Gas Exchange in Humans

Topic 6.4

6.6.1: Ventilation, Gas Exchange and Cellular Respiration

Plasma Membrane Structure

Table 1: The roles of ventilation, gas exchange and cellular respiration.

The step numbers in the table correspond to the roman numerals in Figure 2 shown above.

Step Description
I Ventilation brings fresh air into the alveolus and removing stale air from the alveolus. This mechanism provides contact between the fresh air and the respiratory surface of the alveolus which allows for gas exchange to occur. Ventilation ensures that there is a high concentration of O2 and a low concentration of CO2 in the alveolus.
II Gas exchange in the lungs occurs in the alveolus where there is an exchange of O2 and CO2 between lung and the environment. The lung takes up O2 where it diffuses from the alveolus into the lung capillary and releases CO2 where it diffuses from the capillary into the alveolus.
III Gas exchange in the tissues occurs between the capillaries, extracellular fluid and the cells where there is an exchange of O2 and CO2. In the tissues cells undergo cellular respiration where O2 is used up and CO2 is produced. Therefore, O2 diffuses from the capillaries into the extracellular tissue fluid and then into the cells while CO2 diffuses from the cells into the extracellular fluid and then into the capillaries.
IV Cellular respiration occurs in all cells of the body. The cells require O2 in order produce ATP and CO2 from food molecules (i.e., glucose).

6.6.2: Why do we need ventilation?

Movement of Oxygen and Carbon Dioxide in the Alveolus

6.6.3: What features of the alveolus that help in gas exchange?

Features alveolus

Feature Role in gas exchange
i The alveolus has a large surface area which allows more oxygen to diffuse in and more carbon dioxide to diffuse out in the same period of time.
ii The alveolus is made up of a single layer of flattened cells which is essential to keep the diffusion distance for oxygen and carbon dioxide short, therefore, diffusion occurs quickly.
iii The alveolus is covered in a thin film of moisture, therefore, the oxygen and carobn dioxide will diffuse into this layer and keep their respective concentrations high within the thin film. High concentration gradients for these gases will ensure diffusion occurs quickly.
iv The alveolus contains a dense network of capillaries which helps make diffusion more efficient.

6.6.4: Structures of the human lung

Can you label the structures shown below in the figure?

Double click on the smiley face to show the labels and click on smiley face to hide the labels.

breathing link breathing animation
lungstructureblank

6.6.5: Lung structures and the mechanism of ventilation of the lung

Structure Inhalation Exhalation
Intercostal muscles External muscles contract while the internal muscles relax moving the rib cage up and out. External muscle relax while internal muscles contract moving the ribcage down and in.
Diaphragm Contracts becoming flatter and moving down. Relaxes becoming dome-shaped. Stomach muscles also contract pushing up on the diaphragm.
Thorax volume Increases due to flattening of the diaphragm. Decreases due to doming of the diaphragm and contracting of stomach muscles.
Thorax pressure Decreases below atmospheric pressure due to increase in thorax volume. Increases above atmospheric pressure due to decrease in thorax volume.
Air flow Air moves into the lungs from outside of the body until the pressure inside the lungs rises to atmospheric pressure. Air moves out of the lungs to outside of the body until the pressure inside the lungs falls to atmospheric pressure.
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