RESPIRATORY SYSTEM

Respiration as a process that occurs at two different levels. One level is the level of the cell. Here, in the mitochondria of eukaryotic cells, respiration requires oxygen, releases carbon dioxide, and produces large amounts of ATP. This level of respiration is called internal respiration, or cellular respiration.
The other level at which respiration occurs is the level of the organism. Here, an organism must get oxygen into its cells (and thereby into the mitochondria) and carbon dioxide back out. This level of respiration is called external respiration because the exchange of gases takes place with the external environment. External respiration involves the respiratory system.
A respiratory system is a group of organs working together to bring about the exchange of oxygen and carbon dioxide with the environment.

The Human Respiratory System
Respiratory Structures
The human respiratory system consists of the nose, nasal cavity, pharynx, larynx, trachea, smaller conducting, passageways, and lungs.
Air enters the body through the mouth or nose. Air entering the nose passes into the nasal cavity. The nasal cavity is richly supplied with arteries, veins, and capillaries, which bring nutrients and water to its cells. As air passes back from the nasal cavity, it enters the pharynx. The pharynx is located in the back of the mouth and serves as a passageway for both air and food. From the pharynx, the air moves into the trachea (windpipe), this leads directly into the lungs. All these passageways provide a direct connection between the outside air and some of the most delicate tissues in the body. Therefore, these passageways must filter out dust, dirt, smoke, bacteria, and a variety of other contaminants found in ordinary air.
The first filtering is done in the nose. The nasal airways are lined with hair and kept moist by mucous secretions. The combination of hair and mucus helps to filter out all sorts of solid particles from the air that passes through the nose. The moisture in the nose helps to humidity the air, increasing the amount of water vapor the air entering the lungs contains. This helps to keep the air that enters the nose from drying out the lungs and other parts of the respiratory system. When air enters the respiratory system through the mouth, much less filtering is done. Therefore, it is generally better to take in air through the nose.
At the top of the trachea is the larynx. The larynx is made up of several pieces of cartilage. The larynx produces sound, it is sometimes known as the voice box.
From the larynx, the air passes downward into the chest cavity through the trachea. The walls of the trachea are made up of C – shaped rings of tough, flexible cartilage. These rings of cartilage protect the trachea, make it flexible, and keep it from collapsing or over expanding. The cells that line the trachea produce mucus. This mucus is swept out of the air passageway by tiny cilia on other cells. In this way, particles trapped in the mucus are carried to the upper part of the trachea and swept down into the digestive system.
The trachea divides into two branches, the right and left bronchi (singular: bronchus). Each bronchus enters the lung on its respective side.