Identify the endocrine glands, their hormones and target organs, compare negative and positive feedback in hormonal regulation, and analyze the stress response and endocrine imbalances

Learning goal: Describe the endocrine system as glands that secrete hormones which produce responses in target cells or organs; identify the main glands (pituitary, hypothalamus, thyroid, thymus, parathyroid, pineal, pancreas, adrenal, ovaries and testes) with their hormones and targets; explain how the system regulates metabolism, maintains homeostasis, regulates growth and development and controls reproduction; compare negative feedback (blood glucose, calcium, blood pressure, temperature) with positive feedback (oxytocin in childbirth, hemostasis); analyze the endocrine stress response; and explain homeostatic imbalances including hyper- and hypo-function of glands, type 1 and type 2 diabetes, gigantism and dwarfism.

Two systems of control. The body has two ways to coordinate its parts. The nervous system sends fast, brief electrical signals along wires to specific targets. The endocrine system broadcasts chemical signals, hormones, into the blood; they reach every cell but act only on target cells that carry a matching receptor, and their effects begin more slowly and last longer, from minutes to days. The endocrine system consists of glands that secrete hormones directly into the blood (endocrine glands have no ducts, which distinguishes them from exocrine glands such as sweat and salivary glands, which secrete through ducts onto a surface). Some organs, such as the pancreas, do both.

How a hormone acts. A hormone is a chemical messenger that binds a specific receptor protein on or in its target cell; a cell without the receptor ignores it, which is how one chemical in the blood can affect only the thyroid or only the kidney. Water-soluble hormones, most of which are built from amino acids (peptides such as insulin, and amines such as epinephrine), cannot cross the membrane; they bind receptors on the surface and trigger second messengers inside, altering the activity of enzymes within seconds to minutes. Lipid-soluble hormones, the steroids made from cholesterol by the adrenal cortex and gonads and the thyroid hormones, pass through the membrane, bind receptors inside the cell and change which genes are expressed, so their effects take hours but persist. The amount of hormone in the blood depends on how fast the gland secretes it and how fast the liver and kidneys remove it.

The glands and their hormones. The hypothalamus, a part of the brain, is the link between the two control systems: it makes releasing and inhibiting hormones that travel through a short set of blood vessels to the anterior pituitary and control its output, and it makes two hormones, antidiuretic hormone (ADH) and oxytocin, that are stored in and released from the posterior pituitary. The pituitary gland hangs below the hypothalamus. Its anterior lobe secretes growth hormone, which stimulates growth of bone and muscle, largely through the liver; thyroid-stimulating hormone; adrenocorticotropic hormone (ACTH), which drives the adrenal cortex; follicle-stimulating and luteinizing hormones, which control the ovaries and testes; and prolactin, for milk production. Its posterior lobe releases ADH, which makes the kidneys conserve water, and oxytocin, which contracts the uterus in childbirth and releases milk. The thyroid gland in the neck secretes thyroid hormone (T3 and T4), which needs iodine and sets the metabolic rate of nearly every cell, and calcitonin, which can lower blood calcium. The four small parathyroid glands on its back surface secrete parathyroid hormone (PTH), which raises blood calcium. The thymus behind the sternum secretes thymosins that help immune cells called T lymphocytes mature; it is large in childhood and shrinks after puberty. The pineal gland in the brain secretes melatonin, which rises in darkness and helps set the daily cycle of sleep and waking. The pancreas contains clusters of endocrine cells, the islets, with beta cells secreting insulin, which lowers blood glucose, and alpha cells secreting glucagon, which raises it. Each adrenal gland sits on a kidney: its outer cortex secretes cortisol, which raises blood glucose during stress and suppresses inflammation, and aldosterone, which makes the kidneys retain sodium and water and so supports blood pressure; its inner medulla secretes epinephrine and norepinephrine, the hormones of the fight-or-flight response. The ovaries secrete estrogen and progesterone and the testes testosterone; these control the development of the reproductive organs and secondary sex characteristics, the reproductive cycles and, with growth hormone, the growth spurt and closure of the epiphyseal plates at puberty. The diagram lists the glands with a hormone, its target and its effect.

Four kinds of regulation. Through hormonal release the endocrine system results in regulating metabolism (thyroid hormone sets the rate; insulin, glucagon and cortisol manage fuel), maintaining homeostasis (calcium, water, sodium, glucose, blood pressure and temperature are all held by hormones), regulating growth and development (growth hormone, thyroid hormone and the sex hormones together determine size, timing of puberty and maturation of the nervous system) and controlling reproduction (the pituitary and gonadal hormones run the cycles and pregnancy).

Negative feedback, worked through the thyroid. Most hormones are held at their level by negative feedback, in which the hormone's own effect turns down its production. When thyroid hormone in the blood falls, the hypothalamus releases thyrotropin-releasing hormone, the anterior pituitary responds with thyroid-stimulating hormone, and the thyroid secretes more thyroid hormone. As the level rises, thyroid hormone acts back on the hypothalamus and pituitary to reduce their signals, so secretion falls. The variable is held in a band. Other loops work the same way. Blood glucose: after a meal, insulin lowers it; between meals, glucagon raises it. Calcium: when it falls, PTH stimulates osteoclasts to release calcium from bone, makes the kidneys reclaim calcium and activates vitamin D so the gut absorbs more; when it rises, PTH release stops and calcitonin can act. Blood pressure: when it falls, the kidneys start a chain that ends in aldosterone, which retains sodium and water and restores volume, while ADH conserves water when the blood becomes concentrated. Temperature: the hypothalamus, described in the homeostasis lesson, uses the nervous system for fast responses and thyroid hormone to adjust heat production over days in a cold climate.

Positive feedback, compared. In positive feedback the hormone's effect increases its own release until an end point is reached. During childbirth, the baby's head stretches the cervix; stretch receptors signal the hypothalamus; the posterior pituitary releases oxytocin; oxytocin strengthens uterine contractions; stronger contractions stretch the cervix more and release more oxytocin. The loop ends with delivery, which removes the stretch. Hemostasis is the other standard example: activated platelets release chemicals that activate more platelets, so the plug grows rapidly until the vessel is sealed; this is a local chemical loop rather than an endocrine one, but it shows the same amplifying logic. Negative feedback stabilizes; positive feedback completes.

The stress response. When a threat is perceived, two endocrine responses follow in order. The alarm phase is immediate: the sympathetic nervous system fires and, through it, the adrenal medulla secretes epinephrine and norepinephrine into the blood. Within seconds, heart rate and blood pressure rise, the bronchioles widen, the pupils dilate, the liver releases glucose, blood is diverted from the gut and skin to the skeletal muscles and the brain, and sweating begins (sweat glands are driven mainly by sympathetic nerves directly, not by the hormones): the physiological reactions felt as a pounding heart, dry mouth, cold clammy hands and sharpened attention. If the stress continues, the resistance phase follows over minutes to hours: the hypothalamus secretes a releasing hormone, the anterior pituitary secretes ACTH, and the adrenal cortex secretes cortisol, which makes glucose from proteins and fats to keep fuel available. Aldosterone, also from the adrenal cortex, helps hold blood pressure up by retaining sodium, but it is controlled mainly by angiotensin II (from the renin-angiotensin system, triggered by low blood pressure) and by high blood potassium; ACTH plays only a minor part. These are useful for a short emergency. Stress that lasts for weeks keeps cortisol high, and prolonged high cortisol suppresses the immune system, promotes high blood pressure and breaks down muscle and bone, which is why chronic stress harms health. Analyzing a stressful moment means naming which reactions came from the medulla's fast hormones and which from the cortex's slower ones.

Homeostatic imbalances. Endocrine disorders are usually hyperfunction, too much hormone, or hypofunction, too little, and the symptoms follow from the hormone's normal job. Hyperthyroidism, often caused by an immune protein that stimulates the thyroid as if it were TSH, raises the metabolic rate: weight loss, rapid heartbeat, heat intolerance and, in some, bulging eyes. Hypothyroidism lowers it: fatigue, weight gain, cold intolerance; where the diet lacks iodine, the thyroid cannot make its hormone, the pituitary keeps secreting TSH, and the gland enlarges into a goiter; in an infant, untreated hypothyroidism impairs growth and brain development, which is why newborns are screened. Diabetes mellitus is a failure of glucose regulation. In type 1 the immune system destroys the beta cells, so no insulin is made; muscle and fat cells take up little glucose (they depend on insulin for most of their uptake, though cells such as neurons do not) and the liver keeps releasing glucose, so blood glucose stays high; insulin must be supplied. In type 2, more common and linked with excess body fat and inactivity, cells respond poorly to insulin (insulin resistance), and the beta cells cannot keep up; it is managed by diet, activity and medicines. In both, prolonged high glucose damages vessels, nerves, kidneys and the retina. Growth hormone disorders depend on timing: excess in childhood, before the epiphyseal plates close, causes gigantism, with very tall stature; excess in adulthood causes acromegaly, thickening of the bones of the hands, feet and face; deficiency in childhood causes pituitary dwarfism, short stature with normal proportions, treatable with growth hormone if found early. A deficiency of ADH, by contrast, causes the kidneys to lose large volumes of dilute urine. None of this is medical advice; each disorder is the predictable result of a named hormone being too high or too low.

Misconceptions. "Hormones affect every cell they reach" ignores receptors: a hormone acts only where its receptor is. "Adrenaline is the stress hormone" is half the story; epinephrine is the fast alarm hormone and cortisol the slow resistance hormone. "Type 2 diabetes means no insulin" confuses it with type 1; in type 2 insulin is present but the cells respond poorly. "Positive feedback is harmful" mistakes direction for value; childbirth depends on it.

Investigation: analyzing a stress response. Recall or read a hypothetical account of a sudden threat, for example a near miss while crossing a road. List every physiological reaction experienced (heart racing, sweating, trembling, dry mouth, sharpened vision, later tiredness). For each, identify the aspect of the endocrine system that created it: the adrenal medulla's epinephrine and norepinephrine for the immediate reactions, the adrenal cortex's cortisol, which peaks roughly 15 to 30 minutes after the threat and keeps fuel available, for later changes such as hunger, and note which reactions were nervous rather than hormonal (for example, the tiredness felt as the alarm fades is largely the sympathetic response subsiding). A safe classroom measurement is heart rate before and after a mildly startling but harmless event such as an unexpected loud clap, recorded with the person's consent. The analysis shows the pattern; it cannot show hormone concentrations, which require blood tests, and it cannot separate nervous from endocrine contributions to the same reaction.

Worked negative feedback loop, calcium: A hypothetical blood calcium falls slightly below its normal band after a day with little dairy. The parathyroid glands detect the fall and secrete PTH. PTH stimulates osteoclasts to release calcium from bone, makes the kidneys reclaim calcium that would have been lost in urine and activates vitamin D so the intestine absorbs more from food. Blood calcium rises back into its band, and as it does the parathyroids reduce PTH secretion. If calcium overshoots, the thyroid can release calcitonin to slow osteoclasts. The response (raising calcium) opposes the disturbance (falling calcium): negative feedback. Chronic over-secretion of PTH would strip calcium from bone, weakening it, which shows how a hyperfunction disorder follows from the hormone's normal role.

Worked stress-response analysis: A hypothetical student steps off a curb and a cyclist swerves past. Reactions listed: heart pounding within a second, palms sweating, trembling hands, a dry mouth, a jolt of alertness, and, twenty minutes later, hunger and tiredness. Assignment: the pounding heart, sweating, trembling, dry mouth and alertness are the alarm phase, produced by the sympathetic nervous system and the adrenal medulla's epinephrine and norepinephrine; the sweating is the most purely nervous of these, because sympathetic nerves act on the sweat glands directly. Twenty minutes later cortisol from the adrenal cortex is near its peak, keeping glucose available from stored fuel, and cortisol is known to stimulate appetite, so the hunger fits the slower hormonal response; the tiredness is mostly the sympathetic alarm fading, a nervous change rather than a hormone acting. (A single scare triggers only the start of the cortisol response; the full resistance phase belongs to stress that continues for hours or longer.) The analysis cannot say how much each hormone rose, because that needs blood samples.

Worked diabetes comparison: Two hypothetical patients both have high fasting blood glucose. Patient A is a lean 12-year-old whose beta cells have been destroyed by the immune system and whose blood contains almost no insulin: type 1 diabetes; the missing hormone must be replaced. Patient B is a 55-year-old with excess body fat whose blood insulin is actually high but whose muscle and liver cells respond to it poorly: type 2 diabetes, insulin resistance; the loop is intact but the target cells are deaf to the signal. Same symptom, different failure in the loop: A lacks the hormone, B lacks the response.

Worked growth hormone timing: Suppose a pituitary tumor causes excess growth hormone. If it begins at age 8, the epiphyseal plates are still open, so the long bones lengthen for years and the result is gigantism. If it begins at age 40, the plates closed long ago, so the bones cannot lengthen; instead the bones of the hands, feet, jaw and brow thicken, which is acromegaly. Same hormone excess, different outcome, decided by whether the growth plates can still respond.

Representation task: Using models or illustrations, produce a labeled figure of the body showing the hypothalamus, pituitary, pineal, thyroid, parathyroid, thymus, adrenal glands, pancreas, ovaries and testes, and for each gland draw an arrow to its main target cell or organ labeled with the hormone and its effect. Then draw the thyroid negative feedback loop and the oxytocin positive feedback loop side by side, marking on each where the hormone's effect turns its own release down or up.

Independent performance: Write an analysis of a stressful situation (real or hypothetical) and an explanation of two endocrine imbalances. Success criteria: (a) at least six physiological reactions are listed and each is assigned to the adrenal medulla (alarm), the adrenal cortex (resistance) or the nervous system, with the hormone named; (b) the report explains why the two phases differ in speed using the difference between the medulla's amine hormones and the cortex's steroid hormones; (c) two imbalances from hyper- or hypothyroidism, type 1 diabetes, type 2 diabetes, gigantism, acromegaly and dwarfism are each explained as too much or too little of a named hormone, or a weak target-cell response to it, acting on a named target; (d) one negative feedback loop and one positive feedback loop are compared in a table with the variable, the hormone, the response and how each loop ends; (e) the report states one measurement the analysis could not make without a blood test.