Endocrine System

The endocrine system is a collection of ductless glands that work independently and secrete chemical substances called hormones directly into the blood to regulate body functions. By definition it is a system of ductless glands that secrete hormones directly into the blood, and those hormones are non-nutrient chemicals that act as intercellular (chemical) messengers and are produced in only trace (very small) amounts.

In short, hormones are chemical messengers secreted by glands into the blood, they act on specific target organs, and they work in tiny amounts — which is exactly what separates ductless endocrine glands from the duct-bearing exocrine glands described next.

The body's endocrine glands are spread from the brain down to the reproductive organs, and the standard diagram of the system labels each one in turn. Knowing this full list of glands that make up the endocrine system is itself a common exam point.

• The pituitary gland — the master gland, just below the hypothalamus.
• The pineal gland — deep in the centre of the brain.
• The thyroid and parathyroid glands — in the neck.
• The thymus — near the heart.
• The pancreas — behind the stomach.
• The adrenal glands — one on top of each kidney.
• The ovary — in the female.
• The testicle (testis) — in the male.
• The placenta — acts as an endocrine gland during pregnancy.

Glands are sorted into three kinds according to their way of secretion, and the simplest way to remember the difference is by the duct: endocrine glands have NO ducts while exocrine glands HAVE ducts. A few organs do both jobs at once and are therefore called mixed glands.

• Examples: sweat glands, salivary glands, mammary gland, lacrimal gland and mucous gland.
• Functions: regulate body temperature (sweat), lubrication, lactation (milk), help in digestion and help in reproduction.

• They secrete hormones into the blood flowing through them, so the secretion can act at distant parts of the body far from the gland.
• They have no ducts and their hormones act on distant target organs.
• Examples: pineal, pituitary, pancreas, ovaries, testes, thyroid, parathyroid, hypothalamus and adrenal glands.

• A mixed gland is both endocrine + exocrine; the two examples are the liver and the pancreas.
• They secrete some products (bile, pancreatic juice) into the gut through ducts (exocrine) and other substances (hormones) directly into the bloodstream (endocrine).
• The liver secretes bile (via the bile duct) and also hormones into the blood.
• The pancreas secretes pancreatic juice (via the pancreatic duct) and also hormones (insulin, glucagon) into the blood.
• Liver / pancreas diagram labels: gallbladder, liver, pancreas, pancreatic duct, pancreatic tumor and small intestine.

At the base of the brain sit two structures that command the whole system. The hypothalamus contains groups of neurosecretory cells called nuclei that produce hormones, while the pituitary gland just below it is the master gland because its hormones in turn switch the other endocrine glands on and off.

• It makes two types of hormone: releasing hormones (which stimulate) and inhibiting hormones (which inhibit).

The crucial correction is that the anterior lobe manufactures GH, ACTH, TSH, FSH, LH, prolactin and MSH, while the posterior lobe makes nothing itself — it merely stores and releases oxytocin and ADH, which the hypothalamus produces.

Each endocrine gland sits at a fixed place and pours out its own hormones to control a particular process, and this is the single most valuable table for the exam. The thyroid is the largest endocrine gland of the body, while the liver is the largest gland overall.

The corrected facts that matter most: the adrenal gland is two glands in one — a cortex giving cortisol, aldosterone and sex steroids, and a medulla giving adrenaline and noradrenaline — while the pancreas balances blood sugar through insulin (β-cells) and glucagon (α-cells), and the thyroid makes both the metabolism hormones T3/T4 and the calcium-lowering calcitonin. Together these glands maintain balance, growth, metabolism and reproduction, releasing their hormones directly into the bloodstream.

A gland can fall ill in two opposite ways: it may secrete too little of its hormone (hyposecretion) or too much (hypersecretion), and each fault produces its own well-known disorder. These disease names are among the most heavily asked endocrine facts.

The headline pairs: too little growth hormone gives dwarfism, too much gives gigantism or acromegaly; a lack of iodine swells the thyroid into a goitre; too little insulin causes diabetes mellitus; and the adrenal cortex gives Addison's disease (under) and Cushing's syndrome (over).

The source pairs the endocrine glands with the body's digestive enzymes, because several glands (mouth, stomach, pancreas, intestine) also pour out enzymes that chemically break down food. Each enzyme acts on a specific nutrient.

• The pancreas secretes its enzymes into the small intestine, where small-intestine enzymes complete digestion into absorbable forms.
• These enzymes are essential for proper digestion and absorption.

Beyond the big tables, a handful of single-hormone facts recur so often they deserve their own list, linking a familiar everyday idea to the exact hormone or gland behind it.

Salt is iodised precisely because the body needs iodine to build thyroxine, and without it the thyroid swells into a goitre — one of the cleanest cause-and-effect chains in the topic.