Endocrine System: Glands, Hormones & Target Organs

What you'll learn

The endocrine system controls long-term body processes through chemical messengers called hormones. This topic appears frequently in CXC CSEC Biology papers, particularly in Section B structured questions and multiple-choice items testing knowledge of specific glands, their hormones, and effects on target organs. You must be able to identify major endocrine glands, explain how hormones regulate processes like blood glucose and water balance, and compare endocrine and nervous system coordination.

Key terms and definitions

Hormone — A chemical messenger produced by endocrine glands and transported in the bloodstream to target organs where it causes specific physiological effects.

Endocrine gland — A ductless gland that secretes hormones directly into the bloodstream (examples: pituitary, thyroid, pancreas, adrenal glands).

Target organ — A specific organ or tissue that contains receptor cells sensitive to a particular hormone and responds to its presence.

Homeostasis — The maintenance of a constant internal environment within an organism, often regulated by hormone feedback mechanisms.

Negative feedback — A control mechanism where a change in a condition triggers a response that counteracts the original change, returning the system to its normal state.

Insulin — A hormone secreted by the pancreas that lowers blood glucose levels by promoting glucose uptake into cells and conversion to glycogen.

Glucagon — A hormone secreted by the pancreas that raises blood glucose levels by stimulating the breakdown of glycogen to glucose in the liver.

Adrenaline — A hormone produced by the adrenal glands that prepares the body for rapid action during stress or danger (the "fight or flight" response).

Core concepts

Structure and function of the endocrine system

The endocrine system consists of specialized glands distributed throughout the body. Unlike exocrine glands (which have ducts), endocrine glands are ductless and secrete hormones directly into the bloodstream. This allows hormones to travel throughout the body but only affect cells with specific receptors.

Major endocrine glands tested in CXC CSEC Biology:

Pituitary gland — Located at the base of the brain; called the "master gland" because it controls other endocrine glands
Thyroid gland — Located in the neck; produces thyroxine
Pancreas — Located behind the stomach; produces insulin and glucagon
Adrenal glands — Located on top of each kidney; produce adrenaline and other hormones
Ovaries (females) — Produce oestrogen and progesterone
Testes (males) — Produce testosterone

Comparison of endocrine and nervous systems

Both systems coordinate body activities, but they operate differently. This comparison appears regularly on CXC papers:

Nervous system:

Uses electrical impulses transmitted along neurons
Response is rapid (milliseconds to seconds)
Effect is short-lived and localized
Transmits information via nerve cells

Endocrine system:

Uses chemical messengers (hormones) transported in blood
Response is slower (seconds to hours)
Effect is long-lasting and widespread
Transmits information via the bloodstream

Blood glucose regulation

Regulation of blood glucose concentration is a classic example of homeostasis through negative feedback. The pancreas contains specialized cells called Islets of Langerhans that monitor blood glucose levels continuously.

When blood glucose is too HIGH (after eating a meal rich in carbohydrates):

Beta cells in the pancreas detect elevated glucose
Pancreas secretes insulin into the bloodstream
Insulin travels to target organs (liver, muscles, other cells)
Insulin causes:
- Increased glucose uptake by cells for respiration
- Conversion of glucose to glycogen for storage (glycogenesis) in liver and muscles
- Increased protein and fat synthesis
Blood glucose level decreases to normal (approximately 90 mg/100 cm³)

When blood glucose is too LOW (during exercise, between meals, or overnight):

Alpha cells in the pancreas detect reduced glucose
Pancreas secretes glucagon into the bloodstream
Glucagon travels to the liver (primary target organ)
Glucagon causes:
- Breakdown of glycogen to glucose (glycogenolysis)
- Conversion of amino acids and fats to glucose (gluconeogenesis)
Blood glucose level increases to normal

This is negative feedback because the response opposes the original change. In Trinidad and Tobago, where diabetes prevalence is significant, understanding insulin function becomes especially relevant for public health education.

Diabetes mellitus

Diabetes is a disease where blood glucose regulation fails. Two types exist:

Type 1 Diabetes:

Pancreas produces insufficient or no insulin
Usually develops in childhood or adolescence
Requires daily insulin injections
Cannot be prevented

Type 2 Diabetes:

Body cells become resistant to insulin, or pancreas produces insufficient amounts
Usually develops in adults (though increasingly seen in young people)
Associated with obesity and poor diet
Can often be managed through diet, exercise, and medication

Symptoms of untreated diabetes:

Excessive urination (polyuria) — kidneys cannot reabsorb all the glucose, which draws water out
Excessive thirst (polydipsia) — due to water loss
Glucose in urine (normally absent)
Weight loss despite good appetite
Fatigue and weakness

The role of adrenaline

Adrenaline (also called epinephrine) is produced by the adrenal medulla (inner part of adrenal glands) during stress, fear, anger, or excitement. This hormone produces the "fight or flight" response.

Effects of adrenaline on target organs:

Heart — increases heart rate and force of contraction (more blood pumped)
Blood vessels — causes vasoconstriction in skin and digestive system, vasodilation in muscles (redirects blood to muscles)
Liver — stimulates conversion of glycogen to glucose (provides energy)
Lungs — increases breathing rate and dilates airways (more oxygen intake)
Pupils — dilates pupils for better vision
Skin — stimulates sweating and causes hair to stand up
Digestive system — slows digestion (non-essential during emergency)

A student in Jamaica being chased by an aggressive dog would experience these coordinated changes, preparing the body for rapid physical action.

Control of water balance (osmoregulation)

The pituitary gland produces antidiuretic hormone (ADH), which regulates water content in the body by controlling water reabsorption in the kidneys.

When the body is dehydrated (after exercise in hot Caribbean sun):

Receptors in the hypothalamus detect high blood concentration (low water content)
Pituitary gland releases more ADH
ADH travels to kidneys (target organ)
Kidney tubules become more permeable to water
More water is reabsorbed into the blood
Small volume of concentrated (dark) urine is produced
Blood concentration returns to normal

When the body has excess water (after drinking several glasses of water):

Receptors detect low blood concentration (high water content)
Pituitary gland releases less ADH
Kidney tubules become less permeable to water
Less water is reabsorbed into the blood
Large volume of dilute (pale) urine is produced
Blood concentration returns to normal

This is another example of negative feedback maintaining homeostasis.

Reproductive hormones

In females:

Oestrogen — produced by ovaries; causes development of secondary sexual characteristics (breast development, wider hips, pubic hair); repairs and thickens uterine lining after menstruation
Progesterone — produced by ovaries; maintains uterine lining during pregnancy

In males:

Testosterone — produced by testes; causes development of secondary sexual characteristics (deepening voice, facial hair, muscle development, sperm production)

These hormones control the menstrual cycle and are essential for reproduction, topics that connect to human reproduction units in CXC CSEC Biology.

Thyroid hormones

The thyroid gland in the neck produces thyroxine, which controls metabolic rate (the speed of chemical reactions in cells).

Effects of thyroxine:

Regulates growth and development in children
Controls basal metabolic rate (BMR)
Affects body temperature regulation
Influences mental alertness

Abnormal thyroid function:

Hypothyroidism (too little thyroxine) — causes weight gain, tiredness, slow heart rate, cold sensitivity
Hyperthyroidism (too much thyroxine) — causes weight loss, restlessness, rapid heart rate, heat intolerance
Goitre — swelling of thyroid due to iodine deficiency; historically seen in regions with iodine-poor diets, less common now due to iodized salt

Worked examples

Example 1: Structured question on blood glucose regulation (6 marks)

Question: A student ate a large meal of white rice and provision at 12:00 pm. Blood samples were taken at intervals and glucose levels measured.

(a) Name the hormone that would increase in the blood shortly after this meal. (1 mark)

(b) Name the gland that produces this hormone. (1 mark)

Answers:

(a) Insulin ✓

(b) Pancreas ✓

(c)

Insulin is transported in the bloodstream to target organs/cells ✓
It increases the uptake of glucose by cells (for respiration) ✓
It stimulates the conversion of glucose to glycogen ✓
Glycogen is stored in the liver and muscles ✓

(Award any 4 marks from acceptable responses)

Example 2: Multiple choice question

Question: Which of the following correctly matches a hormone with its source and function?

Hormone	Source	Function
A. Insulin	Pancreas	Raises blood glucose
B. Glucagon	Liver	Lowers blood glucose
C. Adrenaline	Adrenal gland	Prepares body for action
D. Thyroxine	Pituitary	Controls water balance

Answer: C ✓

Example 3: Extended response on homeostasis (8 marks)

Question: Explain how the body responds to a decrease in blood glucose concentration. Include the role of negative feedback in your answer.

Answer:

Alpha cells in the pancreas detect low blood glucose concentration ✓
The pancreas secretes the hormone glucagon ✓
Glucagon is transported in the blood to the liver (target organ) ✓
Glucagon stimulates the breakdown of glycogen to glucose (glycogenolysis) ✓
Glucose is released into the bloodstream ✓
Blood glucose concentration increases back to normal ✓
This is negative feedback because the response (increasing glucose) opposes the original change (low glucose) ✓
The system returns to its set point/normal level ✓

(Award up to 8 marks for a complete, well-explained answer)

Common mistakes and how to avoid them

Mistake: Confusing insulin and glucagon functions (stating insulin raises blood glucose). Correction: Remember: INsulin helps glucose go IN to cells (lowers blood glucose); glucagon releases glucose from glycogen stores (raises blood glucose).
Mistake: Stating hormones travel along nerves or that the nervous system uses hormones. Correction: Hormones travel only in the bloodstream. The nervous system uses electrical impulses along neurons, not chemical messengers.
Mistake: Naming the liver or muscles as the gland that produces insulin. Correction: The pancreas produces both insulin and glucagon. The liver and muscles are target organs that store glycogen.
Mistake: Describing positive feedback when explaining blood glucose regulation. Correction: Blood glucose regulation uses negative feedback — the response always opposes the initial change to maintain homeostasis.
Mistake: Writing "adrenaline is released when you are calm" or confusing it with other hormones. Correction: Adrenaline is released during stress, fear, excitement, or danger (fight or flight response). Be specific about which body changes it causes.
Mistake: Stating that diabetes is caused by eating too much sugar or only affects overweight people. Correction: Type 1 diabetes is an autoimmune condition, not caused by diet. Type 2 diabetes is associated with obesity and poor diet but also has genetic factors. Be precise about the biological mechanisms (insufficient insulin or insulin resistance).

Exam technique for "The Endocrine System: Glands, Hormones and Target Organs"

"Name" or "State" questions (1 mark each) require only the term. Write "insulin" or "pancreas" — no explanation needed. These appear frequently for glands and hormones.
"Explain" questions (3-6 marks) require step-by-step mechanisms. For blood glucose regulation, include: detection → hormone secretion → transport in blood → effect on target organ → result. Use connecting words like "causes," "stimulates," "resulting in."
Comparison questions ("Compare the nervous and endocrine systems" — 4-6 marks) require at least three points of difference. Structure your answer clearly: "The nervous system uses electrical impulses while the endocrine system uses chemical messengers. The nervous system produces rapid responses while the endocrine system produces slower responses..." Give paired comparisons for full marks.
Diagram questions often show the pancreas, kidneys, or pituitary-hypothalamus system. Read labels carefully and ensure your answers match the specific structure or process shown. When asked to "label," use precise anatomical terms tested in the syllabus.

Quick revision summary

The endocrine system uses hormones (chemical messengers) secreted by ductless glands and transported in blood to target organs. Key examples: pancreas produces insulin (lowers blood glucose) and glucagon (raises blood glucose); adrenal glands produce adrenaline (fight or flight); pituitary produces ADH (water balance). These systems maintain homeostasis through negative feedback loops where responses oppose initial changes. Hormones act more slowly but have longer-lasting effects than nervous system responses. Know specific gland-hormone-target organ-effect relationships for exam success.