Endocrine System Flashcards
Classification of Hormones
• Endocrine system consists of Glands that secrete Hormones (signaling molecules that are secreted directly into the bloodstream to induce change in gene expression or cellular functioning in distant target tissues).
- Peptide Hormones: Made up of amino acids, and must bind to extracellular receptors (as peptide hormones are charged and cannot pass through plasma membrane) and induce conformational change in G Protein-Coupled Receptor to initiate Signaling Cascade with possible Amplification (increase in signal strength at each step). Peptide hormone is extracellular First Messenger, and common intracellular Second Messengers are cAMP, IP₃, Ca²⁺. • Effects are rapid but short-lived, as second messengers are transient.
- Names often end in: -in, -ine.
- Steroid Hormones: Derived from cholesterol, and must bind to intracellular/intranuclear receptors (as steroid hormones can easily cross plasma membrane) and induce conformational change (dimerization) in hormone-receptor complex, which binds directly to DNA, to increase or decrease transcription of particular genes. Must be transported through bloodstream by proteins (such as Albumin) in inactive form; must dissociate from carrier to function.
- Effects are slow but long-lived, as steroid hormones participate in gene regulation.
- Names often end in: -one, -ol, -oid.
- Amino Acid-Derived Hormones: Derived from one or two amino acids with a few additional modifications.
- Epinephrine and Norepinephrine (Catecholamines) bind to GPCRs, like peptide hormones with fast but short-lived effects. Think adrenaline rush.
- Triiodothyronine and Thyroxine (Thyroid Hormones) bind intracellularly, like steroid hormones with slow but long-lived effects. Think metabolic rate regulation.
Hypothalamus
• Hypothalamus is the bridge between nervous and endocrine systems and is regulated by Negative Feedback (hormone/product later in pathway inhibits hormones/enzymes earlier in pathway).
- Hypothalamus controls the Anterior Pituitary Gland (or Hypophysis) through paracrine release of Tropic Hormones into the Hypophyseal Portal System.
- H: Gonadotropin-Releasing Hormone (GnRH) -> AP: Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH).
- H: Growth Hormone-Releasing Hormone (GHRH) -> AP: Growth Hormone (GH).
- H: Thyroid-Releasing Hormone (TRH) -> AP: Thyroid-Stimulating Hormone (TSH).
- H: Corticotropin-Releasing Factor (CRF) -> AP: Adrenocorticotropic Hormone (ACTH).
- Hypothalamus also releases Prolactin-Inhibiting Factor (PIF) or Dopamine, which decreases Prolactin secretion; prolactin is released by anterior pituitary in the absence of PIF.
- Anterior Pituitary also releases Endorphins without input from hypothalamus.
• Hypothalamus controls the Posterior Pituitary Gland through neurons that send their axons from the hypothalamus directly into the posterior pituitary, which release Oxytocin and Antidiuretic Hormone (ADH) into circulation. These hormones are synthesized in the hypothalamus and simply released from the posterior pituitary gland.
Anterior Pituitary Gland
• FLAT PEG: Tropic Hormones (FSH, LH, ACTH, TSH) and Direct Hormones (Prolactin, Endorphins, GH).
- FSH and LH (stimulated by GnRH) act on Gonads (Testes and Ovaries).
- ACTH (stimulated by CRF) acts on Adrenal Cortex.
- THS (stimulated by TRH) acts on Thyroid.
- Prolactin: Stimulates milk production in the memory glands of females. Secretion of prolactin is decreased due to the release of dopamine (PIF) from hypothalamus. Expulsion of placenta causes dopamine levels to drop, removing block on prolactin secretion. Nipple stimulation (from newborn latching on to breast) causes posterior pituitary to release Oxytocin (for milk letdown) and causes hypothalamus to stop releasing dopamine onto anterior pituitary to allow secretion of Prolactin (for milk production).
- Endorphins: Decrease the perception of pain (natural painkillers).
- Growth Hormone (GH): Promotes growth of bone and muscle. This sort of growth is energetically expensive, so GH prevents glucose uptake in tissues that are not growing and stimulates breakdown of fatty acids to increase overall availability of glucose. In children, Gigantism (abnormal growth at epiphyseal plates) can result from excess GH and Dwarfism can result from GH deficit. In adults, Acromegaly (abnormal growth of small bones) may result.
Posterior Pituitary
• Receives and releases direct hormones produced by hypothalamus.
- Antidiuretic Hormone (ADH): Secreted in response to low blood volume (as sensed by baroreceptors) or increased blood osmolarity (as sensed by osmoreceptors); acts at the level of the collecting duct to increase the permeability of the collecting duct to water and thus to increase reabsorption of water from the filtrate back into circulation. This results in greater retention of water, which increases both blood volume and blood pressure and decreases blood osmolarity.
- Oxytocin: Secreted during childbirth to allow for coordinated contraction of uterine smooth muscle and during suckling to promote milk ejection through contraction of smooth muscle in breasts. Also involved in bonding behavior. Unusual in that it has positive feedback loop; the release of oxytocin promotes uterine contraction, which promotes more oxytocin release, which promotes stronger uterine contractions, and so on, to aid delivery.
Thyroid Gland
• Controlled by Thyroid-Stimulating Hormone (TSH) from anterior pituitary.
- Triiodothyronine (T₃) and Thyroxine (T₄): Produced by iodination of Tyrosine in Follicular Cells. Responsible for setting the basal metabolic rate; increased amounts of T₃ and T₄ lead to increased cellular respiration and increased turnover of glucose, fatty acids, and proteins. High levels of thyroid hormones inhibits TSH and TRH synthesis and thus prevents excessive secretion of T₃ and T₄. Iodine deficiency or inflammation of thyroid causes Hypothyroidism (reduced secretion of thyroid hormones), which can lead to Cretinism (intellectual disability and developmental delay) as thyroid hormones are required for appropriate neurological and physical development in children. Tumor or thyroid overstimulation causes Hyperthyroidism (excess production of thyroid hormones), which can lead to heightened activity level, weight loss, increased body temperature, and increased heart and respiratory rate (opposite effects of hypothyroidism).
- Calcitonin: Produced by Parafollicular Cells (C-Cells) in response to high calcium levels in blood. Acts to decrease plasma calcium levels by increasing calcium excretion from kidneys, by decreasing calcium absorption from the gut, and by increasing storage of calcium in the bone. Calcitonin and PTH are antagonistic to each other.
Parathyroid Glands
• Parathyroid Hormone (PTH): Produced by parathyroid glands in response to low calcium levels in blood. Acts to increase plasma calcium levels by decreasing calcium excretion (and promoting phosphate excretion) from kidneys, by increasing calcium and phosphate absorption from the gut (by activating required Vitamin D), and by increasing bone resorption (to free up stored calcium and phosphate). PTH has little effect on phosphate (as its absorption and excretion somewhat cancels). PTH secretion is inhibited by rising calcium levels. Calcitonin and PTH are antagonistic to each other.
Adrenal Cortex
• Outer portion of adrenal glands that secretes Corticosteroids (Glucocorticoids, Mineralocorticoids, and Cortical Sex Hormones).
- Glucocorticoids (Cortisol and Cortisone): Cortisol and Cortisone raise blood glucose levels by increasing gluconeogenesis and decreasing protein synthesis. These glucocorticoids can also decrease inflammation and immunological activity. Cortisol is “stress hormone” as it is released during times of physical or emotional stress (increased blood sugar provides ready source of fuel in case body needs to quickly react to dangerous stimuli). CRF from hypothalamus -> ACTH from anterior pituitary -> Glucocorticoids from adrenal cortex.
- Mineralocorticoids (Aldosterone): Aldosterone increases sodium reabsorption in Distal Convoluted Tubule and Collecting Duct of Nephron in the Kidneys (and promotes excretion of potassium and hydrogen in urine). Water follows sodium, so aldosterone also increases blood volume and blood pressure (without change in plasma osmolarity, as sodium and water flow together). Primarily under the control of the Renin-Angiotensin-Aldosterone System, in which decreased blood pressure causes Juxtaglomerular Cells of kidneys to secrete Renin, which cleaves Angiotensinogen (inactive protein) to Angiotensin I (active form), and then ACE in lungs converts Angiotensin I to Angiotensin II, which stimulates adrenal cortex to secrete aldosterone. Once blood pressure is restored, renin release declines via negative feedback mechanism.
- Cortical Sex Hormones (Androgens and Estrogens): Adrenal androgens important in females, but insignificant in males as testes produce them in large amounts. Adrenal estrogens important in males, but insignificant in females as ovaries produce them in large amounts.
Adrenal Medulla
• Inner portion of adrenal glands that secretes sympathetic hormones Epinephrine and Norepinephrine, both of which are Catecholamines and are centered around the fight-or-flight response.
- Epinephrine and Norepinephrine: both increase heart rate, dilate the bronchi, and shunt blood flow to the skeletal muscle, heart, lungs, and brain (systems that would be used in sympathetic response). Vasoconstriction decreases blood flow to the gut, kidneys, and skin. Epinephrine increases glycogenolysis in both liver and muscle and increases basal metabolic rate.
- Epinephrine and Norepinephrine control short-term stress responses (such as fight/flight), while Cortisol controls long-term stress responses (such as illness/injury).
Pancreas
• Has both exocrine and endocrine functions; exocrine tissue produces digestive enzymes, while endocrine tissue (cluster of cells in Islets of Langerhans) secretes metabolic hormones. Islets contains three distinct hormone-producing cell types: Alpha Cells, Beta Cells, Delta Cells.
- Glucagon: Secreted by Alpha Cells in response to low glucose levels (during fasting) to increase glucose production by triggering glycogenolysis, gluconeogenesis, and catabolic processes such as degradation of proteins and fats. CCK and Gastrin from GI tract also increase glucagon release. Glucagon release inhibited by high blood glucose levels.
- Insulin: Secreted by Beta Cells in response to high glucose levels (after meal) to induce muscle and liver cells to take up glucose and store it as glycogen for later use. Thus, insulin stimulates glycolysis, glycogenesis, and anabolic processes such as fat and protein synthesis. Excess insulin causes Hypoglycemia. Insulin underproduction or insensitivity results in Hyperglycemia and Diabetes Mellitus; Type I (insulin-dependent) caused by insulin underproduction due to autoimmune destruction of beta cells, and Type II (non-insulin-dependent) caused by receptor-level resistance to effects of insulin due to genetics, obesity, and poor diet.
- Somatostatin: Secreted by Delta Cells to inhibit secretion of both insulin and glucagon.
Gonads
- Testes secrete testosterone in response to stimulation by gonadotropins (LH and FSH) to cause sexual differentiation of the male during gestation and to promote secondary sex characteristics in males, such as axillary and pubic hair, deepening of the voice, and muscle and bone growth.
- Ovaries secrete estrogen and progesterone in response to gonadotropins (LH and FSH) to develop female reproductive system during gestation and to promote secondary sex characteristics in females, such as axillary and pubic hair, breast growth, and body fat redistribution. These two steroid hormones also govern the menstrual cycle and pregnancy.
Pineal Gland
• Receives projections directly from the retina and responds to decreases in light intensity by releasing Melatonin, which is involved in regulation of Circadian Rhythms and is partially responsible for sensation of sleepiness.
Other Organs
- Kidney: Produces Erythropoietin in response to low oxygen levels in blood, which stimulates production oferythrocytes in bone marrow.
- Heart: Produces Atrial Natriuretic Peptide (ANP) when atria are stretched from excess blood volume, which promotes excretion of sodium and water (functionally antagonistic to aldosterone, as blood volume and pressure are lowered with no change in blood osmolarity).
- Thymus: Produces Thymosin, which stimulates development and differentiation of T-cells. Thymus atrophies by adulthood and thymosin levels drop.
Summary of Hormones
