The immune system is the body's defense against bacteria, viruses and other pathogens that may be harmful. For example, one of the glands in the pancreas - called the endocrine gland - releases hormones called insulin and glucagon to regulate blood sugar. These hormones, in turn, travel to different tissues and regulate various bodily functions, such as metabolism, growth and sexual function, according to Johns Hopkins Medicine. The endocrine system consists of a network of glands that secrete hormones into the blood. The liver and pancreas also play a role in the digestive system because they produce digestive juices filled with enzymes to break down the components of your food, such as carbohydrates, fats and proteins, according to the National Institute of Diabetes and Digestive and Kidney Diseases. It includes the mouth, esophagus, stomach, small intestine, large intestine, rectum, and anus. The digestive system consists of a series of connected organs that together, allow the body to break down and absorb food, and remove waste. According to the Cleveland Clinic, the human body's network of blood vessels, veins and capillaries is over 60,00 miles (around 96,560 kilometres) long. It consists of the heart, blood, blood vessels, arteries and veins. The job of the circulatory system is to move blood, nutrients, oxygen, carbon dioxide, and hormones, around the body.

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