Grade 12 · Life Processes in Plants & Animals · Lesson 4

Human Endocrine System

Hormones are the body's chemical messengers — slower and longer-lasting than nerve impulses. Click through the major glands to see what each one releases and why it matters.

What is the endocrine system?

The endocrine system is a network of glands that produce and secrete hormones — chemical messengers released directly into the bloodstream. Hormones travel throughout the body but only affect specific target organs or tissues, which have receptors that recognise that particular hormone.

Hormone: a chemical substance, produced by an endocrine gland and transported in the blood, that alters the activity of one or more specific target organs.
A hormone travelling in the bloodstream binds only to receptors on target organ cells, while passing non-target cells unaffected
Why one hormone in the blood only affects specific target cells

Nervous vs endocrine control

The body has two communication systems working together: the fast, electrical nervous system, and the slower, chemical endocrine system. Each is suited to different jobs — nerves for split-second reactions, hormones for sustained, body-wide changes.

Nervous systemEndocrine system
Signal typeElectrical impulseChemical (hormone)
Transmitted viaNeurons (nerve cells)Bloodstream
Speed of responseVery fast (milliseconds)Slower (seconds to days)
Duration of effectShort-livedLong-lasting
Area affectedPrecise, localisedCan be widespread
Nervous control shown as a fast electrical impulse travelling down a neuron to a muscle, compared with endocrine control shown as hormones diffusing through the bloodstream to reach target cells
Nervous control vs endocrine control

Major endocrine glands and their hormones

Several glands throughout the body secrete hormones with specific roles:

Diagram of the human body showing the location of the major endocrine glands: hypothalamus, pituitary, thyroid, adrenal glands, pancreas, testes and ovaries
Major endocrine glands and where they're located

How hormones reach their target

Once secreted, a hormone diffuses into nearby capillaries and is carried around the body dissolved in the blood plasma. It only produces an effect in cells with the correct receptor for that hormone — the target cells. This is why a single hormone circulating everywhere in the blood can still have a very specific effect.

Master gland: the pituitary gland is controlled by the hypothalamus (part of the brain), linking the nervous and endocrine systems — together they form the body's main regulatory network.

Endocrine vs exocrine glands

Not all glands work the way endocrine glands do. It's important to distinguish the two types:

Endocrine glandsExocrine glands
DuctsDuctlessHave ducts
How secretions travelHormones released directly into the bloodstreamSecretions released directly into a cavity, onto a surface, or to a specific target organ (not into the blood)
ExamplesThyroid, pituitary, adrenal glands, pancreas (hormone-producing part)Sweat glands, salivary glands, pancreas (enzyme-producing part)
The pancreas is unusual — it is BOTH an endocrine gland (secreting insulin/glucagon directly into the blood) AND an exocrine gland (secreting digestive enzymes via a duct into the small intestine).
Close-up of the pancreas showing its endocrine function (ductless secretion into the bloodstream) alongside its exocrine function (ducted secretion of digestive enzymes)
The pancreas: endocrine and exocrine function side by side

Gland Explorer

Click a gland on the body diagram (or the list) to see its hormone and effect.

0/6
Review the explanations above.
Answer in your exercise book.
Question 1 · (2 marks)
Define the term "hormone".
Question 2 · (4 marks)
Compare the nervous and endocrine systems in terms of signal type, speed of response, and duration of effect.
Question 3 · (3 marks)
Explain why the pituitary gland is called the "master gland".
Question 4 · (3 marks)
Describe the role of adrenaline in the "fight-or-flight" response.
Question 5 · (3 marks)
Name the gland that secretes insulin and glucagon, and state the general role of these two hormones.
Question 6 · (6 marks)
Blood glucose concentration was measured every hour after a carbohydrate-rich meal, for a healthy person and for a person with untreated diabetes:

Time after meal (hours)01234
Healthy person — glucose (mmol/L)5.07.86.55.25.0
Untreated diabetic — glucose (mmol/L)5.59.510.810.29.0
(a) Calculate the rate of change of blood glucose concentration for the healthy person between hour 1 and hour 2 (in mmol/L per hour).
(b) Compare the overall pattern of the diabetic person's glucose data to the healthy person's over the 4 hours.
(c) Using the data and your knowledge of negative feedback, explain why the healthy person's blood glucose returns to close to 5.0 mmol/L by hour 4, while the untreated diabetic's glucose remains high throughout.