6.4 Gaseous exchange

6.4 Gas exchange

Fundamental questions: How is breathing achieved? What are the adaptations of the alveoli for gaseous exchange? 


Lesson one: Breathing

There are three introductory challenges to this lesson

Objectives: Understand how breathing is produced using muscles.

Key words:

  • Ventilation: the bulk movement of air between the lungs and the environment.
  • Cellular Respiration: the release of energy from sugars (happens in cells).
  • Gas exchange: the movement of gases (in this context oxygen and carbon dioxide), between the alveoli and the capillaries
  • Alveoli: air sacs in the lungs that facilitate gas exchange.
  • Cartilage: a flexible connective tissue made mostly from a structural protein called collagen.


The respiratory system is designed to facilitate both ventilation and gas exchange. When you take in a breath, air moves sequentially from:

  1. The mouth and nose
  2. Sinuses and the pharynx (the back of the nose and the throat respectively)
  3. Trachea
  4. Left and right bronchi (bronchus singular)
  5. Left and right bronchioles
  6. Alveoli in the lungs, where gas exchange occurs.
imagecredit: organsofthebody.com

imagecredit: organsofthebody.com


The lungs are covered with a membrane called the pleural membrane. This protects the lungs and reduces friction as they expand and contract. Lungs are spongy tissues, because they comprise mostly tiny air sacs called alveoli, as well is capillaries, connective tissues, and small air tubes called bronchioles. 

imagecredit: ptinterest.com

imagecredit: ptinterest.com


Gas exchange between the alveoli and the capillaries occurs by diffusion from high –) low concentration gradients. The alveoli are designed to facilitate this exchange. Inhaled air and exhaled air are compared in the table below.

Q) Compare  inhaled and exhaled air and suggest reasons for the patterns shown.

image credit: quora.com

image credit: quora.com


How does breathing work? Breathing, or ventilation, is the bulk exchange of air between the lungs and the atmosphere. Ventilation is achieved by altering the volume of the thorax (thorax = part of chest including lungs), and therefore the pressure inside the lungs. The alteration of the volume of the thorax is controlled by muscles; principally the diaprhagm, internal and external intercostal muscles, and the diaphragm.


Volume of thorax increases —-) pressure in lungs drops —–) negative pressure draws air in


Volume of thorax decreases—–)pressure in lungs rises —–) positive pressure pushes air out

The Ribs.

In order to increase the thoracic cavity, the ribs also move upwards and outwards to some extent. This is made possible by the fact that out of the 12 ribs, the upper ten are attached to the sternum by flexible costal cartilage. Ribs 11 and 12 are actually not attached to the sternum, which allows for even more movement.

Rib movement:

The rib bones have two important layers of muscle, the internal intercostal muscles and the external intercostal muscles. These help to move the ribs upwards and outwards (inhalation), and downwards and inwards (exhalation). The two layers of muscle act antagonistically (in opposition).




Summary Discussion link:



A.) Sketch on a human body outline the respiratory system. Label the pathway of air for a human body for inhalation and exhalation, using two colour coded pencils.

B) Build a table showing how the different muscles of the thorax (diaphragm and intercostal muscles) achieve the pressure changes indicated above.

Bonus extension: Research the following respiratory conditions and share with the class; asthma, pleurisy, emphysema, bronchitis, pleural effusion.


Lesson two: The Alveoli

Objectives: understand the structure of alveoli. Understand how alveoli facilitate gas exchange


  • Alveoli: air sacs in the lungs that facilitate gas exchange.
  • Pneumocytes 1: cells in the alveoli which are adapted for gas exchange
  • Pneumocytes 2: cells in the alveoli lining which are adapted to reduce friction
  • Surfactant: a soapy liquid produced by pneumocytes 2 that reduces friction

Gas exchange only occurs across the lining of the alveoli. Gas exchange occurs between the alveoli and capillaries.

Where are the alveoli?


image credit: ibbiology blogspot

image credit: ibbiology blogspot


Skill: Label an alveolus 

image credit: IB Oxford course companion

image credit: IB Oxford course companion

Activities: Labelling challenge

A. Copy the image and label in as much detail as possible

image credit: Claudins

image credit: Claudins

 B. https://highered.mheducation.com/sites/dl/free/0072351136/23466/chap15labeling03.html

Essay question prepare an essay outline for the following question – How are alveoli adapted for their role in gaseous exchange:

Alveoli are adapted to their role of gas exchange in the following ways: 

  • Pneumocytes type 1 are very thin epithelial cells (0.1 micrometers), maintaining a small diffusion distance
  • Pneumocutes type 2 secrete a surfactant, keeping the surface moist which enhances diffusion
  • The surfactant also reduces surface tension, and stops the alveolar walls from sticking together
  • Capillaries maintain high concentration gradients for gas exchange
  • Both types of cell are highly permeable to gases

Studying spirograms

A spirogram is a graphic produced by a spirometer, which measures breathing or ventilation. A spirometer can be used in an investigation into breathing rates (this is not to be confused with a respirometer which measures cellular respiration).

Task: Understand how a spirometer can be used in class (practical skills). Make some personal notes on how this is done.

A Spirogram 

image credit:medical dictionary

image credit:medical dictionary


Tips when reading a spirogram:

  • the difference between the peak and a trough, represent the volume of a breath
  • Dividing the number of peaks by the time, will give you the breathing rate
  • Exercise has the effect of making the peaks larger and closer together
  • Oxygen consumption can be calculated by multiplying breathing rate x volume x 0.25

Screen Shot 2016-10-31 at 10.39.08 AM

Diseases of the lung tissue:

Emphysema: causes alveolar walls to break down, this reduces the surface area available for gas exchange. Smaller alveoli are replaced with larger alveoli, with thicker walls. This increases the distance required for diffusion and therefore reduces the efficacy.

Lung cancer: A tutor forms in the lungs, often from pneumocytes

Bronchitis: Failure of the ciliary escalator, resulting in build-up of mucus in the lungs

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