Circulatory control.

This film shows the essential differences between artery and vein structure and the main causes of changes of blood volume in the limbs. The first part of the film explains how the arteries work. An intertitle says 'Pulsations of the heart are felt in the arteries', and blood is shown moving through an artery. An intertitle says 'A beef artery shows elastic structure', and a hand is shown holding a cow artery , squeezing it and filling it with water. Next, a demonstration shows that elastic tubes like arteries absorb unevenness of flow whereas rigid tubes do not. An intertitle then says 'The pressure of the blood in arteries can be measured', and a doctor is seen taking a male patient's blood pressure. An illustration shows where the artery is in the arm and how the blood pressure machine works; pressure is increased until the flow of blood stops, and then pressure is reduced until blood begins to flow again. At this point, blood pressure is read. The next part of the film explains how veins work. A hand is seen holding a cow's vein and filling it with water. When this is done, water shoots back out of the vein, and an intertitle explains 'Valves keep the blood from flowing backward in the veins', and the valves are shown in detail. 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.