The main tissue of the circulatory system is blood. It carries food and oxygen to the cells of the body. It carries waste products away from the cells. Blood flows* inside tubes called blood vessels.
The main organ of the circulatory system is the heart. The heart pumps blood through the blood vessels. The circulatory system works with the respiratory system. Blood circulates through the lungs. In the lungs it exchanges carbon dioxide and oxygen.
The circulatory system works with the digestive system. Blood circulates through the small intestines. In the small intestines it absorbs* nutrients from food that was eaten and digested.
The circulatory system works with the excretory system. Blood circulates through the kidneys. The kidneys filter out waste that is removed from the body in urine.
The circulatory system also helps our bodies maintain a constant* temperature. As blood flows past cells heat moves from the blood to the cells or from the cells to the blood. This constant exchange of heat keeps all the body parts at the same temperature. This is why people who have circulation problems seem to always have cold feet.
