The endocrine system is a body-wide communication network made of glands that release hormones into the blood. These hormones help control growth, metabolism, reproduction, stress responses, sleep cycles, and water balance. It matters because many body processes depend on the right hormone being released at the right time and in the right amount.
When endocrine signals are too high or too low, disorders such as diabetes, thyroid disease, or growth problems can occur.
Endocrine glands include the hypothalamus, pituitary, thyroid, parathyroids, adrenal glands, pancreas, ovaries, and testes. Hormones travel through the bloodstream, but they only affect target cells that have the correct receptors. Compared with the nervous system, endocrine signaling is usually slower but often lasts longer.
Many endocrine pathways use negative feedback, where the result of a hormone signal helps shut down or reduce further hormone release.
Key Facts
- Hormones are chemical messengers secreted by endocrine glands into the bloodstream.
- Only target cells with the correct receptor can respond to a hormone.
- Negative feedback helps keep hormone levels stable, such as high blood glucose triggering insulin release.
- Blood glucose change = glucose after a meal - glucose before a meal.
- Insulin lowers blood glucose by helping cells take up glucose, while glucagon raises blood glucose by signaling the liver to release glucose.
- Nervous system signals are usually fast and short-lived, while endocrine signals are usually slower and longer-lasting.
Vocabulary
- Endocrine gland
- An organ or tissue that releases hormones directly into the bloodstream.
- Hormone
- A chemical messenger that travels through the blood and changes the activity of target cells.
- Target cell
- A cell that has the specific receptor needed to detect and respond to a hormone.
- Receptor
- A protein on or inside a cell that binds a specific hormone and starts a response.
- Negative feedback
- A control process in which the output of a system reduces the original stimulus to help maintain balance.
Common Mistakes to Avoid
- Thinking hormones affect every cell equally is wrong because only cells with matching receptors can respond to a specific hormone.
- Calling the pancreas only a digestive organ is incomplete because it also acts as an endocrine gland by releasing insulin and glucagon into the blood.
- Confusing endocrine signaling with nervous signaling is wrong because nerves send rapid electrical and chemical messages to specific locations, while hormones travel through the blood and often act more slowly.
- Assuming negative feedback means a harmful response is wrong because negative feedback usually means the body reduces a change to maintain homeostasis.
Practice Questions
- 1 A student's blood glucose is 90 mg/dL before lunch and 135 mg/dL after lunch. What is the change in blood glucose, and which pancreatic hormone would most likely increase to help bring it down?
- 2 A hormone concentration drops from 48 units/mL to 30 units/mL after a feedback response. By how many units/mL did it decrease, and what percent decrease is this?
- 3 Explain why a hormone can travel throughout the entire bloodstream but only cause a response in certain organs or tissues.