Chapter 11 Human Physiology

Endocrine System Overview

Introduction

  • The function of the endocrine system is to provide a method of communication across the body
  • The four structural components of the endocrine system are the endocrine glands, hormones, blood, and target cells
    • The effectors of the endocrine system are the target cells
  • Endocrine glands: Ductless organs or groups of cells that secrete hormones directly intothe blood or other body fluids
  • Hormonal control systems help regulate homeostasis, stress responses, growth, and reproductive function
  • True: T or F, A single gland can secrete more than one hormone

Structural Classes of Hormones

  • The three major structural classes of hormones are amines, peptides/proteins, and steroids
  • Amine Hormones
    • Amine: Hormones derived from the amino acid tyrosine
    • Amine hormones include thyroid, catecholamines, and dopamine
    • Thyroid hormones: Amine hormones from the thyroid gland
    • Catecholamines: Amine hormones form the adrenal medulla, such as epinephrine
    • Dopamine: An amine hormone secreted by the hypothalamus
  • Peptide/Protein Hormones
    • Peptide and protein hormones are the most numerous class of all hormones
    • Peptide and protein hormones are primarily polar molecules
    • Synthesis
    • Polypeptides are packaged into preprohormones by the rough ER, then into prohormones by the Glogi apparatus, and then the hormone is secreted by exocytosis
  • Steroid Hormones
    • Steroids: Hormones synthesized from cholesterol by gonads, adrenal cortex, and the placenta
    • Steroid hormones are secreted via diffusion through the plasma membrane once they are made
    • Steroid hormones circulate in the plasma and they are bound to plasma proteins (aka albumin)

Hormone Synthesis

  • Adrenal Glands: Paired glands that sit atop the kidneys and are known for hormone synthesis
    • The adrenal glands have two separate areas, the adrenal medulla, and the adrenal cortex
    • Adrenal medulla: A modified sympathetic ganglion that releases catecholamines in resposne to sympathetic activation
    • The adrenal medulla releases about 20% norepinephrine and 80% epinephrine, which bind to adrenergic receptors
    • Adrenal cortex: Outer region of the adrenal gland that produces steroid hormones
    • Aldosterone: A mineralocorticoid that regulates sodium, potassium, and hydrogen ions to help with water balance; relelased by the adrenal cortex
    • Cortisol: A glucocorticoid that regulates metabolism of glucose and other nutrients to deal with stress response; released by the adrenal cortex
    • Adrostenedione: A less potent version of testosterone, used in sexual development; released by the adrenal cortex
  • The gonads (the testes and ovaries)
    • Gonads: Produce steroids that are important for sexual development and reproductive function
    • Testes: Mainly secrete testosterone and small amounts of estrogens; often converted to estradiol in target tissues by way of aromatase
    • Aromatase helps convert testosterone into estradiol
    • Ovaries: Secrete estradiol (estrogen) and small amounts of testosterone, as well as progesterone
    • Progesterone can be secreted by the corpus lteum in ovulation or the adrenal cortex

Hormone Signaling & Action

Hormone Transport in Blood

  • The structural components of the endocrine system are endocrine glands, hormones, blood, and the target cells (effectors)
  • Catecholamines & protein hormones are water soluble and are easily transported by being dissolved in the plasma after being released during exocytosis
  • Steroid hormones are nonpolar, so they attach to plasma proteins (in a hormone-protein complex) to travel through the blood after being secreted via diffusion
    • Most hormones are bound to carrier proteins, but a very small amount are not. This free hormone concentration is what ends up binding to receptors in target cells.
    • Some of the protein bound hormones in the blood dissacociate to reach receptor cells
  • Plasma hormone concentration is affected by the rate of secretion into the blood and the rate of removal from the blood
    • Rate of Removal
    • Hormone removal from the blood is called hormone clearance
    • Hormones that bind to the receptor end up decreasing the free hormone concentration
    • Hormone clearance occurs in the liver and the kidneys
    • In the blood the rate of removal of free proteins happens by enzymatic breakdown or binding
      • Catecholamines take minutes to hours to break down (free hormones, so not as protected)
      • Steroids and thyroids can stay from hours to days (why? they’re bound to transporters so they’re protected)
    • Hormones can be activated by metabolism to increase binding to receptors

Hormone Action

  • All tissues are exposed to circulating hormones, but only ones with the hormone receptor will respond
    • Non-polar chemical messengers can bind to intracellular receptors after diffusing through the cell membrane
    • Polar chemical messengers must have receptors built into the cell membrane
  • Methods of regulating hormonal response
    • Number of receptors (if a gland oversecretes, the target cells would down-regulate the receptor building so there isn’t too much taken in)
    • Permissiveness: An up-regulation of the number of receptors for one hormone due to the presence of the second hormone, allowing for a larger response
    • Epinephrine and thyroid combined have way more fatty acids released
  • Cellular effects of polar hormones
    • Catecholamines and peptide hormones cannot cross the cell membrane, so the receptor must be built into the cell membrane
    • Most polar hormones activate a second messenger system upon binding to a receptor
    • Polar hormones have fast, non-genomic effects (aka changing enzyme activity)
    • Non-genomic: Effects of a hormone that have short-term effects, and the components are already built within the cell
    • Less frequently, polar hormones can have longer-lasting genomic effects that do affect transcription
  • Cellular effects of non-polar hormones
    • Steroid and thyroid hormones bind to intracellular receptors after diffusing through the cell membrane
    • Steroid and thyroid hormones form a hormone-receptor complex when the hormone binds inside of the cell
    • The hormone-receptor complex of non-polar hormones act as a transcription factor
    • Less frequently, non-polar hormones can bind to plasma membrane receptors to exert non-genomic effects

Control of Hormone Secretion

  • Hormone secretion is regulated by changes in plasma concentration of subsances, neurotransmitters released by neurons synapsing upon endocrine cells, or other hormones
  • Humoral Control: A modality of control for homone secretion where ion/nutrient concentrations within the blood act as the stimulus for hormone release
  • Neural Control: A modality of control for homone secretionl where neurotransmitter release from the autonomic nervous system can influence hormone release from many endocrine glands
  • Hormonal Control: A modality of control for homone secretion where one hormone can signal the release of a second hormone from a different endocrine gland
    • Tropic: The hormone that singals the release of a second hormone from a different endocrine gland

Hormone System Regulation & Endocrine Disorders

The Hypothalamus & Pituitary Gland

  • The hypothalamus is part of the diencephalon & works with the pituitary gland
  • Infundibulum: The pituitary gland and the hypothalamus are connected by this tissue.
    • The infundibulum includes axons and mood vessels to connect the hypothalamus and the pituitary gland
  • The anterior pituaitary glands have more emphasis on blood vessels for hormone transport
  • Hypothalamus controls the release of anterior pituitary hormones via release of hypophysiotropic hormones
    • Hypophysiotropic refers to hormones from the anterior pituitary gland that causes the release of another hormone
    • Portal system: This modality includes veins are in between two separate capillary beds are portal veins that make up the system that supports hypophysiotropic hormone transport
    • Pros of the portal system include that it’s faster & less diluted instead of travelling all the way through the heart and wait for a wrap around back to effectors

Three-Hormone System

  • The first hormone in a three-hormone system is released from the hypothalamus and has a hypotropic effect on the anterior pituitary gland
  • The second hormone in a three-hormone system is released from the anterior pituitary gland and effects the third gland
  • The third hormone in a three-hormone system is released from a gland, that targets the effector
  • Hypophysiotropic Hormone Symmary (3-Hormone System Examples)

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  • Hypophysiotropic hormone secretion is controlled by neural inputs & influenced by sensory info, biological rhythms, and developmental stages
  • Long-Loop: The third hormone in a three-hormone system can have a negative feedback on the hypothalumus or the anterior pituitary, also called negative feedback
    • Short-loop: The second hormones can exert negative feedback on the hypothalumus, called _ negative feedback

Mechanisms of Endocrine Disorders

Hyposecretion issues

  • Hyposecretion: There is a reduction in the amount of the hormone within the plasma, causing an impaired response
  • Primary hyposecretion: The reduction of hormones in the plasma is caused by the hormone-producing gland (the third hormone in sequence), caused by destruction of the gland, enzyme deficiency, or dietary deficiency
  • Secondary hyposecretion: There is too little stimulation by a tropic hormone, causing an unsafe reduction in hormone concentration; the second hormone is not doing enough so the final hormone cannot be released
  • If there are high levels of the tropic hormone, the hyposecretion is probably primary
  • If there are low levels of the tropic hormone, the hyposecretion is probably secondatry

Hypersececretion issues

  • Hypersecretion: Increased circulating hormone concentrations
  • Primary hypersecretion: The gland is producing too much hormone
  • Secondary hypersecretion: Excessive stimulation by the tropic hormone on the final hormone producer

Hyporesponsiveness issues

  • Hyporesponsiveness: The cell has a diminished response to hormonal inputs, despite a proper amount of hormone being present
  • Hyporesponsiveness can be caused by not enough receptors, dietary issues, or the signal transduction mechanisms

Hyperresponsiveness issues

  • Hyperresponsiveness: The cell has a very much increased response to hormonal inputs, although hormone levels are normal