This how it works covers How It Works: Hormones Travelling in Blood to Target Organs within Human Endocrine System for GCSE Biology. Topic 4: Human Endocrine System It is section 5 of 11 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.
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.
Practice questions for Human Endocrine System
Which gland is known as the 'master gland' because it controls other endocrine glands?
Compare how the nervous system and the endocrine system coordinate responses in the body. [3 marks]