Key ideas for breathing and gas exchange

Inspiration

• Diaphragm contracts and becomes flatter
• External intercostal muscles contract and pull ribs upwards
• Volume of chest cavity increases
• Pressure in chest cavity drops below atmospheric pressure
• Air moves into lung

Expiration

• Diaphragm relaxes and is pushed upwards by digestive organs
• External intercostal muscles relax and ribs drop
• Volume of chest cavity decreases
• Pressure in lungs increases above atmospheric pressure
• Air moves out of the lungs

(Internal intercostal muscles only contract during forced expiration)

A spirometer measures lung capacity and breathing rate - it can measure the following

• Tidal volume - the volume of air moved in and out of the lungs during each breath at rest. It is normally around 0.5dm3
• Vital Capacity - The largest volume of air that can be moved into and out of the lungs in one breath. It is normally around 5dm3
• Residual Volume - the volume of air that remains in the lungs even after forced expiration. normally around 1.5dm3
• Inspiratory Reserve Volume - the volume of extra air that can be taken in above the amount of normal tidal volume
• Expiratory Reserve Volume - the volume of extra air that can be moved out above the amount of normal tidal volume

The main airways are the Trachea, Bronchi and Bronchioles

Airways should have the following features

• Large enough to allow sufficient air flow without obstruction
• Must divide into smaller airways to deliver air to all the alveoli
• Must be strong enough to not collapse when air pressure is low
• Must be flexible to allow movement
• Must be able to stretch and recoil

Airways are adapted for their function in the following ways

• Incomplete rings of cartilage in the trachea and bronchi offer structural support while allowing movement
• Goblet cells produce mucus to trap particles such as pollen or bacteria
• Ciliated epithelium have cilia which move to waft the mucus to the top of the throat
• Smooth muscle in all of the airways allow it to constrict the lumen, restricting airflow
• Elastic fibres cause recoil that will dilate the airways to their original size

The lungs are adapted in several ways for efficient gas exchange. Some adaptions increase the rate of diffusion by maintaining a diffusion gradient, some shorten the diffusion pathway

• Large surface areas (more space for diffusion) provided by the alveoli
• Barrier that is permeable to oxygen and carbon dioxide (plasma membranes are the only barriers)
• The walls of the alveoli which are only one cell thick (reduced diffusion distance)
• The walls of the cappilaries which are only one cell thick (reduced diffusion distance).
• The cells in the walls of the alveoli and capillaries are flattened squamous epithelial cells (reduced diffusion distance).
• The capillaries are in close contact with alveoli (reduced diffusion distance).
• The capillaries are narrow, squeezing blood cells against their walls (reduced diffusion distance).
• Ventilating the lungs keeps levels of oxygen high and carbon dioxide low in the lungs (maintains gradient).
• Constant blood flow keeps levels of carbon dioxide high and oxygen low in the capillaries (maintains gradient)