The endocrine system is composed of several hormone releasing glands located in different parts of the body. Examples of these organs are the hypothalamus, pituitary, pineal, thyroid, parathyroid, adrenal gland, and gonads. They secrete hormones which play a role in maintaining homeostasis (state of equilibrium).
Major Endocrine Glands in the Body
Some organs such as the heart, skin, and kidneys also secrete hormones despite having their own functions with other body systems. The heart secretes atrial natriuretic peptide in response to high blood pressure and regulates it. The skin secretes calcitriol (together with the kidneys and liver) to promote calcium absorption in the intestines, and the kidneys release renin which plays a role in the renin-angiotensin-aldosterone system (RAAS) for blood pressure regulation. RAAS is a mechanism that's activated when our blood pressure is low, facilitating an increase to hopefully supply our major organs with enough blood. These organs like the heart and the kidneys are called secondary endocrine organs because of the role of their hormones.
The endocrine system has so many functions depending on the action of the hormone to target sites. In general, the endocrine system regulates the following:
Reproduction
growth and development
water, electrolyte and nutrient balance
cellular metabolism and energy balance
and it also plays a role in immune defences.
What are hormones?
Hormones are chemical messengers. They are released directly to the bloodstream by endocrine glands. They travel in circulating blood until they reach their target site or target cell. They are called target sites because the cells have specific receptors where particular hormones can bind and initiate a response. Hormones therefore are chemicals that come from different types of chemical class such as amines, peptides, protein, and steroids.
Hormones travelling to target sites
Hormones are secreted in response to “highs” and “lows” in the body to sustain a state of equilibrium. The body needs to maintain a normal range of its electrolyte levels, temperature, blood pressure and many others for our cells and organs to function properly. For example, when the calcium levels in our blood is low, the parathyroid glands release parathyroid hormone to allow the release of calcium in the bone transferring it to the circulatory system thus normalising its level in the blood. Calcium needs to be within normal range for the body’s cells to function properly, and so are the other electrolytes. The endocrine glands therefore are like communication systems responding to different types of stimulus by secreting hormones depending on the body’s need.
How different are hormones to neurotransmitters?
Both hormones and neurotransmitters are chemicals however neurotransmitters are the chemicals released by nerve cells. They travel between synapses or by a synapse and an effector organ like muscles (neuromuscular junctions) and they move through diffusion whereas hormones are chemicals released by ductless glands. These hormones travel to the bloodstream in response to either some highs or lows in the body (e.g. high or low calcium, high or low blood pressure and etc.). Endocrine glands are ductless glands which makes it distinct from other glands such as exocrine glands. Examples of exocrine glands are sweat and mucus glands which contain duct systems.
Synapse and neurotransmitters
in the Neuromuscular Junction
Feedback Mechanisms at a Glance.
The endocrine system works on either the negative or positive feedback mechanism. Feedback mechanisms are mechanisms in our body that respond to changes in homeostatic environment usually promoting a return of stable internal situation. Most of the time, the body works on negative feedback mechanisms to maintain a state of equilibrium however in less common instances, positive feedback mechanisms occur.
Negative feedback mechanism is activated by counteracting the highs or lows in the body. As an example, our pancreas secrete insulin in response to high glucose levels in the blood which then allows glucose to be taken by cells thus then lowering its levels in the blood. This counteracting effect is due to negative feedback mechanism.
Positive feedback mechanism on the other hand is a mechanism that promotes what is already high rather than inhibiting it. It is less common and a good example of this is during childbirth. When the baby stretches the uterine muscles during labour and puts pressure on pudendal nerves, this creates a stimulus causing contractions in the uterine muscles. These contractions stimulate the release of oxytocin from the posterior pituitary gland. Oxytocin then responds by promoting further contraction of the uterus until the baby is out. This is an example of positive feedback mechanism.
The Hypothalamus-Pituitary Axis
The hypothalamus is an endocrine gland as well as a central nervous system organ. It has so many functions such as temperature regulation, maintenance of energy, stress control and many others. A lot of the hormones secreted by the hypothalamus are releasing hormones and inhibiting hormones, which means that the hormones released either promote the release of the pituitary hormones or inhibits it. The hypothalamus works hand in hand with the pituitary gland especially in maintaining homeostasis that is why the hypothalamus is dubbed as the master switch and the pituitary as the master gland.
Located below the hypothalamus is the pituitary gland which contains an anterior lobe and a posterior lobe. The anterior lobe, also called the adenohypophysis, is highly vascular while the posterior lobe contains extensions of neural stalks from the hypothalamus. The posterior lobe is also called neurohypophysis.
The anterior lobe produces its own hormones which are released in response to the trophic (stimulating) or inhibitory hormones of the hypothalamus whereas the posterior lobe gets its hormones from the hypothalamus. The posterior lobe do not produce its own hormones but rather just stores it and releases when needed. The two hormones stored in the posterior lobe are oxytocin and antidiuretic hormone (vasopressin).
The Pineal Gland
The Pineal gland is a pea-shaped small gland located in the brain. It secretes melatonin. This hormone is believed to be involved in the sleep-wake cycle or circadian rhythm. What makes you wake up at the same time each morning even if you forgot to set the alarm is because of melatonin. The pineal gland also secretes other hormones like histamine and dopamine which plays a role in mood regulation.
The Thyroid Gland
The thyroid gland is a butterfly shaped organ located just below the cricoid cartilage and wraps around the trachea. The two large lobes are connected by the isthmus. The glands receive its blood supply from the inferior and superior thyroid arteries. The glands are composed of epithelial cells arranged as hollow vesicles. This is called a follicle, which is the functional unit of the thyroid gland. It is within these follicular cells that the hormones triiodothyronine (T3) and thyroxine (T4) are formed. It is called T3 because it has 3 iodine molecules attached to its chemical structure, and T4 having four iodine molecules. Humans do not produce iodine and the source are usually from diet which can be found in seafood, table salt, and some vegetables.
The thyroid gland also secretes calcitonin in response to high blood calcium levels. Its action is on lowering the calcium levels in the blood by allowing calcium to be absorbed in the bone. Calcitonin is produced by the parafollicular cells of the thyroid gland.
Parathyroid Gland
These 4 small glands are embedded in the thyroid glands. They produce parathyroid hormone which has a role in increasing blood calcium levels. When the calcium in the blood is low, this stimulates the parathyroid gland to release PTH which then promotes the bones to release calcium thus increasing the calcium levels in the blood. Calcium is an electrolyte essential for nerve conduction and muscle contraction. Any increase or decrease will lead to disturbances in nerve and muscle function in which hypocalcaemia (low blood calcium) usually result to a condition called tetany which refers to muscle spasms due to low calcium.
Pancreas
The pancreas is an endocrine organ as well as an exocrine one. Only a small portion of the pancreas is endocrine and the rest are involved in secreting enzymes such as lipases and proteases to aid in the digestion of food.
The endocrine part of the pancreas includes the cells located in the islet of Langerhans. Within these “islets” (because they look like islands on a microscope) are different types of cells called the alpha, beta, and delta cells. The alpha cells secrete glucagon. Its release is stimulated by low blood glucose levels. Glucagon, when released, promotes the conversion of glycogen in the liver to glucose through a process called glycogenolysis. Glucagon also promotes glucogenesis which is the production of glucose from other sources like protein and fat.
The secretion of glucagon by the alpha cells of the pancreas usually happens when our body is in a fasting state. The conversion of adipose tissue to fatty acids produce ketone bodies which can be acidic. Ketones are then used as an energy source for organs like the muscles and the brain especially when the body is in a fasting state since thesy need a continuous supply to function properly.
The beta cells of the pancreas release insulin in response to high blood sugar levels. The action of insulin is to allow glucose to be utilised by the cells for energy, which then results to the lowering of blood glucose levels. Insulin also promotes the conversion of excess glucose in the blood to glycogen which is then stored in the liver. In addition, excess blood glucose also gets stored as fats when insulin is released.
The delta cells produce somatostatin which inhibits the release of insulin and glucagon. It is released when there is a need to inhibit either insulin or glucagon.
Adrenal Glands
The adrenal glands (both right and left) are located on top of each kidney. It contains an outer layer called the adrenal cortex and an inner layer which is the adrenal medulla. The hormones of the adrenal cortex are stimulated by the adrenocorticotropic hormone of the anterior pituitary gland.
The adrenal cortex has 3 parts: an outer zone (zona glomerulosa), middle zone (zona fasciculate), and an inner zone (zona reticularis).
Zona glomerulosa (outer zone of adrenal cortex) – secretes aldosterone which acts on the distal convoluted tubule of the kidneys thus increasing the reabsorption of sodium and water in the body. Aldosterone is secreted when there is low blood pressure. In addition, it is also secreted in response to the renin angiotensin-aldosterone system which plays a role in blood pressure regulation.
Zona fasciculata (middle zone of the adrenal cortex) – This layer secretes glucocorticoids (primarily cortisol), a steroid based hormone. Glucocorticoids are released in response to long-term stress. It functions by increasing blood sugar levels by stimulating the liver to form more glycogen which fill it up. This then causes the glucose to be shunted into the bloodstream causing hyperglycaemia. Glucocorticoids also has other effects in the body which includes the breakdown of triglycerides in the adipose tissues. It also has anti-inflammatory effects because it suppresses the immune system by inhibiting white blood cell production making the person more prone to infection. Chronic stress can lead to exhaustion if not managed. Initially, glucocorticoids will promote the adaptation to stress together with the other adrenal hormones such as adrenaline and noradrenaline. If the stress continues in the long term, this can lead to exhaustion making the person fatigued and tired. This is well represented in Hans Selye’s General Adaptation Syndrome wherein there is that line of resistance that enables adaptation to stress but can lead to a stage of exhaustion when stress is not managed in the long run.
Zona Reticullaris (innermost zone of the adrenal cortex) – This secretes androgens such as testosterone and oestrogen. They play a role in the development of secondary sex characteristics. In women, testosterone also has role in the development of muscle and bone, as well as sexual arousal and libido. Oestrogen in men helps control sex drive and also promotes the production of sperm.
The Adrenal Medulla
The adrenal medulla is located in the inner portion of the adrenal gland. It secretes the catecholamine hormones adrenaline (epinephrine) and noradrenaline (norepinephrine) in response to acute stress, sometimes termed as the fight or flight response. These hormones promote an increase in heart rate, blood pressure and respiratory rate allowing the cells and tissues of the body to utilise more oxygen and energy to adapt to acute stress. The fight and flight response happens when our body senses danger (e.g. responding to a burning building to escape the premises). This response activates the sympathetic nervous system which explains the increase in vital signs like heart rate and blood pressure. Adrenaline and noradrenaline also increases blood glucose by promoting glycogenolysis (breakdown of glycogen stores from the liver) and gluconeogenesis (formation of glucose). This allows more glucose to be utilised by the cells.
The Gonads
These refer to sex cells: testes (for males) and ovaries (for females). These organs are parts of the reproductive system but they also secrete hormones thus they are also endocrine. The testes secretes testosterone which is involved in the development of male secondary sex characteristics (growth of facial hair, increase in muscle size, deepening of voice & etc.) while the ovaries secrete the hormones oestrogen and progesterone. Oestrogen participates in the development of female secondary sex characteristics (broadening of hips, enlargement of breast & etc.) and progesterone is involved in the menstrual cycle.
“Hypers” and “Hypos”
The endocrine system works by maintaining homeostasis. It ensures that our body functions within normal levels of electrolytes, fluid volume, metabolic rate and many others. In some instances, certain pathologies or trauma can take place which may be genetic, or related to tumour growth, surgery, or autoimmune disorders. This can lead to either an increase or decrease of a specific hormone. Signs and symptoms are usually an exaggeration of the hormone function if an increase in hormone production occurs (hyper-) and the opposite may be expected for decreased hormone production (hypo-). For example, in hyperthyroidism, one can expect a fast metabolic rate leading to intolerance to heat while for hypothyroidism, a slow metabolic rate and intolerance to cold may be expected.
Understanding the endocrine system, its glands, the hormones it secretes as well as the function of each of these hormones will assist clinicians in understanding the different disorders of the endocrine system. The endocrine system is a system that works collaboratively with other body systems such as the cardiovascular, respiratory and gastrointestinal. Knowing how these other systems work will help one appreciate why such physiologic reactions can occur.
Disclaimer:
The content in this blog is for informational purposes only and should not be taken as medical advice. It is always best to consult your doctor for medical questions.
If you are a healthcare provider, the content here should not be used to make any diagnosis, give advice or prescribe treatment as this blog is for informational and educational purposes only. Healthcare is an everchanging field and each patient is unique. It is your responsibility as a healthcare provider to always refer to current care standards and practices.
References:
Marieb, E. and Keller, S., 2018. Essentials of Human Anatomy & Physiology. 12th ed. Pearson Education Limited
Germann, W. and Stanfield, C., (2005). Principles of Human Physiology: Second Edition. San Francisco: Pearson Education, Inc.
https://www.hopkinsmedicine.org/health/conditions-and-diseases/hormones-and-the-endocrine-system
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