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Endocrine System Anatomy & Physiology

Endocrine System Anatomy & Physiology

Nestled within our bodies is an intricate network of glands that house our hormones and play a crucial role in maintaining harmony and balance. This network of hormone secreting glands is known as the Endocrine, or hormone system, and its influence on our wellbeing is held in a delicate balance of feedback loops, cascades, and body system inputs. 

Endocrine system function may not be on your radar. This system, as important as it is, does not get as much attention as immunity, digestion, or respiration. This is because imbalances within this system are not as easily recognizable, and they tend to creep up on us, unless of course you happen to be raising a teenager. Besides puberty, menopause, and PMS it is not often that we can feel our hormones “raging.”  Hormones act more slowly than neurotransmitters giving us more time to acclimatize to changes over time. Issues with this system can often be vague and difficult to define.  

The endocrine system is a network of glands throughout the body that secrete hormones into the bloodstream. This chemical messaging system orchestrates crucial functions such as metabolism, growth and development, and reproduction. There are many factors that can throw the endocrine system and our hormones out of balance. Increased cortisol (a stress hormone) is a major contributor, as well as diet, environmental toxins, medications, sleep quality, life stages, and genetics.   

Imbalance in steroid hormones such as testosterone, estrogen, and progesterone can lead to irregular periods, mood swings, low energy, or reduced sex drive. Thyroid imbalances can cause fatigue, weight & mood fluctuations and difficulty sleeping. Pancreatic imbalances show up as fluctuations in glucose levels, and insulin sensitivity.   

We know how common hormone imbalances can be, and we want to give you the tools you need to better understand how this complex system works.  

 

Anatomy of the Endocrine System

 

The Endocrine Glands 

Hypothalamus: The hypothalamus links the endocrine and nervous systems together and drives the endocrine system through neural and hormonal commands sent to the pituitary.  This gland maintains homeostasis, the HPA axis (stress response), body temperature, and Circadian rhythms, and participates in emotional and physical behaviors (thirst and hunger).  

Pituitary gland: The pituitary gland receives signals from the hypothalamus and is often called the “master gland”. This gland has two lobes, the posterior and anterior lobes. The posterior pituitary is made up of neural cells and produces two hormones: antidiuretic hormone and oxytocin. The anterior pituitary is comprised of endocrine cells and produces six hormones and affects bone and soft tissue, adrenal glands, thyroid gland, testes/ovaries, corpus luteum, and the breasts. They are all stimulating hormones. Hormones of the Pituitary:  

  • Growth hormone (GH): A metabolic hormone that affects growth of skeletal muscle and the long bones of the body. It stimulates target cells to grow and divide.  
  • Prolactin (PRL): Structurally similar to GH it is only known to target breast tissue by stimulating milk production after childbirth and throughout breastfeeding.  
  • Adrenocorticotropic hormone (ACTH): Regulates the endocrine activity of the adrenal cortex.  
  • Thyroid-stimulating hormone (TSH): Controls the growth and activity of the thyroid gland.  
  • Gonadotropic hormones: Regulates the activity of the gonads (ovaries and testes).  
  • Follicle-Stimulating hormone (FSH): Stimulates the development of the follicles in the ovaries; triggering them to produce estrogen and ready eggs for ovulation. In men FSH stimulates the development of sperm in the testes.  
  • Luteinizing hormone (LH): Triggers ovulation and the production of progesterone and small amounts of estrogen from the follicle. In men LH stimulates testosterone production by cells within the testes. 

Pineal gland: The pineal gland is a small pinecone-shaped gland in the brain that produces Melatonin. Large amounts of melatonin are produced at night making us drowsy and production decreases during daylight hours. Melatonin plays a vital role in our circadian rhythms 

Thyroid gland: The thyroid gland is located at the base of the throat just below the Adam's apple and is comprised of two lobes. It is critical to the healthy development and maturation of vertebrates and regulates metabolism. Thyroid hormone is two hormones: thyroxine (T4) and triiodothyronine (T3). Thyroid hormone controls the rate at which glucose is converted to energy in the body and controls normal tissue growth and development. A third hormone, calcitonin, is produced by the parafollicular cells in the connective tissues between the follicles (where the other two hormones are produced). Calcitonin controls calcium deposition in the bones, therefore decreasing blood calcium levels.  

Parathyroid glands: The parathyroid glands are tiny glands located on the back surface of the thyroid gland. They produce parathyroid hormone (PTH) which maintains proper blood calcium levels. It works in contrast to calcitonin as it causes increased levels of calcium to stay in the blood. It also stimulates the kidneys and intestines to absorb calcium.  

Thymus Gland: Located behind the sternum, the thymus gland is large in infants and decreases in size and activity with age. The thymus gland produces thymosin and other hormones important in the development of white blood cells (T-lymphocytes) part of the immune response.  

Adrenal glands: The adrenal glands are made up of two parts: the cortex and medulla. The adrenal cortex is made of endocrine tissue and the adrenal medulla is nervous system tissue. The adrenal cortex produces three groups of steroid hormones called corticosteroids: mineralocorticoids, glucocorticoids, and sex hormones. Mineralocorticoids (mainly aldosterone) control mineral levels in the blood, especially sodium and potassium.  

Glucocorticoids including cortisone and cortisol maintain normal cell metabolism and aid the body’s response to long term stressors. They can also reduce the pain response by inhibiting pain producing molecules called prostaglandins. The adrenal cortex also produces male and female sex hormones in small amounts throughout life. The adrenal medulla produces Catecholamines (epinephrine and norepinephrine) which increase heart rate, blood pressure, and blood glucose levels while stimulating the opening of lung passages. Catecholamines prepare the body for short term stress events with the “fight or flight” response.  

Pancreas: The endocrine portion of the pancreas are called the islets of Langerhans, and they can be found scattered throughout the exocrine tissue of the pancreas. The islets of Langerhans are comprised of alpha and beta cells which produce glucagon and insulin, respectively. The alpha cells are stimulated to produce glucagon when blood glucose levels are low. Beta cells produce insulin when blood glucose levels are high. Insulin causes cells to allow more glucose to pass across the cell membranes, reducing blood glucose levels, whereas glucagon acts as an antagonist to insulin. Glucagon stimulates the liver to break down stored glycogen into glucose and release it into the blood stream when blood glucose levels are low.  

Gonads: The female gonads (ovaries) are located within the region of the pelvis and the male gonads (testes) are outside the pelvis in a scrotal sac. They produce steroid hormones that affect growth and regulate reproductive cycles and behaviors. The major categories of gonadal steroids are androgens, estrogens, and progestins, all of which are found in both males and females but in differing amounts. Estrogens and progesterone stimulate development of female sex characteristics at puberty including breast development and changes in the uterine lining (menstrual cycle). Progesterone takes over as the dominant sex hormone during pregnancy and prevents the uterine lining from being shed while the baby develops and prepares the breast tissue for lactation. The male gonads produce androgens (mostly testosterone) which stimulate the maturation of male sexual organs for reproduction, the development of secondary sexual characteristics, continuous production of sperm, and the male sex drive.  

Hormone-Producing Tissues/Organs 

Hormones are produced in fatty tissues in several places in the body and in the walls of the kidneys, heart, stomach, and small intestine besides the endocrine glands.  

The placenta is a hormone producing organ that forms within the uterus during pregnancy. This organ acts as a source for respiration, nutrition, and excretion for the developing fetus and produces steroid hormones that maintain a pregnancy and prepare for delivery. It produces human chorionic gonadotropin (hCG) that keeps the ovaries producing progesterone and estrogen to stop the uterine wall from being shed (menses) during pregnancy. The placenta also produces human placental lactogen (hPL) which works with progesterone and estrogen to prepare the breast tissue for lactation. The final hormone produced by the placenta is Relaxin. This hormone relaxes the joints in a pregnant person's body so that the pelvic ligaments and pelvic symphysis can more easily relax during birth.  

Endocrine System Physiology

The endocrine system controls reproduction, growth and development, maintaining a balance of electrolytes, water, and nutrients in the blood, defending the body against stress, and controlling metabolism, and energy levels. It essentially has an important role in every aspect of the body.  

All this control comes from hormones produced by ductless glands directly into the bloodstream. These hormones are either amino acid molecules (proteins, peptides, and amines) or steroid molecules (made from cholesterol).  

How Hormones Act in the Body 

You might wonder how hormones can be floating everywhere in the bloodstream but only act on specific cells or tissues. Hormones bind with proteins on or in target cells called “binding sites”. If a cell does not have a site where a specific hormone can bind, that cell will not react to the hormone.  

Hormones mostly act on cells to increase or decrease their existing activity or metabolic action rather than creating new actions. They primarily stimulate mitosis, increase/decrease secretions, activate/deactivate enzymes, change the permeability of cell membranes, or activate production of proteins or enzymes within cells.  

Hormone Release  

Glands are triggered to release hormones into the bloodstream by negative feedback loops or endocrine gland stimuli. Negative feedback loops are the most common method that hormone release is regulated. Endocrine gland stimuli can be neural, hormonal, humoral. Hormonal stimuli are the most common. Endocrine glands are triggered to release their hormones by other hormones from other glands. Occasionally nerves will trigger hormone release; like in the stress response of the adrenals. Humoral stimuli occur when blood levels of certain nutrients or ions trigger the release of hormones.  

The HPA Axis & The Stress Response 

The HPA axis is When the brain perceives a threat or acute stressor, the hypothalamus releases a hormone called corticotropin-releasing hormone (CRH), which signals the pituitary gland to release adrenocorticotropic hormone (ACTH). ACTH then travels through the bloodstream to the adrenal glands, prompting them to release cortisol, a stress hormone. This hormone cascade is known as the sympathetic nervous system “fight or flight” response. Once stress levels decrease, the HPA axis feedback loop signals the system to reduce cortisol production and return to a state of balance and calm.   

“Rest and digest” is the alternative to fight or flight. Both responses are divisions of the autonomic nervous system known as the parasympathetic (rest & digest) and sympathetic (fight or flight) divisions. Ideally, we spend most of our day in our parasympathetic rest and digest response hanging out, unphased by life.   

It is normal to trigger a fight or flight response; it is a defense mechanism designed to keep us safe; driving is a common trigger, as is being surprised or startled. The fight or flight response causes vasoconstriction, which means blood vessels constrict to prioritize blood flow to essential organs like the heart and muscles. The eyes dilate, the pulse and breath quicken, and sweat increases. Because blood flow prioritizes the muscles and heart, critical body systems like the immune, urinary and digestive systems are all suppressed when in fight or flight.  

Chronic or prolonged exposure to experiences that trigger the fight or flight response means a constant suppression of immune, digestive and kidney function, leading to many of the common discomforts associated with these body systems. Also, look for signs of adrenal fatigue, characterized by low energy, difficulty coping with stress, sleep disturbances, cravings for salty or sugary foods, dizziness or lightheadedness, weight fluctuations, and mood changes.   

Sex Hormones

The hypothalamus-pituitary-gonadal (HPG) axis orchestrates the production and regulation of sex hormones. In response to signals from the hypothalamus, the pituitary gland releases luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which stimulate the gonads (testes in males, ovaries in females) to produce sex hormones—primarily testosterone in males and estrogen and progesterone in females.  

The stress response can significantly impact sex hormone balance. In times of chronic stress, the body may prioritize the production of stress-related hormones like cortisol over sex hormones. Elevated cortisol levels due to the overactive stress response can suppress the production of LH and FSH, which in turn affects the secretion of sex hormones. This disruption can lead to imbalances such as low testosterone in males and disrupted menstrual cycles and irregular ovulation in females.  

Thyroid Imbalance 

Thyroid hormone governs metabolism, energy regulation, and overall growth and development. The thyroid gland produces two primary hormones: thyroxine (T4) and triiodothyronine (T3). The hypothalamus-pituitary-thyroid (HPT) axis orchestrates the release and conversion of these hormones.   

The stress response can profoundly impact thyroid function. During times of prolonged stress, the body's increased production of cortisol can inhibit the enzyme responsible for the conversion of T4 to T3, leading to a scenario where T4 is not efficiently converted into its active form, T3. This disruption can result in a condition known as "low T3 syndrome," characterized by decreased levels of active thyroid hormone and potential symptoms of fatigue, weight gain, and cognitive impairment.

The Endocrine System’s Role in Detoxification 

The endocrine system plays a crucial role in the intricate process of detoxification within the human body. While often associated with hormone regulation, this system also significantly influences the body's ability to eliminate toxins and maintain overall health. The liver, a key player in detoxification, is heavily influenced by the endocrine system. Hormones released by various endocrine glands help orchestrate the liver's detoxification pathways, enabling the breakdown and elimination of harmful substances.  

Disruptions in hormonal equilibrium, often caused by factors like prolonged stress, can hinder the body's detoxification processes. Specifically, the stress hormone cortisol can interfere with the liver's detoxification functions, altering the normal breakdown and clearance of toxins. The impact of an imbalanced endocrine system on overall detoxification exemplifies Doctor Morse’s message: revitalize all body systems, including the endocrine system, to ensure the proper removal of toxins and harmful acidosis from the body.   

 

*FDA warning: This statement has not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. 

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