External respiration is a fundamental biological process that facilitates the exchange of gases between the body and its external environment. This process primarily occurs within the lungs, where oxygen from the inhaled air is taken up by the blood, and carbon dioxide, a waste product, is released from the blood to be exhaled. External respiration is essential for sustaining life, as it ensures a continuous supply of oxygen for cellular functions and the efficient removal of carbon dioxide.
The Mechanics of Air Movement
The physical act of breathing, known as pulmonary ventilation, involves the movement of air into and out of the lungs. This process is driven by changes in pressure within the thoracic cavity, which are governed by Boyle’s Law. Boyle’s Law states that in a closed space, the pressure and volume of a gas are inversely related; as volume increases, pressure decreases, and vice versa.
During inhalation, or inspiration, the diaphragm, a dome-shaped muscle located below the lungs, contracts and flattens, moving downward. Simultaneously, the external intercostal muscles, situated between the ribs, contract, pulling the rib cage upward and outward. These coordinated muscle actions increase the volume of the thoracic cavity, leading to a decrease in the air pressure inside the lungs compared to the atmospheric pressure outside the body. Consequently, air flows from the higher pressure outside the body into the lower pressure within the lungs until the pressures equalize.
Conversely, exhalation, or expiration, is typically a passive process during quiet breathing. The diaphragm and external intercostal muscles relax, causing the thoracic cavity to decrease in volume. This reduction in volume increases the pressure within the lungs, making it higher than the external atmospheric pressure. As a result, air is expelled from the lungs. During more forceful exhalation, such as during physical activity, additional muscles like the internal intercostals and abdominal muscles can contract to further decrease thoracic volume and push air out more rapidly.
Gas Exchange at the Alveoli
The actual exchange of oxygen and carbon dioxide takes place in the alveoli, tiny air sacs within the lungs, surrounded by a dense network of capillaries. The structure facilitating this exchange is the respiratory membrane, an extremely thin barrier separating the air in the alveoli from the blood in the capillaries. This membrane is composed of alveolar epithelial cells, capillary endothelial cells, and their fused basement membranes. Its thinness, approximately 0.5 to 1 micrometer, allows for efficient gas diffusion.
Gas exchange across the respiratory membrane occurs through simple diffusion, driven by differences in the partial pressures of oxygen and carbon dioxide. Partial pressure refers to the pressure exerted by a single gas within a mixture of gases. Oxygen has a higher partial pressure in the alveoli (around 104 mmHg) than in the deoxygenated blood entering the pulmonary capillaries (around 40 mmHg). This pressure gradient drives oxygen from the alveoli into the blood.
At the same time, carbon dioxide has a higher partial pressure in the deoxygenated blood (around 45 mmHg) compared to the air in the alveoli (around 40 mmHg). This gradient drives carbon dioxide from the blood into the alveoli for exhalation. Once oxygen enters the bloodstream, approximately 98% of it binds reversibly to hemoglobin, a protein found in red blood cells, for transport throughout the body.
External Respiration Versus Internal Respiration
It is important to distinguish external respiration from internal respiration, as they represent different stages of gas exchange. External respiration specifically refers to the exchange of oxygen and carbon dioxide between the external environment and the blood within the lungs’ alveoli.
In contrast, internal respiration involves the exchange of gases between the blood and the body’s cells and tissues. This occurs at the cellular level throughout the body, where oxygen is released from the blood to individual cells for metabolic processes, and carbon dioxide, a byproduct of these processes, is picked up by the blood. Cellular respiration, distinct from both external and internal respiration, is the metabolic process within cells where oxygen is used to break down nutrients to produce energy, with carbon dioxide and water as waste products.