describe the features of the endocrine system with reference to the hormones ADH, glucagon and insulin (see 14.1.8, 14.1.9 and 14.1.10)

Control and Coordination in Mammals

Objective

Describe the features of the endocrine system with reference to the hormones ADH, glucagon and insulin (see 14.1.8, 14.1.9 and 14.1.10).

1. Overview of the Endocrine System

The endocrine system is a network of glands that secrete chemical messengers called hormones directly into the bloodstream. These hormones travel to distant target cells and regulate a wide range of physiological processes.

• Glands involved: hypothalamus, pituitary, thyroid, parathyroid, adrenal cortex & medulla, pancreas (islets of Langerhans), gonads, and specialised cells in the kidney.
• Mode of action: Hormones bind to specific receptors on or in target cells, triggering a cascade of intracellular events that alter cell function.
• Regulation: Most hormones are regulated by negative feedback loops; a rise in the hormone’s effect reduces its further secretion.
• Speed of response: Slower than neural signalling but longer lasting, allowing coordination of growth, metabolism, water balance and reproduction.

2. Hormones of Interest

2.1 Antidiuretic Hormone (ADH)

Source: Synthesised in the hypothalamus and stored in the posterior pituitary.

Target organ: Kidneys (collecting ducts).

Primary action: Increases water re‑absorption by inserting aquaporin‑2 channels into the apical membrane of collecting‑duct cells, reducing urine volume and concentrating the urine.

Regulation: Plasma osmolality is the main stimulus; osmoreceptors in the hypothalamus detect increased osmolality and trigger ADH release. Low blood volume (via baroreceptors) also stimulates release.

Clinical relevance: Deficiency leads to diabetes insipidus (excessive dilute urine); excess can cause water intoxication.

2.2 Glucagon

Source: Alpha cells of the pancreatic islets of Langerhans.

Target organ: Liver (also kidney and adipose tissue).

Primary action: Stimulates glycogenolysis and gluconeogenesis, raising blood glucose levels.

Regulation: Low blood glucose (< 4 mmol L⁻¹) triggers secretion; high glucose and insulin inhibit release.

Clinical relevance: Used therapeutically in severe hypoglycaemia; chronic excess can contribute to hyperglycaemia in diabetes mellitus.

2.3 Insulin

Source: Beta cells of the pancreatic islets of Langerhans.

Target organs: Liver, skeletal muscle, adipose tissue.

Primary action: Promotes glucose uptake (via GLUT4 translocation), glycogen synthesis, lipogenesis and protein synthesis; lowers blood glucose.

Regulation: Elevated blood glucose (> 5 mmol L⁻¹) after a meal stimulates release; glucagon, sympathetic activity and somatostatin inhibit secretion.

Clinical relevance: Deficiency or resistance leads to diabetes mellitus type 1 (autoimmune destruction of β‑cells) or type 2 (insulin resistance).

3. Comparison of ADH, Glucagon and Insulin

4. Integrated Feedback Example: Blood Glucose Regulation

The balance between insulin and glucagon maintains blood glucose within a narrow range (≈ 4–6 mmol L⁻¹). The process can be summarised by the following equations:

\$\text{When }[Glucose] \uparrow \Rightarrow \text{Insulin secretion} \uparrow \Rightarrow \text{Glucose uptake} \uparrow\$

\$\text{When }[Glucose] \downarrow \Rightarrow \text{Glucagon secretion} \uparrow \Rightarrow \text{Hepatic glucose output} \uparrow\$

These opposing actions constitute a classic negative‑feedback loop: the deviation from the set point triggers a hormonal response that restores the set point.

5. Key Points to Remember

1. The endocrine system uses hormones as chemical messengers that act over longer periods than nerve impulses.
2. ADH primarily controls water balance; glucagon and insulin regulate carbohydrate metabolism.
3. All three hormones are regulated by negative feedback mechanisms based on the physiological variable they control.
4. Disorders of these hormones underlie common clinical conditions such as diabetes mellitus and diabetes insipidus.