Homeostasis and the Endocrine System

Homeostasis, Hormones and the Endocrine System

• Normal body conditions include a temperature of 37°C, 0.1% blood glucose and a pH of 7.35
• Homeostasis is how the body maintains a constant internal environment using receptors, coordinating centers, and effectors

Homeostasis Components

• Receptor sends a message about conditions to a coordinating center
• The coordinating center relays information to an effector
• Effector functions to restore normal balance
• An example of components working together; Low oxygen levels in the blood

Dynamic Equilibrium and Feedback

• Dynamic equilibrium is a stable state with fluctuating limits

• Examples of fluctuating limits include blood glucose, body temperature, blood pressure, and blood pH values

• Negative feedback activates a mechanism, and conditions are restored; an example is a thermostat

• Positive feedback reinforces change, the controlled variable shifts away from the steady state and allows a specific event to occur, then the feedback stops

• Examples of positive feedback include platelets and blood clotting, and oxytocin and labor

Hormones

• Hormones are chemicals released by cells that affect cells in other parts of the body, and they speed up or slow down bodily processes
• Endocrine hormones are secreted by endocrine glands directly into the blood
• Endocrine glands which secrete hormones include the Hypothalamus, Pituitary, Thyroid, Parathyroid, Pancreas, Adrenal gland, and Testes/ovaries
• Hormones can be classified by activation site
• Non-target hormones affect many cells or tissues in the body
• Insulin is an example non-target hormone, it is produced by islets of Langerhans in the pancreas, and is secreted when blood sugar is high.
• Human growth hormone (hGH) is produced by the pituitary gland and stimulates growth of the body
• Epinephrine or adrenaline is produced in the adrenal medulla and accelerates heart rate and body reactions during fight-or-flight
• Hypothalamus regulates hormones released by the pituitary through nerve stimulation
• Endocrine glands stimulated by the pituitary secrete chemicals that affect nerve activity of the hypothalamus
• Hormones only affect some cells
• Water-soluble hormones act outside the cell, bind to receptor sites on cell membrane and activate enzymes in cytoplasm
• Fat-soluble hormones or steroids act inside the cell, binding with receptors in cytoplasm for specific protein creation
• Hormones must be regulated, for example epinephrine
• The pituitary gland at the base of the brain is connected to the hypothalamus, controls other endocrine glands, and produces and stores hormones
• Releasing hormone is a peptide produced by the hypothalamus that stimulates the pituitary to release a hormone
• Inhibiting factor stops pituitary secretions
• Recombinant DNA is used to synthetically create hormones

Anterior vs Posterior Lobe Hormones

• Anterior lobe hormones include Thyroid-stimulating (TSH), Adrenocorticotropic (ACTH), Human growth (hGH), Follicle-stimulating (FSH), Lutenizing (LH), Prolactin (PRL), Melanocyte-stimulating (MSH)
• Posterior lobe hormones include. Oxytocin and Antidiuretic (ADH)

Hormones that Affect Blood Sugar

• Blood sugar is affected by the pancreas and adrenal glands
• The pancreas produces digestive enzymes and hormones
• Islets of Langerhans produce insulin and glucagon
• Glucagon, produced by the alpha cells promotes the conversion of glycogen to glucose when blood sugar is low
• Insulin is released when blood sugar levels increase, produced by beta cells and causes muscle, liver, and other organs to be permeable to glucose

Diabetes

• Diabetes occurs when body does not produce enough, or any insulin and is unable to properly use the insulin it makes

• Untreated diabetes can cause blindness, kidney failure, nerve damage, and limb amputation, as well as high urine volumes, thirst, feeling tired, and acetone breath

• After eating, no insulin results in constantly high blood sugar; kidneys then excrete glucose and large amounts of water

• There are three types of diabetes

  • Type 1 (juvenile diabetes) where the pancreas does not produce insulin, which accounts for 10% of cases and requires insulin intake to live
  • Type 2 (adult-onset) has decreased insulin production or ineffective use of insulin by the body, accounts for 90% of cases and is controlled with diet, exercise, and sulfonamides
  • Gestational diabetes is 2-4% of pregnancies and increases risk of type 2 in mother and child

• Islet cell transplants could be used for Type 1, however, immune rejection is an issue due to Rejection drugs are toxic and harmful to islet cells

• The Edmonton Protocol is where three drugs are combined

• Islet cells are transplanted into liver, the liver regenerates, and islet cells work

Adrenal Glands

• Adrenal glands are located above each kidney and comprised of two glands in one shell for each
• The adrenal medulla is the inner gland, and produces epinephrine (adrenaline) and norepinephrine (initiates fight of flight)
• The adrenal cortex is the outer region, and produces steroids
• Glucocorticoids regulate electrolytes, water balance and blood glucose levels
• Mineralocorticoids regulate carbohydrate, lipid, and protein metabolism, as well as inhibits release of ACTH
• Sex hormones affect development of growth and sex organs
• Cortisol stimulates conversion of amino acids to glucose by the liver, which helps stress recovery and is a glucocorticoid
• ACTH promotes cortisol release, also called corticotropin, and goes to adrenal cortex where mineralocorticoids and glucocorticoids go to liver and muscle
• Aldosterone is a mineralocorticoid that increases sodium retention and water reabsorption by kidneys

Hormones That Affect Metabolism

• The Thyroid Gland is a two-lobed gland at the base of the neck

• The thyroid gland regulates metabolic processes and produces triiodothyronine, thyroxine, and calcitonin

• 60% of used glucose is released as heat, and 40% is converted to ATP

• Thyroxine - T4 – increases metabolism, regulates growth and accounts for 65% of secretions

• Triiodothyronine - T3- increases metabolism and regulates growth, and contains three iodine atoms

• Calcitonin acts on bone cells to lower calcium levels in the blood

• Thyroid activity is regulated by negative feedback and controlled by Thyroid-releasing hormone (TRH)

• The regulatory process; Hypothalamus – TRH – anterior pituitary – TSH – thyroid – T3/T4 - hypothalamus

• Goiter is an enlargement of thyroid gland due to low iodine

• Parathyroid is comprised of four small pea-sized glands in thyroid that responds directly to chemical changes in surroundings to increase calcium levels in blood and lowers phosphate levels

• If the parathyroid is removed then tetanus results

• PTH acts on the kidneys and intestines to absorb calcium; PTH acts on bones to release calcium

• High calcium causes it to collect in blood vessels or form kidney stones

• PTH activates vitamin D, while low vitamin D causes rickets and bones develop improperly

• hGH stimulates elongation of the skeleton

• Dwarfism or gigantism may result from hGH deficiency

Hormones Affecting Water and Ion Balance

• Antidiuretic hormone (ADH) causes kidneys to increase water reabsorption
• Osmoreceptors are receptors in the hypothalamus that detect changes in blood and extracellular fluids
• Sweating results in concentrated blood which causes ADH production
• 85% of water is reabsorbed by the nephron with 15% loss if no ADH is present
• ADH makes upper part of distal tubule and collecting duct permeable to water
• Diabetes insipidus results in excessive urine production, up to 16 L/day

RAAS

• The Juxtaglomerular apparatus (JGA) a unit near glomerulus that controls renin release in response to blood pressure changes
• Low blood pressure causes release of renin, and angiotensinogen converts to angiotensin
• Angiotensin constricts blood vessels and stimulates release of aldosterone
• Aldosterone increases sodium reabsorption, by acting on distal tubule and collecting duct to increase sodium reabsorption, resulting in blood pressure increase
• ADH responds to increase in osmotic pressure of blood, for example, dehydration
• RAAS responds to low blood volume but osmotic pressure remains the same, for example, diarrhea or hemorrhage

Adjustment to Stress

• When body is stressed it releases epinephrine, cortisol, and glucagon, and decreases insulin

• Epinephrine mobilizes carbohydrate and fat energy stores, increases blood glucose and fatty acids, and accelerates heart rate and respiratory system

• Cortisol mobilizes energy stores, converts protein to glucose, and elevates amino acids, glucose, and fatty acids in the blood

• Glucagon converts glycogen to glucose

• Insulin decreases breakdown of glycogen in liver

• RAAS is activated and ADH increases

• Prolonged exposure to stress alters the body

• Prostaglandins are hormones that act on the cell that produced them and are created when tissue is damaged or stressed

• Prostaglandins cause Inflammation, increased blood flow, platelets to clot, fever, and pain perception

• Aspirin works by blocking enzymes for prostaglandin production

• Anabolic Steroids are substances that mimic muscle-building traits of testosterone, resulting increased strength, and more lean muscle, Without affecting the cardiovascular system

Steroid Effects

• Steroid effects include premature growth plate fusion, mood swings, acne, bad breath, high blood pressure, liver disease, and cancer
• Men may experience baldness, breast development, shrinking testes, and reduced sperm count
• Women may experience facial hair, breast reduction, and changes to their menstrual cycle
• Beta blockers slow the heartbeat
• Erythropoietin (EPO) affect red blood cell production
• hGH decreases fat and promotes muscle development