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Unit 5 - Endocrine System - Full Textbook Notes

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BIOL 373
Heidi Engelhardt

Unit 5: Endocrine System  Endocrinology: the study of hormones Hormones  Hormones are chemical messengers secreted into the blood by specialized cells o Loose definition: a chemical secreted by a cell or group of cells into the blood for transport to a distant target, where it exerts its effect at very low concentrations  Functions that fall under hormonal control are: o Growth and development o Metabolism o Regulation of the internal environment (temperature, water balance, ions) o Reproduction  They act on their target cells in one of 3 ways: 1. By controlling the rates of enzymatic reactions 2. By controlling the transport of ions or molecules across cell membranes 3. By controlling gene expression and the synthesis of proteins  [1849] A.A. Berthold performed the first classic experiment in endocrinology o He removed rooster testes and observed that the birds had smaller combs, less aggressiveness, and less sex drive than un-castrated roosters o If the testes were surgically replaced, normal male behaviour and comb development resumed  Since they weren’t connected to nerves, Berthold concluded that the glands must be secreting something into the blood which affected the entire body  [1889] Charles Brown-Séquard injected himself with extracts made from bull testes ground up in water o physicians from all around the world began to test the effects of endocrine extracts on patients> aka organotherapy o the extract on Brown-Séquard most likely had a placebo effect because testosterone is a hydrophobic steroid that cannot be extracted by an aqueous preparation o his research opened the door to hormone therapy  [1891] a woman was treated for low thyroid hormone levels with glycerin extracts of sheep thyroid glands> first successful case of organotherapy  The classic steps for identifying an endocrine gland: o Remove the suspected gland: equivalent to inducing a state of hormone deficiency; if the gland does produce hormones, the animal should start to exhibit anatomical behavioural, or physiological abnormalities o Replace the hormone: done by placing the gland back in the animal or administering an extract of the gland; should eliminate the symptoms of hormone deficiency o Create a state of hormone excess: take a normal animal and implant an extra gland or administer extract from the gland to see if symptoms characteristic of hormone excess appear o Once a gland is identified as a potential source of hormones, scientists purify extracts of the gland to isolate the active substance> they test for hormone activity by injecting animals with the purified extract and monitoring for a response o Hormones identified by this technique are sometimes known as classic hormones  These include hormones of the pancreas, adrenal glands, pituitary, and gonads> all discrete endocrine glands that could be easily identified and surgically removed  Hormone is coined from the Greek phrase “to excite or arouse”  Molecules that act as hormones are secreted not only by classic endocrine glands but also by isolated endocrine glands, by neurons (neurohormones), and by cells of the immune system (cytokines)  Secretion is the movement of a substance from inside a cell to the extracellular fluid or directly into the external environment o Ectohormone: molecules secreted into the external environemt o Pheromones: specialized ectohormones that act on other organisms of the same species to elicit a physiological or behavioural response  Some fish secrete alarm pheromones; ants release trail pheromones to attract fellow workers to food sources  Sex pheromones are used to attract members of the opposite sex for mating> found throughout the animal kingdom o Some studies show that human axillary (armpit) sweat glands secrete volatile steroids related to sex hormones that may serve as human sex pheromones  A hormone must be transported by the blood to a distant target cell o Candidate hormones> expected to be hormones but cannot be fully accepted  Usually identified as factors  Growth factors: a large group of substances that influence cell growth and division  being studied to determine if they meet all the criteria for hormones  most are autocrines and paracrines > they do not seem to be widely distributed o a molecule may act as a hormone when secreted from one location, but as a paracrine or autocrine signal when secreted from a different location  Hormones act at concentrations in the nanomolar to picomolar range o Some targets aren’t considered hormones because they must be present in high concentrations for a change or effect to be present o Cytokines are not considered as hormones because peptide cytokines are synthesized and released on demand, in contrast to classic peptide hormones, which are made in advance and stored in the parent endocrine cell  All hormones bind to target cell receptors and initiate biochemical responses o These responses are the cellular mechanism of action of the hormone o The affects of hormones may vary in different tissues or at different stages of development o A hormone may have no effect in a particular cell o The variable responses of a cell to a hormone depends primarily on the cell’s receptor and signal transduction pathways  Signal activity by hormones and other chemical signals must be of limited duration if the body is to respond to changes in its internal state o Hormones in the bloodstream are degraded (broken down) into inactive metabolite by enzymes found primarily in the liver and kidneys o The metabolites are then excreted in either the bile or the urine o The rate of hormone breakdown is indicated by a hormone’s half life in the circulation, the amount of time required to reduce the concentration by one-half o Hormones bound to target membrane receptors are terminated in several ways:  Enzymes present in the plasma degrade peptide hormones bound to receptors  In some cases, the receptor-hormone complex is brought into the cell by endocytosis and the hormone is then digested in lysosomes  Intracellular enzymes metabolize hormones that enter cells Location Hormone Primary Main Effect(s) Target(s) Pineal gland  Melatonin A  Brain, other  Circadian rhythms; immune tissues functions; antioxidant Hypothalamus  Trophic hormones P  Anterior  Release or inhibit pituitary N pituitary hormones Posterior  Oxytocin P  Breast and  Milk ejection; labour and delivery; pituitary N uterus behaviour  Vasopressin P  Kidney  Water reabsorption Anterior  Prolactin P  Breast  Milk production pituitary G  Growth Hormone (  Liver,  Growth factor secretion somatotropin) P many tissues  Growth and metabolism  Corticotropin P  Adrenal cortex  Cortisol release  Thyrotropin P  Thyroid gland  Thyroid hormone synthesis  Follicle-stimulating hormone P  Gonads  Egg or sperm production; sex hormone production  Luteinizing hormone P  Gonads  Sex hormone production; egg or sperm production Thyroid gland  Triiodothyronine and thyroxine  Many tissues  Metabolism, growth, and A development  Calcitonin P  Bone  Plasma calcium levels (minimal effect in humans) Parathyroid  Parathyroid Hormone P  Bone, kidney  Regulates plasma Ca2+ and gland phosphate levels Thymus gland  Thymosin, thymopoietin P  Lymphocytes  Lymphocyte development Heart C  Atrial natriuretic peptide P  Kidneys  Increases Na+ excretion Liver C  Angiotensinogen P  Adrenal cortex,  Aldosterone secretion; increases blood vessels blood pressure  Insluin-like growth factors P  Many tissues  growth Stomach and  Gastrin, cholecystokinin,  GI tract and  Assist digestion and absorption of small intestine secretion, and others P pancreas nutrients C Pancreas G  Insulin, glucagon,  Many tissues  Metabolism of glucose and other somatostatin, pancreatic nutrients polypeptide P Adrenal cortex  Aldosterone S  Kidney  Na+ and K+ homeostasis G  Cortisol S  Many tissues  Stress response  Androgens S  Many tissues  Sex drive in females Adrenal  Epinephrine, norepinephrine A  Many tissues  Fight-or-flight response medulla N Kidney C  Erythropoietin P  Bone marrow  Red blood cell production  1,25 Dihydroxy-vitamin D3  Intestine  Increases calcium absorption (calciferol) S Skin C  Vitamin D3 S  Intermediate  Precursor of 1,25 dihydroxy-vitamin form of D3 hormone Testes (male) G  Androgens S  Many tissues  Sperm production, secondary sex characteristics  Inhibin P  Anterior  Inhibits FSH secretion pituitary Ovaries  Estrogen, progesterone S  Many tissues  Egg production, secondary sex (female) G characteristics  Inhibin P  Anterior  Inhibits FSH secretion pituitary  Relaxin P  Uterine muscle  Relaxes muscle Adipose  Leptin, adiponectin, resistin  Hypothalamus,  Food intake, metabolism, Tissues C other tissues reproduction Placenta  Estrogen, progesterone S  Many tissues  Fetal, maternal development (pregnant  Chorionic  Many tissues  Metabolism females only) C somatomammotropin P  Chorionic gonadotropin P  Corpus luteum  Hormone secretion N- neurons G- glands C- endocrine cell
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