Biology (0610) | Gas exchange in humans

Hello IGCSE Biologists! Let's Master Gas Exchange!

Welcome to the fascinating world of human breathing! This chapter, "Gas exchange in humans," explains one of the most vital processes in your body: how you get the oxygen needed for survival and how you get rid of waste carbon dioxide.

Don't worry if the anatomy seems complex at first. We will break down the tubes, muscles, and tiny air sacs into clear, easy-to-understand sections. By the end, you'll know exactly how every breath works, from your nose down to your bloodstream!

1. Understanding Gas Exchange: The Big Goal

The primary purpose of the breathing system is to carry out Gas Exchange. This is the movement of gases between the blood and the external environment (air).

We need oxygen (O₂) for aerobic respiration (to release energy), and we need to remove the waste product, carbon dioxide (\(CO_2\)).

What is Diffusion? (A Quick Reminder)

Gas exchange relies entirely on diffusion. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration (down the concentration gradient).

Analogy: Think of spraying air freshener in one corner of a room. Eventually, the scent particles spread everywhere, moving from where they were highly concentrated (the corner) to where they were less concentrated (the rest of the room).

2. The Respiratory System: Anatomy and Pathway

Air enters through the nose or mouth and travels down a series of tubes to reach the lungs.

Key Components (The Tubes)

• Larynx: The voice box (at the top of the trachea).
• Trachea (Windpipe): The main tube carrying air from the larynx down to the chest.

  It is supported by C-shaped rings of cartilage.

  Function of Cartilage (Supplement)

  Cartilage rings keep the trachea and bronchi permanently open (patent). This is crucial because if the tubes collapsed (especially when you exhale or cough), air flow would stop.

• Bronchi (singular: bronchus): The trachea splits into two large tubes, one leading to each lung.
• Bronchioles: These are smaller, finer tubes branching off the bronchi, spreading deeper into the lung tissue.
• Alveoli (Air Sacs): Tiny air sacs found at the end of the bronchioles where the actual gas exchange happens.

Key Components (The Structures that Move)

The lungs themselves don't have muscle to pull air in. They are moved by the structures surrounding them, located in the chest cavity (thorax).

• Lungs: Contain the bronchi, bronchioles, and alveoli.
• Diaphragm: A sheet of muscle beneath the lungs, separating the thorax from the abdomen.
• Ribs: Form the cage protecting the lungs and heart.
• Intercostal Muscles: Muscles located between the ribs.

  Intercostal Muscles (Supplement)

  There are two sets:

  • External Intercostal Muscles: Lie on the outside; involved in inhalation.
  • Internal Intercostal Muscles: Lie on the inside; involved only in forced exhalation (like blowing out candles).

3. The Gas Exchange Surface: Alveoli Features

The alveoli and the capillaries surrounding them make up the gas exchange surface. To ensure fast and efficient diffusion, this surface has specific adaptations (Core 1):

1. Large Surface Area (SA):

   The lungs contain hundreds of millions of alveoli. If flattened out, the surface area would cover a tennis court!
   Why? A large SA allows a high rate of diffusion to occur simultaneously across many points.

2. Thin Surface:

   The wall of the alveolus and the wall of the capillary are both just one cell thick.
   Why? This keeps the diffusion distance (\B>diffusion path) very short (about 1 µm), speeding up gas exchange.

3. Good Blood Supply:

   The alveoli are covered by a dense network of tiny blood vessels called capillaries.
   Why? The blood constantly removes oxygen and brings in carbon dioxide, maintaining steep concentration gradients for both gases.

4. Good Ventilation with Air:

   The act of breathing (ventilation) constantly replaces the air in the alveoli.
   Why? Bringing in fresh air high in O₂ and removing stale air high in \(CO_2\) helps maintain steep concentration gradients.

Key Takeaway: These four features all work together to maximize the rate of diffusion. If any one fails (e.g., if the surface thickens due to disease), gas exchange becomes inefficient.

4. The Mechanism of Breathing (Ventilation)

Breathing (ventilation) is the mechanical process that moves air into and out of the lungs. It works by changing the volume of the chest cavity (thorax), which changes the pressure inside the lungs.

A. Inspiration (Inhaling)

This is an active process requiring muscle contraction (Supplement 8):

1. The diaphragm muscle contracts and moves downwards (flattens).
2. The external intercostal muscles contract, pulling the ribs upwards and outwards.
3. The combined effect increases the volume of the thorax.
4. This volume increase causes the air pressure inside the lungs to decrease (become lower than the atmospheric pressure).
5. Air is pushed into the lungs, down the pressure gradient.

B. Expiration (Resting Exhaling)

Normal expiration is usually a passive process (doesn't require muscle energy), relying on the elastic recoil of the lungs (Supplement 8):

1. The diaphragm muscle relaxes and moves upwards (becomes dome-shaped).
2. The external intercostal muscles relax, and the ribs move downwards and inwards.
3. The volume of the thorax decreases.
4. This volume decrease causes the air pressure inside the lungs to increase (become higher than the atmospheric pressure).
5. Air is pushed out of the lungs, down the pressure gradient.

Forced Expiration (Extended)

When you exercise vigorously or shout, you use extra muscle effort. The internal intercostal muscles contract powerfully, pulling the ribs down and in further, forcing more air out.

5. Comparing Inspired and Expired Air

The composition of the air we breathe in is very different from the air we breathe out (Core 4, Supplement 9).

Why? Because gas exchange has taken place in the alveoli.

Component	Inspired Air (Inhaled)	Expired Air (Exhaled)	Reason for Difference
Oxygen (\(O_2\))	About 21%	About 16%	O₂ has diffused from the alveoli into the blood.
Carbon Dioxide (\(CO_2\))	About 0.04%	About 4%	\(CO_2\) has diffused from the blood into the alveoli.
Water Vapour	Variable (depends on humidity)	High (Saturated)	Water evaporates from the moist surfaces of the alveoli.
Nitrogen	About 78%	About 78%	Nitrogen is an inert (unreactive) gas and does not exchange.

Investigating Expired Air (Core 3)

We can experimentally show that expired air contains more carbon dioxide using limewater (calcium hydroxide solution).

• If you bubble inspired air through limewater, it remains clear or changes very slowly.
• If you bubble expired air through limewater, it turns cloudy or milky almost immediately.

Conclusion: Expired air contains significantly more carbon dioxide than inspired air.

6. Protecting the Breathing System

The air we inhale is full of dust, smoke particles, and pathogens (like bacteria). The system is protected by three key components (Supplement 11):

1. Goblet Cells: These specialized cells, found in the trachea and bronchi, produce mucus. Mucus is a sticky substance that traps dust and pathogens.
2. Ciliated Cells: These cells line the air passages and have tiny hairs called cilia on their surfaces. The cilia sweep the layer of mucus (with the trapped particles) constantly upwards towards the throat.
3. Mucus: The sticky trap itself.

Did you know? This cleaning mechanism is often called the ciliary escalator or mucociliary escalator. When you swallow, you send the mucus (and trapped dirt) down to the stomach where acid kills the pathogens.

7. Physical Activity and Breathing Control

When you exercise, your breathing rate and depth both increase. This is an essential response to meet the higher energy demands of the muscles.

This process is an excellent example of a feedback mechanism (Core 5, Supplement 10):

1. Increased Respiration: Vigorous physical activity means your muscle cells are doing aerobic respiration much faster to supply ATP energy.
2. Increased Carbon Dioxide: A byproduct of this increased respiration is a faster production of \(CO_2\). This causes the \(CO_2\) concentration in the blood to increase.
3. Detection by Brain: The increase in blood \(CO_2\) concentration is detected by specialized receptors in the brain.
4. Response: The brain sends nervous impulses to the diaphragm and intercostal muscles.
5. Outcome: This leads to an increased rate and greater depth of breathing (deeper and faster breaths). This quickly ventilates the lungs, removing excess \(CO_2\) and bringing in more O₂, ensuring the concentration gradients remain steep and oxygen supply matches demand.

Common Mistake: Students often think the body detects low oxygen. While oxygen levels do drop slightly, the increase in carbon dioxide concentration is the main signal detected by the brain that drives the change in breathing rate during exercise.