What this chapter is about
This chapter explains how the human body obtains oxygen from the atmosphere and releases carbon dioxide, a metabolic waste product. Breathing, also called pulmonary ventilation, is the mechanical process of moving air into and out of the lungs. Gas exchange refers to the diffusion of oxygen and carbon dioxide across respiratory surfaces. Both processes are essential because cellular respiration in mitochondria requires a continuous supply of oxygen to release energy from nutrients.
A Class 11 student meets this chapter after learning about body fluids and circulation. Understanding how oxygen enters the blood and how carbon dioxide is removed completes the picture of how cells receive what they need and dispose of wastes. The chapter covers the human respiratory system's anatomy, the mechanism of breathing, the transport of gases in blood, and the regulation of respiration. Disorders such as asthma, emphysema and occupational lung diseases are also introduced.
After studying this chapter, a student should be able to describe each part of the respiratory tract, explain the pressure changes that cause inhalation and exhalation, calculate lung volumes and capacities, and describe how haemoglobin carries oxygen and how carbon dioxide is transported in plasma and red blood cells.
Key ideas
- Respiratory organs in humans include the nostrils, nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles and alveoli; the alveoli are the sites of gas exchange.
- Breathing involves pressure changes: contraction of the diaphragm and external intercostal muscles increases thoracic volume, lowers intra-pulmonary pressure below atmospheric pressure, and draws air in; relaxation reverses these changes for exhalation.
- Lung volumes (tidal volume, inspiratory reserve volume, expiratory reserve volume, residual volume) and lung capacities (vital capacity, total lung capacity, functional residual capacity, inspiratory capacity) quantify pulmonary function.
- Partial pressure gradients drive diffusion: oxygen moves from alveolar air (pO₂ ≈ 104 mm Hg) to deoxygenated blood (pO₂ ≈ 40 mm Hg); carbon dioxide moves in the opposite direction.
- Haemoglobin (Hb) binds oxygen cooperatively; the oxygen-haemoglobin dissociation curve is sigmoidal and shifts right under low pH, high pCO₂ or high temperature, releasing oxygen to active tissues.
- Carbon dioxide transport occurs in three forms: dissolved in plasma (about 7 %), as bicarbonate ions (about 70 %), and bound to haemoglobin as carbamino-haemoglobin (about 23 %).
- Regulation of breathing is primarily by the respiratory centre in the medulla oblongata, influenced by chemoreceptors sensitive to blood pCO₂, pH and, to a lesser extent, pO₂.
Formulas and facts to remember
1. Tidal volume (TV): volume of air inspired or expired during normal breathing; about 500 mL in a healthy adult. 2. Inspiratory reserve volume (IRV): additional air that can be forcibly inhaled after a normal inspiration; about 2500–3000 mL. 3. Expiratory reserve volume (ERV): additional air that can be forcibly exhaled after a normal expiration; about 1000–1100 mL. 4. Residual volume (RV): air remaining in lungs after maximum exhalation; about 1100–1200 mL. 5. Vital capacity (VC) = TV + IRV + ERV: maximum air a person can breathe out after deepest inhalation; roughly 3500–4500 mL. 6. Total lung capacity (TLC) = VC + RV: total volume of air lungs can hold; about 5000–6000 mL. 7. Oxygen-haemoglobin equation: Hb + O₂ ⇌ HbO₂ (oxyhaemoglobin); equilibrium favours HbO₂ at high pO₂ (lungs) and favours release of O₂ at low pO₂ (tissues). 8. Bicarbonate formation in RBC: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻; catalysed by carbonic anhydrase inside red blood cells.
Worked examples
### Example 1: Calculating vital capacity
A student uses a spirometer and records the following: tidal volume = 480 mL, inspiratory reserve volume = 2600 mL, expiratory reserve volume = 1000 mL. Calculate the vital capacity.
Solution
Vital capacity = TV + IRV + ERV VC = 480 + 2600 + 1000 = 4080 mL
The student's vital capacity is 4080 mL, which is within the normal range for a healthy young person.
### Example 2: Direction of gas diffusion
In the alveoli, the partial pressure of oxygen is 104 mm Hg and that of carbon dioxide is 40 mm Hg. In the blood arriving at the pulmonary capillaries, pO₂ is 40 mm Hg and pCO₂ is 45 mm Hg. Describe the direction of diffusion for each gas.
Solution
Gases diffuse from regions of higher partial pressure to lower partial pressure.
- Oxygen: pO₂ in alveoli (104 mm Hg) > pO₂ in blood (40 mm Hg). Therefore oxygen diffuses from alveolar air into the blood.
- Carbon dioxide: pCO₂ in blood (45 mm Hg) > pCO₂ in alveoli (40 mm Hg). Therefore carbon dioxide diffuses from blood into alveolar air.
These gradients ensure that blood leaving the lungs is oxygenated and has lost excess carbon dioxide.
### Example 3: Effect of exercise on the dissociation curve
During a 400-metre race, a runner's active leg muscles produce large amounts of carbon dioxide and lactic acid. Explain how this affects oxygen delivery to the muscles.
Solution
Increased CO₂ and lactic acid lower the local pH (more acidic). A lower pH shifts the oxygen-haemoglobin dissociation curve to the right, which is called the Bohr effect. At any given partial pressure of oxygen, haemoglobin now has a lower affinity for oxygen. Consequently, more oxygen is released from haemoglobin to the actively respiring muscle cells, meeting their increased demand during exercise.
Common mistakes
- Confusing breathing (a mechanical, physical process) with cellular respiration (a biochemical process in mitochondria) → remember that breathing supplies the oxygen that cellular respiration uses.
- Believing that lungs actively expand on their own → the lungs expand passively because the diaphragm and intercostal muscles enlarge the thoracic cavity, lowering pressure inside.
- Thinking that oxygen is transported mainly as dissolved gas in plasma → about 97 % of oxygen is carried bound to haemoglobin, not dissolved.
- Adding residual volume when calculating vital capacity → vital capacity does not include residual volume; it is TV + IRV + ERV only.
- Assuming expiration is always an active process → quiet expiration is largely passive, caused by elastic recoil of lungs; forceful expiration requires internal intercostal and abdominal muscles.
Quick revision
- Alveoli are the thin-walled sacs where gas exchange occurs; their large surface area and moist lining aid diffusion.
- Inhalation occurs when intra-pulmonary pressure drops below atmospheric pressure due to expansion of the thoracic cavity.
- Vital capacity = tidal volume + inspiratory reserve volume + expiratory reserve volume.
- Haemoglobin carries about 97 % of blood oxygen; its sigmoidal curve allows efficient loading in lungs and unloading in tissues.
- About 70 % of carbon dioxide is transported as bicarbonate ions formed inside red blood cells.
- The medulla oblongata's respiratory centre adjusts breathing rate mainly in response to blood pCO₂ and pH.