Hormones and how they work
The endocrine system uses hormones, chemical messengers carried in the blood, to coordinate growth, metabolism, reproduction, and responses to stress.
Endocrine versus nervous control
The nervous system acts in milliseconds through targeted electrical signals; the endocrine system acts more slowly, over seconds to days, but its effects last longer and reach the whole body. The two are linked: the hypothalamus controls much of the endocrine system.
What hormones are
Hormones are chemicals secreted by endocrine glands into the blood. They affect only target cells that have matching receptors, like a key fitting particular locks. A single hormone can affect several tissues in different ways, depending on the receptors and pathways in each cell.
Two chemical classes
Water-soluble hormones, such as insulin, adrenaline, and most protein hormones, cannot cross the cell membrane; they bind surface receptors and trigger second messengers, such as cyclic AMP, inside the cell. Lipid-soluble hormones, the steroids (cortisol, oestrogen, testosterone) and thyroid hormones, cross the membrane and bind receptors inside, directly changing gene expression.
Speed and duration
Surface-receptor signalling acts quickly, within seconds to minutes, by modifying existing proteins. Steroid signalling is slower, over hours, because it changes which proteins are made, but its effects last longer. This explains why adrenaline works almost instantly while steroid medications take time.
Control of hormone release
Hormone release is triggered by three kinds of stimuli: humoral (changing blood levels, such as glucose triggering insulin), neural (nerves stimulating a gland, such as sympathetic nerves triggering adrenaline), and hormonal (one hormone triggering another gland). Negative feedback keeps hormone levels within range.
Negative feedback in hormones
When a hormone's effect is achieved, it signals the control centre to reduce release. Thyroid hormone, for example, suppresses the hypothalamus and pituitary hormones that stimulate the thyroid. This maintains steady levels, like a thermostat for chemistry.
Hormone breakdown
Hormones are removed from the blood by the liver and kidneys, which is why liver or kidney disease can raise hormone levels. Each hormone has a characteristic half-life, from minutes for adrenaline to days for thyroid hormone.
Worked example
A person eats a sugary snack. Blood glucose rises (a humoral stimulus), so pancreatic beta cells release insulin, a water-soluble hormone. Insulin binds receptors on muscle and fat cells, which move glucose transporters to their surfaces, taking up glucose. As glucose falls, insulin release falls: negative feedback.
Key takeaways
Hormones are blood-borne chemical messengers acting on cells with matching receptors. Water-soluble hormones use surface receptors and second messengers, acting quickly; steroid and thyroid hormones enter cells and change gene expression, acting slowly but lastingly. Release is triggered by humoral, neural, or hormonal stimuli and is controlled by negative feedback.
Flashcard
Chemical messenger carried in blood
Check your understanding
Why does cortisol take longer to act than adrenaline?