Biology · Chapter 05

🫁 Respiratory System

How you breathe — alveoli and gas exchange.

💡 How You Breathe

Breathing happens because of a muscle called the diaphragm at the bottom of your lungs.

Inhalation (breathing in):

1. Diaphragm contracts and moves DOWN
2. Ribcage expands OUT
3. Lung volume increases → air pressure inside drops
4. Air rushes in from outside (higher pressure)

Exhalation (breathing out): everything reverses. Diaphragm relaxes UP, ribcage moves IN, air pushed out.

Gas exchange happens in alveoli — tiny air sacs (~300 million in each lung). They're surrounded by capillaries. O₂ diffuses from alveoli into blood; CO₂ diffuses from blood into alveoli.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP (energy)

This is cellular respiration — happens in every cell's mitochondria, using the oxygen you breathe in.

Human lungs — alveoli and bronchi structureWikimedia Commons / CC BY-SA 3.0

Hemoglobin carrying oxygen in red blood cellsWikimedia Commons / CC BY-SA 3.0

💨 The numbers

• Normal breathing rate: 12-20 breaths per minute
• Tidal volume (normal breath): 500 ml
• Lung capacity (max): ~6 litres in men, ~4.5 L in women
• Total alveolar surface area: ~70 m² (about the size of a badminton court!)
• Oxygen carried per litre of blood: ~200 ml (90% bound to haemoglobin)

🩸 Hemoglobin — the oxygen carrier

Red blood cells contain hemoglobin — an iron-containing protein that binds O₂. Each haemoglobin molecule can carry 4 O₂ molecules.

That's why iron deficiency causes anaemia — fewer healthy RBCs, less O₂ delivery, you feel tired.

⚠️ Smoking & lungs

Tobacco smoke damages alveoli (causing emphysema), paralyzes cilia (so dust builds up), and contains 70+ cancer-causing chemicals. Lung capacity drops 30-50% in heavy smokers.

🎬

Breathing Mechanism Live

Animation

Watch lungs inflate and deflate. Diaphragm and ribcage drive the entire process.

🔬

Compare Breathing Conditions

Interactive

See how breathing rate and oxygen need change in different situations.

Breaths per minute12-16

Oxygen needLow (250 ml/min)

Heart rate60-80 bpm

At rest, the body needs minimal oxygen. Breathing is slow and shallow.

Practice (NCERT): Why do we feel breathless after running?

When you run, muscles need more energy. They use up oxygen faster than the lungs can supply, so they switch to anaerobic respiration (without oxygen), which produces lactic acid. The body needs to repay this "oxygen debt" — so you breathe heavily after exercise to bring in extra oxygen and clear the lactic acid. That's why you keep panting even after stopping.

Practice (NEET): Why does breathing become difficult at high altitudes?

At high altitudes, the atmospheric pressure is lower, so the partial pressure of oxygen in inhaled air is also lower. Less O₂ enters the alveoli, less binds to haemoglobin in blood. This leads to hypoxia (low oxygen in tissues). The body compensates by breathing faster (hyperventilation) and producing more RBCs over time. People living in mountains (Tibet, Andes) have evolved more RBCs to cope.