Endocrine Glands: The Producers of Chemical Messengers — ICSE Class 10 Biology Important Questions

Answer: (b) Pituitary.

The pituitary secretes trophic hormones that control other endocrine glands, so it is called the master gland; it is itself regulated by the hypothalamus.

Answer: (b) Insulin.

The $\beta$β-cells of the islets of Langerhans secrete insulin, which lowers blood glucose; the $\alpha$α-cells secrete glucagon, which raises it.

Answer: (b) Goitre.

Without enough iodine the thyroid cannot make thyroxine and enlarges as a swelling in the neck called simple (endemic) goitre; iodised salt prevents it.

Answer: (a) Oxytocin. Oxytocin from the pituitary causes contraction of the uterine muscles at childbirth and the release of milk. ADH makes the kidney tubules reabsorb more water.

Answer: (a) The islets of Langerhans act as an endocrine part (insulin, glucagon into blood) while the acini act as an exocrine part (pancreatic juice via a duct), so R correctly explains A.

A hormone is a chemical messenger secreted in very small amounts by an endocrine (ductless) gland directly into the blood, which carries it to a distant target organ where it regulates a specific activity.

Distinguishing property: A hormone is poured into the blood and acts on a distant organ and is not reused, whereas a digestive enzyme is poured through a duct onto the food at its site of action and only speeds up a chemical reaction there.

(i) Gland: pituitary gland; Hormone: ADH (antidiuretic hormone, vasopressin), which makes the kidney tubules reabsorb more water, so less urine is passed. (ii) Gland: adrenal gland (adrenal medulla); Hormone: adrenaline (epinephrine), which prepares the body for emergency or fight-or-flight.

Endocrine glands act on distant target organs through the blood, while exocrine secretions act locally at or near the site of release.

The islets of Langerhans of the pancreas secrete two antagonistic hormones that keep blood glucose steady:

1. Insulin (from $\beta$β-cells) is released when blood glucose is high; it makes the liver and muscles take up glucose and store it as glycogen, lowering blood sugar.
2. Glucagon (from $\alpha$α-cells) is released when blood glucose is low; it converts stored liver glycogen back to glucose, raising blood sugar.

This balance keeps the level near normal. A lasting deficiency of insulin causes diabetes mellitus, in which blood glucose stays high and glucose is passed out in the urine (glycosuria).

(i) Hyposecretion of thyroxine in a child causes cretinism: stunted growth, mental retardation, a puffy face and delayed development.

(ii) Hypersecretion of growth hormone in an adult causes acromegaly: abnormal thickening and enlargement of the bones of the hands, feet, jaw and face (the long bones can no longer lengthen).

(iii) Hyposecretion of growth hormone in a child causes pituitary dwarfism: the child remains very short but has normal body proportions and normal intelligence.

Pituitary $\rightarrow$→ growth hormone (STH); controls growth of the whole body and stimulates other endocrine glands.

Thyroid $\rightarrow$→ thyroxine; controls the rate of metabolism and body heat production.

Adrenal (medulla) $\rightarrow$→ adrenaline; prepares the body for emergency by raising heart rate and blood glucose.

Pancreas (islets of Langerhans) $\rightarrow$→ insulin; lowers blood glucose by promoting storage of glucose as glycogen.

A feedback mechanism is a self-regulating loop in which the amount of a hormone in the blood itself controls its own further secretion, so the hormone level stays balanced.

Thyroxine and the pituitary (negative feedback):

1. The pituitary secretes thyroid-stimulating hormone (TSH), which stimulates the thyroid to release thyroxine.
2. When the level of thyroxine in the blood rises, it acts back on the pituitary (and hypothalamus) and switches off further TSH secretion, so the thyroid slows down.
3. When the level of thyroxine falls, this braking is removed, the pituitary secretes more TSH, and thyroxine output rises again.

This is called negative feedback because the product (thyroxine) inhibits the process that made it.

Importance: it prevents both hypersecretion and hyposecretion, keeps metabolism steady, and maintains a constant internal environment (homeostasis).

(i) The most likely disorder is diabetes mellitus.

(ii) The deficient hormone is insulin, secreted by the $\beta$β-cells of the islets of Langerhans of the pancreas.

(iii) Without enough insulin the body cells cannot take up glucose, so blood glucose rises above the kidney threshold; the kidney tubules cannot reabsorb all of it, and the excess glucose is passed out in the urine (glycosuria). This also draws out water, causing frequent urination and thirst.

(iv) It can be managed by regular injections of insulin, together with a controlled sugar-free diet and exercise.