Humans are terrestrial mammals, adapted to life on land. Our respiratory system is designed to extract oxygen from air, a process that differs significantly from how aquatic animals obtain oxygen from water.
Oxygen’s Forms: Air Versus Water
Air contains a much higher concentration of oxygen compared to water. Atmospheric air is approximately 21% oxygen. In contrast, oxygen is only dissolved in water, and its concentration is significantly lower, typically ranging from 4 to 8 parts per million, or about 0.0004% to 0.0008% by volume. This means air has roughly 20 to 250 times more oxygen than the same volume of water.
Furthermore, the oxygen present in a water molecule (H₂O) is chemically bonded to hydrogen and is not in a form that human lungs can utilize. The oxygen our bodies require for cellular processes is diatomic oxygen (O₂), which is what is dissolved in water and available in the atmosphere. Water is also significantly denser and more viscous than air, making it much harder to move across a respiratory surface. Even if our lungs could somehow process dissolved oxygen, the sheer volume of water we would need to move to extract sufficient oxygen would be immense and energetically inefficient.
Human Lungs: Designed for Air
The human respiratory system, especially the lungs, is designed for efficient gas exchange with air. Air enters through the nose or mouth, travels down the trachea, and branches into smaller tubes called bronchi and then bronchioles. These pathways lead to millions of tiny air sacs called alveoli. An adult human has an average of 480 million alveoli, which collectively provide an enormous surface area for gas exchange, estimated to be over 130 square meters (1,399 square feet).
The walls of the alveoli are very thin, often just one cell thick, and are surrounded by a dense network of capillaries. This thin barrier, averaging about 1 micron (0.00004 inches) in thickness, allows oxygen to quickly diffuse from the alveoli into the bloodstream and carbon dioxide to diffuse from the blood into the alveoli to be exhaled. This delicate structure, optimized for the low density and viscosity of air, would not withstand the physical properties of water. Water’s density and surface tension would cause the delicate alveolar membranes to collapse or fill with fluid, preventing effective gas diffusion.
The Physiology of Drowning
Attempting to breathe underwater initiates a sequence of events leading to drowning. The initial reaction to water entering the airway is a gasp and cough, an attempt to expel the foreign substance. As water reaches the larynx, a reflex known as laryngospasm occurs, where the vocal cords involuntarily spasm and close the airway. This reflex can initially prevent water from entering the lungs, but it also blocks air intake.
If the laryngospasm eventually relaxes, or if enough water is inhaled, water displaces air within the alveoli. The presence of water damages the delicate alveolar membranes and washes away surfactant, a substance that keeps the alveoli open. This leads to fluid accumulation in the lungs, a condition called pulmonary edema, which severely impedes or stops gas exchange. Oxygen absorption into the bloodstream decreases, and carbon dioxide removal is hindered, leading to a lack of oxygen in the body’s tissues, known as hypoxia. Hypoxia affects all organs, especially the brain and heart, and if prolonged, it can cause irreversible damage and organ failure.
How Aquatic Life Breathes
Aquatic animals like fish possess specialized respiratory organs called gills that are adapted for extracting dissolved oxygen from water. Gills are typically located on the sides of the fish’s head and consist of numerous fleshy filaments and lamellae, which are thin, comb-like tissues. These structures provide a large surface area for gas exchange, similar in principle to the alveoli in human lungs, but specifically designed for a watery environment.
Water flows into the fish’s mouth and is pumped over the gill filaments, where oxygen diffuses into the fish’s bloodstream. An efficient mechanism called countercurrent exchange enhances this process. In countercurrent exchange, blood flows through the capillaries within the gill filaments in the opposite direction to the water flowing over the gills. This maintains a constant concentration gradient, ensuring that oxygen always moves from the water into the blood along the entire respiratory surface, allowing fish to extract a high percentage of the available dissolved oxygen.