Respiration is the process that gets oxygen to the tissues of the body. In mammals, this involves inhalation through the nose into the lungs, where oxygen enters the blood. Oxygenated blood delivers oxygen to the tissues and removes carbon dioxide (the waste gas produced by metabolism). The end of the respiratory cycle occurs when the carbon dioxide is exhaled. Without the circulatory system (heart and blood vessels), the respiratory system would be ineffective.
Nose and Sinuses
The nose conditions (warms and humidifies) air before it enters the lungs. The sinuses are openings in the skull bones and are connected to the nasal cavity, so more air can be warmed and moistened. The nose and sinuses are lined with mucus-producing cells and cilia. The sticky mucus traps particles in the air (for example, dust or pollen). The cilia, which look like small hairs, push the mucus out of the body.
The air leaves the nose through the pharynx, which funnels food from the mouth to the esophagus and air from the nose to the trachea. Because the trachea and esophagus cross it is possible to “swallow wrong” and get food into the trachea—i.e choke on your food. The trachea is lined with cartilage and with mucus-producing cells and cilia so that particles that get past the nose can be caught in the trachea.
The bottom of the trachea splits into two bronchi that split into progressively smaller bronchi and even smaller bronchioles that carry the air into the lungs. As the tubes get smaller they have less cartilage and more smooth muscle.
At the end of the bronchioles are alveoli, small sacs that are one cell thick. On the other side of the alveoli are capillaries, where blood from the heart passes by and gives up the carbon dioxide it has carried from the body for the oxygen in the lungs.
Gas Exchange in Alveoli
Hemoglobin in red blood cells binds to carbon dioxide and oxygen, but not both at the same time. When the blood passes the oxygen-rich alveoli in the lungs, the carbon dioxide leaves the blood and enters the air sacs, while the oxygen does the opposite. The blood that leaves the lungs and returns to the heart is full of oxygen.
Distribution of Oxygen to the Body
The heart pumps the oxygenated blood returning from the lungs to the rest of the body. Oxygen leaves the blood when it reaches capillaries; in the tissues the oxygen is used to metabolize nutrients and provide energy. At the same time, the carbon dioxide (made when oxygen is used up) returns to the blood so it can be exchanged for oxygen in the lungs.