The Breathing System

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Definitions

Breathing
The physical process of taking air into and out of the lungs.

Respiration
The chemical process of releasing energy from food.

Parts of the Breathing System

Click here for a diagram of the breathing system.

Mouth and nasal passages
Air is
(a) Moistened
(b) Warmed
(c) Filtered by the mucus and the hairs in the nose.

Pharynx
Region at the back of the nose and throat. The epiglottis is a flap of tissue, which closes off the windpipe when swallowing. Just below the epiglottis is the larynx (voice box). It contains two folds of membranes movement of air across these membranes produce sound.

Trachea
The trachea (windpipe) connects the pharynx to the lungs. At the lower end the trachea divides into two smaller tubes the bronchi leading to the lungs. Each bronchus on entering the lung divides repeatedly to give rise to a number of tubes called bronchioles. The bronchioles end in tiny air sacs called alveoli. The trachea and the bronchi are prevented from collapsing by c shaped rings of cartilage.

Lungs
Soft spongy organs where exchange of gases takes place.

Comparing breathed in and breathed out air

The air breathed out differs in the following ways:

Warmer
More moist
More carbon dioxide
Less oxygen
Less dust
Different odour
Different proportion of nitrogen

Mechanism of breathing

Inspiration

The diaphragm and intercostal muscles contract
The diaphragm flattens
The rib cage moves up and out
The volume of the thoracic cavity increases therefore the pressure decreases
The pressure of the air outside the body is now greater than the pressure of the air in the chest
Gas flows from higher pressure to lower pressure
As a result air flows into the lungs
It is an active process (energy is required) because muscles are contracting

Expiration

The diaphragm and intercostal muscles relax
The diaphragm moves up to its relaxed dome position
The rib cage moves down and in
The volume of the thoracic cavity decreases therefore the pressure increases
The pressure of the air in the chest is now greater than the pressure of the air outside the body
Gas flows from higher pressure to lower pressure
As a result air flows out of the lungs
It is a passive process (energy is not required) because muscles are not contracting. Expiration only requires energy during strenuous exercise.

Control of the breathing mechanism
The rate of breathing is controlled by the respiratory centre in the medulla oblongata at the base of the brain. When the level of CO₂ in the blood increases the medulla sends impulses to the intercostal muscles and the diaphragm causing them to contract. Exercise increases the rate of respiration therefore it increases the amount of CO₂ in the blood. As a result the rate of breathing is quickened and deepened.

Experiment 1
To show the effect of exercise on the rate of breathing

Lung capacity

Tidal volume
The amount of air that passes in and out of the lungs during a single breath at rest. This is about 500cm³.

Inspiratory reserve volume
The amount of air over and above the tidal volume that can be inhaled during a deep breath. This can be up to about 2500cm³.

Expiratory reserve volume
The extra amount of air that can be expelled from the lungs after normal expiration, to empty them as much as possible. This can be up to about 1000cm³.

Vital capacity
This is the maximum amount of air that can be taken in ore forced out of the lungs in one breath.
Vital capacity = Tidal volume + Inspiratory reserve volume + Expiratory reserve volume = 500cm³ + 2500cm³ + 1000cm³ = 4000cm³

Residual volume
The amount of air that is left in the lungs after as much air as possible has been forced out. This is 1500cm³ approx.

Total capacity
Vital capacity + residual volume = 4000cm³ + 1500cm³ = 5500cm³

Gaseous exchange in the alveoli

As a result of respiration the cells of the body use up oxygen and produce carbon dioxide.
The carbon dioxide diffuses into the blood stream.
In the lungs the Carbon dioxide diffuses from the blood stream into the alveoli.
In the lungs the oxygen diffuses from the alveoli into the blood stream.
The oxygen then diffuses from the blood stream into the cells of the body.

	Experiment 2 To show that exhaled air contains more carbon dioxide than inhaled air.
	Experiment 3 To demonstrate how the diaphragm affects breathing using the 'artificial lung' experiment.

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