Gas exchange in the human body is the efficient swap of oxygen, which cells need for energy production, for carbon dioxide, a waste product of cellular metabolism. This exchange is necessary to sustain life, ensuring cells receive oxygen and harmful carbon dioxide is removed.
The Path to Gas Exchange
Air from the external environment begins its journey through either the nose or mouth, where it is warmed and moistened. It then travels down the pharynx, commonly known as the throat, which serves as a passageway for both air and food. From the pharynx, air moves into the larynx, or voice box, and then enters the trachea, a tube supported by cartilage rings that prevent it from collapsing.
The trachea branches into two main tubes, the left and right bronchi, each leading to a lung. These bronchi continue to divide into smaller branches called bronchioles, resembling an inverted tree structure. The bronchioles eventually lead to tiny air sacs, bringing the air to the lung tissue.
The Alveolar-Capillary Unit
The actual site of gas exchange in the body is the alveolar-capillary unit within the lungs. This unit consists of microscopic air sacs called alveoli, which are surrounded by a dense network of extremely thin blood vessels known as pulmonary capillaries. There are millions of alveoli in the lungs, estimated to be around 300 million in each human lung, providing a vast surface area for gas exchange.
The walls of the alveoli and the capillaries are thin, each consisting of a single layer of epithelial cells. These two layers, along with a shared basement membrane, form the respiratory membrane, also referred to as the alveolar-capillary membrane or blood-air barrier. This barrier is very thin, typically measuring around 0.2 to 0.5 micrometers, which minimizes the distance gases need to travel. This specialized structure facilitates the rapid and efficient transfer of oxygen and carbon dioxide.
The Process of Gas Movement
The movement of oxygen and carbon dioxide across the alveolar-capillary membrane occurs through a passive process called diffusion. This process is driven by differences in the partial pressures, or concentrations, of these gases. Gases always move from an area of higher partial pressure to an area of lower partial pressure.
In the alveoli, the partial pressure of oxygen is high, around 104 mm Hg, while in the capillary blood, it is lower, about 40 mm Hg. This pressure gradient causes oxygen molecules to diffuse from the alveoli into the capillary blood. Conversely, the partial pressure of carbon dioxide is higher in the capillary blood, approximately 45 mm Hg, compared to the alveoli, where it is about 40 mm Hg. This gradient drives carbon dioxide to diffuse from the blood into the alveoli.
Once in the bloodstream, oxygen primarily binds to hemoglobin within red blood cells, forming oxyhemoglobin, with about 97% transported this way. The remaining oxygen dissolves directly in the blood plasma. Carbon dioxide is transported in the blood in several ways: approximately 70% as bicarbonate ions, about 20-25% bound to hemoglobin as carbamino-hemoglobin, and around 7% dissolved in the plasma. This movement of gases ensures body tissues receive a constant supply of oxygen and carbon dioxide waste is removed.