Hormones and Endocrine Glands

The endocrine system is a network of glands that produce hormones -- chemical messengers secreted directly into the bloodstream. Unlike the nervous system (which uses fast electrical signals), the endocrine system produces slower but longer-lasting responses that regulate growth, metabolism, reproduction, and homeostasis.

Major Endocrine Glands and Their Hormones

Hypothalamus

Links nervous and endocrine systems. Produces releasing/inhibiting hormones that control the pituitary.

Pituitary Gland

"Master gland" -- secretes GH, TSH, ACTH, FSH, LH, ADH, and oxytocin to control other glands.

Thyroid Gland

Produces thyroxine (T₄) -- regulates metabolic rate, growth, and development.

Adrenal Glands

Produce adrenaline (fight-or-flight) and cortisol (stress hormone, glucose regulation).

Pancreas

Produces insulin (lowers blood glucose) and glucagon (raises blood glucose).

Gonads

Ovaries produce oestrogen and progesterone; testes produce testosterone -- regulate reproduction.

Hormones vs. nerves: Nervous signals are fast (milliseconds), short-lived, and targeted to specific cells. Hormonal signals are slower (seconds to hours), longer-lasting, and travel throughout the bloodstream to reach target cells with specific receptors.

Feedback Loops

The endocrine system uses feedback loops to maintain stable internal conditions. Most hormonal regulation involves negative feedback, where the output of a system inhibits further output to prevent overproduction.

Negative Feedback

Stimulus detected

e.g. blood glucose rises

↓

Hormone released

Pancreas secretes insulin

↓

Response corrects change

Cells absorb glucose; level falls

↓

Hormone secretion stops

Normal level restored; inhibition

Positive Feedback

Stimulus detected

e.g. cervix stretches in labour

↓

Hormone released

Pituitary releases oxytocin

↓

Response amplifies change

Uterine contractions intensify

↓

Cycle continues until event

Birth occurs; cycle ends

Key distinction: Negative feedback opposes a change to restore equilibrium (most common). Positive feedback amplifies a change until an endpoint is reached (rare but important, e.g. childbirth, blood clotting).

Homeostasis: Blood Glucose Regulation

Homeostasis is the maintenance of a stable internal environment despite external changes. Blood glucose regulation by the pancreas is a classic example of negative feedback in action.

Insulin and Glucagon: Antagonistic Hormones

Blood Glucose Too High

1. After eating, blood glucose rises.
2. Beta cells in the pancreas detect the rise and secrete insulin.
3. Insulin stimulates cells to take up glucose and the liver to convert glucose to glycogen.
4. Blood glucose falls back to normal; insulin secretion decreases.

Blood Glucose Too Low

1. Between meals or during exercise, blood glucose falls.
2. Alpha cells in the pancreas detect the drop and secrete glucagon.
3. Glucagon stimulates the liver to convert glycogen back to glucose and release it into the blood.
4. Blood glucose rises back to normal; glucagon secretion decreases.

Clinical connection: In Type 1 diabetes, the immune system destroys beta cells, so no insulin is produced. In Type 2 diabetes, cells become resistant to insulin. Both result in chronically elevated blood glucose, which can damage blood vessels, kidneys, and nerves.

Key Vocabulary

Hormone

A chemical messenger produced by an endocrine gland, transported in the blood to target organs where it triggers a specific response.

Homeostasis

The maintenance of a stable internal environment (e.g. temperature, blood glucose, pH) within narrow limits despite external changes.

Negative Feedback

A regulatory mechanism where the output of a system inhibits further output, returning the variable to its set point.

Target Organ

An organ or tissue that responds to a specific hormone because its cells have the appropriate receptors for that hormone.

Worked Examples

1

Explain how blood glucose is regulated after a large carbohydrate-rich meal.

Step 1: Carbohydrates are digested into glucose, which is absorbed into the bloodstream, causing blood glucose concentration to rise above the set point.

Step 2: Beta cells in the islets of Langerhans (pancreas) detect the increase and secrete insulin into the blood.

Step 3: Insulin binds to receptors on liver and muscle cells, stimulating glucose uptake and conversion of glucose to glycogen (glycogenesis).

Step 4: As blood glucose returns to normal, the stimulus for insulin secretion is removed (negative feedback), and insulin release decreases.

2

Describe the role of thyroxine and explain how its secretion is regulated by negative feedback.

Step 1: The hypothalamus detects low thyroxine levels and releases TRH (thyrotropin-releasing hormone).

Step 2: TRH stimulates the anterior pituitary to secrete TSH (thyroid-stimulating hormone).

Step 3: TSH stimulates the thyroid gland to produce and release thyroxine (T₄), which increases metabolic rate in target cells.

Step 4: Rising thyroxine levels inhibit both the hypothalamus and pituitary (negative feedback), reducing TRH and TSH secretion until thyroxine levels fall again.

3

Compare the roles of insulin and glucagon as antagonistic hormones.

Insulin (from beta cells): Lowers blood glucose by promoting cellular glucose uptake and glycogenesis (glucose → glycogen) in the liver. Secreted when blood glucose is high.

Glucagon (from alpha cells): Raises blood glucose by stimulating glycogenolysis (glycogen → glucose) in the liver. Secreted when blood glucose is low.

Antagonistic relationship: They have opposite effects on blood glucose concentration. Working together through negative feedback, they keep blood glucose within a narrow range (~4-8 mmol/L).

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

Which gland is often called the "master gland" because it controls other endocrine glands?

Thyroid gland Adrenal gland Pituitary gland Pancreas

Question 2

When blood glucose levels are too low, which hormone is secreted and what does it do?

Insulin; stimulates cells to take up glucose Thyroxine; stimulates glucose absorption in the gut Adrenaline; increases metabolic rate Glucagon; stimulates the liver to convert glycogen to glucose

Question 3

Which of the following is an example of positive feedback?

Oxytocin stimulating uterine contractions during labour Regulation of body temperature by sweating Regulation of blood glucose by insulin and glucagon Thyroxine regulation by TSH

Question 4

In Type 1 diabetes, why can the body not regulate blood glucose levels?

The liver cannot store glycogen The immune system destroys beta cells, so no insulin is produced Cells produce too much glucagon The hypothalamus fails to detect glucose changes

Question 5

What is the key difference between endocrine signalling and nervous signalling?

Endocrine signalling uses electrical impulses; nervous uses chemical Endocrine signalling is faster and shorter-lasting than nervous signalling Endocrine signalling uses hormones in the blood; it is slower but longer-lasting than nervous signalling There is no difference; both systems work the same way

Human Endocrine System

Hormones and Endocrine Glands

Major Endocrine Glands and Their Hormones

Feedback Loops

Negative Feedback

Positive Feedback

Homeostasis: Blood Glucose Regulation

Insulin and Glucagon: Antagonistic Hormones

Blood Glucose Too High

Blood Glucose Too Low

Key Vocabulary

Hormone

Homeostasis

Negative Feedback

Target Organ

Worked Examples

Explain how blood glucose is regulated after a large carbohydrate-rich meal.

Describe the role of thyroxine and explain how its secretion is regulated by negative feedback.

Compare the roles of insulin and glucagon as antagonistic hormones.

Knowledge Check

Key Concepts Summary