Hormonal communication — OCR A-Level Biology

Endocrine communication is the body's chemical signalling route in which specialised endocrine cells secrete hormones directly into the blood. The hormone travels in the plasma to reach cells anywhere in the body, but only target cells respond because they carry a complementary receptor. This is why one hormone can affect several tissues while leaving others unaffected. Learners should distinguish endocrine signalling from nervous signalling: nerves act quickly over short distances via neurones and neurotransmitters, whereas hormones act more slowly but often for longer and can reach every perfused tissue. A useful method is to trace one example end to end, for example insulin released by pancreatic beta cells into the blood, binding to receptors on liver and muscle cells to increase glucose uptake.

Each adrenal gland sits above a kidney and has two regions with different embryological origins and roles. The outer adrenal cortex is divided into zones that secrete steroid hormones: mineralocorticoids such as aldosterone, which increase sodium reabsorption and so help regulate blood pressure and salt balance; glucocorticoids such as cortisol, which raise blood glucose and help the body respond to long-term stress; and small amounts of androgens. The inner adrenal medulla consists of modified sympathetic nervous tissue and secretes adrenaline and noradrenaline, which raise heart rate, increase blood glucose and prepare the body for fight-or-flight responses. A reliable method is to link each region to its hormone class and its main effect: cortex gives steroid hormones with slower, longer actions; medulla gives catecholamines with rapid actions.

The pancreas is a mixed gland: most of its volume is exocrine tissue, arranged as acini of enzyme-secreting cells draining into ducts, while small clusters of endocrine cells, the islets of Langerhans, sit scattered among the acini. In an islet, alpha cells secrete glucagon and beta cells secrete insulin; these hormones pass straight into the blood, so islet cells lie close to capillaries rather than to ducts. Histology questions ask you to recognise these tissues in a stained section and to link structure to function. Acinar cells appear as groups with dark basal nuclei and granular apical cytoplasm; islet cells form paler, rounded groups with a rich capillary network. Distinguish exocrine from endocrine by the presence or absence of ducts and by the direction of secretion.

This practical skill has two parts: examining a stained pancreas section under the microscope and producing a drawing that shows the endocrine tissue. First survey the section at low power to find the islets of Langerhans, which appear as paler, rounded clusters of cells set among the darker, more numerous exocrine acini. Then switch to higher power to study cell arrangement and the capillary network within an islet. When drawing, use a sharp pencil, draw clear continuous lines without shading, keep the drawing large and in proportion, and label the islet, the surrounding acini and any visible capillaries. Add a title stating the tissue and stain, and record the magnification. Your drawing is assessed on accuracy of proportions, clarity of lines, correct labelling of endocrine tissue and a stated magnification, not on artistic shading.

Blood glucose is regulated by negative feedback centred on the pancreatic islets of Langerhans. When blood glucose rises after a meal, β cells secrete insulin. Insulin binds to receptors on liver and muscle cells, increasing glucose uptake and converting glucose to glycogen (glycogenesis), and it increases respiration of glucose, lowering blood glucose. When blood glucose falls, α cells secrete glucagon. Glucagon acts mainly on liver cells, promoting glycogenolysis (glycogen to glucose) and gluconeogenesis (amino acids and glycerol to glucose), raising blood glucose. Adrenaline from the adrenal glands also raises blood glucose in stress. The liver is the key effector organ. Regulation is self-correcting: the change in glucose concentration triggers the corrective hormone response, and the restored level reduces further hormone secretion.

Diabetes mellitus is a condition in which blood glucose is poorly controlled. In Type 1 diabetes, the β cells of the islets of Langerhans are destroyed (an autoimmune response), so little or no insulin is produced; it typically develops in younger people and is treated with insulin injections. In Type 2 diabetes, insulin is still produced but body cells become less responsive to it (insulin resistance), often linked to obesity and lifestyle; it typically develops later in life and is managed with diet, exercise and medication. Both types cause high blood glucose, but the underlying cause differs: Type 1 is a failure of insulin secretion, Type 2 is a failure of cell response to insulin.

Diabetes mellitus arises when blood glucose control fails. In Type 1 diabetes the pancreas does not secrete enough insulin, so treatment is insulin therapy, commonly injected because insulin is a protein digested in the gut. In Type 2 diabetes receptors respond poorly to insulin or secretion is insufficient; treatment begins with lifestyle measures such as diet and exercise, then drugs such as metformin, which reduce blood glucose, and insulin may be added later. Potential treatments also include monitoring blood glucose, transplanting pancreatic tissue or islet cells, and using insulin pumps or genetically engineered human insulin. Candidates should evaluate these options: insulin relieves symptoms but requires careful dosing to avoid hypoglycaemia, while transplants risk rejection and need immunosuppressants. Treatments manage the condition rather than curing it.