The endocrine system is composed of multiple glands whose primary function is to produce hormones. Hormones are chemical messengers found in the body. The hormones that the endocrine system produces regulate all kinds of body systems. Hormones regulate things like metabolism, growth, development, sexual functions, sleep, and even mood. The glands that make up the endocrine system are the pituitary gland, the thyroid gland, the parathyroid gland, the adrenal glands, the pancreas, the ovaries (found in females) and the testicles (found in males).
One of the key things to understand about the endocrine system is that the glands are placed all throughout the body; however, the hormones that each gland produces are targeted for specific organs or tissues. This means that the hormones have to be transported from point A to point B. This is one way that the endocrine system interacts with other organ systems. In order for most hormones to move from the production gland to another tissue or organ, the hormone has to be transported in the blood. This means that the endocrine system depends on the circulatory system for transport throughout the body.
What is interesting is that the blood flow throughout the body doesn't necessarily go straight from the gland to the targeted cell or tissue that the hormone is designed to affect. Blood circulates everywhere, but hormones don't activate every cell they come in contact with. This is because specific cells have specific "receptors" for that hormone. It is only when the specific hormone binds to its specific receptor that a biological response is triggered.
A more specific interaction between the endocrine system and another body system can be found by examining the pancreas and the digestive system. Acid-filled food will leave the stomach and head into the small intestine. This will trigger the secretion of secretin. Secretin will stimulate the pancreas to flood the intestine with bicarbonate. This will neutralize the acid and stop the secretin stimulus, which will in turn stop the pancreas from secreting bicarbonate.