The Respiratory System
The respiratory system is made up of the nose, mouth, trachea, bronchi, and lungs. On average, a
person breathes in and out 15 times every minute and in one day, more than 10 000 L of air passes in
and out of the lungs.
The purpose of the respiratory system is to provide oxygen to the body while removing the carbon
dioxide waste that the body produces. Through the circulatory system, oxygen is delivered to the tissues
and carbon dioxide is brought back to the lungs.
Structural Features
The structure that deals with the exchange of gasses is the lungs. The other structures are meant to
connect the lungs to the outside. Air enters the mouth or nose, passes through the pharynx (throat), and
travels to the lungs through the trachea (windpipe). The two branches that the trachea splits into are
called the bronchus (plural: bronchi).
As with any surface, the surface of the trachea and bronchi are lined with epithelial tissues that produce
mucus. These epithelial tissues have cilia which act like the bristles of a broom. The mucus acts like a
filter to catch small particles like dust and the cilia sweep the mucus up to be coughed out. The filtered
air is delivered to the lungs through the bronchi.
Since the mouth can let both food and air in, the opening of the trachea has a flap called the epiglottis.
When food is swallowed, the epiglottis closes to prevent food from entering into the trachea. However,
when some food does enter the wrong tube, the body tries to clear it by coughing.
The Trachea
Rings of cartilage, which is a type of connective tissue that is strong yet flexible, are used to support the
trachea. This keeps the tube open so that air can flow through freely.
Gas Exchange
The two bronchi branches split again and again until they end in tiny air sacs called alveoli. Each alveolus
has a very thin wall and the sac itself is surrounded by a network of capillaries. Oxygen and carbon
dioxide move through the two thin walls by diffusion.
Since the blood carried to the lungs will always have a lower concentration of oxygen than the lungs,
oxygen will always diffuse into the bloodstream. The opposite is true for carbon dioxide. CO2
concentrations will always be high in the blood so it will always diffuse into the lungs.
Breathing
Breathing is controlled by the brain through detecting the concentrations of carbon dioxide in the blood.
When CO2 levels are too high, signals are sent to the muscles of your ribs, telling them to contract. This
pulls your chest outwards. Another signal is sent to your diaphragm, a muscle under your lungs. The
diaphragm contracts as well, making it flatten out to give the lungs room to expand.
The Respiratory System
This creates a negative pressure in the chest cavity which is balanced out when air from the outside
floods into the lungs, thus expanding the lungs. The process of taking in air is called inhalation.
Exhalation is when air exits the lungs and is done by relaxing the chest muscles and diaphragm. The
muscles push down on the lungs, thus pushing the air out.
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