Sciences - Co-ordinated (Double) (0654) | Gas exchange in humans

Study Notes: B11 Gas Exchange in Humans

Hello future scientists! This chapter is all about how your body manages to suck in life-giving oxygen and get rid of waste carbon dioxide. This process, called gas exchange, is fundamental to survival because without oxygen, your cells can't perform respiration and release the energy you need to live!

We'll look at the incredible structures that make up your breathing system and how they are perfectly adapted to perform this vital task efficiently.

1. The Human Breathing System (Anatomy)

The breathing system (or ventilatory system) is a network of tubes that transport air deep into the lungs where gas exchange occurs.

The Air Pathway - From Nose to Lung

Air travels down a specific path:

• Larynx (voice box)
• Trachea (windpipe): A tube held open by rings of cartilage (C-shaped rings) to ensure the airway never collapses.
• Bronchi (singular: bronchus): The trachea splits into two tubes, one leading to each lung.
• Bronchioles: The bronchi repeatedly divide into smaller and smaller tubes, like the branches of a tree.
• Alveoli (air sacs): Tiny air sacs found at the very end of the bronchioles. This is where the actual gas exchange happens!

Muscles and Structures Involved in Breathing (Ventilation)

Breathing requires the movement of your chest cavity, controlled by specific structures:

• Ribs: Form the protective cage around the lungs.
• Intercostal Muscles: Muscles found between the ribs. They contract and relax to move the rib cage up/down and outwards/inwards.
• Diaphragm: A large sheet of muscle beneath the lungs. It contracts (flattens) when you breathe in and relaxes (domes up) when you breathe out.
• Lungs: The primary organ for gas exchange, containing millions of alveoli.
• Capillaries: A dense network of tiny blood vessels surrounding the alveoli, essential for transporting gases.

2. The Alveoli: The Gas Exchange Surface

The most important part of the lung is the alveolus (air sac), surrounded by a dense network of capillaries. This structure is perfectly adapted to ensure rapid and efficient gas exchange by diffusion.

Diffusion Reminder: Gas exchange works because molecules naturally move from where they are in high concentration to where they are in low concentration (down a concentration gradient).

• Oxygen concentration is higher in the inhaled air (alveoli) than in the blood (capillaries). Oxygen diffuses from the alveoli into the blood.
• Carbon dioxide concentration is higher in the blood (capillaries—waste product of respiration) than in the inhaled air (alveoli). Carbon dioxide diffuses from the blood into the alveoli.

S5: Adaptations of the Gas Exchange Surface

The alveoli and capillaries have four main features that maximise the rate of gas exchange (Diffusion):

1. Large Surface Area: There are hundreds of millions of alveoli in the lungs.
   Analogy: If you flattened out all the alveoli, they would cover a tennis court! This massive area means more space for diffusion simultaneously.
2. Thin Surface: The wall of the alveolus and the wall of the capillary are both only one cell thick.
   This creates a short diffusion distance, allowing gases to cross quickly.
3. Good Blood Supply: The alveoli are surrounded by an extremely dense network of capillaries (associated capillaries).
   This ensures that blood rich in CO₂ is constantly brought to the lungs, and blood rich in O₂ is constantly carried away, maintaining a steep concentration gradient.
4. Good Ventilation with Air: Breathing (ventilation) constantly replaces the air inside the lungs.
   Inhaling brings fresh, O₂-rich air, and exhaling removes CO₂-rich air, which also helps maintain a steep concentration gradient.

3. Composition of Inspired vs. Expired Air

The air we breathe in (inspired air) is very different from the air we breathe out (expired air). These differences occur because of gas exchange and respiration occurring in the body.

C3 & S6: Key Differences in Gas Composition

C2: Testing for Carbon Dioxide

We can demonstrate the increased carbon dioxide content in expired air using a simple investigation involving limewater.

• Test: If you bubble inspired air (from the environment) through limewater, it remains clear (or goes cloudy very slowly).
• Result: If you bubble expired air (from your lungs) through limewater, it turns cloudy immediately.
• Conclusion: This demonstrates that expired air contains a significantly higher concentration of carbon dioxide (which reacts with the limewater).

4. Breathing and Physical Activity

When you exercise, your breathing rate and depth increase dramatically. This is not because your body needs more oxygen immediately, but because it needs to get rid of the rapidly increasing levels of carbon dioxide.

C4 & S7: The Control Mechanism During Exercise

Physical activity requires your muscles to perform aerobic respiration much faster to meet the high energy demand. This rapid respiration produces large amounts of carbon dioxide as a waste product.

Here is the step-by-step process of how breathing is controlled:

1. Increased Respiration: During exercise, muscle cells respire much faster, producing a large volume of CO₂.
2. CO₂ Concentration Rises: The increased CO₂ diffuses into the blood, causing the CO₂ concentration in the blood to rise.
3. Detection by the Brain: The brain (specifically the medulla oblongata, though you only need to know "the brain" for this syllabus) detects this increased concentration of CO₂.
4. Brain Sends Signals: The brain sends electrical impulses along motor neurons to the breathing muscles (diaphragm and intercostal muscles).
5. Increased Ventilation: These signals cause the muscles to contract more frequently and more forcefully, leading to:
   • An increased rate of breathing (breathing faster).
   • A greater depth of breathing (taking deeper breaths).
6. CO₂ Removed: This increased ventilation quickly removes the excess CO₂ from the blood via gas exchange in the alveoli, returning the blood composition to normal.

Key Takeaway for B11: The gas exchange system is highly efficient due to the specific adaptations of the alveoli, ensuring steep concentration gradients are maintained for both O₂ uptake and CO₂ removal, a process that is automatically regulated by the brain based primarily on blood CO₂ levels.