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Endocrine

The term endocrine refers to all endocrine glands, i.e. all glands that release their secretions (called hormones) into the blood, whereby the site of action can be different from the site of production or, more precisely, the site of secretion. This distinguishes endocrine glands from exocrine glands. In contrast to exocrine secretions, hormones can therefore produce a long-distance effect, even throughout the entire body at the same time. Recent research shows that in addition to the classic remote effects, paracrine (acting in the neighbourhood of the site of production) or autocrine (acting in the cell of origin) glandular tissues also exist.
Due to the complexity of the human endocrine system, only some of the most important endocrine glands and hormones will be mentioned here without attempting to explain all glands, regulatory circuits and interrelationships.

Hypothalamus

The hypothalamus (Greek: „under the thalamus“) is the part of the brain located above the pituitary gland and connected to it by the infundibulum, which together with the pituitary gland represents one of the body’s most important endocrine control centres. The hypophyseal posterior lobe (HHL) is usually considered part of the hypothalamus. The hypothalamus produces many important releasing hormones (liberins) and inhibiting hormones (statins) as well as dopamine and some neuropeptides. As the most important vegetative control centre, it is significantly involved in the vegetative control of the body, which is implemented in various homoeostatic control circuits. Temperature (via thyrotropin (TSH) and thyroglobulin (TRH)), blood pressure, osmolarity (via ADH) of the blood, water intake and food intake (via the inhibitory effect of leptin on neuropeptide Y) are controlled here, as are the circadian rhythm and sleep (via histamine, orexin and the stimulation of melatoin release in the pineal gland). Sexuality is also influenced here.
Liberins and statins of the hypothalamus control the release of the hormones of the anterior pituitary (adenohypophysis); the hormones released by the posterior pituitary (neurohypophysis) are produced by the pituitary gland itself.

Hormones of the hypothalamus

Releasing-Hormone (Liberine):

• TRH (thyrotropin-releasing hormone), also known as thyreoliberin, causes the release of thyrotropin (TSH), which releases thyroxine and triiodothyronine at the thyroid gland, and prolactin.
• CRH (corticotropin-releasing hormone), also known as corticoliberin, causes the release of adrenocorticotropin (ACTH), which releases aldosterone, cortisol and sex hormones in the adrenal cortex.
• GnRH (gonadotropin-releasing hormone), also known as gonadoliberin, causes the release of follicle-stimulating hormone (FSH) and luteinising hormone (LH) in the gonads
• GHRH (growth hormone releasing hormone), also known as somatoliberin, causes the release of somatotropin (growth hormone, GH).
• MSH-RH (melanolibre, liberin to MSH)
• PRL-RH (prolactin-RH, prolactoliberin. Suspected, not proven!)

Statine (Inhibiting-Hormone):

• MSH-IH (Melanostatin, MSH-Statin)
• Dopamine (is both a neurotransmitter and a hormone), suppresses the release of prolactin
• Somatostatin (is a statin to GH), inhibits the secretion of pancreatic enzymes, gastrin and pepsin and reduces blood flow in the splanchnic area; is involved in the initiation of programmed cell death (apoptosis)

Hypophyse (pituitary gland)

1. HVL (anterior pituitary gland, adenohypophysis)

glandotrop:

• FSH (follicle-stimulating hormone), stimulates the gonads.
• TSH (thyroid stimulating hormone, thyrotropin, stimulates the thyroid gland)
• LH (luteinising hormone), stimulates the gonads.
• ACTH (adrenocorticotropic hormone), stimulates the adrenal cortex

not glandotropic:

• MSH (melanocyte-stimulating hormone) or melanotropin from the middle lobe of the pituitary gland
• Prolactin (dopamine is a statin for this),
• STH (somatotropic hormone) or somatotropin (GH or HGH).

2nd HHL (neurohypophysis, posterior lobe of the pituitary gland)

• Oxytocin (is also stored),
• ADH (antidiuretic hormone, vasopressin)

3rd HZL (intermediate pituitary lobe)

• – MSH (melanocyte-stimulating hormone)

Epiphysis (pineal gland)

produces melatonin, which regulates the sleep-wake rhythm and other time-dependent rhythms of the body. In fish, reptiles and amphibians, the epiphysis is still sensitive to light itself. In mammals, the transmission of light information is far more complex. It has been known since 1973 that of all organs apart from the heart, only the kidney is supplied with more blood than the epiphysis.
It is assumed that the epiphysis has a puberty-inhibiting effect.

Thyroid gland (thyroid gland):

The largest endocrine gland in humans, it produces the iodine-containing thyroid hormones triiodothyronine and thyroxine as well as the peptide hormone calcitonin. It can store the required iodine itself. It is important in the regulation of energy balance and cell growth. Calcitocin inhibits osteoclasts and stimulates the incorporation of calcium and phosphate into the bones. In the negative feedback thyrotropic control loop, an increased level of T3, T4 inhibits the release of TSH (which promotes the release of T3, T4) in the HVL and of TRH in the hypothalamus.
The thyroid hormones have an effect on the heart and the circulation. They can lead to an increase in heart rate and blood pressure as well as to vasodilation. They affect the metabolism of sugar, fat and connective tissue by increasing their metabolism. They increase the activity of sweat and sebaceous glands in the skin and the activity of the intestinal motor system. In the nervous system, they lead to increased excitability of the cells. Overall, the effect of thyroid hormones increases the body’s energy consumption and basal metabolic rate. This results in a rise in body temperature.

Parathyroid gland (glandulae parathyroideae)

PTH (Antagonist des Calcitocins), erhöht die Calciumkonzentration des Bluts durch indirekte Aktivierung von Osteoklasten.

Thymus gland

The thymus gland is not an endocrine gland. However, the effect of the peptides produced there has not yet been conclusively clarified: it degenerates after puberty, which is the cause of immunosenescence (deterioration of the immune system with ageing). As a primary or central lymphatic organ, it trains thymocytes from the bone marrow into T lymphocytes, which then migrate to the secondary lymphatic organs (lymphatic follicles, Peyer’s plaques, tonsils, spleen, lymph nodes, vermiform appendix)

liver

• Somatomedin (insulin-like growth factor)
• Prohormon Angiotensinogen
• Thrombopoietin

In addition, the liver reacts to insulin and glucagon and tries to keep the blood sugar level constant (storage of glucose from portal vein blood, controlled release into the blood. Storage of currently superfluous glucose as glycogen, which is released into the blood as glucose by glycolysis when required). Insulin stimulates the liver to store glucose, glucagon to release it. The liver also inactivates steroid hormones. Thrombopoietin stimulates the formation and differentiation of platelet-forming cells, the megakaryocytes. The thrombocytes have a receptor for thrombopoietin that binds it from the bloodstream, which represents the regulatory cycle.

Duodenum

• Secretary, Cholecystokinin (Pankreozymin, ist im Gehirn auch Neurotransmitter). Cholezystokinin löst in der medulla oblongata das Sättigungsgefühl aus, regt die Pankreassekretion an und bewirkt gleichzeitig eine Contraction der glatten Muskulatur der Gallenblasenwand sowie die Erschlaffung des Musculus sphincter oddi und ermöglicht dadurch den Gallenfluss, stimuliert die Peristaltik von Dünndarm und Dickdarm, vermindert die Wirkung von Gastrin und die Sezernierung von HCl. Spielt eine Rolle bei der Angststörung bzw. Auslösung von Panikattacken.
• Secretin is secreted at a pH of the chyme in the duodenum of < 4.5, inhibits gastrin production and thus reduces gastric acid production. It causes the pancreas to release secretions rich in sodium hydrocarbonate and stimulates the release of insulin and somatostatin.

Magen

• Gastrin from the antrum of the stomach and the duodenum promotes the production of Hcl in the stomach. Stretching of the stomach through food, certain proteins in the food, irritation of the vagus nerve, alcohol and caffeine stimulate the release; it is inhibited by somatostatin, secretin, GIP (gastrin inhibiting peptide) and the gastric acid-inhibiting and peristaltic-promoting neurotensin, among others.
• Ghrelin (Growth Hormone Release Inducing), appetite-stimulating hormone; stimulates the release of neuropeptide Y in the hypothalamus, which promotes food intake
• Neuropeptide Y, controls hunger, anxiety, blood vessel contraction, insulin release, gastric motility; acts as a tissue hormone (i.e. paracrine) on the immune system
• Somatostatin (s.o.)
• Histamine (tissue hormone and neurotransmitter, also found in plants and bacteria), messenger substance for tissue swelling in inflammation. Also involved in the regulation of gastric acid production and motility as well as in the central nervous system in the control of the sleep-wake rhythm and appetite control
• Endothelin, strongest known vasoconstrictor, 100 times stronger than noradrenaline; is elevated in CHD, heart failure and arteriosclerosis, also impairs the contractility of the heart, the heart rhythm and blood flow to the kidneys

kidneys

• Renin, from the JGA (juxtaglomerular apparatus), is released at low blood pressure in the afferent arteriole or at low sodium concentrations in the distal tubule. Catecholamines such as dopamine also lead to release. RAAS see below.
• Erythropoietin, stimulates the formation of erythrocytes (as a reminder: around 200 billion erythrocytes are formed per day), apoptosis inhibitor, slight stimulation of megakaryocytes (thrombopoiesis)
• Calcitriol, physiologically active form of the prohormone vitamin D3, acts against osteoporosis, modulates the immune system (improves defence against infections, reduces autoimmune processes), protects against cancer, acts against psoriasis and alopecia areata, promotes sperm motility
• Thrombopoietin (s.o.)

Adrenal glands

• Aldosterone, inhibits sodium excretion and thus increases blood volume, promotes potassium excretion. The increase in potassium in the serum can increase aldosterone synthesis. Very important in the short term when coping with life-threatening stress, chronic increase has multiple pathological effects
• Cortisol (cortisol), the most important stress hormone alongside (nor-)adrenaline, but more sluggish than the latter; activates catabolic metabolic processes and thus provides energy; immunosuppressive, anti-inflammatory, glycolytic, promotes the lipolytic effect of adrenaline and noradrenaline as well as catabolic protein metabolism. Is oxidised in the kidneys and intestines to cortisone, which does not have an antidiuretic effect. CRH and ACTH control the release. Cortisone is released in 7-10 spurts per day, the maximum in the serum is in the morning
• Androgens (andosterone, testosterone, etc.; also from testicles and in small amounts from the ovaries), virilising, make beard growth, deeper voice, stronger muscles)
• (nor-)adrenaline (adrenaline is also called epinephrine; noradrenaline lacks the methyl group of adrenaline). Adrenaline is a vasoconstrictor, particularly in the skin and kidneys, but a vasodilator in the central vessels supplying the muscles. In the heart it is chronotropic (accelerates the pulse), inotropic (increases contraction force) and dromotropic (accelerates conduction). Together with peripheral vasoconstriction, this significantly increases blood pressure. It increases respiration and inactivates vital functions such as digestion in the short term by paralysing peristalsis by relaxing the smooth muscles. The urinary bladder sphincter contracts under adrenaline. It promotes lipolysis, glycolysis and gluconeogenesis and thus increases the blood sugar level. At the same time, it inhibits the effect of insulin and releases glucagon. As a neurotransmitter, adrenaline activates the sympathetic nervous system, which in turn releases more adrenaline and noradrenaline. Increased sweat production, goose bumps, dry mouth and mydriasis (dilated pupils) are also caused by adrenaline. Intravenously, and very rarely intracardially, adrenaline is an important emergency medication. It is also administered intramuscularly in cases of shock or anaphylaxis.
• Noradrenaline acts mainly as a vasoconstrictor of the arterioles and is also a neurotransmitter with the same effect as adrenaline.

Pancreas (islet organ)

• Insulin, promotes the uptake of glucose into the cells and thus (as the only hormone) lowers the blood glucose level. Many hormones raise blood sugar levels, including the direct antagonist glucagon, but also adrenaline, cortisol and thyroid hormones. With insulin as the key, the liver and muscles in particular can absorb large amounts of glucose in a short time and store it as glycogen. Nerve cells and erythrocytes absorb glucose independently of insulin. Insulin inhibits lipolysis, so insulin deficiency leads to increased lipolysis and the formation of ketone bodies (acetone, 3-ketobutyric acid and ß-hydroxybutyric acid). By providing the appropriate enzymes, the brain and muscles can also obtain energy from ketone bodies. For example, with prolonged fasting, it is possible for the brain to manage with 40 mg/dl instead of 120 mg/dl glucose after the change in diet.
  In the liver, fatty tissue and muscles, insulin promotes triglyceride synthesis from food lipids, as well as protein synthesis. Insulin promotes glycogen synthesis and storage in the liver, triglyceride synthesis in the liver and adipose tissue and the storage of amino acids in the muscle; it inhibits hepatic gluconeogenesis and is therefore one of the most important regulators of glucose metabolism. The half-life of insulin in serum is around 5 minutes. It is taken up by cells, but is also broken down in the liver and kidneys. This suggests that blood glucose regulation is faster than it is possible to intervene therapeutically in the long term.
• Glucagon, promotes glycolysis in the liver and is an antagonist of insulin in its effect on glucose, protein and fatty acid metabolism
• Somatostatin,
• PP (pancreatic polypeptide), inhibits pancreatic enzyme and hydrogen carbonate production, intestinal motility and bile flow
• Ghrelin (s.o.)

Ovarien

• Oestrogens, also from the adrenal cortex and testicles; testosterone is also partially converted into oestrogen in fatty tissue
• Gestagens (progesterone and others)

Hoden

• Testosterone

Some important control mechanisms

RAAS (renin-angiotensin-aldosterone system)

As a protease, renin converts the previously inactive angiotensinogen from the liver into angiotensin I. This in turn is converted into angiotensin II by angiotensin converting enzyme (ACE), which is mainly produced in the lungs and has a negative feedback effect on renin formation, thus preventing renin overproduction. Angiotensin II has a very strong vasoconstrictive effect and also promotes the release of aldosterone and antidiuretic hormone (ADH), which is also known as adiuretin or vasopressin. Release stimuli for renin include Reduced blood flow in the Malpighi corpuscle of the kidney, reduced blood pressure in the vas afferens (the arterial vessel leading to the glomerulum, the renal corpuscle), reduced glomerular filtration rate, activation of the sympathetic NS, reduction of Cl ions at the macula densa on the straight part of the distal tubule.

Thyrotropic control loop

The pituitary gland releases the control hormone thyrotropin (TSH), which stimulates the secretion of thyroxine (T4) and triiodothyronine (T3) in the thyroid gland. These thyroid hormones in turn inhibit the production and release of TSH and T4, including TRH, in the sense of a negative feedback loop, so that an equilibrium level of the amount of thyroid hormones in the blood is normally established.