Fish
The circulatory system of fish is very simple, it consists of a heart, blood, and blood vessels also it is a closed system. The heart is made up of an atrium a ventricle and a sinus venous. In fish oxygen deprived blood is carried by the veins to the sinus venosus. The gills act as what lungs are to us they execute the respiratory functions fish go through. Blood facilitates the transport of oxygen and nutrients. It also collects carbon dioxide which is again transported to the heart and then to the gills to be removed from the body.
Snakes
Snakes have a closed circulatory system like all animals in this phylum the function they go through is similar in all animals. Like humans snakes have a left and right atrium but only one large ventricle they also have arteries and veins. The blood they pump provides oxygen to the vital organs with out the heart pumping the blood the animal will die. Even though snakes are cold blooded and need a certain amount of sunlight a day for their body temperature to stay up. Blood is not pumped freely trough out the snakes' body hence the closed system.
Birds
Birds have a very efficient circulatory system that allows them to run, swin and most importantly fly. The circulatory system of a bird is essential in providing oxygen, removing wastes, and maintaing the birds body temperature. The circulatory system transports nutrients, oxygen and carbon dioxide, waste products, hormones, and heat. Birds like most mammals have two ventricles and two atria, with complete separation or oxygenated and de-oxygenated blood. Birds tend to have larger hearts (relative to size and mass) this is necessary because of the high metabolic demands of flight.