IGCSE^TM BIOLOGY

​

Unit 2�Animal Physiology

​

Chapter 3

Breathing and Gas Exchange

BREATHING AND GAS EXCHANGE

• Cellular respiration consumes oxygen and produce carbon dioxide.
• To respire aerobically, cells require a constant supply of oxygen.
• Carbon dioxide produced during respiration is a waste product that must be removed from the body.
• In humans, oxygen and carbon dioxide are exchanged between the blood and the air within the lungs.

Two terms often confused, but referring to fundamentally different biological processes.

​

• Respiration (Chemical Process)
• An oxidation reaction occurring at the cellular level.
• Breaks down food substances like glucose to release usable energy.

​

• Breathing (Physical Mechanism)
• The physical movement of air into and out of the lungs.
• Serves to facilitate continuous gas exchange between air and blood.

​

• Clarifying Terminology: "Gas Exchange System"
• The lungs and related organs are traditionally called the "respiratory system."
• To prevent confusion with cellular respiration, referring to it as the gas exchange system is much more accurate.

​

Respiration vs Breathing

THE STRUCTURE OF THE GAS EXCHANGE SYSTEM

• The Thorax (Chest Cavity)
• The thorax is the chest cavity—the airtight, protected space in the upper part of your body between your neck and your abdomen..
• Formed by the ribcage and underlying muscular floor.

​

• The Diaphragm
• dome-shaped muscular sheet separating thorax from abdomen.
• Features a fibrous central "roof" and muscular outer walls.

• Intercostal Muscles
• Two sets of muscles joining adjacent ribs ('costals' = ribs).
• Work with the ribcage to expand and contract chest volume.

​

Airway Network (The Bronchial Tree)

The air passages of the lungs form a highly branching network.

​

• Airway Pathway
• Nose / Mouth 🡪 Trachea (Windpipe)
• Splits into two Bronchi (one to each lung).
• Branches further into narrower Bronchioles

​

• Alveoli (Microscopic Air Sacs)
• Terminal ends of the bronchioles.
• The actual site of gas exchange with blood capillaries.

​

Anatomy of Respiratory System (Gas Exchange System)

1 ( )

2 ( )

4 ( )

3 ( )

trachea

bronchus

bronchiole

Alveoli

6 ( )

5 ( )

diaphragm

thorax

Ventilation of the Lungs

• Ventilation means “moving of air in and out of the lungs”
• The Golden Rule: Air moves from high pressure to low pressure
• When we change the volume of our thorax, it alters the pressure inside lungs. This causes air to move in or out of the lungs.
• If we increase the volume of the chest, it cause a slight drop in pressure inside the thorax compared with the air outside. Air then enters the lungs (inhalation)
• If decrease the volume of the chest, it makes the opposite occurring. Air exits from the lungs (exhalation)

When you press the bellows firmly,

Inner gas pressure increases.

When you release the bellows,�inner gas pressure decreases.

Inhaling vs Exhaling

• Breathing In (Inhalation)
• Diaphragm: Contracts and flattens down
• Ribs: External intercostals pull ribs up and out
• Chest volume increases 🡪 Pressure drops below outside air�🡪 Air rushes in

​

• Breathing Out (Exhalation)
• Diaphragm: Relaxes into a dome shape
• Ribs: Internal intercostals pull ribs down and in
• Chest volume decreases 🡪 Pressure rises 🡪 Air is forced out
• Lungs are elastic and deflate naturally like a released balloon

Common Trap:

• Lungs do NOT pump air themselves!
• Lungs have no muscles to force air in or out—the ribs, intercostal muscles, and diaphragm do the mechanical work.

1. The ( ventilation | respiration ), which is just the scientific word for breathing, is the physical process of moving air into and out of the lungs.
2. The ( ventilation | respiration ), on the other hand, is a chemical process carried out by cells in order to release energy.
3. What are the two muscles that make the volume of the thorax increase or shrink?
1. neck muscles
2. Intercostal muscles
3. Diaphragm
4. Antebrachial muscles
4. When we decrease the volume of gas container, gas pressure ( increase | decrease ).

​

Gas Exchange in the Alveoli

• What Are Alveoli?
• Microscopic air sacs that look like bunches of grapes
• Wrapped densely in a web of tiny blood capillaries

​

​

​

​

​

• What happens in Alveoli?

​

Built for Maximum Speed:

• Massive surface area: ~700,000,000 alveoli totaling 60 m² (larger than a classroom floor!)
• Ultra-thin boundary: Only two single cell layers thick (less than 0.001 mm)
• Moist lining: Lets gases dissolve before diffusing

Here Oxygen is consumed out

Lung Diseases

bronchitis

Smoking causes these diseases