The endocrine system is responsible for sending "messages" to the various organs and tissues of the body. These signals are supplied by chemical substances of different nature, called hormones, a term coined in 1905 starting from the Greek verb ormao ("substance that stimulates or awakens").
Until recently it was believed that hormones were produced exclusively by the endocrine glands. Today we know that this function also belongs to individual cells or groups of cells, such as neurons or certain cells of the immune system. The heart, for example, despite being a muscle, produces a hormone called atrial natriuretic peptide (PAN), which is secreted in the blood and increases the excretion of sodium at the kidney level. The stomach, adipose tissue, liver, skin and intestines also have the ability to produce hormones.
As a whole, the endocrine system is therefore made up of glands and cells responsible for producing particular substances called hormones.
The activity of the endocrine system is strongly correlated to that of the nervous system. Between the two there is an important anatomical and functional connection, represented by the hypothalamus. Through the pituitary stalk this anatomical formation regulates the activity of the pituitary gland, the most important human endocrine gland.
Placed at the base of the encephalon and the size of a bean, the pituitary or pituitary gland, in turn controls the functioning of many cells, organs and tissues.
In addition to the pituitary gland, the main endocrine glands are:
the thyroid
the parathyroids
the endocrine portion of the pancreas
adrenal glands or capsules
the gonads
the thyme
the epineal gland (epiphysis)
According to the traditional theory the hormones, after being produced by glands or cells, are secreted in the blood (mechanism of endocrine action). From here they are transported to the target tissues, where they perform their function by influencing cellular activity. Today it has been amply demonstrated that some hormones can influence the functionality of the same structures that have produced them (mechanism of autocrine action) or those adjacent (mechanism of action paracrine).
It should be remembered that hormones:
they act in infinitesimal concentrations
to perform their function they need to bind to a specific receptor
Furthermore, a hormone can have different effects depending on the tissue in which it is picked up.
Steroid hormones (androgens, cortisol, estrogens, progesterone, etc.) are lipophilic and as such manage to easily cross the cell membrane, both to enter and exit the target cell. This lipophilicity turns into a big disadvantage when the steroid hormones have to be transported into the bloodstream. Since they are not soluble, they must in fact bind to specific transporter proteins, called carriers, such as albumin or SHBG (sex hormone binding proteins). This bond prolongs the half-life of the hormone, protecting it from enzymatic degradation. Near the target cell the complex protein transporting hormone + must dissolve, since the hydrophobicity of these carriers would prevent their entry into the intracellular environment.
The goal of any steroid hormone is the nucleus, to which it can arrive directly or indirectly, for example by binding to a cytoplasmic receptor. Having arrived here, he regulates gene transcription to direct the synthesis of new proteins.
Peptide hormones (growth hormone, LH, FSH, parathormone, insulin, glucagon, erythropoietin, etc.) are hydrophobic and as such cannot enter the target cells directly. To do this they rely on specific receptors on the cell surface. The receptor hormone complex triggers a series of events mediated by a complex of second messengers.
While steroid hormones directly regulate protein synthesis, the second messengers triggered by peptide hormones modify the functions of already existing proteins.
Cortisol, for example, increases the number of lipases (enzymes responsible for the degradation of triglycerides present in adipose tissue), while adrenaline, with faster action, activates the already existing lipases. For this reason the cell's response to protein-based hormones is generally faster.
With the recent advances in science, all the general discourse made so far has been questioned. In fact, some peptide hormones have been discovered that are able to activate second messengers which, similar to steroid hormones, activate gene transcription, driving the synthesis of new proteins. Thanks to other studies, the existence of membrane receptors for steroid hormones also emerged, capable of activating second messenger systems and stimulating rapid cellular responses.
