Unit 16: Human Endocrine System — Short Questions

A hormone is a regulatory chemical secreted into blood or tissue fluid by an endocrine gland. Blood carries the hormone to every cell in the body, but it shows its effect only on specific target cells that bear receptors for it. Because they carry a chemical instruction from the gland to a distant target cell and cause a specific response there, hormones are referred to as chemical messengers or signalling molecules.

Amino acid-based hormones (peptide, protein, and glycoprotein hormones) are water-soluble and cannot cross the cell membrane, so their receptors are located on the cell surface (plasma membrane). Steroid hormones are lipid-soluble, so they can pass through the cell membrane, and their receptors are located inside the cell, in the cytoplasm or nucleus.

Neurosecretory cells are special nerve cells, found mainly in the hypothalamus, that not only conduct nerve impulses like ordinary neurons but also secrete hormones (e.g., oxytocin, ADH, and the releasing/inhibiting hormones that act on the anterior pituitary).

The anterior pituitary secretes Growth Hormone (GH), Adrenocorticotropic Hormone (ACTH), Thyroid-stimulating Hormone (TSH), Luteinizing Hormone (LH), Follicle-stimulating Hormone (FSH), and Prolactin.

The pituitary gland (mainly its anterior lobe) is called the master gland because it secretes hormones such as TSH, ACTH, LH and FSH that regulate the activity and hormone secretion of other endocrine glands, namely the thyroid, adrenal cortex and gonads.

ADH (antidiuretic hormone) is synthesized by the neurosecretory cells of the hypothalamus. It is transported along the axons of these cells to the posterior pituitary, where it is stored. On appropriate stimulation, the posterior pituitary releases the stored ADH into the blood, where it acts on the kidneys to stimulate water retention.

Parathyroid glands are four small glands attached to the posterior surface of the thyroid gland. They secrete parathormone, which is released when blood calcium level falls. Parathormone stimulates osteoclasts to dissolve calcium crystals in bone matrix and release calcium into blood, stimulates the kidneys to reabsorb calcium from urine, and activates vitamin D, which is needed for calcium absorption from food in the intestine. Over-secretion causes hypercalcemia, bone deformation and soft bones, while under-secretion causes hypocalcaemia, increased neuron excitability and tetany (sustained muscle contraction).

Ovaries secrete estrogen and progesterone. Estrogen is secreted by the Graafian follicle in response to FSH; it causes development of secondary sexual characteristics, regulates the menstrual cycle, repairs the uterine lining after menstruation, triggers ovulation, maintains bone density, and lowers LDL cholesterol. Progesterone is produced by the corpus luteum (and later the placenta); it causes thickening and vascularisation of the uterine wall to maintain pregnancy, suppresses ovulation and inhibits further FSH secretion. Testes secrete testosterone (an androgen) from the interstitial cells; it initiates maturation of male reproductive organs, development of secondary sex characteristics, sex drive, and is necessary for sperm production.

Exocrine vs Endocrine glands Exocrine glands secrete their products into ducts attached to them, while endocrine glands are ductless and secrete hormones directly into blood or tissue fluid.
Hypothyroidism vs Hyperthyroidism: Hypothyroidism is deficiency of thyroid hormones causing growth retardation, lethargy, weight gain, low heart rate and body temperature; hyperthyroidism is excess of thyroid hormones causing weight loss, high blood pressure, heart rate and body temperature.
Parathormone vs Calcitonin: Parathormone (from parathyroid glands) raises blood calcium by releasing it from bone and kidneys; calcitonin (from thyroid gland) lowers blood calcium by promoting its deposition into bone.
Beta vs Alpha cells of pancreas: Beta cells are more numerous and secrete insulin, which lowers blood glucose; alpha cells secrete glucagon, which raises blood glucose.
Insulin vs Glucagon: Insulin is secreted when blood glucose rises and lowers it by increasing glucose uptake by cells and promoting glycogenesis; glucagon is secreted when blood glucose falls and raises it by promoting glycogen hydrolysis in the liver.
Diabetes insipidus vs Diabetes mellitus: Diabetes insipidus is caused by under-secretion of ADH, causing excessive dilute urine and thirst; diabetes mellitus is caused by under-secretion/insensitivity to insulin, causing high blood glucose (hyperglycaemia) and large volumes of glucose-containing urine.
Estrogen vs Progesterone: Estrogen is secreted by the Graafian follicle, develops secondary sexual characteristics and regulates the menstrual cycle/ovulation; progesterone is secreted by the corpus luteum/placenta, thickens and vascularises the uterine wall and maintains pregnancy while suppressing ovulation.
Positive vs Negative feedback: In positive feedback, the product of a process speeds up the action of the controlling system (e.g., oxytocin release during childbirth); in negative feedback, the end product stops/slows the initial step of the process (e.g., insulin and glucagon regulation of blood glucose).

Artificially synthesized steroids (anabolic steroids) are synthetic versions of testosterone that have been widely used by athletes to illegally enhance sports performance by increasing muscle mass, strength, and endurance. However, their use in sports is illegal and banned, and long-term use causes serious health problems: liver damage leading to hepatitis and liver tumours; increased risk of heart disease, stroke, and blood clots; disruption of the body's natural hormone balance, leading to infertility, impotence, and breast development in men; and mental health issues such as depression, anxiety, and aggressive behaviour.

Growth hormone (GH) treatment genuinely improves growth rate in children who have short stature because of an actual GH deficiency. Some companies extend this valid medical fact to claim that GH (or similar products) can increase height or build muscle in anyone with short stature, regardless of whether a true GH deficiency exists. However, not every person of short stature (e.g., dwarf individuals) has GH deficiency, so GH therapy is not appropriate or effective for everyone, and doctors do not support using GH for building muscle or slowing aging in people without a genuine deficiency. Such companies' claims are therefore only valid for a specific deficiency-related condition and are misleadingly generalized to all short-statured people.