What Does The Endocrine System Do?
The system that produces hormones and regulates homeostasis is sometimes overlooked, but its importance cannot be overstated
The primary function of the endocrine system is to allow for rapid or long-term change within the body (therefore cells are able to “talk” with each other in a coordinated fashion). The textbook definition of the endocrine system is “acting over great distances”.
Hormones (messengers) are produced by glands or neurosecretory cells of the hypothalamus. The overlap of the endocrine and nervous systems is particularly exemplified by the chemicals epinephrine and norepinephrine which are produced by the sympathetic nervous system and the adrenal medulla.
Since the effects of hormones are amplified within a cell, only small quantities of any hormone need to be present to exert their effects. Hormones do not seek out their target cells; rather the target cells await the arrival of the hormone.
The 2 Basic Categories of Hormones
Nonsteroid (protein or peptide) Hormones: These can be an amino acid, a peptide, or a protein composed of 1 or more polypeptide complexes.
Target cells have receptor proteins on their plasma membranes that specifically bind non-steroid hormones (similar to the lock-and-key theory for enzymes). The non-steroid hormones are usually referred to as the “first messenger” because they trigger a cascade reaction within the target cell by activating a secondary messenger which carries out the rest of the response (e.g. usually cyclic AMP (cAMP) or Ca2+).
Steroid Hormones: Has the same 4 carbon fused rings, but each hormone has different side chains and groups.
Once the steroid hormone crosses the target cell’s plasma membrane, it binds to a specific hormone-receptor protein. The hormone-receptor complex then binds to the target cell’s DNA which activates particular genes—these genes produce the mRNA strands that code for the proteins that are required by the cell and the body.
Types of Glands in The Endocrine System
Endocrine Glands
These are “ductless” and secrete their hormones directly into the bloodstream for distribution throughout the body.
Exocrine Glands
These have ducts and secrete their products into these ducts for transport to certain body cavities (e.g. salivary glands which produce saliva that is deposited into the mouth).
Types of Hormonal Feedback
Like the nervous system, the endocrine system is important for maintaining homeostasis within the body. A hormone’s production is usually controlled in one of 2 ways:
Negative Feedback
Example: When the blood glucose levels are high, insulin is produced by the pancreas. Insulin causes cells to take up the excess glucose, thereby diminishing the signal of high glucose in the blood. Consequently, the pancreas produces less insulin.
Contrary Hormonal Actions
Many of the hormones in our bodies have a hormone that operates in the opposite manner to its function. For example, insulin is produced when blood glucose levels are high but glucagon is also produced by the pancreas when blood glucose levels are low, as exemplified by figure 5.
Hypothalamus and The Pituitary Gland
The hypothalamus controls and regulates the internal environment of the body (e.g. heart rate, body temp., water balance …) through the autonomic nervous system. It also controls the function of the pituitary gland through a stalk-like projection. It is important to realize that the pituitary gland is composed of 2 separate sections, the posterior pituitary (post. pit.) and the anterior pituitary (ant. pit.). Since the pituitary gland is responsible for producing so many hormones, it is often referred to as the “master gland”.
Posterior Pituitary
The neurosecretory neurons in the hypothalamus produce 2 hormones which are released through the posterior pituitary gland.
Antidiuretic Hormone (ADH) a.k.a. Vasopressin: whose target cells are the collecting tubules of the nephron. ADH’s function is to promote the reabsorption of water. The osmolarity of the blood determines if ADH should or should not be released. Remember that it is the hypothalamus that monitors the blood’s osmolarity.
Oxytocin: has 2 target cells. Firstly, it acts on the uterus which causes the uterine smooth muscles to contract during childbirth. Secondly, oxytocin stimulates the mammary glands to release breast milk when the baby is nursing.
Anterior Pituitary
The anterior pituitary gland is separated from the hypothalamus by a portal system consisting of 2 capillary beds joined by a vein. The hypothalamus “communicates” with the ant. pit. by producing hypothalamic-releasing hormones and hypothalamic-inhibiting hormones. Three of the six hormones produced by the ant. pit. control the release of hormones from other glands:
Thyroid-stimulating hormone (TSH): Promotes the release of the hormone thyroxine from the thyroid gland.
Adrenocorticotropic hormone (ACTH): Promotes the release of cortisol from the adrenal cortex.
Gonadotropic hormones (FSH and LH): Stimulate the gonads to produce their respective sex hormones (testes — testosterone / ovaries — estrogen and progesterone).
The other three are:
Prolactin (PRL): is only produced in females who are nursing and causes the breasts to develop and produce milk.
Melanocyte-stimulating hormone (MSH): causes skin pigments to form.
Growth hormone (GH): promotes skeletal and muscle growth.
Thyroid and Parathyroid Glands
The thyroid gland is the largest in the neck and is attached to the trachea. The parathyroid glands are embedded in the thyroid gland itself. The function of the thyroid gland is to produce thyroxine. Iodine is an essential element in the production of the thyroxine; an absence of iodine in a person’s diet will result in goiter.
What is Thyroxine?
Thyroxine’s function is to increase an individual’s metabolic rate. The hormone does not have a specific target organ; it stimulates all organs of the body to metabolize at a higher rate which causes more glucose to be broken down so that more energy can be utilized.
Calcitonin
Another hormone produced by the thyroid gland is calcitonin. Calcitonin is one of the methods the body uses to maintain calcium ion (Ca2+) homeostasis in the body. Its target cells are the osteoblasts in the bone which take up and deposit the excess Ca2+ (remember that calcium ions are important in neural conduction, muscle contraction, and blood clotting).
Parathyroid Hormone (PTH)
The function of the parathyroid is to produce parathyroid hormone (PTH) which is released when blood Ca2+ levels are low. PTH increases osteoclast activity, which breaks down bone to release the Ca2+ ions into the blood. PTH also causes the kidneys to reabsorb more Ca2+ by activating Vitamin D. Vitamin D then causes the small intestine to absorb more calcium ions.
The Adrenal Glands
The adrenal glands consist of 2 portions: the inner portion called the adrenal medulla and the outer portion known as the adrenal cortex. These structures, like the pituitary, have no physiological connection with one another and are controlled by the hypothalamus. The anterior pituitary gland secretes ACTH which, in turn, causes the adrenal cortex to release cortisol during times of “stress” (e.g. physical trauma, emotional duress, etc). Cortisol breaks down proteins into amino acids which are then converted by the liver to glucose molecules used for energy production. In addition to this, cortisol breaks down fatty acids into usable sugars for the body.
Epinephrine (adrenaline) and norepinephrine produced by the adrenal medulla are also secreted when the body needs to rapidly react to stressful situations. The adrenal cortex hormones, in contrast to the hormones produced by the adrenal medulla, provide a sustained response to the stress. These hormones include glucocorticoids (cortisol) and mineralocorticoids. The adrenal cortex also secretes trace amounts of both sex hormones (estrogen and testosterone) in both males and females.
Mineralocorticoids
The most important mineralocorticoid produced by the adrenal cortex is the hormone aldosterone. The kidney (distal convoluted tubules) is affected by aldosterone which causes it to reabsorb more Na+ and excrete more K+ to help increase the body’s blood pressure. Aldosterone also works in conjunction with the hormone renin which is produced by the kidneys to help raise the body’s blood pressure. It is important to mention that these 2 hormones work independently from the pituitary.
Pancreas
The pancreas has both exocrine and endocrine gland functions. The exocrine functions produce digestive enzymes and juices for the digestive system; the endocrine tissues are called the pancreatic islets (Islets of Langerhans). The hormones these structures produce are:
Insulin: is secreted when blood glucose levels are high, usually after a meal. Insulin stimulates the uptake of glucose primarily in the liver and muscle cells where it is converted to the storage form of glucose called glycogen. Excess glucose is converted into triglycerides by the adipose cells.
Glucagon: is usually secreted between meals when blood glucose levels are lower and stimulates the liver to break down its glycogen stores. The hormone also causes the adipose cells to break down triglycerides into glycerol and fatty acids which are converted by the liver to glucose.
Testes and Ovaries
The gonads in males are known as the testes and produce androgenic hormones (e.g. testosterone). The gonads in females are known as the ovaries and they produce estrogen and progesterone. Both the testes and the ovaries are controlled by secretions from the hypothalamus and the anterior pituitary gland (e.g. gonadotropic hormones).
Testosterone
Has many functions in the body. Testosterone is essential for the normal functioning of the sex organs in males and for the maturation of sperm. It is produced more during adolescence which causes many of the secondary sexual characteristics (e.g. testes maturation, beard growth, and lowering of voice). Muscular development is another one of these characteristics and testosterone derivatives are used by some athletes to enhance their performance—these types of drugs are known as anabolic steroids.
Estrogen and Progesterone
Also have many functions in the body; they are primarily responsible for uterus development and egg maturation. Like testosterone in males, estrogen and progesterone are produced more during adolescence and are associated with many of the secondary sexual characteristics found in females (e.g. widening of pelvic girdle, breast development, and fat deposition). These hormones are also responsible for the monthly menstrual/uterine cycles.
Thank you for reading! I hope you learned about the basics of the endocrine system, the hormones it produces, and how these hormones influence all aspects of our physiology. Make sure to like, comment your thoughts, and subscribe to Science For Humanity! ❤️
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