How It Works: Hormones Travelling in Blood to Target Organs

How It Works: Hormones Travelling in Blood to Target Organs

The endocrine system communicates using hormones — chemical messenger molecules produced and secreted by endocrine glands directly into the bloodstream. Unlike the nervous system, hormones do not travel along specific pathways — they circulate throughout the entire body in the blood. However, only target organs respond to a particular hormone, because only target organs possess the specific receptor proteins on their cell surfaces that can bind to that hormone.

The sequence is: an endocrine gland secretes a hormone → the hormone enters the bloodstream → the hormone is carried to all parts of the body → only cells in the target organ (those with the matching receptor) respond → those cells alter their activity.

This system is slower than the nervous system because hormones must travel through the bloodstream (which circulates at a rate of roughly 5 litres per minute) rather than along dedicated nerve fibres. However, once hormones reach their target, their effects are often long-lasting — hours or even days — and can affect large numbers of cells simultaneously across different tissues.

The pituitary gland as master gland: Located at the base of the brain, the pituitary gland releases hormones that stimulate other glands (such as the thyroid, adrenal glands, and ovaries/testes) to release their own hormones. This creates hierarchical control, with the pituitary acting as a central regulator of much of the endocrine system.