To survive, the body must detect changes and respond to them. A change that the body detects is a stimulus. Receptors (for example in the eyes, ears and skin) detect the stimulus, and effectors (muscles or glands) carry out the response.

The nervous system sends fast electrical impulses along nerve cells called neurons. The pathway is: stimulus → receptor → sensory neuron → central nervous system (brain and spinal cord) → motor neuron → effector → response.

A reflex is a fast, automatic response that does not need you to think first, such as blinking when something comes near your eye. Reflexes protect the body from harm.

The endocrine system is made up of glands that release chemical messengers called hormones into the blood. Each hormone affects particular target organs. Hormonal responses are usually slower than nervous responses, but they last longer.

Important glands include the pituitary (in the brain; controls other glands), the thyroid (in the neck; controls the rate of chemical reactions in the body), the pancreas (controls blood glucose with insulin and glucagon), the adrenal glands (on top of the kidneys; release adrenaline), and the ovaries and testes (release sex hormones such as estrogen and testosterone).

The two systems often work together. When you get a fright, nerves signal the adrenal glands to release adrenaline. Adrenaline increases your heart rate and breathing rate and sends more glucose to your muscles, preparing your body for action.

Homeostasis is keeping the internal environment of the body stable, even when conditions outside change. Body temperature stays close to 37 °C, and blood glucose stays within a narrow range.

Homeostasis works by negative feedback: a change is detected, and the body responds in a way that reverses the change. Too hot: you sweat, and blood vessels near the skin widen so more heat is lost. Too cold: you shiver (muscle contractions release heat), and blood vessels near the skin narrow so less heat is lost.