Describe endocrine control of the internal environment, including the role of hormones, target cells, the hypothalamus and pituitary gland, and the regulation of blood glucose by insulin and glucagon

What this dot point is asking

QCAA expects you to describe how the endocrine system controls the internal environment: what a hormone is, how it acts on a target cell, the role of the hypothalamus and pituitary, and how insulin and glucagon regulate blood glucose. Pancreatic islet diagrams and glucose-time graphs are common stimulus.

The answer

The endocrine system is the slower, longer-acting partner to the nervous system. Hormones are signalling molecules released into the blood and carried to target cells anywhere in the body.

Hormones and target cells

A hormone is a chemical messenger released by an endocrine gland directly into the bloodstream. The blood carries it to all parts of the body, but it acts only on target cells that express the matching receptor.

Two broad chemical classes:

• Steroid hormones (cholesterol-derived; oestrogen, testosterone, cortisol, aldosterone). Lipid-soluble. Diffuse through the plasma membrane and bind intracellular receptors that act as transcription factors. Slow but long-lasting effects (hours to days).
• Peptide and amine hormones (made of amino acids; insulin, glucagon, ADH, growth hormone, adrenaline). Water-soluble. Bind cell-surface receptors and trigger second-messenger cascades inside the cell. Fast but short-lived effects (seconds to minutes).

The specificity of hormonal signalling comes from receptor expression: only cells with the matching receptor respond to a given hormone, even though every cell is bathed in the same blood.

The hypothalamus and pituitary gland

The hypothalamus is the integrator that links the nervous and endocrine systems. It sits at the base of the brain, attached to the pituitary gland.

Posterior pituitary. Stores and releases two hormones made by neurosecretory cells in the hypothalamus:

• ADH (antidiuretic hormone). Increases water reabsorption by the kidney collecting duct (see osmoregulation and excretion).
• Oxytocin. Triggers uterine contractions during labour and milk ejection during breastfeeding.

Anterior pituitary. Produces its own hormones in response to releasing or inhibiting hormones from the hypothalamus, carried in a portal blood system.

• Growth hormone (GH). Promotes growth of bone and soft tissues.
• Thyroid stimulating hormone (TSH). Stimulates thyroid hormone release.
• Adrenocorticotrophic hormone (ACTH). Stimulates cortisol release from the adrenal cortex.
• Follicle stimulating hormone (FSH) and luteinising hormone (LH). Regulate reproduction.
• Prolactin. Stimulates milk production.

Each anterior pituitary axis is regulated by negative feedback: rising hormone levels from the target gland inhibit further release from the hypothalamus and pituitary.

Major endocrine glands and their hormones

Blood glucose regulation

Blood glucose is held around 4 to 6 mmol per L by two antagonistic hormones from the pancreatic islets.

Rising glucose (after a meal).

• Stimulus: blood glucose rises above the set point.
• Receptor and control centre: beta cells of the pancreatic islets detect the rise.
• Effector: insulin is released into the blood. Liver, skeletal muscle and adipose tissue respond.
• Response: insulin binds cell-surface receptors and triggers insertion of GLUT4 glucose transporters into the plasma membrane. Glucose enters cells and is stored as glycogen in liver and muscle (glycogenesis) or converted to fat in adipose tissue. Blood glucose falls back to the set point.

Falling glucose (between meals or during exercise).

• Stimulus: blood glucose falls below the set point.
• Receptor and control centre: alpha cells of the pancreatic islets detect the fall.
• Effector: glucagon is released. The liver is the main target.
• Response: liver glycogen is broken down to glucose (glycogenolysis); amino acids and lactate are converted to glucose (gluconeogenesis). Glucose is released into the blood. Blood glucose rises back to the set point.

Diabetes mellitus. Failure of the insulin pathway.

• Type 1. Autoimmune destruction of beta cells; insulin is absent. Managed by insulin injection.
• Type 2. Cells become resistant to insulin; beta cell function eventually declines. Managed by diet, exercise and oral drugs (sometimes insulin).

In both forms, blood glucose runs high (hyperglycaemia), spills over into urine and damages blood vessels and nerves long-term.

Endocrine vs nervous control

The two systems are integrated through the hypothalamus.

Common traps

Saying hormones act on every cell. Hormones travel everywhere but act only on cells with the matching receptor.

Confusing the two pituitary lobes. The posterior pituitary stores hormones made by the hypothalamus. The anterior pituitary makes its own hormones under hypothalamic control.

Putting glucagon receptors on muscle. Glucagon's main target is the liver. Muscle cells respond to insulin and adrenaline, not glucagon.

Forgetting antagonism. Insulin and glucagon work as an opposing pair; both must be named in a glucose regulation answer.

Cross-link to Year 12 assessment

The endocrine system underpins QCAA Unit 2 questions on disease (autoimmune diabetes is a classic crossover with immunity), and the hypothalamus-pituitary control structure is foreshadowed in Unit 4 reproductive genetics. EA Paper 1 short-response questions on insulin and glucagon are extremely common.

In one sentence

Hormones travel in the blood and act on target cells with matching receptors, with steroid hormones binding intracellular receptors to switch genes on or off and peptide hormones binding cell-surface receptors to trigger second messengers; the hypothalamus integrates nervous and endocrine signals through the pituitary, and the pancreatic islets keep blood glucose around 5 mmol per L by releasing insulin when glucose rises and glucagon when it falls.