How the respiratory system and circulatory system evolved in different organisms? Example, between human and fish.
The respiratory system in living organisms works in tandem with the circulatory system to provide oxygen for the cellular respiration going on the organism's cells. In fish, the circulatory system is what is called a "single circuit" system, and in humans, a "double circuit" system is the norm. For fish, deoxgenated blood enters a single atrium, then a muscular ventricle, then on to an bulbous arteriosis, to the aorta, then to the gills. The gills, in fish, facilitate the gaseous exchange, while lungs serve that purpose in humans. In humans, the circulatory system has a two-chambered atrial unit, which delivers blood to a two-chambered ventricular unit. The right side of the heart in humans receives blood from the body and sends it to the lungs, where the gaseous exchange occurs. The blood returns to the left side of the heart and is then sent to the rest of the body.
Both respiratory systems serve the same purpose, to facilitate gaseous exchange between oxygen intake and carbon dioxide outtake. Gills are used by fish, as their oxygen is dissolved in water, while lungs are used by humans, who breathe air. Both circulatory systems serve the same purpose, to pump blood throughout the organism's body, distributing needed oxygen to all cells and at the same time picking up carbon dioxide for elimination from the organism's body. The main difference here is the two-chambered heart exists in fish, while humans have a four-chambered heart. Fish generally have lower energy requirements than do humans, so that would be a reasonable explanation for why humans need a more complex circulatory system.