The endocrine system and how hormones work

25 min

A message for every cell, read by only a few

Just before a national exam, many students feel their heart race, their mouth go dry and their hands sweat. One hormone, adrenaline, released into the blood from two small glands above the kidneys, reaches every cell in the body within about a minute, yet only some cells respond: the heart, the liver, the muscles of the airways and of blood vessels. How can a chemical carried everywhere act only in particular places? In Senior 3 you met the main endocrine glands. This unit studies hormonal coordination at A-level: how hormones act on their target cells, how their levels are controlled by feedback, and what happens when that control fails.

Definition

Hormone: a chemical messenger made by an endocrine gland and carried in the blood to target cells, where it changes their activity.

Endocrine gland: a ductless gland that secretes hormones directly into the blood. Exocrine gland: a gland that secretes through a duct onto a surface or into a cavity, such as a salivary gland or a sweat gland.

Target cell: a cell with specific receptors for a particular hormone, so that it responds to that hormone.

Second messenger: a molecule inside a cell, such as cyclic AMP (cAMP), that passes on the signal from a hormone that binds to a receptor on the cell surface.

The human endocrine system

Hormones are carried in the blood plasma to every part of the body, but a hormone affects only cells that have receptors with a shape complementary to it. Different target cells may respond differently to the same hormone: adrenaline makes the heart beat faster but relaxes the muscle around the airways.

Two kinds of hormone, two ways of acting

	Peptide and protein hormones (and amines such as adrenaline)	Steroid hormones
Made from	Amino acids	Cholesterol (lipids)
Examples	Insulin, glucagon, ADH, adrenaline	Oestrogen, progesterone, testosterone, cortisol, aldosterone
Solubility	Water-soluble: cannot pass through the phospholipid bilayer	Lipid-soluble: diffuse through the membrane
Where the receptor is	On the cell-surface membrane	Inside the cell, in the cytoplasm or nucleus
How they act	Activate a second messenger (often cAMP), which starts a cascade of enzyme reactions	The hormone–receptor complex acts as a transcription factor, switching genes on or off
Speed of effect	Seconds to minutes	Hours to days

Worked example 1: amplification by a second messenger

Illustrative values: one glucagon molecule bound to a liver cell activates enough adenylyl cyclase to make 100 molecules of cAMP. Each cAMP leads to the activation of 10 protein kinase molecules, and each of these activates 100 enzyme molecules that break down glycogen. Each of those enzymes releases 1 000 glucose molecules a minute. How many glucose molecules are released per minute because of one glucagon molecule?

Multiply the steps: $1 \times 100 \times 10 \times 100 \times 1\,000 = 1 \times 10^{8}$ .
One hundred million glucose molecules a minute from one hormone molecule.

Interpretation: each step of the cascade multiplies the signal. That is why hormones work at tiny concentrations in the blood, and why a small change in hormone level has a large effect.

Worked example 2: why hormone effects can be switched off

Suppose a hormone has a half-life in the blood of 5 minutes (insulin's is of this order). After a meal the pancreas stops secreting it. What fraction of the hormone remains after 20 minutes? Why is a short half-life useful?

20 minutes is $\frac{20}{5} = 4$ half-lives.
Fraction remaining $= \left(\frac{1}{2}\right)^{4} = \frac{1}{16} \approx 6\%$ .
The liver and kidneys break hormones down continuously, so a hormone's level depends on its rate of secretion. If the gland stops secreting, the effect soon fades. This allows the level to be adjusted quickly by feedback (lesson 3).

Warning

Common mistakes

"Hormones travel only to their target organ." They travel in the blood to every organ; only cells with the right receptors respond.
"Endocrine glands have ducts." Endocrine glands are ductless. The pancreas is both: its islets are endocrine, while the rest secretes digestive enzymes through the pancreatic duct (exocrine).
"Steroid hormones bind to receptors on the membrane." They are lipid-soluble and pass through it, binding to receptors inside the cell.

CrashCourse: "Endocrine System, Part 1 - Glands & Hormones: Crash Course Anatomy & Physiology #23". Watch for the difference between water-soluble and lipid-soluble hormones.

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Practice: the endocrine system

Define (a) hormone, (b) target cell.
Distinguish between an endocrine and an exocrine gland, with one example of each.
Explain why the pancreas is described as both an endocrine and an exocrine gland.
Explain why adrenaline affects the heart but not, for example, the cells of the skin's epidermis.
Compare how insulin and oestrogen act on their target cells.
A hormone has a half-life of 10 minutes. What percentage remains 30 minutes after its secretion stops?
Explain the role of cAMP in the action of glucagon.
Suggest why the effects of steroid hormones take longer to appear than the effects of adrenaline.

Show answersHide answers

(a) A chemical messenger secreted by an endocrine gland into the blood and carried to target cells, whose activity it changes. (b) A cell with specific receptors for a hormone, which responds to it.
An endocrine gland is ductless and secretes hormones into the blood (thyroid, adrenal gland); an exocrine gland secretes through a duct (salivary gland, sweat gland).
Its islets of Langerhans secrete insulin and glucagon into the blood (endocrine), while most of its tissue secretes digestive enzymes through the pancreatic duct into the small intestine (exocrine).
Heart muscle cells have receptors complementary to adrenaline; cells without these receptors cannot bind it, so they do not respond.
Insulin is a protein: it cannot cross the membrane, so it binds to a cell-surface receptor and acts through signalling inside the cell. Oestrogen is a steroid: it diffuses through the membrane, binds to a receptor inside the cell, and the complex switches genes on or off in the nucleus.
30 minutes = 3 half-lives: $\left(\frac{1}{2}\right)^{3} = \frac{1}{8} = 12.5\%$ .
Glucagon binds to a receptor on the liver cell surface, activating adenylyl cyclase, which makes cAMP from ATP. cAMP is the second messenger: it activates protein kinase, which starts an enzyme cascade that breaks glycogen down to glucose.
Steroids change gene transcription; time is needed to transcribe mRNA and make new proteins. Adrenaline activates enzymes that are already present, so its effect is almost immediate.

Key points

Endocrine glands are ductless and secrete hormones into the blood; only target cells with complementary receptors respond.
Main glands: hypothalamus, pituitary, pineal, thyroid, parathyroids, thymus, adrenals, pancreatic islets, ovaries and testes.
Peptide hormones bind to surface receptors and act through second messengers such as cAMP, with amplification; steroid hormones enter the cell and change gene expression.
Hormones are continually broken down, so their effects can be switched off when secretion stops.

Sources and credits

7.5.2: Types of Hormones and 7.5.3: How Hormones Work, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for the classes of hormone, receptors, second messengers and the action of steroid hormones. Adapted by Ganzaa.
Video: "Endocrine System, Part 1 - Glands & Hormones: Crash Course Anatomy & Physiology #23", CrashCourse, embedded from YouTube.

Figures 1 and 2, the tables, the worked examples (with illustrative values) and the practice questions were written by Ganzaa.

1A message for every cell, read by only a few

2The human endocrine system

3Two kinds of hormone, two ways of acting

A message for every cell, read by only a few

The human endocrine system

Two kinds of hormone, two ways of acting