Homeostasis & hormones

Homeostasis is keeping a constant internal environment, vital in multicellular animals. It works by negative feedback: a change away from the norm triggers a response that opposes it, bringing conditions back.

Blood glucose is controlled by two hormones: insulin lowers it (after eating), glucagon raises it. In type 1 diabetes the body cannot make insulin (treated with insulin injections); in type 2 the body responds poorly to insulin (linked to lifestyle).

Water content is regulated partly by ADH, which adjusts how much water the kidneys reabsorb. Body temperature is also kept constant by responses such as sweating and shivering.

Hormones are chemical messengers released by endocrine glands and carried in the blood to target organs. Adrenaline prepares the body for action ("fight or flight"). In reproduction, FSH, LH, oestrogen and progesterone control the menstrual cycle; some hormones are used in contraception (hormonal methods vs non-hormonal barrier methods).

• Swapping insulin and glucagon: insulin lowers blood glucose.
• Thinking hormones act fast and locally; they travel in the blood and act more slowly than nerves.