In order for respiration to occur, gases must be exchanged between the human and the environment. Oxygen must be inhaled while carbon dioxide must be exhaled as a waste.
When we inhale, air enters the nose where it is moistened and filtered due to hairs that can trap dust and other particles. Next, air passes into the pharynx, the tube inside the throat. Next, air passes into the trachea, a tube which has cartilaginous rings to strengthen it and to help it remain open. At the top of the trachea are the vocal cords as well.
Next, the trachea branches into two large tubes called the bronchi and air continues on its journey. Each of these bronchi lead to one of the two lungs. Inside our lungs, the bronchi divide into smaller tubes called bronchioles. These continue to get smaller and smaller, with their ends culminating in tiny alveoli or air sacs. These are the organs of gas exchange. They are surrounded by tiny capillaries from the circulatory system which allow the exchange of gases between the alveoli and the blood stream. Oxygen in the air sacs enters the capillaries by diffusion, and can be transported in the circulating blood to the cells, while carbon dioxide in the bloodstream enters the air sacs by diffusion and can be exhaled.
Breathing is regulated by carbon dioxide levels in the blood. Higher levels mean that the brain sends impulses to the diaphragm (a muscular organ between the chest and abdomen) to cause it to contract. When this occurs, the air pressure inside the chest cavity is lower than the surrounding air. This change in pressure pushes air into the nose and it begins its journey to the lungs.
When we exhale, the diaphragm relaxes. This causes the air pressure inside the chest cavity to become higher than in the surrounding air. This pushes air out of the lungs and, eventually, we exhale.
I have included a link to a labeled diagram of the human respiratory system.
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