Cardio-Respiratory System: Breathing and Gaseous Exchange
Breathing (ventilation) and gaseous exchange are the two processes of the cardio-respiratory system that move oxygen into the blood and carbon dioxide out of it: breathing is a mechanical process, and gaseous exchange is how the gases cross between air and blood.
Before you start
Make sure you're comfortable with these topics first:
Method
- Learn to distinguish inspiration from expiration, and within each, resting (passive) breathing from exercise (active/forced) breathing.
- Learn exactly which muscles act in each phase: the diaphragm and external intercostals for normal inspiration; the internal intercostals and abdominal muscles added for forced expiration during exercise.
- Learn the chain of cause and effect for inspiration: the muscles contract, thoracic cavity volume increases, pressure inside the lungs decreases below atmospheric pressure, and air moves in down the pressure gradient.
- Learn the pathway air travels into the lungs: trachea, bronchi, bronchioles, alveoli.
- Learn the three features of the alveoli that make gaseous exchange efficient: a large surface area, moist and one-cell-thick walls, and a dense capillary network, and be able to link each one to the rate of diffusion.
- Learn that gases move by diffusion down a concentration gradient - oxygen from alveolar air (high concentration) into the blood (low concentration), and carbon dioxide from the blood (high concentration) into the alveolar air (low concentration).
- For explain questions, state the mechanism and then its effect, e.g. more surface area allows a faster rate of diffusion, meaning more oxygen enters the blood per breath.
Worked example
Describe what happens to the diaphragm, the intercostal muscles, and the volume and pressure of the thoracic cavity, during inspiration at rest.
- State what happens to the diaphragm: it contracts and flattens, moving downward.
- State what happens to the external intercostal muscles: they contract, pulling the ribs upward and outward.
- Link these to thoracic cavity volume: the combined effect increases the volume of the thoracic cavity.
- Link the volume change to pressure: as volume increases, the air pressure inside the thoracic cavity decreases below atmospheric (outside) pressure.
- State the outcome: air moves from the higher pressure outside the body into the lower pressure inside the lungs, down the pressure gradient, so inspiration occurs.
Practice questions
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Q1Name the muscle that contracts and flattens during inspiration.Show answer
Answer: The diaphragm.
Q2State which extra muscles are used during forced (exercise) expiration, in addition to relaxation of the external intercostals.Show answer
Answer: The internal intercostal muscles (and the abdominal muscles).
Q3Name the site where gaseous exchange takes place in the lungs.Show answer
Answer: The alveoli.
Q4Give one structural feature of the alveoli that speeds up the rate of gaseous exchange.Show answer
Answer: A large surface area (or thin, one-cell-thick walls, or a rich capillary network).
Q5State the direction oxygen moves during gaseous exchange at the alveoli.Show answer
Answer: From the alveoli, where it is at a high concentration, into the blood in the surrounding capillaries, where it is at a low concentration.
Q6Is expiration at rest an active or a passive process?Show answer
Answer: Passive - the diaphragm and external intercostal muscles simply relax.
Q7Name the airway that air passes through directly after the trachea.Show answer
Answer: The bronchi.
Q8State the gas that moves out of the blood and into the alveoli to be exhaled.Show answer
Answer: Carbon dioxide.
Exam-style questions
Written in the style of a GCSE Physical Education exam paper, with a full mark scheme.
Explain how the structure of the alveoli makes them efficient at gaseous exchange.
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Analyse the changes to breathing that occur during vigorous exercise, using a named sporting example, and evaluate how effectively these changes meet the oxygen demands of the activity.
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Free printable worksheet
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