Gas exchange is the process of swapping one gas for another.
It occurs in the alveoli of the lungs.
Oxygen diffuses into the capillaries from the air in the alveoli and carbon dioxide diffuses out of the capillaries and into the air in the alveoli.
Functions of the different parts
The trachea or windpipe is made of muscle and elastic fibres with rings of cartilage.
The cartilage prevents the tubes of the trachea from collapsing.
The trachea is divided or branched into bronchi and then into smaller bronchioles.
The bronchioles branch off into alveoli.
These tubes are lined with mucous-secreting cells and tiny hairs called cilia.
The mucous traps bacteria, dust and viruses.
The cilia beat and create an upward current.
This moves the mucous up and into the oesophagus.
Here it goes to the stomach.
When we clear our throats we force the mucous away from our vocal cords.
This is often called coughing.
It is used to get rid of irritants and excess mucous from our respiratory system.
The lungs are a spongy structure where the exchange of gases takes place.
Each lung is surrounded by a pair of pleural membranes.
Between the membranes is pleural fluid, which reduces friction while breathing.
The bronchi are divided into about a million bronchioles.
The ends of the bronchioles are hollow air sacs called alveoli.
There are over 700 million alveoli in the lungs.
This greatly increases the surface area through which gas exchange occurs.
Surrounding the alveoli are capillaries.
The lungs give up their oxygen to the capillaries through the alveoli.
Likewise, carbon dioxide is taken from the capillaries and into the alveoli.
Body cells use the inhaled oxygen obtained from the alveoli of the lungs.
In turn, they produce carbon dioxide and water, which is taken to the alveoli and then exhaled.
These exchanges occur as a result of diffusion.
In each case the materials move from an area of high concentration to an area of lower concentration.
The alveoli are well suited for the important job they have.
There are about 300,000,000 alveoli per lung!
That means there is a great surface area for gas exchange.
Also, the walls of the alveoli as well as the capillaries are very thin so that the gases can diffuse readily.
When the blood picks up the diffused gases the gases are carried to their destinations.
Most of the oxygen is carried by the haemoglobin in the red blood cells with only a small % dissolved in the plasma.
Carbon dioxide and water are carried in the plasma of the blood.
Breathing
Breathing exchanges gases between the outside air and the alveoli of the lungs.
Lung expansion is brought about by two important muscles, the diaphragm and the intercostal muscles.
The diaphragm is a dome-shaped sheet of muscle located below the lungs that separates the thoracic and abdominal cavities.
The intercostal muscles are located between the ribs.
Nerves from the brain send impulses to the diaphragm and intercostal muscles, stimulating them to contract or relax.
When the diaphragm contracts, it moves down.
The dome is flattened, and the size of the chest cavity is increased.
When the intercostal muscles contract, the ribs move up and outward, which also increases the size of the chest cavity.
By contracting, the diaphragm and intercostal muscles reduce the pressure inside the lungs relative to the pressure of the outside air.
As a consequence, air rushes into the lungs during inhalation.
During exhalation, the reverse occurs.
The diaphragm relaxes and its dome curves up into the chest cavity, while the intercostal muscles relax and bring the ribs down and inward.
The diminished size of the chest cavity increases the pressure in the lungs, thereby forcing air out.
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