Hormones . Chemical substances that are generally released directly into the bloodstream, alone (bioavailable) or associated with certain proteins (which extend their half-life) and have their effect on certain organs or tissues at a distance from where they were synthesized, hence the glands that produce them are called endocrine (endo within). They can act on the same cell that synthesizes it (autocrine action) or on contiguous cells (paracrine action), intervening in cell development.
They intervene in the metabolism
They are released into the extracellular space.
They diffuse into the blood vessels and travel through the blood .
They affect tissues that may be far from the hormone’s point of origin.
Its effect is directly proportional to its concentration.
Regardless of their concentration, they require adequate receptor functionality to exert their effect.
They regulate the functioning of the body.
Stimulating: promotes activity in a tissue. (eg, prolactin).
Inhibitory: decreases activity in a tissue. (eg, somatostatin).
Antagonist: when a pair of hormones have opposite effects to each other, (eg, insulin and glucagon)
Synergist: when two hormones together have a more powerful effect than when they are separated. (ex: hGH and T3 / T4)
Tropic: this is a hormone that alters the metabolism of other endocrine tissue, (eg, gonadotropin serves as a chemical messenger).
The endocrine glands produce and secrete several types of hormones :
- Steroids: soluble in lipids , they easily diffuse into the target cell . It binds to a receptor within the cell and travels to some nuclear DNA gene to stimulate its transcription. In plasma, 95% of these hormones travel coupled to plasma protein transporters.
- Non-steroidal: derived from amino acids. They adhere to a receptor on the membrane, on the outside of the cell . The receptor has an active site in its internal part of the cell that initiates a cascade of reactions that induce changes in the cell. The hormone acts as a first messenger and the biochemicals produced, which induce changes in the cell, are the second messengers.
- Amines: modified amino acids. Ex: adrenaline, noradrenaline.
- Peptides: short chains of amino acids, eg OT, ADH. They are water-soluble with the ability to circulate freely in blood plasma (for which they are rapidly degraded: half-life <15 min). They interact with membrane receptors thereby activating intracellular second messengers.
- Protein: complex proteins . (eg, GH, Pch)
- Glycoproteins: (ex: FSH, LH).
The functions that hormones control include :
- The activities of whole organs.
- Growth and development.
- Reproduction .
- The sexual characteristics.
- The use and storage of energy .
- Blood levels of fluids, salt, and sugar .
Mechanism of action
Stimulation of the endocrine gland causes the release of the hormone, or first messenger, which at the cellular level includes the activity of membrane-bound adenyl cyclase, which leads to the conversion of ATP into c-AMP, the second delivery courier.
The c-AMp in turn influences many enzymatic reactions, membrane permeability, ionic movements, hormone release, etc .; involved in the production of many physiological products and responses. Modulatory effects, including prostaglandins, provide a delicately sensitive system of control for the concentrations and activities of the first and second messengers.
Each cell is capable of producing a large number of regulatory molecules. The classic endocrine glands and their hormonal products are specialized in the general regulation of the organism as well as in the self-regulation of an organ or tissue. The method used by the body to regulate the concentration of hormones is a balance between positive and negative feedback, based on the regulation of their production, metabolism and excretion.
Hormones can be stimulated or inhibited by:
- Other hormones.
- Plasma concentration of ions or nutrients.
- Neurons and mental activity.
- Environmental changes: eg. light, temperature, atmospheric pressure.
- A special group of hormones are the trophic hormones that work by stimulating production by the endocrine glands. Eg. TSH stimulates the release of thyroid hormones in addition to stimulating the growth of the gland. Recently the hunger hormones have been discovered: Ghrelin, Orexin and Peptide Y and their antagonists such as Leptin.
Hormones belong to three groups of compounds:
- Derivatives of amino acids.
Hormones of the pituitary and hypothalamus
The master gland, as the pituitary gland is known, releases hormones that influence other glands to generate specific hormones that the body needs. These are stored in the two lobes of the pituitary.
Thyroid and parathyroid hormones
Thyroxine or tetraiodothyronine (T4) and triiodothyronine (T3) are two hormones of the thyroid gland and their functions include: stimulating metabolism , increasing oxygen consumption and body temperature and intervening in the development of organs and tissues, especially in the nervous and bone systems. The other thyroid hormone is calcitonin, which lowers the levels of calcium and phosphorus in the circulatory flow and inhibits bone resorption.
The only hormone released by the parathyroid glands is parathyroid hormone or parathyroid and is responsible for increasing the concentration levels of calcium and phosphorus in the blood and stimulates bone reabsorption.
Pancreatic and adrenal hormones
The endocrine pancreas (islets of Langerhans) produces two hormones that influence the metabolism of glucose (sugar), according to the needs of the body. One of them is insulin – a hormone produced by beta cells of the islets – which lowers the level of glucose in the blood. And the other is glucagon – a hormone produced by alpha cells – which increases sugar levels, extracting all the glucose stores from the liver that go into the bloodstream. Somatostatin – another pancreatic hormone produced by delta cells – indirectly intervenes in the regulation of glucose, decreasing the secretion of insulin and glucagon.
The medulla of the adrenal glands produces hormones known as catecholamines, among the most important are adrenaline or epinephrine and norepinephrine or norepinephrine.
These are secreted in certain stress situations (fight, fear or flight), so the heart rate accelerates, blood pressure increases, muscle activity is stimulated due to the tension of the muscles and the skin becoming moist due to perspiration. . Two hormones, aldosterone and cortisol, are released in the cortex of this gland. Additionally, the adrenal glands produce small amounts of male and female hormones (androgens, estrogens, and progesterone).
Hormones of the sex glands
The sex glands, or gonads, secrete different hormones. In women, the ovaries manufacture and release estrogens, important for the development of reproductive organs and secondary sexual characteristics (such as growth of the breasts, pubic and axillary hair, and widening of the hips). Also, this hormone is important in the ovarian cycle, because it helps the ovum to develop, mature and if it is fertilized, it implants correctly in the uterus.
Progesterone is another hormone that the ovaries secrete and that also intervenes in the ovarian cycle, exerting a kind of relief with estrogens, since if a pregnancy occurs, it is responsible for maintaining it well. In addition, the ovaries make a hormone called relaxin, which acts on the ligaments of the pelvis and cervix and causes them to relax during childbirth.
The testes, in men, produce and secrete hormones called androgens and correspond to testosterone, androsterone and androstenedione. However, the most important is the first, because it makes sperm and stimulates the development of secondary sexual characteristics. Among the latter, the following stand out: the growth of the prostate, the seminal vesicles, the appearance of hair on the legs, arms, cheek, chest and pubis, and an increase in muscle mass.
There are different ways that hormones work and these are:
Endocrine action: The hormone is synthesized in an organ or gland and is released into the bloodstream, to later bind to specific receptors.
Paracrine action: The hormone acts from endocrine cells to specific receptors in neighboring cells.
Autocrine action: The hormone exerts its action on the same cell.
Aldosterone and cortisol
Aldosterone is a hormone secreted from the outermost cortex of the adrenal gland. Its function is to inhibit the amount of sodium in the urine and to help maintain the volume and pressure of the blood. Cortisol is also produced and released by the cortex of the adrenal gland, but from its middle part. This hormone plays an important role in the metabolism of fats, carbohydrates and proteins. Also, it is essential for the body’s response to stress (physical and emotional).
There are two types of glands that secrete substances that can be hormonal or non-hormonal:
Endocrine glands: Most hormones are made in these glands, which are located in various parts of the body. Their main characteristic is that the hormones they produce are released into the bloodstream and through it are distributed throughout the body, to act on various organs and tissues. The most important are the pituitary, thyroid, parathyroid, pancreas, adrenal glands, and gonads.
Exocrine glands: It refers to those that do not secrete hormones, but that produce substances that are also important for our body. These glands – such as salivary or sweat glands – send these substances (such as saliva, sweat, etc.) through ducts or tubes that lead them to the place where they should act.
The placenta that forms during pregnancy from the membrane that surrounds the fetus assumes endocrine functions of the pituitary and ovaries, which are important in maintaining pregnancy. It secretes a hormone called chorionic gonadotropin, which is present in the urine throughout the gestational process, which is why it is the test for pregnancy tests performed by women. In addition, it produces progesterone and estrogens, chorionic somatotropin, placental lactogen, and lactogenic hormones.