The gas we exhale is carbon dioxide (CO2). When we breathe in, our bodies take in oxygen, which is vital for cellular respiration. This process allows cells to produce energy. As a byproduct of this metabolic process, carbon dioxide is formed, which the body then expels during exhalation. This exchange of gases is crucial for maintaining the proper functioning of our respiratory and circulatory systems. The CO2 we exhale is also essential for plants, as they use it in the process of photosynthesis to produce oxygen and glucose, demonstrating a significant ecological connection between humans and plant life.