Chemical Coordination and Integration

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XI, Chapter 19.

Why do we need a second control system?

Your nervous system can make you jump at a sudden noise in a fraction of a second. But the same body takes about two decades to grow to full height, keeps your blood sugar steady between meals and runs a monthly cycle. Nerves cannot do those jobs, for two reasons. A nerve fibre cannot reach every cell in the body, and cellular functions need continuous, long lasting regulation, not just a quick signal. So the body adds a chemical coordination system that works through the blood. The neural system and the endocrine system together regulate physiological functions.

What is a hormone?

Endocrine glands have no ducts, so they are called ductless glands. They release their secretions straight into the surrounding fluid and blood, unlike exocrine glands, which pour theirs into a duct. The secretions are hormones. The modern definition is broader than the classical one: hormones are non-nutrient chemicals that act as intercellular messengers and are produced in trace amounts. The word "non-nutrient" is the examinable part, because it separates hormones from food molecules. Invertebrates have very simple endocrine systems with few hormones, while vertebrates use a large number of chemicals as hormones, and the human system is the one we study.

The human endocrine system is made of organised glands: hypothalamus, pituitary, pineal, thyroid, parathyroid, thymus, adrenal, pancreas, and the gonads (testis and ovary). In addition, hormones are made by cells in the heart, kidney and gastrointestinal tract.

How a hormone does its job

A hormone only affects cells that carry its own hormone receptor, a protein found only on target tissues. Each receptor fits one hormone, which is why a hormone circulating everywhere acts on only a few places. Receptors are of two kinds, and where the receptor sits depends on the chemical nature of the hormone.

Why it works this way. A cell membrane is made of lipid. Steroid hormones are fat soluble and slip through it easily, so they can go straight to a receptor in the cell and switch genes on or off. Peptides are large and water soluble and cannot cross, so they ring the bell from outside and let a second messenger do the work indoors. That is why the second messenger is called a messenger: it carries the hormone's message the rest of the way. The messengers cAMP, IP₃ and Ca²⁺ are not hormones themselves, and NTA likes to test that mix-up.

Pause and predict: which type of hormone is more likely to change which proteins a cell makes over the next few hours, a peptide or a steroid?

Check your answer: the steroid (and the thyroid hormones). Their receptor complexes interact directly with the genome and regulate gene expression. Peptide hormones act through second messengers on existing cell machinery.

The hypothalamus: where the brain meets the glands

The hypothalamus is the basal part of the diencephalon (forebrain). It has groups of neurosecretory cells called nuclei that make two kinds of hormone.

These hormones travel through a portal circulatory system (a short blood route) to the anterior pituitary. The posterior pituitary, in contrast, is under the direct neural control of the hypothalamus.

The pituitary gland

The pituitary sits in a bony cavity, the sella tursica, and hangs from the hypothalamus on a stalk. It has two parts, and the terminology is a fine source of marks.

Oxytocin and vasopressin are made by the hypothalamus, travel down the axons to the posterior lobe and are only stored and released there.

Pause and predict: a 40 year old man has a pituitary tumour that makes too much growth hormone. His height does not change, but his face and hands look coarser. What is the condition, and why not gigantism?

Check your answer: acromegaly. Gigantism is the over secretion of GH in the growing age. In an adult the long bones can no longer lengthen, so the excess shows up as thickening of facial and hand bones. It is hard to diagnose early and can cause serious complications and premature death if left unchecked.

Beyond the textbook. For the exam: the pituitary secretes seven hormones of its own (six from pars distalis and MSH) and stores two more. In reality: calling the pituitary the "master gland" is a simplification, because the hypothalamus controls it. It also runs on feedback. When thyroid hormone in the blood is high, less TSH is released, and when it is low, more is released. Most hormone systems are balanced in this way, and that is what keeps their levels steady.

Pineal and thymus

The pineal gland sits on the dorsal side of the forebrain and secretes melatonin. It helps regulate the 24 hour (diurnal) rhythm, including the sleep–wake cycle and body temperature, and it also influences metabolism, pigmentation, the menstrual cycle and defence capability. In reality, darkness switches melatonin secretion up and bright light suppresses it, which is why bright screens at night can delay sleep.

The thymus is a lobular gland between the lungs, behind the sternum, on the ventral side of the aorta. Its thymosins drive the differentiation of T lymphocytes, which give cell mediated immunity, and they promote antibody production for humoral immunity. The thymus shrinks in old age, so thymosin output falls and immune responses weaken.

Thyroid and parathyroid: metabolism and calcium

The thyroid has two lobes on either side of the trachea, joined by a thin flap of tissue called the isthmus. Its follicles are lined by follicular cells that make tetraiodothyronine (T₄, thyroxine) and triiodothyronine (T₃), and iodine is essential for this synthesis. Thyroid hormones regulate the basal metabolic rate (BMR), support red blood cell formation, control the metabolism of carbohydrates, proteins and fats, and influence water and electrolyte balance. The thyroid also makes a protein hormone, thyrocalcitonin (TCT), which lowers blood calcium.

Beyond the textbook. For the exam: iodine deficiency causes goitre. In reality: the gland swells for a reason you can now work out. Without iodine the gland makes too little T₄ and T₃, so the pituitary receives less negative feedback and releases more TSH, which keeps stimulating the thyroid to grow. This is why common salt is fortified with iodine.

Four parathyroid glands lie on the back of the thyroid, one pair behind each lobe. They secrete parathyroid hormone (PTH), a peptide whose release is controlled by the level of circulating Ca²⁺.

Together they maintain calcium balance. This pair is an antagonistic pair, like insulin and glucagon below.

Adrenal gland: one gland, two organs

Each adrenal gland sits above a kidney. Its centre is the adrenal medulla and its outer part is the adrenal cortex, and the two make completely different hormones.

The cortex has three layers, from outside to inside: zona glomerulosa (outer), zona fasciculata (middle) and zona reticularis (inner).

Beyond the textbook. For the exam: learn the three layers in order from outside to inside (glomerulosa, fasciculata, reticularis). In reality: the layers make different hormones, and a short rhyme helps: the outer layer makes salt hormones (aldosterone, glomerulosa), the middle layer makes sugar hormones (cortisol, fasciculata) and the inner layer makes sex steroids (reticularis).

Pancreas: the sugar thermostat

The pancreas is both exocrine (digestive enzymes) and endocrine. The endocrine part is the islets of Langerhans: 1 to 2 million islets making up only 1 to 2 per cent of the pancreatic tissue. Two cell types matter.

The two hormones oppose each other and together keep blood glucose steady. Prolonged hyperglycemia leads to diabetes mellitus, in which glucose is lost in the urine and harmful ketone bodies form. Patients are treated with insulin therapy.

Pause and predict: you have not eaten for twelve hours. Which hormone is rising in your blood, and from which cells?

Check your answer: glucagon, from the α-cells. It breaks down liver glycogen and makes new glucose, so blood sugar does not fall too low.

Beyond the textbook. For the exam: diabetes mellitus = prolonged hyperglycemia with glucose in urine, treated with insulin. In reality: it comes in two main forms. In type 1 the body's own immune system destroys β-cells, so insulin is almost absent. In type 2 the cells respond poorly to insulin. The name itself carries a clue: mellitus means honey sweet, because the urine contains sugar.

Do not confuse it with its look alike.

Gonads: testis and ovary

The testis is both a primary sex organ and an endocrine gland. Leydig (interstitial) cells, found between the seminiferous tubules, make androgens, mainly testosterone. Androgens regulate the development, maturation and function of the male accessory sex organs (epididymis, vas deferens, seminal vesicles, prostate gland and urethra). They also stimulate muscular growth, facial and axillary hair, aggressiveness and a low pitched voice, drive spermatogenesis, act on the CNS to influence male sexual behaviour (libido), and have anabolic effects on protein and carbohydrate metabolism.

The ovary is the primary female sex organ and makes one ovum per menstrual cycle. It secretes two groups of steroid hormones. Estrogen comes mainly from the growing follicles. After ovulation the ruptured follicle becomes the corpus luteum, which secretes large amounts of progesterone.

The cycle itself is taught in Human Reproduction.

Hormones from tissues that are not glands

Several organs have a main job elsewhere but also make hormones.

Many other non-endocrine tissues also secrete growth factors, which are essential for normal growth and for tissue repair and regeneration.

Exam traps

60-second revision

Where this chapter connects

• Previous: Neural Control and Coordination explains the neurosecretory cells of the hypothalamus, the link between the two control systems.
• Related in Class XI: Excretory Products and Their Elimination shows ADH, aldosterone and ANF acting on the kidney, and Plant Growth and Development covers plant hormones, which are a separate topic.
• Class XII link: Human Reproduction brings back GnRH, LH, FSH, estrogen, progesterone, oxytocin and prolactin in the menstrual cycle, pregnancy and lactation.