CoverModule 1.0. Homeostasis, Membranes, Electrophysiology and ANS1.1. Homeostasis1.1.1. Homeostasis Defined1.1.2. Homeostatic Control Systems1.1.3. Feedback Response Loop1.2. Cell Transport; Water & Solutes1.2.1. Fluid Compartments1.2.2. Osmosis1.2.3. Diffusion of Solutes1.2.4. Active Transport1.2.5. Bulk Transport1.3. Electrophysiology1.3.1. Ions and Cell Membranes1.3.2. Membrane Potentials1.3.3. Graded Potential1.3.4. Action Potentials1.3.5. Refractory Periods1.3.6. Propagation of an Action Potential1.4. The Synapse1.5. The Autonomic Nervous System1.5.1. Organization of the Nervous System1.5.2. Structural Organization of the ANS1.5.3. The SNS and the PNS1.5.4. The Enteric Nervous System1.5.5. Physiology of the ANS1.5.6. Neurotransmitters of the ANS1.5.7. Receptors of the ANS1.5.8. Actions of the Autonomic Nervous System1.5.9. Table of Actions for the SNS and PNS and Some Common DrugsModule 2.0. Skeletal Muscle and Special Senses2.1. Structural Organization of Skeletal Muscle2.2.1. Neuromuscular Junction, Excitation-Contraction Coupling2.2.2. Muscle Contractures and Cramps2.3. Whole Muscle Contraction, Fiber Type, Fatigue and Muscle Pharmacology2.3.1. Motor Units2.3.2. Factors that Influence the Force of Contraction2.3.2. Factors that Influence the Force of Muscle Contraction2.3.3. Energy Source for Muscle Contraction2.3.4. Skeletal Muscle Fiber Types2.3.5. Fatigue2.3.6. Muscle Pharmacology2.4. Smooth Muscle2.4.1. Smooth Muscle Contraction2.5. Control of Body Movement2.5.1. Voluntary Control of Muscle2.5.2. Reflexes2.6. Taste and Smell2.6.1. Taste2.6.2. The Sense of Smell2.7. Vision2.7.1. Structure of the Eye2.7.2. Focusing Light on the Retina2.7.3. Converting Light to Action Potentials2.7.4. The Retina2.7.5. Phototransduction2.7.6. Receptive Fields2.8. Hearing and Equilibrium2.8.1. The Nature of Sound2.8.2. The Hearing Apparatus2.8.3. Sound Vibrations to Action Potentials2.8.4. The Sense of Balance and EquilibriumModule 3.0. Cardiovascular System3.1. Structure of the Heart3.1.1. Chambers and Circulation3.2. Cardiac Cell Action Potentials3.2.1. Action Potentials in Cardiac Muscle Cells3.2.2. Action Potentials in Cardiac Autorhythmic cells3.2.3. Cellular Mechanisms of Inotropy and Chronotropy3.3. Electrophysiology of Heart Muscle3.3.1. Heart Conduction System3.3.2. Electrocardiogram (ECG)3.3.3. Abnormal ECG - Current of Injury3.4. The Cardiac Cycle3.4.1. Cardiac cycle3.4.2. Cardiac Measurements and Pressure Volume Loops3.5. Blood vessels and Blood Pressure3.5.1. Arteries and Veins3.5.2. Capillaries3.5.3. Blood Pressure Regulation and Shock3.5.4. Capillary Exchange3.5.5. Myogenic and Paracrine Regulation of Vasoconstriction and Vasodilation3.6. Blood3.6.1. Composition of Blood3.6.2. Hematopoeisis3.6.3. Breaking Down Red Blood Cells3.6.4. HemostasisModule 4.0. Urinary and Respiratory Systems4.1. Function and Structure of the Kidney4.1.1. Urinary System Function4.1.2. Functional Anatomy of the Urinary System4.1.3. The Nephron: Functional Unit of the Kidney4.1.4. The Renal Corpuscle: Bowman's Capsule4.2. Physiology of Urine Production4.2.1. Filtration4.2.2. Renal Clearance4.2.3. Tubular Reabsorption4.2.4. Urine Concentration and Dilution4.2.5. Hormonal Regulation of Urine Production4.3. Acid/Base Balance4.3.1. Buffers4.3.2. Acid/Base Disturbances4.4. The Respiratory System4.4.1. Respiratory System Structure and Function4.4.2. Respiratory Membrane4.4.3. Respiratory pressures and Inspriation/Expiration4.4.4. Alveoli and Surfactant4.4.5. Pneumothorax4.4.6. Pressure-Volume Loops and the Work of Breathing4.5. Gas Exchange and Transport4.5.1. Gas Laws4.5.2. Partial Pressure Gradients in the Lung4.5.3. Alveolar Gas Equation4.5.4. Oxygen and Carbon Dioxide Transport in the Blood4.5.5. Alveolar Ventilation4.5.6. Ventilation/Perfusion Ratio4.6. Chronic Bronchitis and Emphysema4.6.1. Respiratory Control by the Medulla Oblongata4.6.2. Chemicals that Regulate VentilationModule 5.0. Digestive, Endocrine and Reproductive Systems5.1. Functional Anatomy of the Digestive System5.1.1. Layers of the Digestive Tract5.1.2. Enteric Nervous System5.1.3. Organs of the Digestive System5.2. Digestion5.2.1. Carbohydrates5.2.2. Proteins5.2.3. Lipids5.2.4. Lipoproteins5.3. Regulation of Digestive Secretions5.4. Endocrine System5.4.1. Overview of the Endocrine System5.4.2. Hormone Receptors5.4.3. Hormones of the Body5.4.4. Other Hormones: Melatonin and Pheromones5.5. The Hypothalamus and Pituitary Gland5.5.1. Structure and Function of the Hypothalamus and Pituitary Gland5.5.2. The Posterior Pituitary5.5.3. The Anterior Pituitary5.5.4. Growth Hormone5.5.5. Prolactin5.5.6. Thyroid Hormones5.5.7. Adrenal Hormones5.6. Pancreas5.6.1. Insulin and Glucagon5.6.2. Diabetes Mellitus5.7. Reproductive System Anatomy5.7.1. Female Reproductive Anatomy5.7.2. Male Reproductive Anatomy5.7.3. Sexual Development at Puberty5.7.4. Male Reproductive Endocrine Axis5.7.5. Spermatogenesis5.7.6. Female Reproductive System: Oogenesis5.7.7. Ovulation and Fertilization5.7.8. The Ovarian Cycle5.7.9. The Uterine Cycle5.7.10. PregnancyAppendix A. GenderAppendix B. The Placebo EffectB.2.1. The Placebo EffectB.2.2. Examples of the Placebo EffectB.2.3. How do Placebos Work?B.2.4. Are Placebos Ethical?B.2.5. How do we validate actual effectiveness of placebosB.2.6. Tips for evaluating scientific evidenceB.2.7. What about Faith Healings

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 (], via Wikimedia Commons  Link:

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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