Gas exchange and respiration
How the lungs are adapted for gas exchange, how we breathe, and how cells release energy by aerobic and anaerobic respiration.
📌 Key points
- Gas exchange surfaces such as alveoli have a large surface area, a thin surface, a good blood supply and good ventilation with air.
- Air passes through the trachea, bronchi and bronchioles to the alveoli. Cartilage keeps the trachea open; goblet cells make mucus and ciliated cells move it away from the lungs.
- Breathing in: the external intercostal muscles contract, moving the ribs up and out; the diaphragm contracts and flattens; the volume of the thorax increases, pressure decreases and air moves in.
- Compared with inspired air, expired air contains less oxygen (about 16% instead of 21%), more carbon dioxide (about 4% instead of 0.04%) and more water vapour. Limewater turns milky faster with expired air.
- Exercise increases the rate and depth of breathing. The brain detects the higher carbon dioxide concentration in the blood and sends impulses to increase breathing.
- Respiration is the chemical reactions in cells that break down nutrient molecules and release energy for metabolism. The energy is used for muscle contraction, protein synthesis, cell division, active transport, growth, nerve impulses and keeping body temperature constant.
- Aerobic respiration uses oxygen: glucose + oxygen → carbon dioxide + water; C6H12O6 + 6O2 → 6CO2 + 6H2O. It takes place in mitochondria.
- Anaerobic respiration releases much less energy per glucose molecule. In muscles: glucose → lactic acid. In yeast: glucose → ethanol + carbon dioxide (used in bread-making and brewing).
- During vigorous exercise lactic acid builds up, causing an oxygen debt. Afterwards, fast heart rate and deep breathing continue so that lactic acid is taken to the liver and broken down by aerobic respiration.
📖 Key terms
- Aerobic respiration
- Reactions in cells that use oxygen to break down nutrient molecules and release energy.
- Anaerobic respiration
- Reactions in cells that break down nutrients without oxygen, releasing less energy.
- Alveolus
- A tiny air sac in the lungs where gas exchange takes place.
- Oxygen debt
- The oxygen needed after exercise to break down the lactic acid made by anaerobic respiration.
- Ventilation
- Moving air into and out of the lungs (breathing).
- Diaphragm
- A sheet of muscle below the lungs that contracts and flattens during breathing in.
✏️ Worked example
At rest a person takes 15 breaths per minute of 0.5 dm3 each. During exercise they take 30 breaths per minute of 1.5 dm3 each. How many times more air do they breathe per minute during exercise?
- Volume per minute at rest = 15 × 0.5 = 7.5 dm3.
- Volume per minute during exercise = 30 × 1.5 = 45 dm3.
- Factor = 45 ÷ 7.5 = 6.
- They breathe 6 times more air per minute, to supply more oxygen and remove more carbon dioxide.
Respiration releases energy – never write that it "produces" or "makes" energy. And breathing (ventilation) is not the same as respiration.
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