We continuously breathe air in and out. Let us examine the mechanism that takes in oxygen and releases carbon dioxide.
Basic Knowledge
Air passage (airway):
Nose and mouth → pharynx → larynx → trachea → bronchi → lungs → alveoli
The bronchi repeatedly branch in the left and right lungs and eventually reach small sacs called alveoli (alveolus). The alveoli are surrounded by capillaries. Oxygen inside the alveoli enters the blood, and carbon dioxide in the blood is released into the alveoli (gas exchange).
Mechanism of breathing movements:
・Inhalation: the diaphragm moves downward and the ribs move upward → the volume of the chest increases and air enters
・Exhalation: the diaphragm moves upward and the ribs move downward → the volume of the chest decreases and air leaves
The lungs themselves have no muscles; they passively expand and contract through the movements of the diaphragm and ribs.
Alveolus (a small sac at the end of a bronchus; the site of gas exchange)
Gas exchange (the exchange of oxygen and carbon dioxide)
Diaphragm (the muscle separating the chest and abdomen; the main muscle of breathing)
Rib (a bone surrounding the chest; moves up and down during breathing)
Trachea and bronchi (air passages that repeatedly branch)
Cellular respiration (a reaction in which organic substances are broken down in cells to obtain energy)
Further Study (Background and Significance)
There are about 300〜600 million alveoli in both lungs combined. If spread out, they would have a surface area of about half a tennis court (about 70m²). This makes it possible to exchange large amounts of gas in a short time.
Breathing includes external respiration (gas exchange between the lungs and outside air) and internal respiration (gas exchange between the blood and tissue cells). Together with cellular respiration (the reaction in which organic substances are broken down with oxygen in the mitochondria inside cells), these processes produce energy:Organic substances + oxygen → CO₂ + water + energy (ATP)
This is the same reaction as respiration in plants. Animals and plants share the basic mechanism for obtaining energy.
The amount of air breathed in one day is about 10,000L or more. An adult at rest breathes about 12〜18 times per minute, and this can exceed 50 times per minute during intense exercise. This is because the demand for oxygen in cells increases, so more oxygen must be taken in and more CO₂ must be released.
- Air passage: nose and mouth→trachea→bronchi→alveoli
- Site of gas exchange: alveoli and capillaries
- Alveoli are small and numerous (large surface area)
- Inhalation: diaphragm down, ribs up
- Exhalation: diaphragm up, ribs down
- The lungs themselves have no muscles
- Cellular respiration=ATP production in mitochondria
Points of Caution (Easily Confused)
① During inhalation, the diaphragm moves down (it contracts and moves downward); during exhalation, it moves up. Do not memorize them in reverse. ② Distinguish the trachea (one tube) from the bronchi (two branches and onward). The trachea divides into bronchi at its end. ③ Gas exchange occurs in the alveoli, between the alveoli and capillaries. It does not occur in the trachea itself. ④ Breathing (movement of the lungs) and cellular respiration (reactions inside cells) are different concepts, but they are connected.
Practice
- What are the sacs at the ends of the bronchi where gas exchange occurs called?
- When you inhale, how do the diaphragm and ribs move?
- Write the equation for cellular respiration briefly.