• BIO 461 Principles of Physiology
  • Module 1.0. Homeostasis, Membranes, Electrophysiology and ANS
  • Module 2.0. Skeletal Muscle and Special Senses
  • Module 3.0. Cardiovascular System
  • Module 4.0. Urinary and Respiratory Systems
  • Module 5.0. Digestive, Endocrine and Reproductive Systems
  • Appendix A. Gender
  • Appendix B. The Placebo Effect
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  • 5.4.3

    Hormones of the Body

    Regulation of Release

    Hormone release is controlled by three general mechanisms:

    Regardless of the mechanism of control, hormone release is typically regulated through negative feedback loops. For example, if blood glucose levels increase, insulin secretion will increase which will lower the levels of glucose in the blood. As glucose levels return toward normal insulin secretion will decrease.

    Negative Feedback Loop using example Glucocorticoids. When hormone levels become elevated, a negative signal is sent to the pituitary gland and hypothalamus to turn down the release of further hormone
    Author: Open Stax; License: [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons  Link: https://cnx.org/resources/531ca03c11f2ff25b845d1dfd86993c058f3424c/1805_Negative_Feedback_Loop.jpg

    Transport: Endocrine hormones are transported to their target tissues via the blood plasma. Since the plasma is mostly water this presents a problem for some hormones. Hydrophilic hormones easily dissolve in the plasma and pose no major problem for transport. Hydrophobic hormones, on the other hand, cannot dissolve in the plasma and so this presents a challenge for their transport. These hormones must bind to plasma proteins produced by the liver in order to be transported. These "carrier" proteins have hydrophobic cores that shield the hormones from the aqueous plasma. However, even for these hydrophobic hormones a small fraction does circulate free in the plasma. We distinguish between the two forms as "bound" and "free" hormone. It is the ratio of bound to free that determines the overall potency of the hormone. The free hormone is the biologically active form and as the levels of free hormone drop, more bound hormone will be released from the transport proteins to maintain the equilibrium ratio of bound to free.

    Life span of a hormone: The only way to turn off an endocrine response is to remove the hormone from the circulation. There are several mechanisms for removing the hormones. These include: removal by the kidneys, removal by the liver, enzymatic destruction of the hormone, and re-uptake and recycling of the hormone. Steroid hormones, for example, are usually removed from the blood by the liver while protein hormones often end up in the urine. We express the life span of a hormone in the blood as its half-life (T ½). Recall that one half-life is the amount of time necessary to remove 50% of the hormone from the circulation. Half-lives for hormones range from a few minutes to several days. The concentration in the blood of hormones with short half-lives tend to fluctuate markedly while the concentrations of hormones with longer half-lives tend to be more constant. Since hormones bound to carrier molecules are shielded from the mechanisms that remove them from the blood their half-lives tend to be longer and their concentrations do not fluctuate as rapidly. 

    The concentrations of hormones in the blood are typically very low, ranging from 10-11 to 10-9 moles/Liter. Thus, hormone receptors must have very high affinities for their particular hormone. The affinity of a hormone for its receptor is a measure of how easily and strongly it binds to the receptor. Hormone affinities are expressed as the dissociation constant (KD) for the hormone. The units of the dissociation constants are molar units (M) and correspond to the concentration of the hormone required to bind exactly one-half of available receptors. The lower the KD number the higher the affinity of the receptor to the hormone.

    There are many different hormones in the body, and making an accurate count is nearly impossible as new hormones are discovered every year. However, there are general groupings that can help distinguish the characteristics of the vast arrays of hormones. One common method of classifying hormones is based on their chemical structure, this type of classification results in three main classes of hormones: peptide/protein hormones, steroid hormones, and amino-acid derived hormones. The hormones within each class have similar functional properties. The table below describes the characteristics of each class of hormones based on the 5 properties define below.

    1. Synthesis: How is the hormone synthesized?  It may be produced on demand or stored for later release.
    2. Mode of release: Is the hormone released from vesicles through exocytosis or simply produced and allowed to diffuse out of the cell.
    3. Transport: How is the hormone transported in the blood? It may circulate free or it may be bound to carrier proteins.
    4. Half-life: How long does the hormone circulate in the blood before being broken down. It may be broken down quickly or it may stay in the circulation for hours or even days. 
    5. Receptor: Which kind of receptors does it interact with (See Table below)

    Classes of Hormones

      Peptide/Protein Steroid Amino-Acid Derived
          Catecholamines Thyroid
    Synthesis Produced as inactive forms and stored in vesicles Made on demand from cholesterol Produced and stored in vesicles Produced and stored as precursor
    Release Exocytosis Diffusion Exocytosis Facilitated diffusion using a carrier
    Transport Dissolved in plasma (water-soluble) Bound to carrier proteins (lipid-soluble) Dissolved in plasma (water-soluble) Bound to carrier proteins (lipid-soluble)
    Circulatory half-life Minutes 100s of minutes Less than a minute Days
    Receptor type Membrane bound receptors Cytosolic or nuclear receptors Membrane bound receptors Nuclear receptors
    Examples Insulin, Growth Hormone Estrogen, Testosterone Epinephrine, Norepinephrine Thyroxine

    As the name suggests, peptide/protein hormones are produced from amino acids and range in size from three amino acids to hundreds of amino acids. This is the most diverse and abundant type of hormone group and is made by tissues located throughout the body. Steroid hormones are always derived from cholesterol (and therefore hydrophobic) and are made by only a few organs, specifically the gonads and adrenal cortex. Hormones categorized as amino-acid derived are created from the amino acid tyrosine (catecholamines and thyroid hormones) or tryptophan (melatonin and serotonin). The tables below list the major endocrine glands, the hormones they produce and the major actions of the hormones.

    Major Endocrine Glands and the Hormones They Secrete

    Hormone Target Tissue Primary Action
    Gonadotropin-Releasing Hormone (GnRH) Anterior Pituitary Stimulate secretion of Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH)
    Thyrotropin-Releasing Hormone (TRH) Anterior Pituitary Stimulate secretion of Thyroid Stimulating Hormone (TSH)
    Corticotropin-Releasing Hormone (CRH) Anterior Pituitary Stimulate secretion of Adrenocorticotrophic Hormone (ACTH)
    Growth Hormone-Releasing Hormone (GHRH) Anterior Pituitary Stimulate secretion of Growth Hormone (GH)
    Growth Hormone-Inhibiting Hormone (GHIH, somatostatin) Anterior Pituitary Inhibit secretion of GH
    Prolactin-Inhibiting Hormone (PIF, dopamine) Anterior Pituitary Inhibit secretion of Prolactin (PRL)
    Anterior Pituitary
    Hormone Target Tissue Primary Action
    Thyroid-stimulating Hormone (TSH) Thyroid Gland Stimulate secretion of Thyroxin (T4) and Triiodothyronine (T3)
    Follicle-stimulating Hormone (FSH) Ovaries and Testes Male: Sperm production Female: Follicle development and Estrogen secretion
    Luteinizing Hormone (LH) Ovaries and Testes Male: Testosterone production Female: Ovulation, Progesterone secretion
    Adrenocorticotrophic Hormone (ACTH) Adrenal Cortex Stimulate secretion of Glucocorticoids (Cortisol)
    Growth Hormone (GH) Most tissues Stimulates tissue growth Regulation of metabolism
    Prolactin Mammary glands and ovaries Stimulates milk production Up-regulation of FSH and LH receptors
    Posterior Pituitary
    Hormone Target Tissue Primary Action
    Oxytocin Uterus and mammary glands Stimulates uterine contractions Stimulates release of milk Social and moral feelings (Brain)
    Antidiuretic Hormone (ADH) (Vasopressin) Kidneys and blood vessels Renal water reabsorption (reduced urine volume) Vasoconstriction
    Thyroid gland
    Hormone Target Tissue Primary Action
    Thyroxine (T4) Whole Body Metabolism and Growth
    Triiodothyronine (T3) Whole Body Metabolism and Growth
    Parathyroid glands
    Hormone Target Tissue Primary Action
    Parathyroid Hormone (PTH) Bone Increase blood calcium
    Hormone Target Tissue Primary Action
    Insulin Skeletal muscle, Adipose tissue, Liver Lowers blood glucose levels
    Glucagon Liver Raises blood sugar levels by stimulating glycogen breakdown and glucose synthesis
    Adrenal glands
    Hormone Target Tissue Primary Action
    Adrenal Cortex: Mineralocorticoids (Aldosterone) Kidney Increased Na+ reabsorption and Excretion, increased water reabsorption
    Adrenal Cortex: Glucocorticoids (Cortisol) Most tissues Increased protein and lipid breakdown Increased glucose production (increased blood sugar) Anti-inflammatory
    Adrenal Cortex: Androgens Many tissues Not as important in males In females stimulates growth of axillary and pubic hair
    Adrenal Medulla: Epinephrine and Norepinephrine Many tissues Increase blood glucose (glycogen breakdown) Fight-or-flight response
    Hormone Target Tissue Primary Action
    Testosterone (Male) Most tissues Male sexual development Spermatogenesis
    Estrogen (Female) Most tissues Female sexual development
    Progesterone (Female) Many tissues Gestation Maternal behavior
    Digestive Tract
    Hormone Target Tissue Primary Action
    Gastrin Parietal Cells Gastric acid secretion
    Cholecystokinin (CCK) Gall bladder, Pancreas, Stomach Release of bile from gall bladder Secretion of digestive enzymes by pancreas Decreased stomach emptying
    Secretin Pancreas, Liver Increased bicarbonate secretion by pancreas and liver
    Gastric Inhibitory Peptide (GIP) Beta cells of pancreas Increased insulin secretion

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