The lungs are a pair of organs in the chest that supply the body with oxygen and remove carbon dioxide (CO2).
Air enters the lungs through the trachea and passes through tubular branches, called bronchi. The bronchi divide into smaller and smaller branches (bronchioles), finally becoming microscopic. These microscopic air sacs are called alveoli.
The walls of the hundreds of millions of alveoli are lined with microscopic capillaries where gas exchange occurs through a semi-permeable membrane. This diffusion is driven by the movement of each gas down the concentration gradient. The partial pressure of CO2 in these blood vessels is higher than in the alveoli, causing it to diffuse from the blood into the air within the alveoli and be exhaled, following the concentration gradient. At the same time, oxygen also follows the concentration gradient down, diffusing from the alveoli into the blood, where its partial pressure is lower.
Oxygen is captured by the hemoglobin molecules within the red blood cells. As the oxygen is bound to the hemoglobin, more oxygen diffuses into the blood. Hemoglobin enables the blood to carry 70 times more oxygen than possible through equilibrium alone. As this process continues, the oxygen concentration in the air within the alveoli decreases and the CO2 concentration increases. During the next breath, the original air is exhaled and replaced with fresh air, reestablishing a favorable concentration gradient for gas exchange.
Breathing is driven by pressure. Contracting the diaphragm pulls the muscle downward in the chest cavity increasing the space the lungs occupy and decreasing the pressure within the lungs. Atmospheric pressure is now greater than the pressure within the lungs causing air to flow in to the lungs through the mouth, nose, and trachea. Relaxing the diaphragm increases lung pressure and forces much of the air out of the lungs.
An entire branch of the circulatory or cardiovascular system is dedicated to the lungs. Deoxygenated blood from the major veins of the body enter the right atrium of the heart. It is pumped into the right ventricle and from there into the pulmonary arteries in the lungs. Following the gas exchange described above, oxygenated blood travels through the pulmonary veins and into the left side of the heart. The oxygenated blood is then pumped throughout the body.
