Hormones are chemical messengers that carry information to various locations in the body. Together with the nervous system, they form the two major regulatory systems that maintain homeostasis.
Basic Knowledge
Endocrine glands are glands that secrete hormones into the blood. They differ from exocrine glands (sweat glands, salivary glands, and digestive glands), which release their secretions onto the body surface or into the digestive tract.
Main endocrine glands and hormones:
・Hypothalamus: various releasing hormones (control the pituitary gland)
・Pituitary gland (anterior lobe): growth hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, and gonadotropins
・Pituitary gland (posterior lobe): vasopressin (antidiuretic hormone) and oxytocin
・Thyroid gland: thyroxine (promotes metabolism) and calcitonin (blood Ca↓)
・Parathyroid glands: parathyroid hormone (blood Ca↑)
・Adrenal medulla: adrenaline (blood glucose↑・heart rate↑)
・Adrenal cortex: glucocorticoids (cortisol) (blood glucose↑・anti-inflammatory) and mineralocorticoids (Na reabsorption)
・Islets of Langerhans in the pancreas: insulin (B cells, blood glucose↓) and glucagon (A cells, blood glucose↑)
・Gonads: ovaries = estrogen and progesterone; testes = testosterone
Comparison of hormones and nerves:
・Transmission speed: nerves = fast (ms), hormones = slow (minutes〜hours)
・Range of action: nerves = local, hormones = broad
・Duration: nerves = short, hormones = long
Insulin (pancreatic B cells, lowers blood glucose, the only blood-glucose-lowering hormone)
Glucagon (pancreatic A cells, raises blood glucose)
Adrenaline (adrenal medulla, blood glucose↑・heart rate↑・during excitement)
Glucocorticoids (adrenal cortex, blood glucose↑・anti-inflammatory)
Thyroxine (thyroid gland, contains iodine, promotes metabolism)
Vasopressin (posterior pituitary, antidiuretic, promotes water reabsorption)
Feedback regulation (a mechanism that keeps hormone concentrations constant)
Further Study
Hormonal chemical structures are classified into three types:
① Peptide hormones (protein type): insulin, glucagon, and growth hormone. They are water-soluble and bind to receptors on the cell membrane.
② Steroid hormones (derived from cholesterol): glucocorticoids and sex hormones. They are fat-soluble, enter cells, bind to nuclear receptors, and directly control gene expression.
③ Amine hormones (derived from amino acids): adrenaline (derived from tyrosine; a catecholamine) and thyroxine (tyrosine + iodine).
Chemically, steroids can pass freely through cell membranes, so their effects are slow and long-lasting. Peptides cannot cross membranes, so they act rapidly through receptors and second messengers such as cAMP.
A typical example of feedback regulation is the hypothalamus-pituitary-thyroid axis:
① The hypothalamus releases TRH (thyrotropin-releasing hormone).
② The anterior pituitary releases TSH (thyroid-stimulating hormone).
③ The thyroid gland secretes thyroxine.
④ When thyroxine concentration rises, negative feedback acts on the hypothalamus and pituitary, suppressing TRH and TSH.
This keeps the blood thyroxine concentration stable. Similar hierarchical regulation occurs in the adrenal cortex (CRH→ACTH→cortisol) and gonads (GnRH→FSH/LH→sex hormones).
Examples of hormonal disorders include diabetes (type I・type II) = insulin deficiency/resistance, Graves' disease = hyperthyroidism, Hashimoto's disease = hypothyroidism, Cushing's syndrome = excess cortisol, and gigantism・acromegaly = excess growth hormone.
- Endocrine glands = secrete hormones into the blood (no ducts)
- Exocrine glands = secrete onto the body surface or into the digestive tract (with ducts)
- Hormones = slow but broad and long-lasting effects
- Nerves = fast but local and short-lasting effects
- Insulin is the only blood-glucose-lowering hormone
- Steroid hormones = intracellular receptors and gene expression
- Peptide hormones = cell-surface receptors and second messengers
- The hypothalamus-pituitary axis = command center of hormonal regulation
Points to Note
① Do not confuse endocrine secretion (into the blood; no ducts) with exocrine secretion (onto the body surface or into the digestive tract; with ducts). Sweat and salivary glands are exocrine glands. ② Adrenaline is secreted by the adrenal medulla, whereas glucocorticoids are secreted by the adrenal cortex. Both raise blood glucose, but their locations and chemical structures differ. ③ Do not reverse insulin (pancreatic B cells, blood glucose↓) and glucagon (pancreatic A cells, blood glucose↑). Associate “B = Blood sugar Brake.” ④ Thyroxine is a thyroid hormone containing iodine (seaweed iodine is a source). Iodine deficiency is uncommon in Japan because seaweed provides iodine, but it can be a problem in landlocked countries.
Practice
- What is the only hormone that lowers blood glucose? Also state where it is secreted.
- What hormone is secreted by the adrenal medulla and raises blood glucose and heart rate?
- Briefly explain the difference between the mechanisms of steroid and peptide hormones.