Alveoli, Ventilation & Gas Exchange in the Lungs
The human gas exchange system is located in the thorax, protected by the ribcage, where ventilation continuously brings fresh oxygen into millions of microscopic air sacs called alveoli and removes waste carbon dioxide. Inhalation and exhalation rely on pressure changes created by the diaphragm and intercostal muscles altering the volume of the thoracic cavity. Examiners frequently test the four structural adaptations of alveoli for rapid diffusion and the step-by-step mechanics of ventilation.
- Pathway of Air into the Lungs
- Air enters through the nose/mouth, travels down the trachea (windpipe, supported by C-shaped rings of cartilage), splits into the left and right bronchi, branches into smaller bronchioles, and terminates in microscopic air sacs called alveoli.
- Inhalation (Inspiration) Mechanism
- An active process: external intercostal muscles contract, pulling the ribcage up and out; the diaphragm muscle contracts and flattens downwards. Thorax volume increases, intrathoracic pressure drops below atmospheric pressure, and air rushes into the lungs.
- Exhalation (Expiration) Mechanism
- At rest, external intercostal muscles relax (ribcage moves down and in) and the diaphragm relaxes (domes upwards). Thorax volume decreases, intrathoracic pressure rises above atmospheric pressure, and air is forced out of the lungs.
Interactive Human Body & Cell Structure Explorer
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Heart (Double Circulatory Pump)
Structure & Function:
Pumps blood in a double circulatory system: the right ventricle pumps deoxygenated blood to the lungs, and the thick muscular left ventricle pumps oxygenated blood at high pressure around the whole body.
Key Exam Mark-Scheme Point:
Exam tip: The left ventricle has a much thicker muscular wall than the right ventricle because it must pump blood at high pressure all the way around the entire body, not just to the lungs.
What happens if this organ fails or is removed?
Without pressure from the heart, blood flow stops immediately. Oxygen and glucose cannot reach tissues, and waste carbon dioxide cannot be removed.
Tip: Use the “Simulate Organ Failure” button above to see how each organ system depends on the others to maintain homeostasis and cellular respiration.
Key Concepts & Exam Definitions (6)
- Pathway of Air into the Lungs
- Air enters through the nose/mouth, travels down the trachea (windpipe, supported by C-shaped rings of cartilage), splits into the left and right bronchi, branches into smaller bronchioles, and terminates in microscopic air sacs called alveoli.
- Inhalation (Inspiration) Mechanism
- An active process: external intercostal muscles contract, pulling the ribcage up and out; the diaphragm muscle contracts and flattens downwards. Thorax volume increases, intrathoracic pressure drops below atmospheric pressure, and air rushes into the lungs.
- Exhalation (Expiration) Mechanism
- At rest, external intercostal muscles relax (ribcage moves down and in) and the diaphragm relaxes (domes upwards). Thorax volume decreases, intrathoracic pressure rises above atmospheric pressure, and air is forced out of the lungs.
- Alveolar Gas Exchange
- Deoxygenated blood arriving at alveolar capillaries has a low oxygen concentration and high CO₂ concentration compared to alveolar air. Oxygen diffuses down its concentration gradient from the alveolus into red blood cells, while CO₂ diffuses out of blood plasma into the alveolus to be exhaled.
- Ciliated Epithelial & Goblet Cells
- Goblet cells lining the trachea and bronchi secrete sticky mucus that traps inhaled dust, dirt, and bacteria. Ciliated epithelial cells have microscopic hair-like cilia that beat rhythmically to sweep the mucus up to the throat to be swallowed and destroyed by stomach acid.
- Breathing Rate Calculation
- Breathing Rate (breaths per minute) = Number of breaths / Time in minutes. During exercise, both breathing rate and breath depth (tidal volume) increase to supply more oxygen for aerobic respiration and remove excess carbon dioxide.
Quick-Fire Flashcards (6)
Click any card to check the answer, then mark it as Known to track your progress.
Describe what happens to the diaphragm and ribcage during inhalation.
Why are the walls of both the alveolus and the surrounding capillary only one cell thick?
How is a steep concentration gradient maintained in the alveoli?
What is the role of cartilage rings in the trachea and bronchi?
Why does exhaled air contain more carbon dioxide and water vapour than inhaled air?
Explain why patients with emphysema feel tired and out of breath during light exercise.
Exam-Style Questions & Mark Schemes (4)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
Explain how the human lungs are adapted for efficient gas exchange. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Millions of alveoli provide a massive total surface area for gas diffusion [1 mark]. 2. The walls of the alveoli and capillaries are each only one squamous cell thick, creating an extremely short diffusion pathway [1 mark]. 3. Each alveolus is surrounded by a dense network of blood capillaries, maintaining a steep concentration gradient as blood flows past [1 mark]. 4. Continuous ventilation (breathing in and out) refreshes the air in the alveoli, keeping alveolar oxygen concentration high and carbon dioxide concentration low [1 mark].
- Q2
Describe the sequence of events that causes air to enter the lungs during inhalation. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. The external intercostal muscles contract, pulling the ribcage upwards and outwards [1 mark]. 2. At the same time, the muscular diaphragm contracts and flattens downwards [1 mark]. 3. These movements increase the volume of the thorax (chest cavity), which causes the pressure inside the thorax to decrease below external atmospheric pressure [1 mark]. 4. Air is drawn into the lungs down the pressure gradient through the trachea and bronchi [1 mark].
- Q3
Explain why a runner's breathing rate and heart rate remain high for several minutes after finishing a 400 m sprint. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. During the 400 m sprint, oxygen supply could not meet demand, so muscles respired anaerobically, producing toxic lactic acid and creating an oxygen debt [1 mark]. 2. A high breathing rate continues after exercise to take in extra oxygen into the blood and expel excess carbon dioxide [1 mark]. 3. A high heart rate continues to pump oxygenated blood rapidly to the muscles and transport lactic acid to the liver [1 mark]. 4. In the liver, the extra oxygen is used to oxidise lactic acid back into glucose and carbon dioxide + water, repaying the oxygen debt [1 mark].
- Q4
A student counts 42 breaths over a 3.5-minute period while resting, and 56 breaths over a 2-minute period during exercise. Calculate the percentage increase in the student's breathing rate. [3 marks]
Click “Check Mark Scheme” to compare your answer, 1. Resting breathing rate = 42 / 3.5 = 12 breaths per minute [1 mark]. 2. Exercise breathing rate = 56 / 2 = 28 breaths per minute [1 mark]. 3. Percentage increase = ((28 - 12) / 12) x 100 = (16 / 12) x 100 = 133.3% (or 133%) [1 mark].
Common Exam Questions (4)
What is the difference between respiration and breathing (ventilation)?
Breathing (ventilation) is the physical, mechanical movement of air into and out of the lungs caused by muscle contractions. Respiration is a chemical reaction occurring inside all living cells (in the mitochondria and cytoplasm) that breaks down glucose to release energy.
How do the diaphragm and intercostal muscles cause inhalation?
During inhalation, the external intercostal muscles contract (pulling the ribcage upwards and outwards) and the diaphragm muscle contracts (flattening downwards). This increases thorax volume, which decreases thorax pressure below atmospheric pressure, drawing air into the lungs.
How are alveoli adapted for efficient gas exchange?
Alveoli have four key adaptations: a huge total surface area (millions of spheroidal sacs), very thin walls (one cell thick for a short diffusion distance), a moist lining (so gases can dissolve), and a dense network of capillaries coupled with ventilation (to maintain a steep concentration gradient).
How does smoking damage the gas exchange system?
Smoking destroys the cilia on ciliated epithelial cells lining the trachea and bronchi, causing mucus and pathogens to build up (smoker's cough and bronchitis). Tar and toxins also break down the walls of the alveoli (emphysema), reducing total surface area for gas exchange and leaving patients breathless.
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