What is a hormone?


Endocrine is a unique system in the human body, which covers the whole body and comprehensively regulates the physiological activities of the human body by secreting various hormones, such as bone growth and development, sexual maturity during puberty, blood sugar concentration, heartbeat rhythm, and so on

At the beginning of the 20th century, British physiologists Starling and Bayliss found in long-term observation that after the dog ate, the food passed through the esophagus and stomach to the small intestine, and the pancreas immediately secreted pancreatic juice and transported it to the small intestine through the duct to participate in digestion. So, how does the pancreas know that food is in the small intestine? They concluded experimentally that the friction of food on the wall of the small intestine causes mucosal cells to secrete a chemical that enters the bloodstream and flows to the pancreas, prompting the pancreas to secrete pancreatic juice. They named the chemical "secretin," a substance that provokes a strong response in organs in small amounts and is called hormones.

In general, hormones reach target cells in other parts of the body through the bloodstream, realize long-distance cell communication, can be regulated by diffusion of interstitial fluid to neighboring cells, and can also bind to receptors on their own cell membranes to regulate themselves. At present, more than 100 kinds of hormones have been discovered, from endocrine glands such as hypothalamus, pituitary gland, pineal gland, thyroid, parathyroid gland, adrenal gland, islets, and endocrine cells in organs such as the heart, vascular endothelium, digestive tract mucosa, liver, kidney, skin and placenta.

Hormones can be divided into three major families according to their chemical structure: amine hormones, peptide or protein hormones, and lipid hormones.

The first family is amine hormones, mainly including thyroxine, catecholamines and dopamine. Taking thyroxine as an example, it participates in the anabolism and catabolism of proteins, sugars and fats, promotes cell division and differentiation, and promotes the growth and development of tissues and organs. If people lack thyroxine in childhood, they will suffer from cretinism, manifested as mental retardation and short stature, and if they lack thyroxine in adulthood, it will lead to a decrease in central nervous system excitability, manifested as slow speech and action, memory loss, indifference and sleepiness.

The second family is peptides or protein-based hormones. They are many types and widely distributed, including growth hormone, prolactin, parathyroid hormone, insulin, glucagon, secretin, angiotensin, leptin and many other hormones. Take growth hormone as an example, it is secreted by the pituitary gland in the brain, acts on bone tissue and internal organs such as liver and kidney, and promotes their growth and development. Growth hormone secretion is highest in youth, gradually decreasing with age, if the secretion of growth hormone in juvenile is insufficient will cause dwarfism, excessive secretion of growth hormone will appear gigantism. In adulthood, because the epiphysis has been closed, the long bones no longer grow, and the cartilage and soft tissues at the end of the limbs can still grow, and excessive auxin will cause people to have symptoms of acromegaly such as thickening fingers and thickening of lips.

The third family is lipid hormones, which are divided into two small families: steroid hormones and docanic acids. Steroid hormones mainly include adrenal corticosteroids and sex hormones. Taking sex hormones as an example, a person's gender is already determined in the embryonic stage, but the emergence of gender characteristics requires sex hormones to promote. The testes are the main site for the synthesis of androgens, including testosterone, dihydrotestosterone, dehydroisandrosterone, and androstenedione, which function to form male signs and maintain spermatogenesis and libido. The ovaries are the main place for the synthesis of estrogen, estrogen is mainly estradiol and estrone, the function is to promote the growth and development of female reproductive organs, maintain a normal physiological cycle.

Various hormones work by binding to the corresponding receptors. The receptors of some hormones are distributed throughout the body and organs, and the effects are very extensive, such as growth hormone, thyroxine, and insulin. The receptors of some hormones are only present in specific target organs and target cells, and their effects are very limited, such as aldosterone.

The amount of hormones in the blood is usually extremely low, measured in micrograms, nanograms or picograms, but its signal transduction link is biomagnified and can produce strong biological effects. Such a powerful hormonal effect, if out of control, can cause environmental disorders in the human body. In fact, hormone secretion is strictly regulated, and the hypothalamus, pituitary gland, and target gland axes play an important role in maintaining hormone secretion homeostasis. The high-level hormones in this system usually have a promoting regulatory effect on the lower endocrine activities, while the lower hormones play a suppressive role in the regulation of the higher endocrine activities, and achieve dynamic balance through multiple feedback mechanisms.