Hormones

A hormone is a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.

Endocrine glands and hormones

An endocrine gland is a ductless organ that produces and secretes hormones directly into the bloodstream.

Endocrine gland	Hormone secreted	Main role/target organ
Adrenal glands	Adrenaline	Prepares the body for action (fight/flight).
Pancreas	Insulin and Glucagon	Insulin lowers blood glucose levels if too high and glucagon raises blood glucose levels if too low. Glucagon is secreted by the pancreas.
Testes	Testosterone	Controls male secondary sexual characteristics.
Ovaries	Oestrogen and progesterone	Controls female secondary sexual characteristics and menstrual cycle.

Adrenaline is released by the adrenal glands during stressful, scary or exciting situations to prepare your body to either fight the danger or flee away from it.

Increased breathing rate. Delivers more oxygen to lungs so it can diffuse into the blood.
Increased heart rate. Pumps blood faster to deliver oxygen and glucose to muscles rapidly.
Increased pupil diameter. Allows more light into eyes to improve visuall claritty and awareness.
Increased blood glucose concentration. Adrenaline stimulates the liver to break down stored glycogen into glucose. This extra glucose is released into the blood to provide energy for increased cellular respiration in working muscles.

Feature	Nervous control	Hormonal control
Form of transmission	Electrical impulses along neurones	Chemical substances in the blood
Speed of action	Very rapid (milliseconds)	Slower (can take minutes, hours, or days)
Duration of effect	Short-lived (stops as soon as impulses stop)	Longer-lasting (effects continue until hormone is broken down)
Area of response	Localized (precise cells/muscles)	Widespread (can affect multiple target organs)