Imagine you have a presentation to give in the next one hour. You find your heart pounding, hands sweating and your body trembling out of fear. So you either find yourself resisting the situation. Or you get the energy to simply fight back and deliver a great presentation. Have you ever wondered what processes are at play for such responses?
Well, such responses are a result of a number of organs releasing chemical messengers called hormones directly into your bloodstream, each performing a specific function. These hormones are responsible for regulating various metabolic activities as well as fight or flight responses.
Now, such responses are necessary to protect you against danger and stressful situations such as trauma, work pressure, surgery etc.
However, exposure to such stress hormones for long periods of time can cause hormonal imbalance, leading to serious disorders.
Thus, glands and their hormones together form part of the endocrine system. This system plays a pivotal role during conditions that demand extensive as well as long lasting regulatory action. Thus, the Endocrine System harmonizes with Nervous System for the body to function properly.
The impulses originating from the nervous system can impact the release of hormones from the endocrine glands in the human body. These hormones in turn play a key role in regulating the growth, development and metabolism of the body.
Therefore, to understand the mechanism of glands and their hormones, we need to know what is endocrine system, various endocrine glands and the hormones released by such glands.
What is Endocrine System?
It is a regulatory system of glands that secrete hormones within specific organs. These hormones are chemical messengers that are released directly into the bloodstream by endocrine glands. Further, the chemical messages are carried by the hormones to different cells in the body that interpret such messages and act accordingly.
Now, the term ‘endocrine’ means the hormones produced by glands are released into the bloodstream in return of the impulses received. The term ‘exocrine’, on the other hand, means the release of secretions via specific glands outside the body. For instance sweat, saliva etc
Let’s now have a look at the different types of endocrine glands and the hormones released by each of them.
Endocrine Glands and Hormones
Hypothalamus is a tiny structure located within the brain that regulates Autonomic Nervous System. Thus, it influences bodily functions including:
- eating and drinking
- blood pressure and heart rate
- sexual functions and behaviour
- body temperature
- sleep wake cycle and
- emotional conditions like fear, anger, pleasure, pain etc.
Since this small structure is a part of the Central Nervous System, the Hypothalamic hormones that it releases are produced by neurons. Furthermore, the amount of hypothalamic hormones released is regulated by the signals received from the other neurons. Therefore we can say that hypothalamus acts as a major link between the nervous and endocrine systems.
This can be explained with the help of an example.
Whenever we as humans are exposed to signals from external environment, various parts of the brain receive such information and send it to hypothalamus. Therefore, the functions performed by hypothalamus gets impacted both by external and internal environment as well as the hormonal activity of other glands.
So, the impulses from the external environment that influence the function of the hypothalamus include temperature, intensity of light, the behavior of other individuals and various other visual, olfactory, auditory and sensory impulses.
On the other hand, the information about the internal environment of the human body gets transmitted from other areas of the brain to the hypothalamus through neurotransmitters.
Thus, the activity of the neurotransmitters controls the release of hormones from the hypothalamus.
Remember, hypothalamic hormones are generally releasing or inhibiting in nature. This is because they typically promote or inhibit the secretion of hormones from the pituitary gland.
Following are the major releasing and inhibiting hypothalamic hormones:
- Corticotropin Releasing Hormone (CRH)
- Gonadotropin Releasing Hormone (GnRH)
- Thyrotropin Releasing Hormone (TRH)
- Growth Hormone Releasing Hormone (GHRH)
2. Pituitary Gland
Pituitary gland is a small marble sized gland located in the brain right below the hypothalamus. This gland consists of two parts:
(i) Anterior Pituitary Gland
The anterior pituitary gland releases hormones that affect either of the following two:
- target glands like adrenal glands, gonads or thyroid gland to target gland hormones and
- target organs
Further, the hormones released by the anterior pituitary gland include:
(a) Adrenocorticotropic Hormone (ACTH)
ACTH acts on the adrenal cortex (outer layer of the adrenal gland) to release cortisol and small amount of male and female sex hormones.
(b) Gonadotropins like Luteinizing Hormone and Follicle Stimulating Hormone (FSH)
Gonadotropins comprise of two molecules that include Luteinizing Hormone(LH) and Follicle-Stimulating Hormone (FSH). These two hormones control the release of male and female sex hormones in ovaries and testes respectively. Further, these two hormones also regulate the production of egg cells and sperm cells.
(d) Thyroid Stimulating Hormone (TSH)
TSH stimulates the thyroid gland which in turn releases thyroid hormone.
(e) Growth Hormone (GH)
The Growth Hormone (GH) is produced in ample quantity in the human body. This hormone plays a great role in regulating growth and development of a human body. For instance, it stimulates the growth of internal organs, muscles, fat tissues, endocrine glands, development of reproductive organs etc.
Thus, the quantity of GH is the highest during childhood and puberty. Whereas, GH levels decline as an individual matures. However, small amount of GH levels are important for the functioning of the human body even during later life.
In addition to this, the GH also regulates carbohydrates, protein and fat metabolism.
Prolactin is a hormone that promotes milk production in females after childbirth as a result of suckling by the young one. In addition to this, the prolactin hormone also plays a central role in other areas like reproductive, metabolic, regulation of immune system and behavioural functions.
The release of prolactin is regulated by a hormone called dopamine produced by the hypothalamus. Thus, dopamine reduces the release of prolactin. This means more the level of dopamine, lesser the production of prolactin.
Furthermore, prolactin itself promotes the release of dopamine thus creating a negative feedback loop. Accordingly, increased prolactin passes impulses to hypothalamus, which further releases dopamine. This release of dopamine results in reduction of prolactin release.
Oestrogen is another hormone that regulates the production of Prolactin. This hormone increases the secretion of prolactin from the pituitary gland. Thus, during pregnancy, increased amount of prolactin is released as a result of increased levels of oestrogen in the blood of pregnant women.
Also during menstruation cycle in women,the level of prolactin in blood increases as oestrogen level is the highest during this period.
(ii) Posterior Pituitary Gland
Posterior pituitary gland is a large collection of axonal projections of neurons located in the hypothalamus. This gland does not produce hormones of its own. Rather, it stores and releases two hormones vasopressin and oxytocin that are produced in the hypothalamus.
Vasopressin is also known as Arginine Vasopressin (AVP). This hormone helps in maintaining water and electrolyte levels in the body. Therefore, the release of AVP hormone helps in conserving water in the body by reabsorbing it from urine in kidneys. Also, the level of sodium in blood as well as blood volume and blood pressure control AVP release from the pituitary gland. This means AVP release is reduced as a result of high blood pressure or increased blood volume.This leads to more release of water in the form of urine leading to lower blood volume and blood pressure.
Oxytocin is a hormone that is produced in the hypothalamus but gets stored in the pituitary gland. One of the major functions of oxytocin is that it promotes contraction of the uterus during childbirth. Thus, the release of oxytocin hormone triggers the uterine muscles to contract to help in childbirth.
In addition to this, oxytocin also promotes lactation. Therefore, this hormone gets released as the baby sucks the nipples which enables the movement of milk , thus letting it come out. This hormone is regulated by the release of oxytocin itself.
For instance, when the uterus starts contracting, oxytocin hormone is released. The release of this hormone further stimulates uterus contraction. As the uterus contracts more and more, more and more of oxytocin is released. However, the release of this hormone stops once the baby is delivered or the baby stops feeding.
3. Adrenal Glands
Adrenal glands are small structures that are located right on top of each of the kidneys. These glands consist of an outer layer known as the ‘Adrenal Cortex’ and an inner layer called and the ‘Medulla’. So, the adrenal cortex releases a number of hormones that are typically referred toas the ‘corticosteroids’.
These ‘corticosteroids’ predominantly include glucocorticoids and mineralocorticoids. Furthermore, it also releases a small amount of sex hormones known as adrenal androgens. But, sex hormones released are very small in number as compared to ones produced by sex organs like testes and the ovaries.
On the other hand the inner adrenal medulla releases two substances, that is, adrenaline and non – adrenaline. These hormones are responsible for fight or flight response during stress.
Now, the predominant glucocorticoid in human body is cortisol. This hormone is responsible for regulating carbohydrate, protein and lipid metabolism. For instance, cortisol increases the amount of glucose level in the blood by triggering a process called gluconeogenesis in the liver.
As the name suggests, gluconeogenesis means release of new glucose molecules in the blood when the body is in a fasting (non – eating) state. Furthermore, cortisol also promotes the formation of a molecule called glycogen in the liver. It is a molecule that stores glucose to help maintain glucose level in the blood when the body is in a fasting state.
Apart from regulating the metabolic activities, cortisol protects the body from dangerous effects of stress factors like injury, emotional stress, surgery, etc .
The predominant mineralocorticoid in humans is aldosterone. This hormone plays a key role in regulating the level of salt and water in the human body.
The main function of this hormone is to conserve sodium and water levels in the blood and excrete potassium through urine from the body. Thus, aldosterone encourages reabsorption of sodium in the kidney along with water, thus leading to less water excretion and increased blood volume.
The gonads include ovaries and testes. These glands perform two important functions. Firstly, ovaries releases ova and testes release spermatozoa . Secondly, the gonads harmonize steroid sex hormones. These hormones are important for the development and functioning of male and female reproductive organs as well as sex characteristics like facial hair in men, lactation, child birth and preparation of pregnancy in women. Furthermore, there are three types of sex hormones including:
Estradiol is a major estrogen that is produced in ovaries in addition to estrone and estriol. Besides ovaries, estrogens are produced in corpus luteum, placenta and the adrenal glands. Here, corpus luteum refers to a collection of cells that is obtained from the follicle which produces ovum during a particular menstrual cycle.
In the case of men and post menopausal women, estrogens are obtained when adrenal, ovarian and testicular androgens undergo conversion.
Thus, the primary function of the estrogen is to regulate the development and functioning of the female genitalia and breasts.
Then, during puberty these estrogens help in:
- the growth of uterus, breasts and vagina
- fat accumulation and distribution in the body, thus giving shape to the female body
- halting the height of an adult, regulating the development of secondary sexual characteristics
Lastly, in the adult women estrogens:
- regulate the menstrual cycle
- help in regulating hormones during pregnancy and lactation and
- maintaining the female libido
These are the hormones released by the ovaries during a certain stage of the menstrual cycle. Furthermore, these hormones are also released in the placenta during most of the pregnancy. Furthermore, progestogens encourage the development of the mammary glands in the female breasts in order to prepare them for lactation. Alos, these hormones bring about the changes in the uterine lining to prepare women for pregnancy. The main progestogen is progesterone. It is a hormone that is secreted by the corpus luteum in the ovaries during the menstrual cycle.
The primary androgenic steroid is testosterone. These hormones are released mainly by the testes. However, small amounts of testosterone is also released from the adrenal glands in both men and women and from the ovaries.
The major function of this hormone is growth and development of male genitalia. In addition to this, the testosterone also helps in protein that leads to increased muscle mass in the body.
Now, the specific functions of testosterone depend upon the various stages in human life.
- This hormone promotes the development of internal and external male genitalia in case case of a fetus.
- Then, during puberty testosterone helps in the growth of the male sex prgans and secondary male characteristics like surge in pubertal growth and terminating growth and adult height, growth of facial, pubic and body hair,deepening of voice etc.
- Finally, in adult males, testosterone helps in maintaining masculinity, libido, sexual potency and production of sperms.
5. Thyroid Gland
The thyroid gland consists of two lobes. One is located right in front of the windpipe, known as trachea. The other one is located below the voice box, known as the larynx.
Now, the thyroid gland releases two hormones, namely thyroxine (T4) and triiodothyronine (T3). Together these hormones are known as thyroid hormones. Furthermore, 90% of the thyroid hormone released in the thyroid gland constitutes T4.
Whereas, T3 is an active hormone that is produced as a result of T4 getting converted into T3 in the liver and kidneys. So, the major function of the thyroid hormone is to increase the metabolism.
For instance the thyroid hormone activates the production of specific proteins in the body that help in generating heat. This is an essential function as the release of such proteins help in maintaining body temperature during cold climates. In addition to this, thyroid hormone also promotes other metabolic activities that involve carbohydrates, proteins and lipids. These are essential for generating energy or proper functioning of the human body. Also, thyroid hormone performs the following functions:
- normal bone growth and maturation
- normal development of teeth, skin and hair follicles
- proper functioning of nervous, cardiovascular and gastrointestinal systems
6. Parathyroid Glands
These are four small sized glands located right behind the thyroid gland. THese glands produce parathyroid hormone (PTH). The major role of these hormones is to increase the level of calcium in the blood, thereby helping to maintain the quality of bones and sufficient supply of calcium in the body.
In particular, PTH lead to reabsorbing calcium from urine and release of phosphate in urine. Also, PTH encourages the release of calcium from bones as well as bone resorption. Both these lead to increase in the level of calcium in the blood. Finally, PTH also encourages the absorption of calcium from food.
So, as far as the regulation of PTH is concerned, the release of these hormones is not controlled by the pituitary hormones. Rather, it the calcium level in the body that regulates PTH. Therefore, low level of calcium leads to stimulating the PTH release. Whereas high PTH level inhibit the same.
Pancreas is a gland that is situated in the abdomen right behind the stomach. This gland performs two primary functions. Firstly, the pancreatic cells produce various digestive enzymes that are released in the gut. These enzymes help in digesting the food effectively. Thus, here the pancreas act as the exocrine gland.
The second function that the pancreas performs includes the releasing of two hormones – insulin and glucagon. These hormones are released into the blood and play an important role in regulating glucose levels in the blood. Thus, here the pancreas act as an endocrine gland.
Insulin is the only hormone that lowers the level of glucose in the blood. However, it promotes the formation of molecules (like glycogens, proteins and lipids) that store energy and reduces the break down of those stored nutrients.
Thus, insulin targets those organs that perform the function of storing energy. These organs include liver, muscles and adipose tissue. Following are the metabolic effects caused by insulin:
- Insulin promotes glucose uptake into cells and converts it into glycogen;
- Helps in breaking down glucose and inhibits gluconeogenesis
- Also, it promotes the transmission of amino acids into cells, synthesizes protein in muscle cells. Thus, reducing the amount of amino acids available for gluconeogenesis in liver.
- Furthermore, insulin increases the activity of synthesizing fat in liver and adipose tissue. Thus reducing glycerol and inhibiting gluconeogenesis
Hence, there are various factors that help in regulating the secretion of insulin. These include blood glucose levels, glucagon and hormones like GH, glucocorticoids and thyroid hormones.
Glucagon is a pancreatic hormone that increases the level of glucose in blood. For instance, glucagon stimulates the breakdown of glycogen and gluconeogenesis in liver as well as lipid and protein breakdown.
Furthermore, the secretion of glucagon is regulated by the same factors which are responsible for controlling insulin release. However, the effect of such factors in regulating glucagon is opposite to that of insulin.
Therefore, an increase in the level of blood glucose increases insulin release but inhibits the secretion of glucagon. This means that there should be a balance between the activities of insulin and glucagon as it helps in maintaining blood glucose levels.
Any interruption in such a balance may lead to disorders like diabetes.