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 Healingstest chapter

Adrenal Hormones

Image by BYU-Idaho Spring 2015
Image by BYU-I Student Jared C. 2015

The adrenal glands are paired organs that sit atop either kidney. The adrenal gland is composed of two distinct structures, an outer cortex and an inner medulla, both of which produce hormones. The cortex secretes a variety of steroid hormones, including cortisol, aldosterone and androgens. The medulla produces primarily epinephrine (80%) and norepinephrine (20%) and embryologically is an extension of the sympathetic nervous system.

The adrenal cortex produces various hormones, all of which are derived from cholesterol. Collectively these hormones are referred to as adrenal steroids. The adrenal cortex can be subdivided into three zones or layers. Starting from the outside and moving in, these zones are called the zona glomerulosa, the zona fasciculata, and the zona reticularis. The zona glomerulosa produces a number of hormones that are collectively known as mineralocorticoids, the most common is aldosterone. The production of aldosterone is controlled by angiotensin II and/or extracellular potassium. The zona fasciculata produces the glucocorticoids. The most common glucocorticoid in humans is cortisol. The zona reticularis produces androgens such as DHEA and androstenedione, which contribute to the male secondary sex characteristics. (androgens will be discussed in the unit on reproduction).

Cortisol plays a role in mobilization or breakdown of carbohydrates (hence the name glucocorticoid), fats, and proteins. Cortisol production is stimulated by adrenocorticotropic hormone (ACTH) from the anterior pituitary gland. Like growth hormone, it is secreted in a circadian pattern with highest levels occurring in the early morning. Other factors that stimulate cortisol secretion include hypoglycemia and stress. The control of cortisol release by adrenocorticotropic hormone is through a pathway called the hypothalamic-pituitary-adrenal pathway and is categorized as a classical negative feedback pathway. The pathway begins with the release of CRH from the hypothalamus in response to physiological stress or hypoglycemia. CRH enters the pituitary portal system and induces the release of ACTH which acts to increase the synthesis and release of cortisol by cells in the zona fasciculata of the adrenal cortex. Cortisol diffuses out of the cell and is immediately bound by the carrier protein corticosteroid-binding globulin. Cortisol interacts with cytoplasmic receptors on target tissue. Cortisol acts in a negative feedback fashion to decrease ACTH secretion. Cortisol is anabolic in the liver and catabolic towards other tissues. The effects of cortisol are listed below:

  1. Gluconeogenesis: cortisol stimulates glucose production by the liver from other sources such as amino acids or glycerol. Some of this glucose is released into the blood and some of it is stored as glycogen.
  2. Protein catabolism: cortisol stimulates the breakdown of protein to amino acids in skeletal muscle, mainly to provide amino acids for glucose production by the liver.
  3. Lipid catabolism: Cortisol stimulates the breakdown of fat into glycerol and fatty acids. Fatty acids will be used as energy and glycerol as a substrate for the liver. Sounds like a good weight loss idea? Actually, probably not, as cortisol favors the breakdown of fat from arms and legs but deposits fat in the torso.
  4. Immuno-suppressant: Cortisol suppresses inflammation by decreasing antibody production, decreasing white blood cell migration and decreasing pro-inflammatory signaling from immune cells. Hence the logic behind a cortisol injection into an inflamed joint.
  5. Brain function: Cortisol can cause mood changes and memory alterations. As may be evident, cortisol is a hormone that is essential for life; however, abnormal levels can cause problems. Excessive cortisol production results in hypercortisolism whereas low levels of cortisol results in hypocortisolism.


Excessive cortisol release can result from a tumor in the adrenal cortex which would result in excessive cortisol secretion. It could result from a tumor in the pituitary gland that would produce excessive amounts of ACTH or a tumor in the hypothalamus that would produce excessive amounts of CRH. In either event the adrenal gland would secrete excessive amounts of cortisol (Cushing's syndrome). The most common cause of Cushing's syndrome is the over-administration of glucocorticoids prescribed to treat some other condition such as an autoimmune disorder like rheumatoid arthritis or as an immunosuppressant after organ transplant. Looking at the actions of cortisol from above then you might be able to predict what excessive cortisol might do: hyperglycemia (too much glucose), muscle breakdown, fat breakdown and re-deposition in the face and trunk, depression and learning difficulties.

Symptoms of Cushing’s (Hypercortisolism)
By Mikael Häggström (Own work) [CC0], via Wikimedia Commons Link:'s_syndrome_(vector_image).svg


Loss of cortisol production is usually accompanied by a total loss in the function of all the layers of the adrenal cortex. The condition is known as Addison' disease and although rare is usually due to an autoimmune disease that destroys the adrenal cortex. Patients usually suffer from problems not necessarily associated with cortisol like hyperkalemia (excess potassium) or hyponatremia (low sodium) due to the loss of aldosterone. However, other symptoms associated with loss of cortisol may also be present. These include: fatigue, muscle weakness, fever, and difficulty standing up, diarrhea, sweating, mood changes and muscle pains. The adrenal insufficiency can also result in increased ACTH production since the normal negative feedback control is lost. The excess ACTH is associated with increased skin pigmentation due to the cross reactivity of ACTH with receptors for melanocyte stimulating hormone.

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