What this chapter is about
This chapter introduces the endocrine system, which works alongside the nervous system to coordinate body functions. While neural coordination provides rapid, short-lived responses, chemical coordination through hormones produces slower but longer-lasting effects. A Class 11 student learns how various glands release hormones into the blood, how these chemical messengers reach target organs, and how they regulate processes such as growth, metabolism, reproduction, and maintaining internal balance.
The chapter covers the location and function of major endocrine glands in humans: the hypothalamus, pituitary, pineal, thyroid, parathyroid, thymus, adrenal glands, pancreas, and gonads. Students understand the concept of feedback mechanisms that keep hormone levels within normal ranges. The relationship between the hypothalamus and pituitary gland is emphasised, as this connection links the nervous and endocrine systems.
After studying this chapter, a student should be able to name the principal endocrine glands, list the hormones each produces, describe the effects of these hormones on target tissues, explain disorders caused by hyposecretion or hypersecretion, and understand how the body maintains hormonal balance through negative feedback.
Key ideas
- Hormones are non-nutrient chemicals produced in trace amounts by endocrine glands; they are released directly into the blood and act on specific target organs that have appropriate receptors.
- The hypothalamus is the link between the nervous and endocrine systems; it produces releasing hormones and inhibiting hormones that control the anterior pituitary.
- The pituitary gland (hypophysis) has two lobes: the anterior pituitary secretes growth hormone (GH), thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinising hormone (LH), and prolactin; the posterior pituitary stores and releases oxytocin and antidiuretic hormone (ADH, also called vasopressin).
- The thyroid gland secretes thyroxine (T₄) and triiodothyronine (T₃), which regulate basal metabolic rate, and calcitonin, which lowers blood calcium levels.
- The adrenal glands have an outer cortex producing corticosteroids (glucocorticoids, mineralocorticoids, and small amounts of androgens) and an inner medulla producing adrenaline and noradrenaline for the fight-or-flight response.
- The pancreas is both an exocrine and an endocrine gland; its islets of Langerhans contain alpha cells secreting glucagon and beta cells secreting insulin, together regulating blood glucose.
- Negative feedback is the main mechanism by which hormone levels are controlled: a rise in the hormone's effect inhibits further release, preventing overproduction.
- Some tissues not traditionally called glands also produce hormones: the heart secretes atrial natriuretic factor (ANF), the kidney produces erythropoietin, and the gastrointestinal tract releases gastrin, secretin, and cholecystokinin.
Formulas and facts to remember
1. Hormone action on target cells requires specific receptors; peptide hormones bind to membrane receptors and act via second messengers, whereas steroid hormones enter cells and bind to intracellular receptors to influence gene expression.
2. Hypothalamo-hypophyseal portal system: blood vessels carry releasing and inhibiting hormones from the hypothalamus directly to the anterior pituitary.
3. Iodine is essential for thyroid hormone synthesis; deficiency causes goitre (enlarged thyroid).
4. Diabetes mellitus results from insufficient insulin secretion or reduced sensitivity to insulin, leading to high blood glucose (hyperglycaemia).
5. Addison's disease is caused by hyposecretion of adrenal cortex hormones; Cushing's syndrome results from hypersecretion of cortisol.
6. Gigantism occurs when excess GH is secreted during childhood; acromegaly occurs when excess GH is secreted in adults.
7. Cretinism is caused by thyroid hormone deficiency during infancy, leading to stunted growth and mental retardation.
8. Calcitonin and parathyroid hormone (PTH) have opposite effects: calcitonin lowers blood calcium; PTH raises it.
Worked examples
### Example 1: Identifying the gland and hormone from a symptom
Situation: A 10-year-old child shows unusually rapid linear growth, with height well above average for age. Which gland and hormone are most likely involved?
Step-by-step reasoning:
1. Linear growth in children is mainly regulated by growth hormone (GH), also called somatotropin. 2. GH is secreted by the anterior pituitary gland. 3. Excessive GH secretion before the closure of epiphyseal plates causes gigantism. 4. Therefore, the gland involved is the anterior pituitary, and the hormone is growth hormone.
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### Example 2: Understanding feedback regulation of blood glucose
Situation: After eating a meal rich in carbohydrates, blood glucose level rises from about 90 mg per 100 mL to 140 mg per 100 mL. Describe how the body restores normal glucose levels.
Step-by-step reasoning:
1. The rise in blood glucose is detected by beta cells in the islets of Langerhans of the pancreas. 2. Beta cells respond by secreting more insulin into the blood. 3. Insulin promotes glucose uptake by muscle and adipose tissue and stimulates glycogen synthesis in the liver. 4. As cells absorb glucose and the liver stores it as glycogen, blood glucose falls. 5. When glucose returns to about 90 mg per 100 mL, insulin secretion decreases — this is negative feedback.
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### Example 3: Distinguishing between two thyroid disorders
Situation: Patient A has a slow heart rate, feels cold often, and gains weight easily. Patient B has a rapid heart rate, sweats excessively, and loses weight despite eating well. Identify the likely thyroid condition in each case.
Step-by-step reasoning:
1. Thyroid hormones (T₃ and T₄) increase basal metabolic rate. 2. Low metabolic rate (slow heart, cold intolerance, weight gain) suggests hypothyroidism — underproduction of thyroid hormones. Patient A likely has hypothyroidism. 3. High metabolic rate (rapid heart, excessive sweating, weight loss) suggests hyperthyroidism — overproduction of thyroid hormones. Patient B likely has hyperthyroidism, possibly Graves' disease.
Common mistakes
- Confusing the anterior and posterior pituitary → Remember: anterior pituitary synthesises its own hormones; posterior pituitary only stores and releases hormones made by the hypothalamus.
- Mixing up insulin and glucagon functions → Insulin lowers blood glucose; glucagon raises it. Think: insulin = into cells, glucagon = glucose out of stores.
- Believing adrenaline comes from the adrenal cortex → Adrenaline (epinephrine) is secreted by the adrenal medulla, not the cortex.
- Forgetting that the pancreas is both exocrine and endocrine → Exocrine part secretes digestive enzymes into the duodenum; endocrine part (islets) releases hormones into blood.
- Assuming all hormones act via the same mechanism → Peptide hormones act through membrane receptors and second messengers; steroid hormones enter cells and affect gene expression directly.
Quick revision
1. Hormones are chemical messengers released by endocrine glands into the blood to act on distant target organs with specific receptors.
2. The hypothalamus controls the pituitary; the pituitary is called the master gland because it regulates many other endocrine glands.
3. Thyroid hormones regulate metabolism; their deficiency causes goitre, cretinism (in infants), or myxoedema (in adults).
4. Insulin (beta cells) lowers blood glucose; glucagon (alpha cells) raises it — together they maintain glucose homeostasis.
5. Adrenal medulla hormones (adrenaline, noradrenaline) prepare the body for emergency (fight or flight); adrenal cortex hormones regulate metabolism, salt balance, and some sex characteristics.
6. Negative feedback loops prevent overproduction of hormones by reducing secretion when hormone levels or their effects rise.