Endocrine System

  • along with the nervous system is responsible for allowing communication between the cells of the body

  • does not rely on complex system of cells —> only uses the existing circulatory system to transport the chemical messengers, hormones

  • time for a hormonal response is slower than nervous response but the effect is 10x longer

  • endocrine + nervous systems overlap in the brain —> pituitary gland and hypothalamus

    • hypothalamus - stimulates the pituitary gland via nerves or hormones

    • pituitary gland - affects hypothalamus with hormones

  • environmental factors have effects on hormone production like the effect of light, the effect of diet

Hormones

  • means “set in motion”

  • chemical regulators produced by cells of endocrine glands in one part of the body, that affect cells in other parts of the body

  1. Target hormones vs. Non-target hormones

    1. target hormones affect specific cells in the body

    2. non-target hormones have broad affects in the body

  2. Steroid hormones vs Non-steroid (protein) hormones

    1. steroid hormones - lipid-based hormones include steroid hormones such as testosterone + estrogen, lipid-based hormones can diffuse across cell membranes + bind to receptor proteins to activate specific genes, causing changes in the cell

    2. protein hormones - water-soluble hormones include amino acid based hormones such as epinephrine, human growth hormone, thyroxine + insulin, they cannot diffuse across the cell membrane, they bind to a receptor protein on the surface of the target cell and start a cascade of reactions inside the target cell

  3. Tropic hormones vs Non-tropic hormone

    1. tropic hormones have other endocrine glands as their target

    2. non-tropic hormones have body cells as their target

Endocrine glands - ductless organs producing their messengers + secreting them directly into the bloodstream

Exocrine glands - produce their chemicals + excrete them into a duct

Similarities Between Endocrine and Nervous Systems

  • some nervous system tissues secrete hormones

  • several chemicals function as both neurotransmitters + hormones

  • both nervous + endocrine systems include responses regulated by negative feedback loops

  • regulation of several physiological processes involves both the nervous + endocrine systems working together

Feedback Mechanisms

  • organisms use feedback mechanisms to regulate their internal environment in response to changes to the internal + external environment

  • negative feedback mechanisms - maintain homeostasis by returning the system back to its original set point

  • positive feedback mechanisms - amplify responses and processes in biological organisms, the variable initiating the response is moved further away from initial set point

Hypothalamus + Pituitary Gland

  • pituitary gland is the “master gland” and releases at least 8 hormones

  • posterior lobe of pituitary gland is considered part of the nervous system

    • does not produce any hormones

    • stores + releases hormones produced by the hypothalamus

  • anterior lobe is a true hormone synthesizing gland which is stimulated by releasing and release-inhibiting hormones from the hypothalamus

Insulin + Blood Sugar Regulation

  • the pancreas have an exocrine + endocrine function

    • exocrine - secretes digestive enzymes via ducts into small intestine

    • endocrine - secretes insulin + glucagon directly into the bloodstream

  • Islets of Langerhans produce the hormones —> insulin in beta cells and glucagon in alpha cells

  • both insulin + glucagon are regulated by negative feedback mechanisms

  • increase of glucose will trigger beta cells to secrete insulin, where it circulates throughout the body and acts on specific receptors to make target cells (muscle and liver cells) more permeable to glucose, as glucose levels in blood return to homeostasis insulin secretion slows

  • low blood glucose stimulates the alpha cells to release glucagon, glucagon stimulates the liver to convert glycogen back into glucose which is released into the blood

Glucose Imbalance

  • diabetes mellitus results when the body does not produce enough insulin or does not respond properly to insulin

  • results in high blood sugar levels —> hyperglycemia

  • without insulin, cells remain relatively impermeable to glucose + cannot obtain enough from the blood

  • short term results in fatigue, fat + protein metabolism, and glucose in urine

  • long term can lead to blindness, kidney failure, nerve damage, and gangrene

  • diabetes is one of the leading causes of death in North America

  1. Type 1 Diabetes

    1. insulin dependent or juvenile diabetes

    2. caused by lack of insulin production in pancreas (hereditary)

    3. treated with insulin injections + rigid blood monitoring

    4. since insulin is a protein it has to be injected or it will be digested

    5. must monitory both hypoglycemia and hyperglycemia

    6. experimental treatments such as Islet transplants and gene therapy

  2. Type 2 Diabetes

    1. insulin dependent

    2. caused by decreased insulin production or too much glucose produced by the liver (not enough compensation by pancreas) or insulin resistance (body cells stop responding to insulin)

    3. diagnosed later in life

    4. people who are overweight have a greater chance of developing type 2

    5. controlled with diet, exercise and oral medications

    6. 90% of diabetes are type 2

  3. Gestational Diabetes

    1. during pregnancy mother develops symptoms

    2. mother is at a greater risk later to get type 2

Growth Hormone

  • stimulates the liver + other tissues to secrete insulin-like growth factor IGF-1

  • IGF-1 stimulates bone growth, amino acid uptake and protein synthesis in muscle + other tissues

  • as long as growth hormone is secreted at appropriate levels for the age of the organism there are no ill effects

  • Growth Hormone Disorders:

    • hyposecretion of growth hormone during childhood can lead to short stature (pituitary dwarfism)

    • hypersecretion of growth hormone during childhood can lead to gigantism

Thyroid Gland + Metabolism

  • TRH, TSH, Thyroxine (T4)

  • a releasing hormone (TRH) from the hypothalamus targets anterior pituitary to release TSH

  • TSH targets the thyroid gland to release thyroxine (T4) which increases the rate the body metabolizes fats, proteins and carbohydrates for energy

  • targets the heart, skeletal muscles, liver + kidneys to increase rate of cellular respiration

  • negative feedback loop

  • Thyroid Gland Disorders:

    • goiters if too little iodine in the diet, thyroid swells

      • iodine is required to synthesize thyroxine

      • swelling is due to continued stimulation by TSH, but no T4 is made, thus causing an increase in thyroid size in an attempt to male more thyroxine

    • hypothyroidism occurs when extremely low quantities of thyroxine are produced

      • if it occurs in childhood (cretinism), it leads to abnormal mental + physical development, growth retardation

      • in adults, symptoms include cold, fatigue, slow pulse, puffy skin, hair loss, weight gain

    • hyperthyroidism is overproduction of thyroxine which stimulates metabolism

      • symptoms include anxiety, insomnia, heat intolerance, irregular heartbeat, weight loss

      • Grave’s disease is a severe state of hyperthyroidism when the body’s immune system attacks the thyroid

      • treated with medication or removal/eradication of part of the thyroid

Calcitonin

  • released by the thyroid + stimulates uptake of calcium into bones lowering the concentration in the blood

  • calcium is essential for healthy teeth + skeletal development, also used in blood clotting, nerve conduction, and muscle contraction

Parathyroid Hormone (PTH)

  • released in response to falling concentration of calcium in the blood

  • stimulates bone cells to break down bone material + kidneys to reabsorb calcium from the urine to increase concentration of calcium in the blood

  • brittle bones may be caused by hyperparathyroidism, causes loss of Ca2+, it is not the same as osteoporosis that can be lessened using a calcitonin spray

Adrenal Gland

  • there are 2 adrenal glands on top of the kidneys that are composed of an inner layer (medulla) and an outer layer (cortex)

  • Adrenal Gland Disorders:

    • Addison’s disease

      • occurs when the adrenal cortex does not produce enough hormones

      • caused by the gradual destruction of the adrenal cortex by the body’s immune system

      • symptoms include dark tanning of the skin

    • Cushing’s syndrome

      • caused by body’s prolonged exposure to cortisol

      • symptoms include upper body obesity, round face, thick neck, and thin arms + legs

      • can be treated with radiotherapy + cortisol inhibiting drugs

Short Term Stress

  • adrenal medulla regulates short term stress response

  • release epinephrine + norepinephrine which increases breathing rate, heart rate, blood pressure, blood flow to the heart + muscles, conversion of glycogen to glucose in liver

  • effects of these hormones are similar to those caused by stimulation of the sympathetic nervous system, but hormonal effects last 10x longer

  • release epinephrine + norepinephrine are fast because they are under nervous system control

  • emotional stress leads to epinephrine release, whereas cold leads to norepinephrine release

Long Term Stress

  • long term stress triggers the release if adrenocorticotropic hormone (ACTH) from the anterior pituitary

  • ACTH targets the adrenal cortex to release cortisol

  • cortisol (glucocorticoids) increases blood sugar + suppresses the immune system response (anti-inflammatory)

  • adrenal cortex also release aldosterone (mineralocorticoids) that increase blood pressure + sex hormones (gonadocorticoids) that supplement the hormones produced by the glands

Water Regulation

  • aldosterone released by adrenal cortex plays a role in the reabsorption of sodium in the kidneys which results in the reabsorption of water and the increase in blood pressure

  • there are multiple ways in which aldosterone regulated using ACTH, angiotensin, blood volume and Na+/K+ levels in the blood

  • antidiuretic hormone (ADH) is produced in the hypothalamus and released by the posterior pituitary gland and targets the nephrons of the kidneys, causing tubules to become more permeable to water

  • results in water reabsorption and increase in blood volume and pressure