the body.
   • \(O_2\) diffuses from the alveoli into the blood.
2. Carbon Dioxide (\(CO_2\)) Exchange:
   • Concentration of \(CO_2\) is High in the blood arriving from the body (it's a waste product of respiration).
   • Concentration of \(CO_2\) is Low in the Alveoli.
   • \(CO_2\) diffuses from the blood into the alveoli to be breathed out.

Section 4: Composition of Inhaled vs. Exhaled Air

Because gas exchange is happening constantly, the air you breathe out is very different from the air you breathe in.

Don't worry, the percentages below are approximations, but the trends are what matter most!

Common Mistake to Avoid: Students often think all the oxygen is used up. This is incorrect! We only use about a quarter of the oxygen in the air we inhale. That's why mouth-to-mouth resuscitation works—there is still plenty of oxygen left in the exhaled air to save a life.