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 Endocrine System


Endocrine System

 

 

Content

Immune System Functions
Innate Immunity
Acquired Immunity
Antibodies
Types of T cells
Types of Hormones
Hormone Concentrations in the Blood
Control of Hormone Release
Humoral/neural/hormonal Stimuli
Hypothalamus
Pituitary ( Hypophysis)
Anterior Pituitary Gland (Adenohypophysis)
Posterior Pituitary Gland (Neurohypophysis)
Feedback Control of Endocrine Secretion
Thyroid Glands
Parathyroid Glands
Adrenal (cortex and Medula) Glands
Pancreas
Regulation of Glucose Metabolism During Exercise
Regulation of Fat Metabolism During Exercise
Hormonal Effects on Fluid and Electrolyte Balance
Hormones and Stress

 

 

The endocrine system is made up of ductless glands called endocrine glands that secrete chemical messengers called hormones into the bloodstream or in the extracellular fluid.

A hormone is a chemical substance made and secreted by one cell that travels through the circulatory system or the extracellular fluid to affect the activities of cells in another part of the body or another nearby cell.

 

Functions of the Endocrine System

  • Maintains the internal environment in the body (the optimum biochemical environment)
  • Influences metabolic activities
  • Integrates and regulates growth and development.
  • Controls, maintains and instigates sexual reproduction, including gametogenesis, coitus, fertilization, fetal growth and development and nourishment of the newborn

 

Endocrine Glands

  • Pituitary
  • Thyroid
  • Parathyroid
  • Adrenal
  • Pineal Gland
  • Thymus Gland
  • Hypothalamus (has both neural functions and releases hormones)
  • Pancreas (produces both hormones and exocrine products)
  • Gonads (produce both hormones and exocrine products)
  • Other tissues and organs also produce hormones – adipose cells, cells of the small intestine, stomach, kidneys, and heart

 

Endocrine vs. Nervous System

Nervous System
Endocrine System
Neurons release neurotransmitters
Endocrine cells release hormones
A neurotransmitter acts on specific cell right next to it.
Hormones travel to another nearby cell or act on cell in another part of the body.
Neurotransmitters have their effects within milliseconds.
Hormones take minutes or days to have their effects.
The effects of neurotransmitters are short-lived.
The effects of hormones can last hours, days, or years.
Performs short term crisis management
Regulates long term ongoing metabolic function
Neurotransmitter acts on specific cell right next to it.
Hormone can travel to another nearby cell or it can act on another part of the body.

 

Note, however that these two systems coordinate their activities: certain parts of the nervous system stimulate or inhibit the release of hormones (e.g. hypothalamus) and in turn, certain hormones can stimulate or inhibit the flow of nerve impulses.

The nervous system modifies the stimulation of endocrine glands and their negative feedback mechanisms. The nervous system can override normal endocrine controls

 

 

Hormones vs Neurotransmitters

 

Hormones

  • Are chemical substances secreted by cells into the extracellular fluids
  • Regulate the metabolic function of other cells
  • Have lag times ranging from seconds to hours
  • Tend to have prolonged effects
  • Circulate to all tissues but only activate cells referred to as target cells
  • Target cells must have specific receptors to which the hormone binds.
  • These receptors may be intracellular or located on the plasma membrane.

Target Cell Activation

  • Activation depends on 3 factors:
    1. Blood levels of the hormone
    2. Relative number of receptors on the target cell
    3. Affinity of those receptors for the hormone
  • Up-regulation – target cells form more receptors in response to the hormone
  • Down-regulation – target cells lose receptors in response to the hormone

Mechanism of Hormone Action

  • Hormones produce one or more of the following cellular changes in target cells
  • Alter plasma membrane permeability
  • Stimulate protein synthesis
  • Activate or deactivate enzyme systems
  • Induce secretory activity
  • Stimulate mitosis

Types of Hormones

  • Amino Acid Derivatives
    These are hormones that are modified from the amino acid called tyrosine. They include the thyroid hormones and the hormones of the adrenal medulla (epinephrine and norepinephrine).  These hormones are water soluble.
  • Peptide Hormones
    These hormones consist of chains of amino acids that vary in size from 3 amino acids (TRH) to 191 amino acids (GH). These hormones are water soluble.
  • Lipid Derivatives
    These include hormones that are steroids (e.g. estrogen and testosterone) and fatty acids derivatives (e.g. prostaglandins). These hormones are lipid soluble.

 

How Hormones Work

In general, hormones can act on a target cell in 1 of 2 ways:

1. Activate Second Messengers (Involves regulatory G proteins)
(This is how amino acid-derived, peptide, and fatty acid-derived hormones work)

1. The hormone binds to a G protein-linked receptor on the cell membrane; t he hormone acts as a first messenger.2. The binding of the hormone to the G protein-linked receptor activates a second messenger such as cAMP. 3. The second messenger than activates or inactivates enzymes in the cell

Click here for an animation that will help you understand how hormones that bind to G protein-linked receptors on the surface of the cell activate second messenger systems.

 

 

2. Activate Genes
(This is how steroid and thyroid hormones work)

1. Steroid hormones and thyroid hormones pass directly through the cell membrane of target cells.

2. They bind to receptors in the cytoplasm or in the nucleus. (Thyroid hormones also bind to receptors in the mitochondria.)

3. If they bind to receptors in the cytoplasm, the hormone-receptor complex then enters the nucleus.

4. In the nucleus the hormones directly either turn genes “on” or turn genes “off.” That is they either cause the gene to start making a protein by transcription and translation or not.

Thyroid hormones that go to the mitochondria increase the rate of ATP production in the cell.

Click here for an animation that will help you understand how hormones that bind to intracellular receptors (activate second messengers) work.

Hormone Concentrations in the Blood

  • Hormones circulate in the blood in two forms – free or bound
  • Steroids and thyroid hormone are attached to plasma proteins
  • All others are unencumbered
  • Concentrations of circulating hormone reflect:
    - Rate of release
    - Speed of inactivation and removal from the body
    - Hormones are removed from the blood by:
      - Degrading enzymes
      - The kidneys
      - Liver enzyme systems

Control of Hormone Release

  • Hormone release is controlled by negative feedback systems
  • Hormone levels vary only within a narrow desirable range
  • Hormones are synthesized and released in response to humoral, neural, and hormonal stimuli

Humoral Stimuli

  • Some hormones are secreted in direct response to changing blood levels of ions and nutrients
  • Example: Concentration of calcium ions in the blood
    • Declining blood Ca2+ concentration stimulates the parathyroid glands to secrete PTH (parathyroid hormone)
    • PTH causes Ca2+ concentrations to rise and the stimulus is removed

 

Neural Stimuli -

  • ANS efferent nerve fibers stimulate hormone release
  • Ex: Preganglionic sympathetic nervous system (SNS) fibers stimulate the adrenal medulla to secrete catecholamines.

 

 

Hormonal Stimuli

  • Some hormones are released in response to hormones produced by other endocrine organs
  • Ex: Many hypothalamic hormones stimulate the anterior pituitary to release hormones. The pituitary hormones then stimulate other target organs to secrete still other hormones

Major Endocrine Organs

 

 

Hypothalamus

  • Is a part of the brain located in the diencephalon, inferior to the thalamus. 
  • Is made up of neurons and neuroglial cells.
  • Produces several different hormones:

1. Releasing Hormones

  • These stimulate the anterior pituitary gland to release a specific hormone (e.g. GRH-GH)

2. Inhibiting Hormones

  • These stimulate the anterior pituitary gland to not release a specific hormone (e.g. GRIH-GH)

3. Antidiuretic Hormone (ADH) (also called vasopressin)

  • Antidiuretic hormone conserves body water by reducing the loss of water in urine.
  • This hormone signals the collecting ducts of the kidneys to reabsorb more water and constrict blood vessels, which leads to higher blood pressure and thus counters the blood pressure drop caused by dehydration.

4. Oxytocin

  • Stimulates the smooth muscle of the uterus to contract, inducing labor.
  • Stimulates the myoepithelial cells of the breasts to contract which releases milk from breasts when nursing.
  • Stimulates maternal behavior.
  • In males it stimulates muscle contractions in the prostate gland to release semen during sexual activity
  • The releasing and inhibiting hormones made by the hypothalamus reach the anterior lobe of the pituitary gland DIRECTLY by a special set of blood vessels called the hypophyseal portal system.
  • The hypothalamus makes antidiuretic hormone (ADH) and oxytocin in the cell bodies of neurons and then the hormones are transported down the axons which extend into the posterior pituitary gland. The posterior pituitary gland The posterior pituitary gland stores and later releases the hormones as needed.

 

Click here for an animation on the relationship of the hypothalamus to the anterior and posterior pituitary glands and on the relationship of the hormones made in the hypothalamus (ADH, oxytocin, releasing hormones, and inhibiting hormones) to the anterior and posterior pituitary glands.

 

Releasing and Inhibiting Hormones

Pituitary (also called Hypophysis)

  • Is a two-lobed organ that secretes nine major hormones
    • Neurohypophysis – posterior lobe (neural tissue) receives, stores, and releases hormones (oxytocin and antidiuretic hormone) made in the hypothalamus and transported to the posterior pituitary via axons.
    • Adenohypophysis – anterior lobe, made up of glandular tissue.  Synthesizes and secretes a number of hormones.
  • The hypothalamus sends releasing hormones to the anterior pituitary that stimulates the synthesis and release of hormones from the anterior pituitary gland
  • The hypothalamus also sends inhibiting hormones that shut off the synthesis and release of hormones from the anterior pituitary gland
  • The pituitary gland releases nine important peptide (protein) hormones
  • All nine peptide hormones bind to membrane receptors and use cyclic AMP as a second messenger

 

Anterior Pituitary Hormones

 

Hormones of the Anterior Pituitary Gland (Adenohypophysis)

1. Growth Hormone (GH or somatotropin)

  • GH produced by somatotropic cells of the anterior lobe
  • Stimulates most cells, but target bone and skeletal muscle
  • Stimulates the liver and other tissues to secrete insulin-like growth factor I (IGF-I or somatomedin)
  • IGF-I stimulates proliferation of chondrocytes (cartilage cells), resulting in bone growth.
  • GH stimulates cell growth, replication, and protein synthesis through release of IGF-I.
  • Direct action promotes lipolysis to encourage the use of fats for fuel and inhibits glucose uptake
  • Antagonistic hypothalamic hormones regulate GH
    • Growth hormone–releasing hormone (GHRH) stimulates GH release
    • Growth hormone–inhibiting hormone (GHIH or somatostatin ) inhibits GH release

Growth Hormone Regulation

 

2. Thyroid Stimulating Hormone (TSH or Thryotropin)

  • Travels to the thyroid gland (target cells) where it stimulates the release of thyroid hormones in response to low temperatures, stress, and pregnancy
  • Thyrotropin releasing hormone (TRH) from the hypothalamus promotes the release of TSH
  • Rising blood levels of thyroid hormones act on the pituitary and hypothalamus to block the release of TSH

3. Adrenocorticotropic Hormone (ACTH or Corticotropin)

  • Travels to the adrenal gland (target cells) where it stimulates the release of corticosteroids (such as cortisol) in the adrenal cortex.
  • Corticotropin-releasing hormone (CRH) from the hypothalamus promotes the release of ACTH in a daily rhythm
  • Internal and external factors such as fever, hypoglycemia, and stressors can trigger the release of CRH

4. Follicle Stimulating Hormone (FSH)

  • Travels to the gonads (target cells) and stimulates sperm or egg cell production and maturation and estrogen secretion
  • Gonadotropin-releasing hormone (GnRH) from the hypothalamus promotes the release of FSH during and after puberty

5. Leutinizing Hormone (LH)

  • Travels to the ovaries in females (target cells) and stimulates ovulation, maturation of follicles (together with FSH) and stimulates the corpus luteum to secrete progesterone. 
  • In males LH travels to the testes (target cells) to stimulate secretion of testosterone.
  • LH is also referred to as interstitial cell-stimulating hormone (ICSH)

Click here for an animation that will help you to understand how GnRH, FSH, and LH are involved in the female reproductive cycle.

6. Prolactin (PL)

  • Travels to the mammary glands (target cells) and stimulates the development of mammary glands to produce milk.
  • In males scientists think prolactin influences the sensitivity of cells in the testes (interstitial cells) to the effects of luteinizing hormone (LH)
  • Prolactin-releasing hormone (PRH) from the hypothalamus stimulates the release of prolactin
  • Prolactin-inhibiting hormone (PIH) from the hypothalamus inhibits the release of prolactin
  • Blood levels rise toward the end of pregnancy, suckling stimulates PRH release and encourages continued milk production

Hormones of the Posterior Pituitary Gland (Neurohypophysis)

  • The neurohypophysis contains axons from neurons in the hypothalmus

1. Antidiuretic Hormone (ADH or vasopressin)

  • Made by neurons of the supraoptic nucleus in the hypothalamus
  • Signals the collecting ducts of the kidneys to reabsorb more water and constrict blood vessels, which leads to higher blood pressure and thus counters the blood pressure drop caused by dehydration or other reasons

Click here for an animation that describes how the hypothalamus releases antidiuretic hormone (ADH or vasopressin) which then acts on other organs to have its effects. The animation is titled Hypothalamus and Antidiuretic Hormone.

2. Oxytocin

  • Made by neurons of the paraventricular nucleus of the hypothalmus
  • Stimulates the smooth muscle of the uterus to contract, inducing labor
  • Stimulates the myoepithelial cells of the breasts to contract which releases milk from breasts when nursing.
  • Stimulates maternal behavior.
  • In males it stimulates muscle contractions in the prostate gland to release semen during sexual activity.

 

Posterior Pituitary Hormones

 

Feedback Control of Endocrine Secretion

There are 3 levels of integration

  1. Hypothalamic stimulation–from CNS
  2. Pituitary stimulation–from hypothalamic trophic hormones
  3. Endocrine gland stimulation–from pituitary trophic hormone

Click here for an animation that provides an example of positive and negative feedback control of the reproductive hormones.

The animation is followed by practice questions. Click here for even more practice questions.

 

Thyroid Glands

  • The thyroid gland contains numerous thyroid follicles that release 2 hormones: thyroxine (T4) and triiodothyronine (T3)
  • Thyroid hormones are held in storage but eventually attach to thyroid binding globulins (TBG); some are attached to transthyretin or albumin
  • Thyroid hormones regulate metabolism; they diffuse into target cells and bind to mitochondria, thereby increasing ATP productio; they also bind to receptors activating genes that control energy utilization
  • Thyroid hormones increase protein synthesis, and promote glycolysis, gluconeogenesis, and glucose uptake
  • Thyroid hormones are necessary for normal growth as they stimulate release of GH from the anterior pituitary
  • Thyroid hormones are very important for brain development
  • C Cells in between the thyroid follicles produce calcitonin.
  • Calcitonin decreases the concentration of calcium in the blood where most of it is stored in the bones; it stimulates osteoblast activity and inhibits osteoclast activity, resulting in new bone matrix formation.
  • which help regulate calcium concentration in body fluids

Click here for an animation that describes the structure of the thyroid gland, how thyroid hormones are made, the functions of calcitonin and thyroid hormones, and the effects of hypo- and hyperthyroidism.

Thyroid Hormone Feedback with details Thyroid Hormone Feedback Overview

 

Parathyroid Glands

  • The parathyroid glands are four or so masses of tissue embedded posteriorly in each lateral mass of the thyroid gland
  • Parathyroid hormone (PTH) is the most important endocrine regulator of calcium and phosphorus concentration in extracellular fluid
  • PTH has the opposite effect of calcitonin.
  • PTH stimulates osteoclasts which increases blood calcium levels.
  • PTH causes reabsorption of Ca+2 from kidneys so it is not excreted in the urine
  • PTH stimulates synthesis of calcitriol (hormone made in the kidney which the active form of Vitamin D which increases Ca+2 absorption from small intestine

Click here for an animation that describes the structure of the parathyroid glands, the function of parathyroid hormones, and the effects of hypo- and hyperparathyroidism.

Parathyroid Glands

Calcium Regulation

 

Adrenal Glands

  • The adrenal glands are located superior to each kidney.
  • Each adrenal gland has a pyramid shape.
  • Each adrenal gland has an inner medulla and outer cortex:
    Adrenal Cortex
    Adrenal Medulla

 

Adrenal Gland Location

 

Adrenal Cortex

  • Makes and secretes over 30 different steroid hormones (collectively called corticosteroids)
  • The adrenal cortex has 3 regions (zones) that each make a major type of hormones:


1. Mineralocorticoids (e.g. aldosterone)

  • Stimulates the kidneys to reabsorb sodium if blood pressure drops
  • It also secretes (eliminates) potassium


2. Glucocorticoids (e.g. cortisol)

  • These hormones help you to cope with stress
  • Cortisol increases the level of sugar in the blood by stimulating the production of glucose from fats and proteins (gluconeogenesis)
  • It also reduces swelling
  • In large doses, cortisol inhibits the immune system.
  • It stimulates gluconeogenesis, mobilization of free fatty acids, glucose sparing. Also acts as an anti-inflammatory agent

3. Gonadocorticoids (e.g.testosterone, estrogens, and progesterone )
  • The adrenal gland also makes small amts of the sex hormones (mostly androgens (testosterone) and lesser amounts of estrogens and progesterone)
  • Scientists not certain what role these hormones play; but know that when over secreted they can cause problems

Adrenal Medulla

  • secretes the hormones epinephrine and norepinephrine when stimulated by sympathetic neurons of the autonomic nervous system (ANS)
  • Both epinephrine and norepinephrine contribute to the bodies' "fight or flight" response, just like the sympathetic nervous system.
  • They have the same effects as direct stimulation by the sympathetic NS (increase heart rate, breathing rate, blood flow to skeletal muscles, and concentration of glucose in the blood), but their effects are longer lasting
  • Norepinpehrine is similar to epinephrine, but it is less effective in the conversion of glycogen to glucose.
  • ~75 - 80% epinephrine
  • ~25-30% norepinephrine

Click here for animation that describes adrenal gland structure and hormones,

How the adrenal glands respond to stress

 

 

Pancreas

  • Located along the lower curvature of the small intestine (duodenum)
  • The pancreas contains both exocrine and endocrine cells
  • The exocrine portion secretes digestive enzymes into the duodenum via the pancreatic duct
  • The endocrine portion has clusters of endocrine cells within the pancreas called pancreatic islets or Islets of Langerhans
    • Alpha cells secrete glucagon
    • Beta cells secrete insulin
  • Glucagon increases the levels of glucose in the blood by stimulating the liver to breakdown glycogen into glucose during fasting or starvation
  • Insulin lowers blood glucose by increasing the rate of glucose uptake and utilization
    Glucagon raises blood glucose by increasing the rates of glycogen breakdown and glucose manufacture by the liver

Click here for an animation that describes pancreas structure and hormones.

 

Location of pancreas

Regulation of glucose levels in blood

 

Regulation of Glucose Metabolism During Exercise

  • Glucagon secretion increases during exercise to promote liver glycogen  breakdown (glycogenolysis)
  • Epinephrine and norepinephrine further increase glycogenolysis
  • Cortisol levels also increase during exercise for protein catabolism for later gluconeogenesis
  • Growth Hormone mobilizes free fatty acids
  • Thyroxine promotes glucose catabolism
  • As intensity of exercise increases, so does the rate of catecholamine (epinephrine and norepinephrine) release for glycogenolysis
  • During endurance events, the rate of glucose release very closely matches the muscles' need
  • When glucose levels become depleted, glucagon and cortisol levels rise significantly to enhance gluconeogenesis
  • Glucose must not only be delivered to the cells, it must also be taken up by them.  That job relies on insulin. Exercise may enhance insulin’s binding to receptors on the muscle fiber.
    Up-regulation (receptors) occurs with insulin after 4 weeks of exercise to increase its sensitivity (diabetic importance).

 

Regulation of Fat Metabolism During Exercise

  • When low plasma glucose levels occur, the catecholamines are released to accelerate lypolysis
  • Triglycerides are reduced to free fatty acids by lipase which is activated by:
    • Cortisol
    • Epinephrine
    • Norepinephrine
    • Growth Hormone

Hormonal Effects on Fluid and Electrolyte Balance

  • Reduced plasma volume leads to release of aldosterone which increases Na+ and H2O reabsorption by the kidneys and renal tubes.
    Antidiuretic Hormone (ADH) is released from the posterior pituitary when dehydration is sensed by osmoreceptors, and water is then reabsorbed by the kidneys.

Click here for an animation on the release of ADH in response to decreased blood volume. The animation is followed by practice questions.

 

Hormones and Stress 

  • Stress = any condition that threatens homeostasis
  • GAS (General Adaptation Syndrome) is our bodies response to stress-causing factors
  • Three phases to GAS:
    • Alarm Phase (immediate, fight or flight, directed by the sympathetic nervous system)
    • Resistance Phase (dominated by glucocorticoids)
    • Exhaustion Phase (breakdown of homeostatic regulation and failure of one or more organ systems)

 

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This material is based upon work supported by the Nursing, Allied Health and Other Health-related Educational Grant Program, a grant program funded with proceeds of the State’s Tobacco Lawsuit Settlement and administered by the Texas Higher Education Coordinating Board.