Biology: Chapter 3

Notes on Breathing and Gas Exchange

Ventilation of the Lungs


  • Diaphragm muscle contracts.
  • Diaphragm flattens.
  • External intercostal muscles contract.
  • Ribs are pulled up and out.
  • Volume of thorax increases
  • Pressure inside thorax decreases.
  • Air moves down the trachea.
  • Lungs inflate.


  • Diaphragm mucles relax.
  • Diaphragm becomes dome-shaped.
  • External intercostal muscles relax.
  • Ribs are pulled down and in.
  • Volume of thorax decreases.
  • Pressure inside thorax increases.
  • Air moves up the trachea.
  • Lungs deflate.
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How Alveoli Make Gas Exchange More Efficient

  • Thin water lining - the oxygen and carbon dioxide molecules dissolve in the water and can diffuse in solution form.
  • Large surface area - more gas molecules can diffuse in a given time.
  • Walls of alveoli and capillaries are one cell thick (squamous epithelium) - there is a shorter distance for diffusion so gas exchange is faster.
  • Many capillaries in contact with alveolus wall - there is a bigger supply of blood so there is a higher rate of gas exchange in a given time.
  • Continuous flow of blood - The blood always has a low concentration of oxygen, so oxygen can always go down a concentration gradient. An equilibrium cannot be reached.
  • Ventilation of lungs - The alveoli always have a high concentration of oxygen, so oxygen can always go down a concentration gradient. An equilibrium cannot be reached.
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Effects of Smoking


  • Mucus clogs up the air passages because the cilia in the trachea and bronchi are destroyed by the chemicals in the cigarette smoke.
  • The bronchial tree becomes infected and the smoker develops a smoker's cough. Breathing becomes difficult due to the blocked air passages.


  • The alveoli walls break down and remerge, reducing surface area require for gas exchange, therefore, the blood carries less oxygen.
  • The smoker begins to cough and wheeze. In serious cases, the person is incapable of walking or other forms of exercise. Due to the lack of oxygen, the person needs a supply with them at all times. The deficiency of oxygen can affect the brain.
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Effects of Smoking

Lung Cancer

  • Lung cells mutate and divide to form a tumour. These cause no physical pain so they are not discovered until it is too late. If found in the early stages, it can be removed. If not, the tumour can spread to other tissues in the body.
  • Pleural fluid fills the pleural cavity, preventing the lungs from inflating and deflating properly so it aches to breathe. Other symptoms are weight loss, loss of appetite, pain when swallowing and subsequently, fatigue and a hoarse throat.
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Components of Smoke


  • The tar remains in the lungs and can destroy the lung walls as it sticks to the cilia, preventing them from sweeping away mucus. This means that bacteria is trapped in the trachea so it can lead to infection. The tar stains the lungs, fingers and teeth and can even cause mouth cancer.


  • Nicotine stimulates adrenline production, giving the smoker a sensation of happiness. When the effects of nicotine wear off, the smoker becomes restless until they have another cigarette to feel 'happy' again. The adrenaline released causes the blood pressure to rise, preventing blood flow to the heart, giving the smoker shortness of breath.

Carbon Monoxide

  • When inhaled, the carbon monoxide combines with the haemoglobin in the blood to make carboxyhaemoglobin. This means the blood carries less oxygen so cells do not receive enough oxygen so they do not function properly.
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