Class XII · Second Year · Sindh / BIEK · Chapter 18

Chemical Coordination.

After a sugary meal your blood glucose climbs, yet within an hour it settles back to normal — and you never felt a thing. No nerve fired the order. Instead a tiny gland poured a chemical messenger into your blood that travelled silently to your liver and muscles. That is chemical coordination: the body running itself with hormones.

1 · Two ways the body coordinates

For all its parts to work as one, an animal must coordinate — detect changes and respond in an orderly way. Mammals do this through two linked systems:

The nervous system — fast, electrical impulses carried along neurons to precise targets, giving brief, immediate responses.
The endocrine (hormonal) system — slower chemical messengers (hormones) carried in the blood to wide-ranging targets, giving longer-lasting effects.

The two are not rivals; they work together, and the hypothalamus of the brain is the bridge that links nervous control to hormonal control.

Feature	Nervous control	Hormonal (chemical) control
Messenger	Nerve impulse (electrical)	Hormone (chemical)
Pathway	Along neurons	In the bloodstream
Speed	Very fast (milliseconds)	Slower (seconds to hours)
Target	Precise — one muscle or gland	Widespread — any cell with the receptor
Duration	Short-lived	Often long-lasting

2 · What hormones are

A hormone is a chemical messenger, secreted by an endocrine gland directly into the blood, that is carried to target organs where it produces a specific effect — even in very small amounts.

Endocrine vs exocrine Endocrine ("ductless") glands pour their hormones straight into the blood (e.g. thyroid, adrenal). Exocrine glands release their products through a duct onto a surface (e.g. salivary glands, sweat glands). The pancreas does both — it is endocrine (insulin, glucagon) and exocrine (digestive enzymes via a duct).

Together, all the endocrine glands make up the endocrine system. Because hormones travel everywhere in the blood but act only where the right receptor exists, the body can send one chemical to a single chosen tissue.

3 · The hypothalamus & pituitary — the master gland

The pituitary gland hangs beneath the brain, controlled by the hypothalamus just above it. The pituitary is called the master gland because many of its hormones switch other glands on and off. It has two lobes:

Anterior (front) lobe secretes: GH (growth hormone — body growth), TSH (thyroid-stimulating hormone — drives the thyroid), ACTH (drives the adrenal cortex), and FSH & LH (control the gonads and reproduction).
Posterior (back) lobe stores and releases two hormones made in the hypothalamus: ADH (antidiuretic hormone — makes the kidney save water) and oxytocin (contracts the uterus in birth and releases milk).

Master, but not the boss The pituitary controls other glands, yet it is itself controlled by the hypothalamus — the true link between the nervous system and the endocrine system. The hypothalamus turns nervous signals into hormonal commands.

4 · The thyroid & parathyroid glands

The thyroid gland lies in the neck, in front of the windpipe. Stimulated by TSH from the pituitary, it secretes thyroxine, a hormone that contains iodine and sets the body's metabolic rate — the speed at which cells release energy. Thyroxine also controls growth and mental development.

Embedded in the back of the thyroid are four small parathyroid glands. They secrete parathormone (PTH), which raises blood calcium by releasing calcium from bone and conserving it in the kidney — essential for nerves, muscles and bones.

Iodine & the thyroid Thyroxine cannot be made without iodine. A diet short of iodine leaves the thyroid unable to make enough thyroxine, so it swells into a goitre — which is why table salt in many countries is iodised.

5 · The adrenal glands

One adrenal gland sits on top of each kidney. Each has two parts that secrete different hormones:

Adrenal medulla (inner part) — secretes adrenaline (epinephrine), the "fight-or-flight" hormone. In fear, anger or stress it raises heart rate and breathing, widens the pupils, diverts blood to the muscles and raises blood glucose, readying the body for sudden action.
Adrenal cortex (outer part) — secretes cortisol (a glucocorticoid) which helps the body cope with long-term stress and raises blood glucose, and aldosterone, which controls salt and water balance.

Adrenaline — nervous meets hormonal Adrenaline is the perfect bridge: a nervous alarm signal triggers the adrenal medulla, which then floods the blood with a hormone so the whole body responds at once.

6 · The pancreas — islets of Langerhans

The pancreas lies below the stomach. Scattered through it are clusters of endocrine cells, the islets of Langerhans, which contain two cell types making two opposing hormones:

β (beta) cells secrete insulin — it lowers blood glucose.
α (alpha) cells secrete glucagon — it raises blood glucose.

These two hormones, pulling in opposite directions, are how the body keeps blood glucose steady — the topic of the next section.

7 · Blood-glucose regulation (negative feedback)

Cells need a steady supply of glucose; too high or too low is dangerous. The normal level is about 90 mg per 100 cm³ of blood. The pancreas holds it there by negative feedback, using insulin and glucagon as the two correcting hormones.

When blood glucose rises (after a meal)

The β cells of the islets detect the high glucose and release insulin.
Insulin makes liver and muscle cells take up glucose and store it as glycogen (and body cells respire more glucose).
Blood glucose falls back toward the set point — and as it does, insulin release slows. The change is reversed.

When blood glucose falls (fasting, exercise)

The α cells detect the low glucose and release glucagon.
Glucagon makes the liver break glycogen back down into glucose and release it into the blood.
Blood glucose rises back toward the set point. The change is again reversed.

glucose high → insulin → glycogen stored → glucose falls
glucose low → glucagon → glycogen broken down → glucose rises

Why "negative" feedback Each hormone's effect opposes the change that triggered it, so the glucose level never runs away — it oscillates gently around the set point. The liver is the store; insulin and glucagon are antagonistic (opposite) hormones.

8 · How a hormone acts on its target

A hormone reaches every tissue in the blood, yet acts only on its target cells. The reason is the receptor — a protein, on the cell surface or inside the cell, whose shape exactly fits that one hormone, like a key in a lock.

The hormone travels in the blood and binds to its specific receptor on a target cell.
This binding switches on a response inside the cell — often by triggering a "second messenger" that activates enzymes (for protein hormones like insulin and adrenaline).
Cells without the matching receptor ignore the hormone completely.

Specificity Because the response depends on the receptor, the same hormone in the same blood can change one tissue and leave its neighbour untouched. No receptor, no effect.

9 · Hormonal disorders

If a gland makes too much or too little of its hormone, a disorder follows. Two are essential for the Sindh / BIEK exam:

Disorder	Gland / hormone	What happens
Diabetes mellitus	Pancreas — too little insulin (or cells resistant to it)	Glucose is not stored, so it stays high in the blood and spills into the urine; the patient is thirsty and tires easily. Treated with diet and insulin injections.
Goitre	Thyroid — too little thyroxine, usually from iodine deficiency	The thyroid enlarges into a visible swelling in the neck; metabolism slows. Prevented by iodised salt.
Dwarfism / Gigantism	Pituitary — too little / too much GH in childhood	Stunted growth, or excessive growth.

Why chemical coordination matters

Hormones explain why a single missing chemical can change a whole life — a child short of growth hormone stays small, a thyroid short of iodine swells, a pancreas short of insulin gives diabetes. They show how the body coordinates slowly and widely, where nerves cannot reach, and how the simple rule of negative feedback keeps glucose, water, salt and temperature all within safe limits.

In one minute

The body coordinates by nerves (fast, electrical, precise) and hormones (slower, chemical, widespread); the hypothalamus links the two.
A hormone is a chemical messenger from an endocrine (ductless) gland carried in blood to target cells with the matching receptor.
Key glands: pituitary (master — GH, TSH, ADH, oxytocin, FSH/LH), thyroid (thyroxine), parathyroid (PTH), adrenal (adrenaline, cortisol), pancreas (insulin & glucagon from the islets of Langerhans), gonads.
Blood glucose is held steady by negative feedback: high → insulin stores glycogen in the liver; low → glucagon breaks glycogen down.
Disorders: diabetes (too little insulin), goitre (too little thyroxine / iodine).

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