By Roxann Schroeder | Updated Aug 30, 2022
Air enters the nose, is warmed and humidified, then travels down the trachea to the lungs. In the alveoli—tiny air sacs surrounded by capillaries—oxygen diffuses into the blood while carbon dioxide, a by‑product of cellular metabolism, diffuses out and is expelled during exhalation.
Oxygenated blood is pumped by the heart throughout the body. As it reaches the capillaries of tissues, oxygen leaves the bloodstream and enters cells, where it fuels metabolic reactions that produce energy and carbon dioxide. This CO₂ returns to the lungs via the bloodstream and is finally released into the atmosphere.
Metabolism encompasses all biochemical reactions in the body. Some reactions, like glycolysis, break down nutrients to generate energy, while others, such as protein synthesis or membrane formation, consume energy to build complex molecules. Every cell stores energy from nutrients to power these processes.
Adenosine triphosphate (ATP) carries high‑energy phosphate bonds. During metabolic reactions, ATP is hydrolyzed to ADP, releasing energy that drives biosynthetic pathways. Conversely, cellular respiration—oxidation of food molecules—adds a phosphate to ADP, regenerating ATP.
Digestion breaks down food into absorbable molecules: sugars into monosaccharides, proteins into amino acids, and fats into fatty acids. Transport proteins shuttle these nutrients across intestinal cells into the bloodstream. Cells then uptake these molecules via specific transporters, incorporating them into metabolic pathways.
