Gaseous exchange in humans:
humans can exchange their gasses through lungs

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Airways

The primary job of the human respiratory system is gas exchange - to bring in fresh oxygen and expel carbon dioxide. The airways are pipes that carry oxygen-rich air to your lungs and carbon dioxide (a waste gas), out of your lungs. The airways include your:

• Nose and linked air passages called nasal cavities
• Mouth
• Larynx or voice box
• Trachea or windpipe
• Tubes called bronchial tubes or bronchi, and their branches^[1]

Air initially enters the respiratory system through the nasal and oral cavities. Both of these passageways meet at the same place- the pharynx. The pharynx then leads to the larynx, and down to the trachea. The trachea then branches in two different paths, known as the left and right primary bronchi. These small tubes carry the fresh oxygen deep into the lungs

The nose and mouth draw in the cold, dry air; this is where the cilia in the nose and mucus membrane come in. The cilia filter out fine particles from the air to keep them from entering and damaging the sensitive lung material. While the mucus membrane also traps fine particles, it warms and moistens the air coming in too. Generally the human body will draw its air from the nose, but in some cases it will take air in via the mouth. This is generally due to a blockage of the nasal passages, preventing the air to travel down without interference. It is potentially hazardous to breathe in with the mouth as the air does not receive as much filtration prior to reaching the lungs as it would if it was drawn in through the nose. As this air travels down the pharynx it gets filtered further. The same mucus which was produced by the goblet cells in the nasal passages has been swallowed and has coated the walls of the pharynx.

The air then reaches the larynx. This is a small tube that is attached to the pharynx and is comprised mainly of tough cartilage. Its main function is to guide the air down towards the lungs but it also helps in speech. It is a primary factor in determining how one’s voice sounds. The tip of the larynx also is covered by a small sheath of tissue called the epiglottis. This covering keeps food and water from entering into the lungs by closing instantly as one swallows food or water. The larynx then leads to the trachea, a cartilage-ringed tube that branches off into two bronchi

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Lungs

Main Article: Lungs

The bronchi of the lungs branch off into several bronchioles. All these structures share in the function to trap and remove particles and foreign matter from the respiratory system as they too are coated with mucus and covered with cilia to help the body remove particles. As the bronchioles continue to divide hundreds of times over they meet their ending as a small air filled pocket. These pockets are the alveoli.

 They serve as the end point in the body for the fresh air and transfer the oxygen directly into the blood stream via the thousands of tiny capillaries that are wrapped around the alveoli. These capillaries quickly pick up the fresh oxygen and transfer it directly to the heart where it is then redistributed throughout the entire body. Not only does the alveolus play a vital role in the diffusion of oxygen, but it also serves to remove the body of carbon dioxide waste gas. This carbon dioxide is a byproduct of cellular functions and is constantly being produced in the body. Thus, the body must rid itself of this poison swiftly and efficiently. It does this via diffusion again. The carbon dioxide travels through the bloodstream and back to the capillaries intertwined with the alveoli. It then transfers into the alveoli and is removed from the body by exhalationThe bronchi branch out into the lungs, getting thinner and sparser as they go. Eventually they get thin enough as they are about to end, they can then be called bronchioles. These bronchioles are extremely thin, air-sac structures that harbor the precious oxygen before diffusion into the blood stream occurs. These small sacs are known as the alveoli. There are hundreds of small capillaries around the alveoli; they serve to transport the oxygen into the blood and the carbon dioxide out of the blood.

Process of inspiration and expiration. 

Process of inspiration (breathing in)

1. external intercostal muscles contract
2. ribs and sternum move up and out
3. width of thorax increases front to back and side to side
4. diaphragm contracts
5. diaphragm moves down, flattening
6. depth of thorax increases top to bottom so the...
   • volume of thorax increases.
   • pressure between the pleural surfaces decreases.
   • lungs expand to fill thoracic cavity.
   • air pressure in alveoli is less than atmospheric pressure.
   • air is forced in by the higher external atmospheric pressure.

As the lungs fill with air the stretch receptors send impulses to the expiratory part of the respiration centre to end breathing in.

Process of expiration (breathing out)

1. External intercostal muscles relax
2. ribs and sternum move down and in
3. width of thorax decreases front to back and side to side
4. diaphragm relaxes
5. diaphragm moves up
6. depth of thorax decreases top to bottom. So the ...
   • volume of thorax decreases.
   • pressure between the pleural surfaces increases.
   • lung tissue recoils from sides of thoracic cavity
   • air pressure in alveoli is more than atmospheric pressure.
   • air is forced out.

As the air leaves, the stretch receptors are no longer stimulated. The inhibition of breathing in (via the expiratory part of the centre) stops so breathing in can start aga