2.3 Endocrine System & Homeostasis

Hormones and the Endocrine System

The endocrine system uses chemical messengers called hormones to coordinate the body. Unlike exocrine glands, which release secretions through ducts (for example, sweat or saliva), endocrine glands are ductless and secrete hormones directly into the bloodstream. The blood carries each hormone to target cells that have matching receptors. This is why endocrine control is slower but more widespread than nervous control.

Only cells with the right receptor respond to a given hormone, which is how a chemical broadcast through the whole bloodstream can still produce a specific effect. Hormones fall into two broad chemical groups: steroid hormones (such as cortisol) are lipid-based and can enter cells directly, while protein-based hormones (such as insulin) bind receptors on the cell surface.

Key Glands and Hormones

Memorize this core table — diploma questions frequently link a hormone to its gland, target, and effect.

The pituitary is called the "master gland" because its hormones control other glands, yet it is itself directed by the hypothalamus — the bridge between the nervous and endocrine systems.

Negative Feedback

Homeostasis is the maintenance of a stable internal environment. Its main tool is negative feedback: when a variable moves away from its set point, the body responds in a way that reverses the change and brings it back.

Thyroid control is a clear example. The hypothalamus signals the pituitary to release TSH, which tells the thyroid to release thyroxine. When thyroxine rises high enough, it inhibits the hypothalamus and pituitary, reducing further TSH. This self-limiting loop keeps thyroxine within range. Most hormone systems work this way; positive feedback, which amplifies a change (as in childbirth), is the exception.

Blood-Glucose Regulation

The pancreas keeps blood glucose near its set point using two antagonistic hormones from the islets of Langerhans:

• After a meal (glucose high): beta cells release insulin. Insulin lowers blood glucose by helping body cells take up glucose and by stimulating the liver to store glucose as glycogen (glycogenesis).
• Between meals (glucose low): alpha cells release glucagon. Glucagon raises blood glucose by stimulating the liver to break glycogen back into glucose (glycogenolysis).

This is a textbook negative feedback pair: each hormone opposes the other to hold glucose steady. Remember the direction — insulin in, glucose down.

Diabetes Mellitus

When blood-glucose control fails, the result is diabetes mellitus, marked by high blood sugar.

• Type 1 — the immune system destroys the insulin-producing beta cells, so the body makes little or no insulin. It is managed with insulin injections.
• Type 2 — the body still makes insulin, but cells become resistant to it. It is linked to lifestyle factors and often managed with diet, exercise, and medication.

In both types, cells cannot take up glucose normally, so glucose stays high in the blood and spills into the urine — a classic diagnostic sign.

Connect this back to feedback: in a healthy person, rising glucose is detected and corrected automatically. In diabetes, the detection-and-correction loop is broken either because the insulin signal is missing (Type 1) or because target cells ignore it (Type 2). Exam questions often ask you to predict that, without effective insulin, blood glucose remains dangerously elevated after eating.

Thermoregulation

The body also holds temperature near 37 degrees C by negative feedback, coordinated by the hypothalamus.

• Too hot: blood vessels in the skin dilate (vasodilation) to lose heat, and sweat glands release sweat that cools by evaporation.
• Too cold: skin vessels constrict (vasoconstriction) to conserve heat, and shivering generates heat through muscle activity.

This is another reversing loop: a rise in temperature triggers cooling, and a fall triggers warming, returning the body toward its set point.

Nervous vs Endocrine Control

A favourite comparison question contrasts the two control systems:

The two systems work together — the hypothalamus is the key link — so the body can both react instantly (nervous) and sustain long regulation such as growth and metabolism (endocrine).

A good way to remember the difference: the nervous system is like a phone call to one person (fast, direct, short), while the endocrine system is like a mass mailing (slower to arrive, reaches everyone with the right "address," effects last longer). Many diploma questions describe a scenario and ask which system is responsible — match instant, brief, targeted responses to the nervous system and slow, sustained, body-wide responses to the endocrine system.