TL;DR. The endocrine system is a network of glands that produces hormones — chemical messengers that control growth, metabolism, the stress response and reproduction in the body. In this article we look at each major gland in turn: what it produces, how its activity is regulated, and why all of it matters for people who train, too.
What the endocrine system is and how it works
The endocrine system is made up of ductless glands, which release their output not through ducts but directly into the blood. From there, hormones reach target cells throughout the body. The main endocrine glands are the pituitary gland, the thyroid gland and parathyroid glands, the adrenal glands, the pancreas, and the gonads (the testes in men and the ovaries in women). The digestive organs and kidneys also have an endocrine function — they produce substances that regulate the activity of other glands.
The whole system is closely linked to the nervous system. The key link is the hypothalamus, which controls the pituitary gland, and the pituitary in turn controls most of the other endocrine glands. Hormone balance is maintained mainly by the principle of negative feedback: when enough of a hormone reaches its target through the blood, a signal is sent back to the producing gland and secretion slows down. The glands are also influenced by the autonomic nervous system and by the levels of substances circulating in the bloodstream, such as glucose and amino acid concentrations.
The pituitary gland — the control center of the endocrine system
The pituitary gland is a small gland that sits in the sphenoid bone at the base of the skull, connected to the hypothalamus. Despite its modest size, it controls the activity of most endocrine glands. The pituitary has three parts: the anterior lobe (adenohypophysis), the posterior lobe (neurohypophysis), and a thin intermediate lobe between them.
The anterior lobe and its hormones
The anterior lobe is a richly vascularized cluster of cells made up of three cell types: basophils, acidophils (eosinophils) and neutrophils (chromophobe cells). Each of them produces different hormones.
The glandotropic hormones of the anterior lobe act on other endocrine glands:
- Corticotropin (ACTH) — stimulates the production and release of cortisol in the adrenal cortex. It comes from a larger precursor molecule (proopiomelanocortin), which also gives rise to lipotropins (which stimulate lipolysis in adipose tissue), melanocyte-stimulating hormone and β-endorphins (pain-relieving effect during physical exertion, influence on eating behavior and thermoregulation).
- Thyrotropin (TSH) — stimulates the synthesis and secretion of thyroid hormones in the thyroid gland and promotes growth of the gland.
- Gonadotropins — follicle-stimulating hormone (FSH) and luteinizing hormone (LH) control the maturation of germ cells and the function of the gonads. FSH promotes the maturation of follicles in the ovary and of sperm cells in the testes; LH triggers rupture of the follicle and formation of the corpus luteum.
The non-glandotropic hormones of the anterior lobe act directly:
- Growth hormone (GH). Promotes the transport of amino acids into cells and boosts protein synthesis, stimulates lipolysis in adipose tissue (the body using its fat stores for energy) and limits glucose uptake. Thanks to the last two effects, under the influence of growth hormone the body uses its carbohydrate stores more sparingly. The direct effect is on metabolic processes; the indirect effect works through insulin-like growth factors I and II, which are produced in the liver.
- Prolactin (PRL). In women, it stimulates milk production in the mammary glands and increases the number of FSH and LH receptors in the ovaries, strengthening the action of these hormones.
The posterior and intermediate lobes
The posterior lobe, or neurohypophysis, develops from the axon endings of large hypothalamic neurons and functions as an endocrine gland. It releases into the blood antidiuretic hormone (ADH, also known as vasopressin), which regulates water reabsorption in the kidneys, and oxytocin. Both are synthesized in the cell bodies of neurons located in the supraoptic and paraventricular nuclei of the hypothalamus and are carried along the axons to the posterior lobe. The intermediate lobe is a thin layer that produces intermedin and has little endocrine significance.
The hypothalamus itself produces two groups of peptide hormones that control the anterior lobe: releasing hormones stimulate the release of a particular hormone, and inhibiting hormones suppress it. They reach the anterior lobe of the pituitary through the hypothalamic–hypophyseal portal system, which consists of two capillary networks connected in series. The posterior lobe is controlled directly by nerve impulses through the hypothalamic–hypophyseal nerve tract.
The adrenal glands in the endocrine system: cortex and medulla
The adrenal glands are paired glands located above the upper pole of the kidneys. Each gland has two parts, which differ in both origin and function. The cortex (the outer layer) produces steroid hormones — aldosterone, cortisone and hydrocortisone. The medulla (the inner part) develops from the same embryonic origin as the postganglionic neurons of the sympathetic nervous system, and produces adrenaline and noradrenaline.
The adrenal cortex — three layers, three hormone groups
Cortical hormones are steroids (corticosteroids) by chemical structure. The cortex is divided into 3 layers, and the cells in each produce different hormones:
- Zona glomerulosa (the outer, thin layer) produces mineralocorticoids, the most important being aldosterone. It stimulates the reabsorption of sodium, chloride and, indirectly, water back into the blood in the distal renal tubules, while at the same time promoting the excretion of potassium and free hydrogen ions into the urine.
- Zona fasciculata (the middle, thicker layer) produces glucocorticoids, the most important of which is cortisol (corticosterone in rats). These stimulate gluconeogenesis in the liver (using amino acids), inhibit glucose use in peripheral tissues and raise blood sugar. In adipose tissue they stimulate lipolysis, releasing fatty acids for energy. As for proteins, they promote the breakdown of structural proteins but stimulate the synthesis of enzyme proteins. Glucocorticoids also amplify the effect of adrenaline and noradrenaline on vascular smooth muscle (the permissive effect), suppress inflammation and allergic reactions, and reduce antibody synthesis.
- Zona reticularis (deep inside the gland) produces weaker androgens, mainly dehydroepiandrosterone (DHEA) and, to a small extent, testosterone. In men, most testosterone still comes from the testes, but male sex hormones are also present in women.
The hypothalamus, the anterior pituitary and the zona fasciculata of the adrenal cortex together form the hypothalamic-pituitary-adrenocortical system. Its main task is to mobilize the body’s reserves by increasing glucocorticoid secretion and to trigger general adaptive responses in stressful situations and under physical training load. Blood cortisol levels follow a well-defined circadian rhythm that depends on the time of day, not on sleeping habits. An acute stressor raises cortisol quickly, but repeated exposure to the same stressor produces an increasingly weaker response.
The adrenal medulla — “fight or flight”
In embryonic terms, medullary cells are similar to cells of the sympathetic nervous system — they have no dendrites or axons, but they are innervated by cholinergic nerve fibers. Functionally, the sympathetic nervous system and the adrenal medulla are regarded as one whole, the sympathoadrenal system.
The medulla produces two catecholamines — adrenaline (epinephrine), about 80 %, and noradrenaline (norepinephrine), about 20 %. Noradrenaline is also produced by the postganglionic nerve endings of the sympathetic nervous system, which is where most of the noradrenaline circulating in the blood comes from — there it acts as a neurotransmitter. The precursor of both catecholamines is the amino acid L-tyrosine.
Catecholamines stimulate glycogenolysis in the liver and muscles and gluconeogenesis in the liver, raising blood glucose levels. In adipose tissue they intensify lipolysis and release fatty acids into the blood. The heart beats faster and harder, and vasoconstriction occurs in the skin, kidneys, digestive tract and genitals, while the blood vessels of skeletal and heart muscles dilate. Breathing deepens, the smooth muscles of the intestines relax, and the body’s overall metabolic rate and heat production increase (especially under the influence of adrenaline). Adrenaline also heightens alertness and causes strong mental arousal.
The sympathoadrenal system ensures the rapid release of catecholamines during emotional stress and physical exertion. The response is fast because the adrenaline produced is stored in the adrenal cells. The release of catecholamines is also stimulated by a drop in blood glucose concentration. The effect of a single surge is short-lived — the hormones quickly lose their biological activity.
The thyroid and parathyroid glands
The thyroid gland, also known as the thyroid, is the largest endocrine gland and has a rich blood supply. It sits in the neck, on the front of the 4 upper tracheal cartilages, and consists of a left and a right lateral lobe and a pyramidal lobe. The gland’s basic unit is the thyroid follicle — a small round vesicle whose wall is a single layer of cuboidal epithelial cells. The follicles are filled with colloid containing the protein thyroglobulin, and parafollicular cells lie in the loose connective tissue between them.
Starting from L-tyrosine, the follicles produce two iodine-containing hormones: the biologically more active triiodothyronine (T3, about 10 %) and thyroxine (T4, about 90 %). The hormones are stored in the follicles bound to thyroglobulin, so the thyroid holds a considerable reserve. Thyroxine is converted into triiodothyronine in the liver and kidneys — the vast majority, about 80 %, is synthesized outside the thyroid gland.
Thyroid hormones affect almost all tissues: in some they promote growth processes, in others they stimulate oxidation and metabolic rate. They are essential for the normal development of bones, teeth, hair and nervous tissue, and are especially important for physical and mental development in childhood. Overproduction is accompanied by a rise in heart rate, blood pressure and body temperature; in deficiency the effect is the opposite.
Parafollicular cells produce the peptide hormone calcitonin, which stimulates calcium deposition in the bones and lowers the calcium concentration in the blood. Calcitonin secretion is physiologically stimulated by a rise in blood calcium concentration. Parafollicular cell function declines with age, more so in women, which causes blood calcium levels to fall — this is one of the mechanisms behind osteoporosis.
The parathyroid glands are four small round bodies on the back of the right and left lobes of the thyroid gland. They produce parathyroid hormone (a peptide), whose action is aimed at raising blood calcium concentration and lowering phosphate ion levels. Parathyroid hormone stimulates the release of calcium and phosphates from the bones, the reabsorption of calcium into the blood and the excretion of phosphate into the urine in the kidneys, and the absorption of dietary calcium and phosphate into the blood in the intestine. Parathyroid hormone secretion is stimulated primarily by a drop in blood calcium concentration, while a high plasma calcium level suppresses it. Calcitonin and parathyroid hormone work as antagonists.
Thyroid function is controlled by pituitary thyrotropin, which in turn is under the control of the hypothalamic releasing hormone. By negative feedback, T3 and T4 suppress both the release of thyrotropin-releasing hormone in the hypothalamus and thyrotropin secretion in the pituitary. The hypothalamus, the adenohypophysis and the thyroid follicles form the hypothalamic-pituitary-thyroid system, whose job is to keep thyroid hormone levels in the blood normal both at rest and under stress.
The gonads in the endocrine system
The gonads produce steroid hormones, which are responsible for the development of secondary sex characteristics and for reproductive capacity.
The testes and male sex hormones
The male gonads are the testes (testicles), whose main function is producing sperm cells (spermatozoa). The testes contain Leydig interstitial cells, in which testosterone is produced. Testosterone receptors are found in tissues throughout the body. Testosterone is responsible for the development of the male gonads and of secondary sex characteristics. In addition, it and other androgens have a strong anabolic (protein-synthesis-promoting) effect, which is why men have more muscle mass than women. Androgens also stimulate erythropoiesis, i.e. the formation of red blood cells, influence bone growth (calcium deposition) and are linked to certain psychological traits, such as aggression. Testosterone production rises sharply in puberty and stays relatively high into old age.
The ovaries and the ovarian-menstrual cycle
The ovaries are located in the lesser pelvis and produce different hormones in the different phases of the ovarian-menstrual cycle. The cycle lasts 28 days on average and is divided into three phases.
- Follicular phase. The follicle matures and the egg cell ripens. The dominant hormones are the estrogens produced in the follicle, which are responsible for the development of secondary sex characteristics and prepare the egg for fertilization. Estrogens also affect blood lipid metabolism (women are better protected against cardiovascular disease), increase the blood’s ability to clot, promote osteoblast activity in bone tissue (an effect on bone density) and inhibit longitudinal bone growth. Menstruation ends in the early follicular phase, and the functional layer of the uterine lining begins to develop under the influence of estrogen. By negative feedback, estrogen inhibits FSH production, preventing new follicles from maturing, and by positive feedback it stimulates its own production in the follicle. In the late follicular phase, estrogen production reaches its maximum.
- Ovulation. The mature egg is released from the follicle under the combined action of LH and FSH, 16–24 hours after LH concentration peaks. Ovulation takes place alternately in each ovary, on average every 28 days.
- Luteal phase. Lasts about 14 days. The ruptured follicle forms the corpus luteum, which starts producing mainly progesterone, while estrogen synthesis continues as well. Progesterone prepares the uterine lining for a fertilized egg and raises basal body temperature during ovulation. Progesterone also affects the psyche (premenstrual syndrome and the depression typical of late pregnancy are linked to it). If fertilization occurs, a corpus luteum of pregnancy develops, whose hormones prevent the release of new eggs and prevent uterine contractions. If fertilization does not occur, the corpus luteum regresses, hormone levels fall and menstruation begins — the functional layer of the uterine lining is shed with bleeding.
The anterior pituitary regulates the gonads by means of the gonadotropic hormones (FSH and LH). In women their secretion is cyclical; in men it is continuous and at a low level. The surge in LH at ovulation is linked to positive feedback, and the rapid drop that follows is caused by the sharp rise in progesterone.
The pancreas and blood sugar
The pancreas is a relatively large organ behind the stomach, on the posterior wall of the abdominal cavity. Most of its cells work as a digestive gland, producing pancreatic juice, whose most important components are sodium bicarbonate and digestive enzymes. These neutralize the acidic stomach contents and break down the main components of food in the small intestine before absorption.
Only 1–2 % of the cells in the pancreas synthesize hormones. They are located in the islets of Langerhans — small clusters of cells surrounded by capillaries and innervated by both the sympathetic and the parasympathetic nervous system. The islets contain three types of cells: A (α) cells make up 20–25 %, B (β) cells 60–75 % and D (Δ) cells 5–15 % of all endocrine cells.
- Glucagon (A cells). Has a strong effect on the liver — it stimulates glycogenolysis and gluconeogenesis, increasing the release of glucose into the blood. It boosts fatty acid metabolism in the liver (resulting in greater ketone body production and a rise in their concentration in the blood) and, to a lesser extent, lipolysis in adipose tissue.
- Insulin (B cells). Its targets are the liver, muscle and adipose tissue, and the satiety center in the hypothalamus. Insulin’s main effect is lowering blood glucose levels: it promotes glucose accumulation in the liver (by activating glucokinase, which phosphorylates glucose), activates glycogen synthase and inhibits phosphorylase, stimulates the conversion of glucose into fatty acids and promotes glucose transport into the cells of peripheral tissues. Glucose uptake by nerve cells, erythrocytes and resting muscles does not depend on insulin. In addition, insulin stimulates the storage of fatty acids as triglycerides, suppresses lipolysis and has an anabolic effect on protein metabolism — it promotes protein synthesis.
- Somatostatin (D cells). Acts locally — it inhibits the synthesis and secretion of glucagon and insulin in A and B cells. Somatostatin is also synthesized in the hypothalamus, from where it is released into the blood and acts on the anterior pituitary as a growth hormone-inhibiting hormone.
The secretion of pancreatic hormones is governed primarily by blood glucose and amino acid levels, as well as by other hormones. The main job of insulin and glucagon working together is to keep blood sugar stable under changing conditions. A rise in blood amino acid concentration boosts the activity of both A and B cells: insulin promotes the active transport of amino acids into cells and protein synthesis, while glucagon draws amino acids into gluconeogenesis in the liver. Somatostatin, adrenaline and noradrenaline suppress insulin secretion, whereas the gastrointestinal hormones (gastrin, secretin, cholecystokinin) stimulate it. Increased parasympathetic nervous system activity stimulates insulin secretion and increased sympathetic activity suppresses it — but neural control is still of secondary importance; the primary factor is the direct effect of glucose on B cells.
The endocrine system and training: a practical summary
If you train seriously, it helps to know how the main hormones respond to training and recovery. Growth hormone and testosterone are clearly anabolic — they promote protein synthesis and muscle mass growth. Cortisol is catabolic — chronically high levels break down muscle protein, yet acutely it mobilizes energy for training. Insulin and glucagon keep blood sugar in balance, which in turn affects both endurance and post-workout recovery.
Adrenaline and noradrenaline give you a quick burst of energy in heavier sets and in competition, directing blood flow to the muscles and raising your heart rate. Thyroid hormones largely determine your resting metabolic rate, and therefore how much energy you burn over the course of a day. Chronic lack of sleep, long-term stress and a calorie deficit all suppress the functioning of the whole system — the thyroid gland and the gonadal axis suffer in particular. Quality sleep, adequate energy and protein intake, and a periodized training load are the boring foundations that keep the endocrine system in balance better than any tricks.
FAQ about the endocrine system
What is the difference between an endocrine and an exocrine gland?
An endocrine gland releases its product (hormones) directly into the blood, from where it reaches the whole body. An exocrine gland releases its product through ducts, either onto the body surface or into hollow organs — for example sweat glands, salivary glands or the digestive cells of the pancreas. The pancreas is a good example of an organ with a mixed function: most of its cells are exocrine (pancreatic juice), but the islets of Langerhans (1–2 %) are endocrine.
What is negative feedback in the endocrine system?
Negative feedback is the main mechanism that keeps hormone levels stable. When there is enough of a hormone, or of the result of its action, in the blood, a signal is sent back to both the hypothalamus and the gland that produces the hormone, and secretion slows down. For example, high T3 and T4 levels suppress the release of thyrotropin in the pituitary, and the thyroid gland no longer receives the signal to produce more.
How does stress affect the endocrine system?
In acute stress, the sympathoadrenal system is activated first and adrenaline and noradrenaline are released into the bloodstream — the rapid “fight or flight” response. Over a longer period, the hypothalamic–pituitary–adrenocortical system is activated and cortisol levels rise. Cortisol mobilizes energy reserves, but chronically high levels suppress the immune system, promote catabolic processes and, over time, can also disrupt the functioning of the thyroid gland and the gonads.
Author: EKFL
Source: WHO – physical activity.
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