Endocrine Glands by A.Griguolo

Generality

The endocrine glands are the glands of the human body which produce hormones and which exploit the direct release in the blood of the latter, to spread them where their activity is required.

Constituting the so-called endocrine system, the endocrine glands can be divided into two categories: the category of the major endocrine glands and the category of minor endocrine glands.

In the first category, they belong: hypothalamus, pituitary gland, epiphysis, thyroid, parathyroid glands, adrenal glands, pancreas and gonads; to the second category, instead, they belong: heart, stomach, intestine, kidneys, thymus, adipose tissue and placenta, in women.

Short review of what a gland is

In medicine, the word " gland " identifies an organ, a tissue (ie a grouping of cells) or a simple single cell, having the ability to produce a particular substance, which in technical jargon is called secretion .

What are Endocrine Glands?

The endocrine glands are the glands of the human body that pour the product of their secretory activity directly into the blood, so as to exploit the circulatory stream for delivery to the destination area of the aforementioned product.

The endocrine glands are hormone- producing glands. Hormones are biological substances given the task of transporting a certain signal from a cell, a tissue or an organ to another cell, another tissue or another organ, in order to cause a certain functional response ; the hormones, therefore, are a kind of messengers for the exchange of information between the various parts of the human organism.

Did you know that ...

In the biological field, hormones are defined by the expression of chemical messengers.

Endocrine Glands and Endocrine System

Taken together, the endocrine glands constitute the so-called endocrine system .

By definition, the endocrine system is the set of organs, tissues and cells that produce hormones.

The proper functioning of the endocrine system - therefore of the endocrine glands - is fundamental to the well-being of the human organism and to the management of some crucial phases of an individual's life, such as puberty, pregnancy in women, moments of strong physical stress and / or psychic etc.

Endocrine Glands and Exocrine Glands: the differences

The endocrine glands are opposed, in a sense, to the exocrine glands .

The exocrine glands are the glands of the human body that release their secretion, usually by means of a special expulsion channel, on the epithelial surface of the skin (thus outside the human body) and on the epithelial surface of some hollow internal organs, as for example the mouth, stomach and bronchi.

Deepening: what else differentiates the endocrine glands from the exocrine glands?

Unlike most exocrine glands, the endocrine glands lack an excretory duct, that is, a channel used for expulsion of the secretion.

The endocrine glands, moreover, do not need it, since - as previously stated - they opt for the direct passage to the blood of the product of their secretion.

Meaning of the term "endocrine"

The terms "endocrine", "endocrine" etc. derive from the two words of the ancient Greek language "endo" ("ἔνδον") and "krino" ("κρίνω"), which mean, respectively, "inside" (or "inside") and "secrete".

Consequently, the literal meaning of "endocrine glands" is "glands that secrete inwards" and has the intent to underline the peculiarity of this glandular typology of pouring its own secret into an internal fluid like blood, rather than for example on the surface of the skin (as in the case of exocrine glands).

The contrast between endocrine glands and exocrine glands is also in the etymology used to indicate these two categories of glands; in fact, the term "exocrine" literally means "which secrete from the outside" (the word "eso" takes the place of "endo" and means "external").

What are they?

In the human body, the endocrine glands are varied.

According to a convention that aims to simplify its consultation, these fundamental anatomical structures can be divided into two broad categories: the category of the major endocrine glands and the category of minor endocrine glands .

The category of the major endocrine glands includes only organs, for which the secretion of hormones is one of the main activities or even the only activity promoted.

The category of minor endocrine glands, on the other hand, includes organs and tissues, for which the secretion of hormones is a secondary activity to another main one (primary activity).

Major Endocrine Glands: details

The so-called major endocrine glands are: the hypothalamus, the pituitary gland, the epiphysis, the thyroid, the parathyroid glands, the adrenal glands, the pancreas and the gonads .

HYPOTALAMUS

Located inferiorly to the thalamus, the hypothalamus is the organ of connection between the central nervous system and the endocrine system; in fact, through the release of a series of hormones, it connects the brain with the nearby pituitary gland.

Among the hormones secreted by the hypothalamus and intended to interact with the pituitary gland, are:

GnRH, which is the release hormone for the two pituitary gonadotropins LH and FSH;
TRH, which is the release hormone for pituitary TSH thyrotropin;
GHRH, which is the hormone-releasing hormone for pituitary growth hormone GH;

CRH, which is the release hormone for pituitary hormones ACTH (adrenocorticotropic hormone) and MSH (melanotropic hormone);
Dopamine . The hypothalamic dopamine serves to inhibit the release of the hormone prolactin by the pituitary gland;
Somatostatin . Hypothalamic somatostatin is used to inhibit the release of pituitary hormones GH and TSH.

PITUITARY GLAND

Located below the diencephalon, in the immediate vicinity of the hypothalamus, the pituitary or pituitary gland is the most representative endocrine gland of the endocrine system; in fact, it has the task of secreting the beauty of 9 different hormones, which serve to direct the hormonal activity of other endocrine glands - specifically, thyroid, adrenal gonads - and to control processes and biological mechanisms fundamental to life, such as lactation, water metabolism, body growth, blood pressure, body temperature, etc.

The pituitary is a small organ, anatomically divided into two main portions: the anterior pituitary and the posterior pituitary .

Under the hypothalamus directive, the anterior pituitary provides for the secretion of:

Growth hormone (or GH). It is the fundamental hormone for the statural and ponderal development of the human being; moreover, it is used for the deposition of calcium in bone tissue, the proliferation of cartilage cells, the process of protein synthesis and the increase in muscle mass.
Adrenocorticotropic hormone (or ACTH). It is the hormone that regulates the production of cortisol by the adrenal glands.
Thyrotropin (or TSH). It is the hormone that stimulates the hormonal activity of the thyroid.
Gonadotropin LH ( luteinizing hormone ) and FSH ( follicle-stimulating hormone ). They are the hormones that, in women, regulate the formation of follicles and the production of sex hormones by the ovaries (female gonads), while, in humans, they promote spermatogenesis and control the production of sex hormones by the testicles (gonads male).
Prolactin . It is the hormone that, in women, guarantees the development of the breast in puberty and ensures the lactation process in the postpartum phase.
Melanotropic hormone (or MSH ). It is the hormone responsible for acting on the skin's melanocytes, to regulate the pigmentation of the latter.

Still under the control of the hypothalamus, the posterior pituitary gland deals with the secretion of:

Antidiuretic hormone (or ADH or vasopressin ). It is a hormone with a fundamental role in regulating blood plasma volume.
Oxytocin . It is a hormone that plays a pivotal role in women, as it acts on the breast and on the uterus.

EPIPHYSIS

Located at the level of the diencephalon, the epiphysis is the endocrine gland responsible for the secretion of melatonin, ie the hormone used to regulate the sleep-wake cycle during the day.

THYROID

Located in the anterior region of the neck, at the base of the throat, the thyroid is the endocrine gland which, upon stimulation by the pituitary TSH, produces 3 hormones: triiodothyronine (also called T3), thyroxine (also called T4) and calcitonin .

Through its hormones, the thyroid regulates:

Body metabolism (for example, affects oxygen consumption; controls the synthesis and degradation of cholesterol; stimulates lipolysis and lipogenesis; increases glycogenolysis and gluconeogenesis, etc.);
Skeletal and cerebral development, starting from fetal life;
Protein synthesis;
The development of the skin, the hair apparatus and the genital organs;
The heartbeat;
Body temperature;
The hematopoiesis.

PARATHYROIDS

Generally 4 in number and located behind the thyroid, the parathyroids are the endocrine glands used to produce the parathyroid hormone ( PTH ), which is fundamental to regulate the blood levels of calcium and phosphorus.

Therefore, parathyroids are related to bone health.

adrenal

Located above the kidneys, the adrenal glands are the endocrine glands which are responsible for the secretion of the hormones adrenaline, noradrenaline, aldosterone and cortisol (NB: adrenaline and noradrenaline are catecholamines, while aldosterone and cortisol are steroids).

Through these hormones, the adrenal glands are involved in processes and mechanisms, such as:

The acute response to sudden and stressful events;
The production of androgens, in humans;
Sexual desire, in women;
Blood pressure control;
Maintaining sodium and potassium levels in the blood;
Heart rate control.

Did you know that ...

The adrenal glands can be divided into two portions: the cortical portion and the medullary portion .

The cortical portion is responsible for the production of catecholamines (adrenaline and noradrenaline), while the medullary portion is responsible for the secretion of steroids (aldosterone and cortisol).

PANCREAS

Placed just behind the stomach, the pancreas is the voluminous endocrine gland, also belonging to the digestive system, which has the important task of secreting the hormones glucagon, insulin, somatostatin, gastrin and pancreatic peptide .

Glucagon: raises blood glucose levels;
Insulin: lowers blood glucose and allows it to enter cells;
Somatostatin: inhibits acid secretion of the stomach;
Gastrin: promotes the production of essential substances for the digestive process (eg: digestive enzymes);
Pancreatic peptide: appears to have an inhibitory role against the exocrine function of the pancreas.

Readers should be reminded that the pancreas is an organ that also plays the role of an exocrine gland; a part of its constituent cells, in fact, is responsible for the production of digestive enzymes, whose release takes place on the inner surface of the duodenum, through the excretory ducts of Wirsung and Bartolini.

GONADS

The gonads are the primary sexual organs of the human reproductive system.

Corresponding to the ovaries in women and to the testes in humans, the gonads produce, under the influence of the pituitary hormones LH and FSH, the gametes and sex hormones .

The gametes are the sex cells necessary for reproduction, while the sex hormones are the biological substances necessary for the development of secondary sexual characteristics and the control of the genital apparatus.

Obviously, it is by virtue of the ability to secrete the sex hormones, that the gonads fall into the list of endocrine glands.

Remember that ...

Estrogens and progesterone are female sex hormones, while testosterone (is the main one), androstenedione, androstenediol, dehydropiandrosterone, androsterone and dihydrotestosterone are the male sex hormones.

Minor Endocrine Glands: the details

In the list of so-called minor endocrine glands, there are: the heart, some organs of the gastrointestinal tract ( stomach and intestine ), the kidneys, the thymus, the adipose tissue and the placenta in women.

HEART

The heart is included in the list of endocrine glands, as it produces the hormone called atrial natriuretic peptide .

Atrial natriuretic peptide helps control the levels of water, sodium and potassium present in the body.

STOMACH AND INTESTINE

Stomach and intestine are also two endocrine glands, because they produce small amounts of the aforementioned gastrin, of the hormone secretin etc.

KIDNEYS

The kidneys participate in the composition of the endocrine system, because they secrete the hormones erythropoietin and renin .

Erythropoietin serves to regulate erythropoiesis (ie the production of red blood cells), while renin is involved in the control of blood pressure.

THYME

An organ of the immune system, the thymus is also an endocrine gland, because it produces hormones that are used to promote the T lymphocyte maturation process.

ADIPOSE FABRIC

Adipose tissue participates in the endocrine system through its ability to produce the hormones leptin and resistin .

Leptin has a role in the regulation of ingestion, calorie expenditure and appetite; resistin, on the other hand, is implicated in insulin resistance (NB: knowledge on the physiological role of resistin is scarce).