Bile is a green fluid secreted by the liver that helps in breaking down large fat molecules into smaller globules to ease it's digestion process. The actual digestion of these small fat globules is done by the intestinal juices, which convert fats into fatty acids and glycerol.

Saliva contains the enzyme salivary amylase. It breaks down starch into simpler molecules such as maltose. Starch is a polysaccharide made up of many units of glucose. Maltose, on the other hand is a disaccharide produced when starch is broken down by amylase.

In fishes and reptiles, the heart is two-chambered and there is mixing of oxygenated and deoxygenated blood. They are cold blooded (the body temperature varies according to the surrounding temperature), so energy is required only for locomotion and hence less amount of oxygen is sufficient in them. Birds and mammals are warm blooded, so more energy is required to keep up the body temperature. To maintain high energy requirements of the body, more oxygen supply is required. Therefore, the heart has four chambers and oxygenated blood and deoxygenated blood is kept strictly separated. Also, complexities in the body structure and different mechanisms have increased with evolution. Therefore the mechanism of circulation has also evolved.