The cow's vein is dissected to show how the valves work. An animation shows the valves working to stop blood flowing backwards. The third part of the film is about vasomotor control. Two hands are seen on a table, and rings are put on the ring fingers of each hand. One hand is submerged in hot water, and the ring cannot then be removed. The other hand is submerged in cold water, after which the ring comes off easily. An illustration shows how cold shrinks the volume of the blood. Next, a laboratory demonstration shows changes in the volume of blood. A man lies on a bed with his hand in a belljar, which is attached to a tube of water. An intertitle explains, 'If the hand grows larger, water will be forced into the tube. If the hand grows smaller, water will be sucked back from the tube'. The man falls asleep, and as surface blood vessels enlarge during sleep, the water level in the tube rises. Then the man wakes up and the water level goes back down. The film ends with the intertitle 'A uniform body temperature in heat or cold is maintained by circulatory control', and images of men working in a boiler room and a man playing with dogs in the snow.