Chapter 16 – The Endocrine System

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

Endocrine System

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

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

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

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

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

Pituitary gland hormones

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

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

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

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

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 Parafollicular cells (C cells) produce calcitonin

Thyroid hormones Thyroxine (T4) and Triiodothyronine (T3) Calcitonin #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

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

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

Adrenal cortex microscopic anatomy 3 layers/zones (superficial to deep) Zona glomerulosa Produces mineralcorticoids Zona fasciculata Produces mainly glucocorticoids, some gonadocorticoids Zona reticularis Produces mainly gonadocortcoids, some glucocorticoids

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

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

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

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

Pancreatic hormones Glucagon Insulin 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

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

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 Production of sperm