Respiratory System: Part 2
The digestive and circulatory systems are mostly out of our control – with our respiratory system
however, we are given a lot more control of the process.
Our brain acts as the respiratory control centre to co-ordinate breathing movements and regulate the
breathing rate. It also monitors the volume of air in the lungs and the gas levels in the blood.
Two structures control the air pressure inside our lungs:
1. Diaphragm – a dome-shaped layer of muscle that separates the region of the lungs from the
region of the stomach and liver.
2. Rib Muscles – muscles found between the ribs and along the inside surface of the rib cage.
Inhalation begins when… the external intercostal muscles and the diaphragm contract.
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This action expands the rib cage upward and outward, and the floor of the chest cavity moves
downward.
Since the chest cavity is airtight, its volume increases and pressure decreases.
 As the air pressure in the cavity decreases,
the walls of the lungs are drawn outward and the
lungs expand
 This expansion causes the air pressure in
the lungs to be lower than the air pressure outside
the body.
 Since air moves from regions of higher
pressure to regions of lower pressure, air rushes
into the lungs from the external environment.
The opposite muscle movements expel air from the lungs
Exhalation begins when… the diaphragm and the rib muscles relax.
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This action brings the rib cage downward and inward, and the floor of the chest cavity moves
back up.
Since the chest cavity is airtight, its volume decreases and pressure increases.
 As the air pressure in the cavity increases,
the walls of the lungs are drawn back in and the
volume of the lungs decreases.
 This decrease in lung volume causes the air
pressure inside the lungs to be higher than the air
pressure outside the body.
 Since air moves from regions of higher
pressure to regions of lower pressure, air rushes
out of the lungs and into the external
environment.
Vital Capacity – the maximum volume of air that can be moved into and out of the lungs in one breath.
PASSAGE OF AIR THROUGH THE HUMAN RESPIRATORY SYSTEM
1. Air enters the respiratory system primarily through the nostrils, and also through the mouth
(especially during strenuous exercise).
 Inside the nasal passages at the back of the nose, air is warmed, moistened, and
cleansed of dust and other small particles.
2. The warm, moist, cleaned air passes from the nasal passages through the pharynx (throat).
3. At the base of the pharynx, behind the tongue, is the entrance to the trachea (windpipe).
 This opening is called the glottis. The glottis can be closed by the epiglottis (when a
bolus of food slides down the pharynx)
4. Between the glottis and the trachea is the larynx (voice box)
 This is where the vocal cords are located. They are usually held apart during breathing,
and move closer together to make sounds (as they vibrate)
5. The trachea runs about 10-12cm from the throat to the middle of the chest, where it splits into
two branches called bronchi (singular – bronchus)
 One bronchus enters each lung.
o The right lung has 3 lobes
o The left lung has 2 lobes (to make room for the heart)
6. Inside the lungs, each bronchus subdivides many times to form a network of microscopic
tubules called bronchioles.
7. Each bronchiole eventually ends in a grape-like cluster of tiny sacs called alveoli (singular
alveolus)
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There are an estimated 500
million alveoli in an average-sized human
lung
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Surrounding each alveolus is a
network of fine capillaries.
o
The walls of the alveoli and
the capillaries are only one cell layer thick
- Oxygen moves into the capillaries
- Carbon Dioxide moves into the
alveoli