In the respiratory system, the body's cells require energy for most of their chemical activities in order to maintain homeostasis. Most of this energy comes from chemical reactions that can only occur in the presence of oxygen. The respiratory system is the system of the human body that allows an organism to breathe and therefore absorb oxygen into the body. It involves absorbing oxygen into the body and releasing carbon dioxide into the atmosphere or environment. Breathing occurs like all other body mechanisms with the help of special tissues, cells and organs. The lungs are the main organs with the process of breathing. Air passes into the lungs to provide oxygen to the body through the inhalation process. Carbon dioxide that is not needed by the human body is eliminated or expelled through the process of exhalation. The two processes occur interchangeably and are collectively referred to as respiration or ventilation. The respiratory system consists of the trachea, lungs, air sacs, diaphragm, bronchi, and bronchioles. The respiratory system provides the route through which oxygen supply from the atmosphere enters the body and also an opposite route for the excretion of carbon dioxide. The exchange of gases between an organism and its external environment is known as external respiration while that between blood and cells is known as internal respiration. First, air is breathed into the body through the mouth and nose. Air travels through the voice box (pharynx) to the trachea (windpipe) and then through the bronchi to the lungs. Carbon dioxide that is not needed must then be exhaled. This happens in a reverse process. However, the air drawn in... into the center of the paper... is compared to that of the surrounding blood. After oxygen molecules move into the blood, they bind to red blood cells and are then transported throughout the body. Blood containing oxygen travels from the lungs to the chambers of the heart. When it reaches the heart, it is pumped to the rest of the body. Oxygenated blood travels through a series of vessels, allowing red blood cells to come into contact with other cells in the body. Red blood cells further transport unwanted carbon dioxide through an opposing system of vessels in the heart. Once they reach the heart, they are then pumped into the lungs and come into contact with the alveoli. Therefore, the respiratory system and the circulatory system interact with each other when red blood cells discharge carbon dioxide into the lungs as a waste product. Exhalation marks the end of the cycle.