Breathing is a fundamental process for all mammals, enabling the exchange of gases that sustain life. This gas exchange is essential for cellular functions; without it, cells cannot produce energy for survival. The intricate system responsible for breathing ensures a steady supply of oxygen to every cell while efficiently removing metabolic waste.
Lungs: The Central Organ
The lungs are the primary organs responsible for respiration in mammals, located within the chest cavity. These organs are protected by the rib cage, intercostal muscles, and the chest wall. The right lung typically has three lobes, while the left lung has two, to accommodate the heart. Their main function involves facilitating the transfer of gases between the air and the bloodstream.
The Path Air Takes
Air enters the mammalian respiratory system through the nose or mouth, where it is warmed and humidified. It then travels into the pharynx, a shared pathway for air and food, and subsequently into the larynx, often called the voice box. From the larynx, air moves down the trachea, commonly known as the windpipe. The trachea branches into two main bronchi, which divide into smaller bronchi and bronchioles within the lungs. This branching system ensures air reaches deep into the lung tissue.
Exchanging Essential Gases
Gas exchange occurs at the end of the smallest airways, within tiny air sacs called alveoli. Each alveolus is surrounded by a dense network of capillaries, which are microscopic blood vessels. Oxygen from the inhaled air in the alveoli diffuses across their thin walls and the capillary walls into the bloodstream. Simultaneously, carbon dioxide, a waste product, diffuses from the blood in the capillaries into the alveoli to be exhaled. This efficient transfer is possible due to the very thin blood-air barrier, typically one cell thick, that separates the air from the blood.
The Muscles of Respiration
The physical act of breathing relies on specific muscles that alter the volume of the thoracic cavity. The diaphragm, a dome-shaped muscle at the base of the lungs, is the primary muscle of quiet inhalation. When the diaphragm contracts, it flattens and moves downward, increasing the volume of the chest cavity. Simultaneously, the external intercostal muscles, between the ribs, contract to pull the rib cage upward and outward, further expanding the thoracic volume. This increase in volume lowers the pressure inside the lungs, drawing air in from the outside environment.
During exhalation, these muscles relax. The diaphragm returns to its dome shape, and the external intercostal muscles relax, allowing the rib cage to move inward and downward. This relaxation, combined with the elastic recoil of the lung tissue, reduces the volume of the thoracic cavity. The decreased volume increases the pressure inside the lungs, forcing air out.