Understanding the Endocrine System: Hormones and Homeostasis

1. Chapter 16 – The Endocrine System

2. Endocrine system • Along with the nervous system, is responsible for homeostasis • Nervous system – fast-acting; short-term effects • Endocrine system – slow-acting; long-term effects • Utilizes chemical messengers called hormones • Gland types • Exocrine – have ducts • Endocrine – ductless when mature • Secrete into surrounding fluid and then enters bloodstream • Carried body-wide; affects distant tissues

3. Endocrine System

4. Structural classification of hormones • Amino acid derivatives • Hydrophilic • Peptide • Hydrophilic • Lipid derivatives • Hydrophobic • Steroids; sex hormones • Synthesized from cholesterol

6. Hydrophilic hormone receptors • Water soluble hormones can no diffuse through plasma membrane; receptors are on cell surface • Transmembrane proteins – span entire membrane • Hormone binds to receptor; receptor changes shape • Shape change activates G protein • Converts to an active form • Starts chain reaction that ultimately activates kinases • Kinases attach phosphate groups to molecules • This either activates the molecule or inactivates it, depending on the specific molecule

7. Hydrophobic hormone receptors • Fat soluble hormones can diffuse through plasma membrane • Receptors are located in cell’s interior (the cytoplasm) • Hormone-receptor complex enters nucleus and binds to DNA to affect gene expression

8. Control of hormone release • Endocrine reflexes • Humoral • Detects and responds to change in concentration of certain molecules • Neural • Signal from nervous system stimulates secretion • Hormonal • Some hormones target other endocrine glands • Tropic hormones (sometimes called trophic hormones) • All can be modified by nervous system

9. Pituitary gland (hypophysis) • Located in the sellaturcica of the sphenoid bone • Attached to the hypothalamus by a stalk called the infundibulum • 2 lobes • Posterior (neurohypophysis) • Anterior (adenohypophysis)

10. Pituitary gland hormones

11. Posterior pituitary (neurohypophysis) • Composed of neural tissue; stores and releases hormones produced by the hypothalamus • Antidiuretic hormone (ADH) • Increases water reabsorption • Aquaporins placed in the collecting ducts of the kidneys • Increases blood volume/pressure • Oxytocin (OXT) • Stimulates smooth muscle contractions • Uterine contractions during childbirth • Milk letdown in lactating females • Durng sexual arousal in males and females

12. Anterior pituitary (adenohypophysis) • Release or inhibition is controlled by hypothalamus • Growth hormone (GH) • Bones and skeletal muscles the major target tissues • Prolactin (PRL) • Stimulates milk production • Melanocyte-stimulating hormone (MSH) • Stimulates melanocytes in skin; contributes to skin pigmentation

13. Anterior pituitary (adenohypophysis) • Tropic hormones • Thyroid-stimulating hormone (TSH) • Thyroid gland • Adrenocorticotropic hormone (ACTH) • Adrenal cortex • Follicle-stimulating hormone (FSH) • Gonads to produce gametes • Leutinizing hormone (LH) • Stimulation of gonadal hormones

14. Thyroid gland • On trachea, inferior to larynx • Butterfly-shaped • 2 lobes connected by isthmus

15. Thyroid gland microscopic anatomy • Follicle cells produce thyroglobin (a glycoprotein) • Simple cuboidal epithelium • Central cavity of follicles is filled with colloid – thyroglobulin with iodine atoms • Forms T3 and T4 hormones • Parafollicularcells (C cells) produce calcitonin

16. Thyroid hormones • Thyroxine (T4) and Triiodothyronine (T3) • #4 and #3 refer to number of iodine atoms attached • Affects most cell in body • Increases basal metabolic rate and heat production • Aids in maintenance of blood pressure • Helps regulate growth and development • Calcitonin • Lowers blood calcium levels • Inhibits osteoclasts

17. Parathyroid glands • Usually 4 on posterior thryoid • Parathyroid hormone • Produced by chief cells • Increases blood calcium levels • Antagonistic to calcitonin • Stimulates osteoclasts • Increases reabsorption of calcium by kidneys • Kidney also converts vitamin D to its active form, which aids in calcium absorption in digestive system

18. Adrenal glands • Also called suprarenal glands due to their location • Adrenal cortex (superficial) and adrenal medulla (deep)

19. Adrenal cortex microscopic anatomy • 3 layers/zones (superficial to deep) • Zonaglomerulosa • Produces mineralcorticoids • Zonafasciculata • Produces mainly glucocorticoids, some gonadocorticoids • Zonareticularis • Produces mainly gonadocortcoids, some glucocorticoids

20. Adrenal cortex hormones • Mineralcorticoids • Regulation of electrolytes in extracellular fluid • Aldosterone • Stimulates sodium reabsorption in distal convoluted tubules of kidneys • Glucocorticoids • Influence energy metabolism • Regulates blood glucose and blood pressure levels • Cortisol (hydrocortisone) • Gluconeogenesis – formation of glucose from non-carbohydrate sources • Excess of glucocorticoids results in oversuppression of inflammatory and anti-immune responses • Gonadocorticoids • Androgens • Male sex hormones • Converted to testosterone in males; estrogen in females • Estrogens

21. Adrenal medulla hormones • Autonomic nervous system • Epinephrine and norepinephrine • “Fight or flight” response • Blood glucose levels rise, increases heart rate, blood directed to cardiac and skeletal muscles • Epinephrine serves as a blood vessel contrictor and a bronchiole dilator

22. Pineal gland • Located at roof of third ventricle in brain • Melatonin • Concentration rises and falls within a 24 hour time period • Peaks at night – causes sleepiness • Affects biological clock/circadian rhythms

23. Pancreas • Posterior to stomach • Microscopic anatomy • Acinar cells • Produce pancreatic secretions for digestive system • Islets of Langerhans • Alpha cells – produce glucagon • Beta cells – produce insulin

24. Pancreatic hormones • Glucagon • Increases blood glucose levels • Glycogenolysis • Breaks down glycogen to glucose • Gluconeogenesis • Synthesizes glucose from non-carbohydrate sources • Releases glucose from the liver • Insulin • Decreases blood glucose levels (antagonistic to glucagon) • Increases glucose transport into cells (primarily adipose and muscle) • Uses glucose for ATP production • Stores as glycogen • Excess stored as fat

25. Thymus • Overlays heart • Produces several peptide hormones • Thymosin, thymopoetin, thymic factor • Development/maturation of T lymphocytes

26. Gonadal hormones • Hormones are steroids (fat-soluble) • Ovaries (female) • Estrogen • Secondary sex characteristics • Maturation of reproductive organs • Progesterone • Along with estrogen: • Breast development and menstrual cycle regulation • Testes (male) • Testosterone • Secondary sex characteristics • Maturation of reproductive organs • Production of sperm