• BIO 265 Anatomy and Physiology II
  • 1.0. MODULE 1: CARDIOVASCULAR SYSTEM
  • 2.0. MODULE 2: BLOOD
  • 3.0. MODULE 3: IMMUNE SYSTEM
  • 4.0. MODULE 4: THE INTEGUMENTARY SYSTEM
  • 5.0. MODULE 5: THE RESPIRATORY SYSTEM
  • 6.0. MODULE 6: THE SKELETAL SYSTEM
  • 7.0. MODULE 7: URINARY SYSTEM
  • 8.0. MODULE 8: DIGESTIVE SYSTEM
  • 9.0. MODULE 9: ENDOCRINE SYSTEM
  • 10.0. MODULE 10: REPRODUCTIVE SYSTEM
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  • 9.1.3

    Hormones of the Body

    Regulation of Release

    Hormone release is controlled by three general mechanisms:

    1. Secretion can be regulated by another endocrine gland. As explained above, the anterior pituitary gland releases tropic hormones that control the release of other hormones.
    2. Secretion can be regulated by some other substance in the blood. Insulin secretion is regulated by the amount of glucose in the blood.
    3. Secretion can be regulated by the nervous system. Epinephrine release from the adrenal gland is controlled by the autonomic nervous system.

    Regardless of the mechanism of control, hormone release is typically regulated through negative feedback loops (Module 1: Bio 264). 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.

    This diagram shows a negative feedback loop using the example of glucocorticoid regulation in the blood. Step 1 in the cycle is when an imbalance occurs. The hypothalamus perceives low blood concentrations of glucocorticoids in the blood. This is illustrated by there being only 5 glucocorticoids floating in a cross section of an artery. Step 2 in the cycle is hormone release, where the hypothalamus releases corticotropin-releasing hormone (CRH). Step 3 is labeled correction. Here, the CRH release starts a hormone cascade that triggers the adrenal gland to release glucocorticoid into the blood. This allows the blood concentration of glucocorticoid to increase, as illustrated by 8 glucocorticoid molecules now being present in the cross section of the artery. Step 4 is labeled negative feedback. Here, the hypothalamus perceives normal concentrations of glucocorticoids in the blood and stops releasing CRH. This brings blood glucocorticoid levels back to homeostasis.

    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://books.byui.edu/-SzwI

    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" hormones. 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 hormones will be released from the transport proteins to maintain the equilibrium ratio of bound to free.

    Lifespan 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 lifespan 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 defined below.

    1. Synthesis: How is the hormone synthesized? It may be produced on demand or stored for later release.

    1. Mode of release: Is the hormone released from vesicles through exocytosis or simply produced and allowed to diffuse out of the cell.

    1. Transport: How is the hormone transported in the blood? It may circulate free or it may be bound to carrier proteins.

    1. 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 circulation for hours or even days.

    1. 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

    Hypothalamus

    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 Adrenocorticotropic 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

    Adrenocorticotropic 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

    Pancreas

    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

    Gonads

    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)

    Gallbladder, Pancreas, Stomach

    Release of bile from gallbladder

    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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