You are viewing a free preview of this lesson.
Subscribe to unlock all 9 lessons in this course and every other course on LearningBro.
The nervous system is the body's fast, electrical coordinating system. But the body has a second coordinating system that works in a completely different way — using chemicals carried in the blood. This is the endocrine system, and the chemical messengers it uses are called hormones. This lesson introduces the main endocrine glands and the hormones they release, explains how hormones travel to their targets, sets out a clear comparison of nervous and hormonal control, and looks at two important hormones in detail — adrenaline and (at Higher tier) thyroxine. This is part of Topic B3 of OCR Gateway Science A, and it underpins the homeostasis lessons that follow.
By the end of this lesson you should be able to name the main endocrine glands and their hormones, explain how hormones reach their target organs, compare nervous and hormonal coordination, and describe the roles of adrenaline and (Higher) thyroxine.
The endocrine system is made up of glands that release hormones directly into the blood. A hormone is a chemical messenger that is:
Because hormones travel in the blood, they reach every part of the body — but they only cause a response in the target organs that have receptors matching that particular hormone. Hormonal effects tend to be slower to start than nervous responses but to last much longer, because the hormone stays in the blood for a while.
Exam Tip: "Hormones travel in the blood to their target organs" is the single most exam-rewarding sentence about the endocrine system. Make sure you say blood (not "nerves") and target organ (the place that responds).
You should be able to locate the main glands on a body map and name a hormone for each. The diagram below shows their positions.
| # | Gland | Main hormone(s) | Principal role |
|---|---|---|---|
| 1 | Pituitary gland | FSH, LH, ADH, growth hormone, TSH | The "master gland" — releases hormones that control other glands |
| 2 | Thyroid | Thyroxine | Controls metabolic rate (how fast reactions happen) |
| 3 | Adrenal glands | Adrenaline | Prepares the body for "fight or flight" |
| 4 | Pancreas | Insulin (and glucagon) | Controls blood glucose concentration |
| 5 | Ovaries (females) | Oestrogen (and progesterone) | Controls the menstrual cycle and female secondary sexual characteristics |
| 6 | Testes (males) | Testosterone | Controls sperm production and male secondary sexual characteristics |
The pituitary gland, in the brain, is called the master gland because the hormones it releases act on other endocrine glands, telling them to release their hormones. For example, the pituitary releases hormones that control the thyroid, the ovaries and the testes. So the pituitary sits at the top of a chain of command, coordinating much of the whole endocrine system. Importantly, the pituitary responds to conditions in the body (and to signals from the brain), so it acts as a link between the nervous system and the endocrine system — which is why hormonal control can be triggered by things you sense, such as stress.
A defining feature of an endocrine gland is that it releases its hormone directly into the blood, with no tube or duct to carry it. This is what allows hormones to travel everywhere in the body. It is worth contrasting this with glands that do have ducts — for example, the salivary glands release saliva down a duct into the mouth, and sweat glands release sweat down a duct onto the skin. These duct-bearing glands are not part of the endocrine system because their products do not enter the blood. The pancreas is unusual in being both: it releases the hormone insulin into the blood (an endocrine role) and releases digestive enzymes down a duct into the small intestine. For Topic B3 the key point is simply that endocrine glands secrete hormones straight into the bloodstream, which is how those hormones reach their target organs.
Exam Tip: The pancreas, ovaries and testes each have a non-endocrine job as well (the pancreas makes digestive enzymes; the ovaries and testes make egg and sperm cells). In a question about the endocrine system, focus on their hormones — insulin, oestrogen and testosterone.
The body's two coordinating systems work in very different ways, and OCR frequently asks you to compare them. Learn this table thoroughly.
| Feature | Nervous coordination | Hormonal (endocrine) coordination |
|---|---|---|
| What carries the message | Electrical impulses along neurones | Chemical hormones in the blood |
| Speed of action | Very fast | Slower |
| How long the effect lasts | Short (brief) | Long-lasting |
| Area affected | A precise, localised target | A larger, more general area (any organ with receptors) |
| How transmitted | Along neurones (nerves) | In the bloodstream |
The pattern to remember is that the nervous system is fast, short-lived and precise, while the endocrine system is slower, longer-lasting and more widespread. The two systems work together to coordinate the body — for example, both are involved in the fight-or-flight response.
Exam Tip: A "compare nervous and hormonal control" question is almost guaranteed somewhere in B3. The four highest-value contrasts are speed (fast vs slow), duration (short vs long), transmission (neurones vs blood) and area (precise vs widespread). Give all four for full marks.
Adrenaline is released by the adrenal glands (which sit just above the kidneys) at times of fear, stress or excitement. It prepares the body for "fight or flight" — that is, for sudden physical action.
Adrenaline travels in the blood to several target organs and produces effects such as:
All of these changes make the body ready to run away from danger or to confront it — which is why the response is called fight or flight. Adrenaline is a good example of a hormone whose effects are spread across several organs at once, something the precise nervous system would find harder to do.
The fight-or-flight response also shows the two coordinating systems working together. When you are startled, the nervous system reacts in a fraction of a second — your heart rate jumps almost immediately. But adrenaline, released a moment later, keeps the heart rate, breathing rate and blood glucose raised for some time afterwards, which is why you can still feel "shaky" several minutes after a fright. The fast nervous response and the longer-lasting hormonal response complement each other: one acts instantly, the other sustains the effect.
Subscribe to continue reading
Get full access to this lesson and all 9 lessons in this course.