Chapter 7

• Food gives us energy, which is needed for life processes such as movement, excretion, growth and reproduction.
• Chemical potential energy

  Energy stored in organic nutrient molecules released in our cells
• Aerobic Respiration

  The release of energy by the breakdown of glucose in the presence of oxygen
• Aerobic Respiration

  • Generates carbon dioxide and water as waste products
  • Generates large amount of energy
• Anaerobic Respiration

  The release of energy by the breakdown of glucose in the absence of oxygen
• Anaerobic Respiration

  • Generates lactic acid as a waste product in humans
  • Releases relatively small amounts of energy
• What Happens During Vigorous Activity

  1. Increased breathing to obtain oxygen

  2. Increased heart rate to supply oxygen
• There is a limit to the increase in breathing rate and heartbeat.
• Aerobic respiration alone is not fast enough to meet the increased energy demand.
• Lactic acid

  Builds up in muscles during vigorous activity
• Oxygen debt

  The amount of oxygen required to eventually remove lactic acid in our body
• Heartbeat and breathing rate will remain high in order to repay the oxygen debt.
• Anaerobic respiration takes place to meet the increased energy demand.
• Breathing

  Involves movement of air in and out of the lungs
• Respiration

  Refers to cellular respiration
• Microorganisms such as yeast respire anaerobically when there is a lack of oxygen.
• Unlike anaerobic respiration in muscle cells, anaerobic respiration in yeast produces carbon dioxide.
• We can use yeast to study anaerobic respiration.
• Aerobic respiration

  • Breaks down glucose in the presence of oxygen
  • Waste products produced: carbon dioxide + water
  • Releases a large amount of energy
• Anaerobic respiration

  • Breaks down glucose in the absence of oxygen
  • Waste products produced: lactic acid
  • Releases a relatively small amount of energy
• Oxygen debt


  • Occurs when there is insufficient supply of oxygen in muscles during vigorous activity
  • Addressed through keeping the heart rate fast and breathing deeper and faster
• Parts of the Human Gas Exchange System

  • External nostrils
  • Nasal passages
  • Pharynx
  • Larynx
  • Trachea
  • Bronchi
  • Bronchioles
  • Alveoli
• Passage of airway into the lungs

  1. Air enters through the two external nostrils

  2. Nostrils lead into two nasal passages

  3. Air flows through pharynx and larynx

  4. Trachea is supported by C-shaped rings

  5. Trachea divides into bronchi

  6. Each bronchus branches into bronchioles

  7. Each bronchiole ends with alveoli
• Alveoli

  • Provide large surface area to volume ratio for gas exchange via diffusion
  • Wall of an alveolus is only one-cell thick for faster diffusion
• A thin film of moisture covers the inner wall of the alveolus, allowing oxygen to dissolve in it.
• Walls of the alveoli are richly supplied with blood capillaries

  to allow the continuous flow of blood

  which maintains the concentration gradient of the gases.
• Gas exchange in the alveolus

  1. Oxygen dissolves into the thin film of moisture

  2. Dissolved oxygen diffuses through the wall of the alveolus and wall of the capillary into the red blood cells

  3. Oxygen combines with hemoglobin in red blood cells to form oxyhemoglobin

  4. Carbon dioxide diffuses from the blood into the alveolar air
• Alveoli

  • Numerous alveoli provide large surface area to volume ratio
  • Wall of an alveolus is one-cell thick
  • Thin film of moisture covers the inner wall
  • Walls are richly supplied with blood capillaries
• Gas exchange occurs through the walls of alveoli
• Numerous alveoli in the lungs provide large surface area to volume ratio for gas exchange via diffusion
• Wall of an alveolus is only one-cell thick, providing a short diffusion distance and hence a faster rate of diffusion
• Thin film of moisture covers the inner wall of the alveolus, allowing oxygen to dissolve in it
• Walls of the alveoli are richly supplied with blood capillaries, maintaining the concentration gradient of the gases
• Cilia (In the membrane of the lumen in the trachea)


  Sweep dust-trapped mucus towards the pharynx to be coughed out or swallowed
• Continuous flow of inhaled and exhaled air is important for gas exchange
• Blood entering lungs

  Has a lower concentration of oxygen and higher concentration of carbon dioxide compared to atmospheric air
• Inspiration

  1. Diaphragm muscles contract and diaphragm flattens

  2. External intercostal muscles contract, internal intercostal muscles relax

  3. Ribs move up and forward, volume of thoracic cavity increases

  4. Lungs expand and air pressure inside the lungs is lower than atmospheric pressure
• Expiration

  1. Diaphragm muscles relax and diaphragm arches upwards

  2. External intercostal muscles relax, internal intercostal muscles contract

  3. Ribs move downwards and inwards, volume of thoracic cavity decreases

  4. Lungs compress and air pressure inside the lungs is higher than atmospheric pressure
• Prolonged exposure of the lungs to irritants can lead to chronic bronchitis
• Chronic bronchitis is characterized by inflamed linings of bronchus that secrete excessive mucus