Chemical Coordination and Integration

The One Thing

The endocrine system coordinates body functions by releasing hormones into the bloodstream, where they act only on target cells possessing the appropriate receptors. Hormonal regulation, particularly through negative feedback, maintains homeostasis in processes including metabolism, growth, reproduction, water balance, blood glucose, and calcium concentration.

Reactions, Processes and Experiments

Key Terms

• Endocrine gland: A ductless gland that releases hormones directly into the bloodstream.
• Hormone: A chemical messenger produced in small amounts by endocrine cells that affects specific target cells or organs.
• Target cell: A cell that has specific receptors for a particular hormone and can respond to it.
• Receptor: A specific protein on or inside a target cell that binds to a hormone and starts a cellular response.
• Hypothalamus: A brain region that links the nervous and endocrine systems and controls the pituitary gland through releasing and inhibiting hormones.
• Pituitary gland: A small endocrine gland controlled by the hypothalamus that regulates several other endocrine glands and body functions.
• Thyroid gland: A gland in the neck that produces thyroid hormones, which regulate metabolism, growth, and development, as well as calcitonin, which helps lower blood calcium.
• Parathyroid glands: Small glands located behind the thyroid that secrete parathyroid hormone, which increases blood calcium levels.
• Adrenal glands: Glands above the kidneys; the cortex produces steroid hormones, while the medulla releases adrenaline and noradrenaline during emergencies.
• Pancreas: A gland with both endocrine and exocrine functions; its islets release insulin and glucagon to regulate blood glucose.
• Gonads: The testes and ovaries, which produce sex hormones and reproductive cells.
• Pineal gland: A gland that secretes melatonin, helping regulate daily biological rhythms and sleep-related cycles.
• Thymus: A gland that supports the development of T-lymphocytes and produces hormones involved in immune system maturation, especially during childhood.
• Hypophysis: Another name for the pituitary gland, located at the base of the brain and connected to the hypothalamus.
• Tropic hormone: A hormone that acts on another endocrine gland and stimulates it to release its hormones.
• Negative feedback: A control mechanism in which the effect of a hormone reduces the stimulus for further hormone release.
• Second messenger: An intracellular molecule, such as cyclic AMP, that carries a signal from a hormone receptor to target-cell machinery.
• Peptide or protein hormone: A water-soluble hormone that usually binds to receptors on the cell membrane and acts through second messengers.
• Steroid hormone: A lipid-soluble hormone derived from cholesterol that usually enters cells and affects gene expression.
• Diabetes mellitus: A disorder caused by insufficient insulin production or ineffective insulin action, leading to abnormally high blood glucose.
• Diabetes insipidus: A condition associated with inadequate antidiuretic hormone action, causing excessive urination and thirst.

Easily Confused

• Endocrine gland and exocrine gland: An endocrine gland releases hormones directly into the bloodstream, whereas an exocrine gland releases substances through ducts; the pancreas has both functions.
• Hypothalamus and pituitary gland: The hypothalamus produces releasing and inhibiting hormones and controls the pituitary gland; the pituitary gland releases hormones that regulate other glands and body functions.
• Anterior and posterior pituitary: The anterior pituitary secretes growth hormone, prolactin, thyroid-stimulating hormone, adrenocorticotropic hormone, follicle-stimulating hormone, and luteinising hormone; the posterior pituitary stores and releases oxytocin and vasopressin.
• Oxytocin and antidiuretic hormone: Oxytocin promotes uterine contractions and milk ejection; antidiuretic hormone increases kidney water reabsorption.
• Thyroxine and calcitonin: Thyroxine, mainly T4, increases basal metabolic rate and supports growth and development; calcitonin lowers blood calcium concentration.
• Calcitonin and parathyroid hormone: Calcitonin lowers blood calcium by promoting calcium deposition in bones; parathyroid hormone raises blood calcium through actions on bones, kidneys, and indirectly the intestine.
• Adrenal cortex and adrenal medulla: The adrenal cortex produces steroid hormones including cortisol and aldosterone; the adrenal medulla releases adrenaline and noradrenaline during emergencies.
• Insulin and glucagon: Insulin lowers blood glucose by promoting uptake and storage; glucagon raises blood glucose by stimulating glycogen breakdown and glucose formation.
• Diabetes mellitus and diabetes insipidus: Diabetes mellitus involves insufficient insulin production or ineffective insulin action and causes high blood glucose; diabetes insipidus involves inadequate antidiuretic hormone action and causes excessive urination and thirst.
• Peptide or protein hormones and steroid hormones: Peptide or protein hormones are water-soluble and usually act through membrane receptors and second messengers; steroid hormones are lipid-soluble, derived from cholesterol, and usually affect gene expression inside cells.
• Nervous and endocrine control: Nervous control is generally rapid and short-lived, whereas hormonal control is usually slower but longer-lasting.
• Hormone and receptor: A hormone is the chemical messenger; a receptor is the specific protein that binds it and initiates the target-cell response.
• Hormone concentration and BMI: Hormone concentration is measured in very small units because hormones act at low concentrations; BMI = body mass in kilograms ÷ (height in metres)^2 and is not a hormone formula.

What Gets Asked

• Questions may require identification of the gland, hormone, target tissue, and physiological effect, such as matching thyroid-stimulating hormone with the thyroid gland or antidiuretic hormone with kidney water reabsorption. Marks are lost by confusing a hormone with its gland or target.
• Questions may ask for the hypothalamus-pituitary pathway and the functions of anterior- and posterior-pituitary hormones. A common error is stating that oxytocin and vasopressin are produced by the posterior pituitary; they are produced in the hypothalamus and stored and released by the posterior pituitary.
• Questions may compare opposing homeostatic hormones, especially insulin and glucagon or calcitonin and parathyroid hormone. Marks are lost by reversing which hormone raises or lowers the relevant blood concentration.
• Questions may ask for a negative-feedback explanation using thyroid hormone levels. The required distinction is that low thyroid hormone stimulates hypothalamic and pituitary signals, whereas adequate thyroid hormone reduces those signals.
• Questions may involve endocrine disorders, including gigantism, pituitary dwarfism, acromegaly, goitre, diabetes mellitus, and diabetes insipidus. Marks are lost by assigning excessive or deficient hormone action to the wrong disorder.
• Questions may ask how hormone structure determines signalling mechanism. Peptide or protein hormones usually bind membrane receptors and use second messengers such as cyclic AMP, whereas steroid hormones usually enter cells and affect gene expression.