How does the human circulatory system, digestive system, and respiratory system work together to keep cells alive? answer in complete sentences using proper spelling and grammar.

The circulatory system is mainly made up of the heart and the blood vessels. It is closely related to the respiratory system which is primarily made up of the lungs. The digestive system is primarily made up of the gastrointestinal tract. This simply means your mouth, esophagus, stomach, small and large intestines, and rectum up until your anus. All the cells in the human body are kept alive by these systems working together in harmony.

The heart pumps blood throughout the whole body via the blood vessels. Blood, particularly red blood cells (RBC), carry oxygen and nutrients inside of them and release them to the other cells in the body. Oxygen comes from the lungs in the respiratory system every time we breathe in air. Nutrients come from the digestive system after food has been broken down to small components that can be utilized by the cells in the body.