1.
Prolactin
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Prolactin, also known as luteotropic hormone or luteotropin, is a protein that is best known for its role in enabling mammals, usually females, to produce milk. It is influential in over 300 separate processes in various vertebrates, Prolactin is secreted from the pituitary gland in response to eating, mating, estrogen treatment, ovulation and nursing. Prolactin is secreted in pulses in between these events, Prolactin plays an essential role in metabolism, regulation of the immune system and pancreatic development. Discovered in non-human animals around 1930 by Oscar Riddle and confirmed in humans in 1970 by Henry Friesen prolactin is a peptide hormone and it is associated with human milk production. In fish it is thought to be related to control of water, Prolactin also acts in a cytokine-like manner and as an important regulator of the immune system. It has important cell cycle-related functions as a growth-, differentiating-, as a growth factor, binding to cytokine-like receptors, it influences hematopoiesis, angiogenesis and is involved in the regulation of blood clotting through several pathways. The hormone acts in endocrine, autocrine and paracrine manner through the prolactin receptor, pituitary prolactin secretion is regulated by endocrine neurons in the hypothalamus. Thyrotropin-releasing factor has an effect on prolactin release, however prolactin is the only adenohypophyseal hormone whose principal control is inhibitory. Several variants and forms are known per species, many fish have variants prolactin A and prolactin B. Most vertebrates including humans also have the closely related somatolactin, in humans, three smaller and several larger variants exist. Prolactin has a variety of effects. Sometimes, newborns secrete a substance from their nipples known as witchs milk. This is in part caused by maternal prolactin and other hormones, Prolactin plays an important role in maternal behavior. Prolactin provides the body with sexual gratification after sexual acts, The hormone counteracts the effect of dopamine and this is thought to cause the sexual refractory period. The amount of prolactin can be an indicator for the amount of satisfaction and relaxation. Unusually high amounts are suspected to be responsible for impotence and loss of libido, elevated levels of prolactin decrease the levels of sex hormones — estrogen in women and testosterone in men. The effects of elevated levels of prolactin are much more variable, in women. Prolactin is sometimes classified as a gonadotropin although in humans it has only a weak luteotropic effect while the effect of suppressing classical gonadotropic hormones is more important, Prolactin within the normal reference ranges can act as a weak gonadotropin, but at the same time suppresses GnRH secretion
2.
Endocrinology
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Endocrinology is a branch of biology and medicine dealing with the endocrine system, its diseases, and its specific secretions known as hormones. Specializations include behavioral endocrinology and comparative endocrinology, the endocrine system consists of several glands, all in different parts of the body, that secrete hormones directly into the blood rather than into a duct system. Endocrinology is the study of the system in the human body. This is a system of glands which secrete hormones, hormones are chemicals which affect the actions of different organ systems in the body. Examples include thyroid hormone, growth hormone, and insulin, the endocrine system involves a number of feedback mechanisms, so that often one hormone will control the action or release of another secondary hormone. If there is too much of the hormone, it may provide negative feedback to the primary hormone. This definition holds for most classical hormones, but there are also paracrine mechanisms, autocrine signals, a neuroendocrine signal is a classical hormone that is released into the blood by a neurosecretory neuron. Peptide hormones and protein hormones consist of three to more than 200 amino acid residues and can have a mass as large as 30,000 grams per mole. All hormones secreted by the pituitary gland are peptide hormones, as are leptin from adipocytes, ghrelin from the stomach, steroid hormones are converted from their parent compound, cholesterol. Mammalian steroid hormones can be grouped into five groups by the receptors to which they bind, glucocorticoids, mineralocorticoids, androgens, estrogens, and progestogens. Some forms of vitamin D, such as calcitriol, are steroid-like and bind to homologous receptors and these organs include the pituitary, thyroid, adrenals, ovaries, testes, and pancreas. An endocrinologist is a physician who specializes in treating disorders of the system, such as diabetes, hyperthyroidism. The diagnosis and treatment of diseases are guided by laboratory tests to a greater extent than for most specialties. Many diseases are investigated through excitation/stimulation or inhibition/suppression testing and this might involve injection with a stimulating agent to test the function of an endocrine organ. Blood is then sampled to assess the changes of the relevant hormones or metabolites, an endocrinologist needs extensive knowledge of clinical chemistry and biochemistry to understand the uses and limitations of the investigations. A second important aspect of the practice of endocrinology is distinguishing human variation from disease, atypical patterns of physical development and abnormal test results must be assessed as indicative of disease or not. Diagnostic imaging of endocrine organs may reveal incidental findings called incidentalomas, Endocrinology involves caring for the person as well as the disease. Most endocrine disorders are diseases that need lifelong care
3.
Reference ranges for common blood tests
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Reference ranges for blood tests are sets of values used by a health professional to interpret a set of medical test results from blood samples. Reference ranges for blood tests are studied within the field of clinical chemistry, blood test results should always be interpreted using the reference range provided by the laboratory that performed the test. A reference range is defined as the set of values 95 percent of the normal population falls within. It is determined by collecting data from vast numbers of laboratory tests, in this article, all values denote blood plasma concentration, which is approximately 60–100% larger than the actual blood concentration if the amount inside red blood cells is negligible. The precise factor depends on hematocrit as well as amount inside RBCs, is usually given with dL as the denominator in the United States, and usually with L in, for example, Sweden. Molar concentration is used to a degree in most of the rest of the world, including the United Kingdom and other parts of Europe and Australia. International units are based on measured biological activity or effect, or for some substances, enzyme activity is commonly used for e. g. liver function tests like AST, ALT, LD and γ-GT in Sweden. Percentages and time-dependent units are used for calculated derived parameters, e. g. for beta cell function in homeostasis model assessment or thyroids secretory capacity. If not otherwise specified, a range for a blood test is generally the venous range. An exception is for acid-base and blood gases, which are given for arterial blood. Still, the values are approximately equal between the arterial and venous sides for most substances, with the exception of acid-base, blood gases. Arterial levels for drugs are generally higher than venous levels because of extraction while passing through tissues, reference ranges are usually given as what are the usual values found in the population, more specifically the prediction interval that 95% of the population fall into. This may also be called standard range, for most substances presented, the optimal levels are the ones normally found in the population as well. More specifically, optimal levels are close to a central tendency of the values found in the population. However, usual and optimal levels may differ substantially, most notably among vitamins and blood lipids, references range may vary with age, sex, race, pregnancy, diet, use of prescribed or herbal drugs and stress. Reference ranges often depend on the method used, for reasons such as inaccuracy, lack of standardisation, lack of certified reference material. Also, reference ranges may be inaccurate when the groups used to establish the ranges are small. Units dont necessarily imply anything about molarity or mass, a few substances are below this main interval, e. g. thyroid stimulating hormone, being measured in mU/L, or above, like rheumatoid factor and CA19-9, being measured in U/mL
4.
Lactation
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Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process can occur with all female mammals, although it predates mammals. In humans the process of feeding milk is also called breastfeeding or nursing, in most species, milk comes out of the mothers nipples, however, the monotremes, egg-laying mammals, lack nipples and release milk through ducts in the abdomen. In only one species of mammal, the Dayak fruit bat, is milk production a normal male function, galactopoiesis is the maintenance of milk production. Oxytocin is critical for the milk let-down reflex in response to suckling, newborn infants often produce some witchs milk. Galactorrhea is milk production unrelated to nursing and it can occur in males and females of many mammal species as result of hormonal imbalances such as hyperprolactinaemia. The chief function of lactation is to provide nutrition and immune protection to the young after birth, in almost all mammals, lactation induces a period of infertility, which serves to provide the optimal birth spacing for survival of the offspring. Progesterone levels drop after birth, this triggers the onset of copious milk production, estrogen stimulates the milk duct system to grow and differentiate. Like progesterone, high levels of estrogen also inhibit lactation, estrogen levels also drop at delivery and remain low for the first several months of breastfeeding. Breastfeeding mothers should avoid estrogen-based birth control methods, as a spike in estrogen levels may reduce a mothers milk supply, prolactin contributes to the increased growth and differentiation of the alveoli, and also influences differentiation of ductal structures. High levels of prolactin during pregnancy and breastfeeding also increase insulin resistance, increase growth factor levels, during lactation, prolactin is the main factor maintaining tight junctions of the ductal epithelium and regulating milk production through osmotic balance. Human placental lactogen – from the month of pregnancy, the placenta releases large amounts of HPL. This hormone is associated with prolactin and appears to be instrumental in breast, nipple. Growth hormone is structurally similar to prolactin and independently contributes to its galactopoiesis. Adrenocorticotropic hormone and glucocorticoids such as cortisol have an important lactation inducing function in animal species. Glucocorticoids play a complex regulating role in the maintenance of tight junctions, thyroid-stimulating hormone and thyrotropin-releasing hormone are very important galactopoietic hormones whose levels are naturally increased during pregnancy. Oxytocin contracts the muscle of the uterus during and after birth. After birth, oxytocin contracts the muscle layer of band-like cells surrounding the alveoli to squeeze the newly produced milk into the duct system
5.
Menstrual period
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The menstrual cycle is the regular natural change that occurs in the female reproductive system that makes pregnancy possible. The cycle is required for the production of ovocytes, and for the preparation of the uterus for pregnancy, up to 80% of women report having some symptoms during the one to two weeks prior to menstruation. Common symptoms include acne, tender breasts, bloating, feeling tired, irritability and these symptoms interfere with normal life and therefore qualify as premenstrual syndrome in 20 to 30% of women. In 3 to 8%, they are severe, the first period usually begins between twelve and fifteen years of age, a point in time known as menarche. They may occasionally start as early as eight, and this onset may still be normal, the average age of the first period is generally later in the developing world and earlier in developed world. The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women and 21 to 35 days in adults, menstruation stops occurring after menopause which usually occurs between 45 and 55 years of age. Bleeding usually lasts around 2 to 7 days, the menstrual cycle is governed by hormonal changes. These changes can be altered by using hormonal birth control to prevent pregnancy, each cycle can be divided into three phases based on events in the ovary or in the uterus. The ovarian cycle consists of the phase, ovulation, and luteal phase whereas the uterine cycle is divided into menstruation, proliferative phase. Stimulated by gradually increasing amounts of estrogen in the phase, discharges of blood flow stop. Follicles in the ovary begin developing under the influence of an interplay of hormones. Approximately mid-cycle, 24–36 hours after the luteinizing hormone surges, the dominant follicle releases an ovocyte, under the influence of progesterone, the uterine lining changes to prepare for potential implantation of an embryo to establish a pregnancy. If implantation does not occur within two weeks, the corpus luteum will involute, causing a sharp drop in levels of both progesterone and estrogen. The hormone drop causes the uterus to shed its lining in a process termed menstruation, menstruation also occurs in some other animals including shrews, bats, and other primates such as apes and monkeys. The average age of menarche is 12–15 and they may occasionally start as early as eight, and this onset may still be normal. This first period occurs later in the developing world than the developed world. The average age of menarche is approximately 12.5 years in the United States,12.7 in Canada,12.9 in the UK and 13.1 years in Iceland, factors such as genetics, diet and overall health can affect timing. The cessation of menstrual cycles at the end of a reproductive period is termed menopause
6.
Menstruation
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Menstruation, also known as a period or monthly, is the regular discharge of blood and mucosal tissue from the inner lining of the uterus through the vagina. The first period usually begins between twelve and fifteen years of age, a point in time known as menarche, however, periods may occasionally start as young as eight years old and still be considered normal. The average age of the first period is later in the developing world. The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women, bleeding usually lasts around 2 to 7 days. Menstruation stops occurring after menopause, which occurs between 45 and 55 years of age. Periods also stop during pregnancy and typically do not resume during the months of breastfeeding. Up to 80% of women report having some symptoms prior to menstruation, common signs and symptoms include acne, tender breasts, bloating, feeling tired, irritability, and mood changes. These may interfere with life, therefore qualifying as premenstrual syndrome. In 3 to 8%, symptoms are severe, a lack of periods, known as amenorrhea, is when periods do not occur by age 15 or have not occurred in 90 days. Other problems with the cycle include painful periods and abnormal bleeding such as bleeding between periods or heavy bleeding. Menstruation in other animals occurs in primates, such as apes and monkeys, as well as bats, the menstrual cycle occurs due to the rise and fall of hormones. This cycle results in the thickening of the lining of the uterus, the egg is released from an ovary around day fourteen in the cycle, the thickened lining of the uterus provides nutrients to an embryo after implantation. If pregnancy does not occur, the lining is released in what is known as menstruation, the first menstrual period occurs after the onset of pubertal growth, and is called menarche. The average age of menarche is 12 to 15, however, it may start as early as eight. The average age of the first period is later in the developing world. The average age of menarche has changed little in the United States since the 1950s, Menstruation is the most visible phase of the menstrual cycle and its beginning is used as the marker between cycles. The first day of bleeding is the date used for the last menstrual period. The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women, the medical definition of menopause is one year without a period and typically occurs between 45 and 55 in Western countries
7.
Libido
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Libido, colloquially known as sex drive, is a persons overall sexual drive or desire for sexual activity. Sex drive is influenced by biological, psychological and social factors, biologically, the sex hormones and associated neurotransmitters that act upon the nucleus accumbens regulate libido in humans. Social factors, such as work and family, and internal factors, like personality and stress. Sex drive can also be affected by conditions, medications, lifestyle and relationship issues. A person who has extremely frequent or a suddenly increased sex drive may be experiencing hypersexuality, while the opposite condition is hyposexuality. A person may have a desire for sex, but not have the opportunity to act on that desire, psychologically, a persons urge can be repressed or sublimated. On the other hand, a person can engage in activity without an actual desire for it. Multiple factors affect human sex drive, including stress, illness, pregnancy, Sexual desires are often an important factor in the formation and maintenance of intimate relationships in humans. A lack or loss of desire can adversely affect relationships. Changes in the desires of any partner in a sexual relationship, if sustained and unresolved. The infidelity of a partner may be an indication that a partners changing sexual desires can no longer be satisfied within the current relationship, problems can arise from disparity of sexual desires between partners, or poor communication between partners of sexual needs and preferences. A person is sex starved or sexually frustrated when they have a libido, sigmund Freud defined libido as the energy, regarded as a quantitative magnitude. Of those instincts which have to do all that may be comprised under the word love. It is the energy or force, contained in what Freud called the id. Freud pointed out that these drives can conflict with the conventions of civilised behavior. Excessive use of ego defenses results in neurosis, a primary goal of psychoanalysis is to bring the drives of the id into consciousness, allowing them to be met directly and thus reducing the patients reliance on ego defenses. Freud viewed libido as passing through a series of stages within the individual. According to Swiss psychiatrist Carl Gustav Jung, the libido is identified as psychic energy, defined more narrowly, libido also refers to an individuals urge to engage in sexual activity, and its antonym is the force of destruction termed mortido or destrudo
8.
Dopamine
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Dopamine is an organic chemical of the catecholamine and phenethylamine families that plays several important roles in the brain and body. It is an amine synthesized by removing a carboxyl group from a molecule of its precursor chemical L-DOPA, dopamine is also synthesized in plants and most multicellular animals. In the brain, dopamine functions as a neurotransmitter—a chemical released by neurons to send signals to nerve cells. The brain includes several distinct dopamine pathways, one of plays a major role in reward-motivated behavior. Most types of rewards increase the level of dopamine in the brain, other brain dopamine pathways are involved in motor control and in controlling the release of various hormones. These pathways and cell groups form a system which is neuromodulatory. Outside the central system, dopamine functions primarily as a local chemical messenger. With the exception of the vessels, dopamine in each of these peripheral systems is synthesized locally. Several important diseases of the system are associated with dysfunctions of the dopamine system. Parkinsons disease, a condition causing tremor and motor impairment, is caused by a loss of dopamine-secreting neurons in an area of the midbrain called the substantia nigra. Its metabolic precursor L-DOPA can be manufactured, and in its pure form marketed as Levodopa is the most widely used treatment for the condition. There is evidence that schizophrenia involves altered levels of dopamine activity, similar dopamine antagonist drugs are also some of the most effective anti-nausea agents. Restless legs syndrome and attention deficit hyperactivity disorder are associated with decreased dopamine activity, dopaminergic stimulants can be addictive in high doses, but some are used at lower doses to treat ADHD. A dopamine molecule consists of a structure with one amine group attached via an ethyl chain. As such, dopamine is the simplest possible catecholamine, a family that includes the neurotransmitters norepinephrine and epinephrine. The presence of a ring with this amine attachment makes it a substituted phenethylamine. Like most amines, dopamine is an organic base, as a base, it is generally protonated in acidic environments. The protonated form is highly water-soluble and relatively stable, but can become oxidized if exposed to oxygen or other oxidants, in basic environments, dopamine is not protonated
9.
Follicle-stimulating hormone
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Follicle-stimulating hormone is a gonadotropin, a glycoprotein polypeptide hormone. FSH is synthesized and secreted by the cells of the anterior pituitary gland, and regulates the development, growth, pubertal maturation. FSH and luteinizing hormone work together in the reproductive system, FSH is a 35.5 kDa glycoprotein heterodimer, consisting of two polypeptide units, alpha and beta. Its structure is similar to those of luteinizing hormone, thyroid-stimulating hormone, the alpha subunits of the glycoproteins LH, FSH, TSH, and hCG are identical and consist of about 96 amino acids, while the beta subunits vary. Both subunits are required for biological activity, FSH has a beta subunit of 111 amino acids, which confers its specific biologic action, and is responsible for interaction with the follicle-stimulating hormone receptor. The sugar portion of the hormone is covalently bonded to asparagine, and is composed of N-acetylgalactosamine, mannose, N-acetylglucosamine, galactose, in humans, the gene for the alpha subunit is located at cytogenetic location 6q14.3. It is expressed in two types, most notably the basophils of the anterior pituitary. The gene for the FSH beta subunit is located on chromosome 11p13, and is expressed in gonadotropes of the cells, controlled by GnRH, inhibited by inhibin. FSH regulates the development, growth, pubertal maturation and reproductive processes of the human body, in both males and females, FSH stimulates the maturation of germ cells. In males, FSH induces Sertoli cells to secrete androgen-binding proteins, in females, FSH initiates follicular growth, specifically affecting granulosa cells. With the concomitant rise in inhibin B, FSH levels then decline in the follicular phase. This seems to be critical in selecting only the most advanced follicle to proceed to ovulation, at the end of the luteal phase, there is a slight rise in FSH that seems to be of importance to start the next ovulatory cycle. Control of FSH release from the gland is unknown. Low frequency gonadotropin-releasing hormone pulses increase FSH mRNA levels in the rat, GnRH has been shown to play an important role in the secretion of FSH, with hypothalamic-pituitary disconnection leading to a cessation of FSH. GnRH administration leads to a return of FSH secretion, FSH is subject to oestrogen feed-back from the gonads via the hypothalamic pituitary gonadal axis. FSH stimulates the growth and recruitment of immature ovarian follicles in the ovary, in early antral follicles, FSH is the major survival factor that rescues the small antral follicles from apoptosis. In the luteal-follicle phase transition period the serum levels of progesterone and estrogen decrease and no longer suppress the release of FSH, the cohort of small antral follicles is normally sufficiently in number to produce enough Inhibin B to lower FSH serum levels. When the follicle matures and reaches 8–10 mm in diameter it starts to secrete significant amounts of estradiol, normally in humans only one follicle becomes dominant and survives to grow to 18–30 mm in size and ovulate, the remaining follicles in the cohort undergo atresia
10.
Luteinizing hormone
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Luteinizing hormone is a hormone produced by gonadotropic cells in the anterior pituitary gland. In females, a rise of LH triggers ovulation and development of the corpus luteum. In males, where LH had also been called interstitial cell–stimulating hormone, each monomeric unit is a glycoprotein molecule, one alpha and one beta subunit make the full, functional protein. Its structure is similar to that of the other glycoprotein hormones, follicle-stimulating hormone, thyroid-stimulating hormone, LH has a beta subunit of 120 amino acids that confers its specific biologic action and is responsible for the specificity of the interaction with the LH receptor. This beta subunit contains an amino acid sequence that exhibits large homologies with that of the subunit of hCG. However, the hCG beta subunit contains an additional 24 amino acids, the different composition of these oligosaccharides affects bioactivity and speed of degradation. The biologic half-life of LH is 20 minutes, shorter than that of FSH, the gene for the alpha subunit is located on chromosome 6q12.21. The luteinizing hormone beta subunit gene is localized in the LHB/CGB gene cluster on chromosome 19q13.32, in contrast to the alpha gene activity, beta LH subunit gene activity is restricted to the pituitary gonadotropic cells. It is regulated by the hormone from the hypothalamus. Inhibin, activin, and sex hormones do not affect genetic activity for the beta subunit production of LH, in females, ovulation, maintaining of corpus luteum and secretion of progesterone. LH supports theca cells in the ovaries that provide androgens and hormonal precursors for estradiol production, at the time of menstruation, FSH initiates follicular growth, specifically affecting granulosa cells. With the rise in estrogens, LH receptors are expressed on the maturing follicle. However another theory of the LH peak is a feedback mechanism from estradiol. The levels keep rising through the phase and when they reach an unknown threshold. This effect is opposite from the negative feedback mechanism presented at lower levels. In other words, the mechanism are not yet clear, the increase in LH production only lasts for 24 to 48 hours. LH is necessary to maintain luteal function for the two weeks of the menstrual cycle. If pregnancy occurs, LH levels will decrease, and luteal function will instead be maintained by the action of hCG, LH acts upon the Leydig cells of the testis and is regulated by gonadotropin-releasing hormone
11.
Sleep
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It is distinguished from wakefulness by a decreased ability to react to stimuli, but is more easily reversed than the state of being comatose. Sleep occurs in repeating periods, in which the body alternates between two distinct modes known as non-REM and REM sleep. Although REM stands for rapid eye movement, sleep affects other brain-body functions, during sleep, most systems are in an anabolic state, helping to restore the immune, nervous, skeletal, and muscular systems. The internal circadian clock promotes sleep daily at night, however, sleep patterns vary among individuals. In the last century, artificial light has substantially altered sleep timing in industrialized countries, the diverse purposes and mechanisms of sleep are the subject of substantial ongoing research. Sleep seems to assist with improvements in the body and mind, Research in the 21st century is investigating whether sleep is a period of maintenance for removing metabolic waste compounds from the brain. Sleep is sometimes confused with unconsciousness, but is different in terms of the thought process. REM and non-REM sleep are so different that physiologists classify them as distinct behavioral states, REM sleep is associated with desynchronized and fast brain waves, loss of muscle tone, and suspension of homeostasis. NREM is considered to be sleep, it shows no prominent eye movement or muscle paralysis. Sleep occurs in cycles of approximately 90 minutes and this rhythm is called the ultradian sleep cycle. Sleep proceeds in cycles of NREM and REM, normally in that order, the American Academy of Sleep Medicine divides NREM into three stages, N1, N2, and N3, the last of which is also called delta sleep or slow-wave sleep. The whole period normally proceeds in the order, N1 → N2 → N3 → N2 → REM, REM sleep occurs as a person returns to stage 2 or 1 from a deep sleep. An adult reaches REM approximately every 90 minutes, REM sleep usually lasts for longer during latter half of sleep than in the part of the sleep episode. There is an amount of deep sleep earlier in the night. Key physiological indicators in sleep include EEG of brain waves, electrooculography of eye movements, simultaneous collection of these measurements is called polysomnography, and can be performed in a specialized sleep laboratory. In other words, sleeping persons perceive fewer stimuli, however, they can generally still respond to loud noises and other salient sensory events. Awakening can mean the end of sleep, or simply a moment to survey the environment, sleepers typically awaken from slow-wave sleep, soon after the end of a REM phase or sometimes in the middle of REM. Internal circadian indicators, along with reduction of homeostatic sleep need, typically bring about awakening
12.
Stress (biology)
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Physiological or biological stress is an organisms response to a stressor such as an environmental condition. Stress is the method of reacting to a challenge. Stimuli that alter an organisms environment are responded to by multiple systems in the body, the autonomic nervous system and hypothalamic-pituitary-adrenal axis are two major systems that respond to stress. The second major physiological stress, the HPA axis regulates the release of cortisol, the SAM and HPA axes are regulated by a wide variety of brain regions, including the limbic system, prefrontal cortex, amygdala, hypothalamus, and stria terminalis. Through these mechanisms, stress can alter memory functions, reward, immune function, metabolism, definitions of stress differ, however, one system proposed by Elliot and Eisdorfer suggests five types of stress. The five types of stress are labeled acute time limited stressors, brief naturalistic stressors, stressful event sequence, chronic stressors, acute time limited stressors involve a short term challenge, while brief naturalistic stressors involve an event that is normal but nevertheless challenging. Stressful event sequences are a stressor that occur, and continue to yield stress into the immediate future, chronic stressors are stressors that involve exposure to a long-term stressor. A distant stressor is a stressor that isnt immediate, there is likely a connection between stress and illness. Theories of the stress–illness link suggest that both acute and chronic stress can cause illness, and several studies have suggested such a link, according to these studies, both acute and chronic stress can lead to changes in behavior and in physiology. Behavioral changes can include smoking, changes in eating habits and physical activity, physiological changes can include changes in sympathetic activation or HPA activity, and immunological function. However, there is variability in the link between stress and illness. The HPA axis regulates many bodily functions, both behavioral and physiological, through the release of glucocorticoid hormones, the HPA axis activity varies according to the circadian rhythm, with a spike in the morning. The axis involves the release of corticotropin releasing hormone and vasopressin from the hypothalamus which stimulates the pituitary to secrete ACTH, ACTH may then stimulate the adrenal glands to secrete cortisol. The HPA axis is subject to feedback regulation as well. The release of CRH and VP are regulated by descending glutaminergic and GABAergic pathways from the amygdala, increased cortisol usually acts to increase blood glucose, blood pressure, and surpasses lysosomal, and immunological activity. Under other circumstances, however, the activity may differ, increased cortisol also favors habit based learning, by favoring memory consolidation of emotional memories. Selye demonstrated that stress decreases adaptability of an organism and proposed to describe the adaptability as a special resource, in recent works, it is considered as an internal coordinate on the dominant path in the model of adaptation. Stress can make the more susceptible to physical illnesses like the common cold
. PMID 18473017.