In medicine, monitoring is the observation of a disease, condition or one or several medical parameters over time. It can be performed by continuously measuring certain parameters by using a medical monitor, and/or by performing medical tests. Transmitting data from a monitor to a distant monitoring station is known as telemetry or biotelemetry. Monitoring can be classified by the target of interest, including: Cardiac monitoring, which refers to continuous electrocardiography with assessment of the patients condition relative to their cardiac rhythm. A small monitor worn by an ambulatory patient for this purpose is known as a Holter monitor. Cardiac monitoring can involve cardiac output monitoring via an invasive Swan-Ganz catheter. Hemodynamic monitoring, which monitors the blood pressure and blood flow within the circulatory system. Blood pressure can be measured either invasively through an inserted blood pressure transducer assembly, or noninvasively with an inflatable blood pressure cuff.
Respiratory monitoring, such as: Pulse oximetry which involves measurement of the saturated percentage of oxygen in the blood, referred to as SpO2, measured by an infrared finger cuff Capnography, which involves CO2 measurements, referred to as EtCO2 or end-tidal carbon dioxide concentration. The respiratory rate monitored as such is called AWRR or airway respiratory rate) Respiratory rate monitoring through a thoracic transducer belt, an ECG channel or via capnography Neurological monitoring, such as of intracranial pressure. There are special patient monitors which incorporate the monitoring of brain waves, gas anesthetic concentrations, bispectral index, etc, they are incorporated into anesthesia machines. In neurosurgery intensive care units, brain EEG monitors have a larger multichannel capability and can monitor other physiological events, as well. Blood glucose monitoring Childbirth monitoring Body temperature monitoring through an adhesive pad containing a thermoelectric transducer.
Monitoring of vital parameters can include several of the ones mentioned above, most include at least blood pressure and heart rate, preferably pulse oximetry and respiratory rate. Multimodal monitors that measure and display the relevant vital parameters are integrated into the bedside monitors in critical care units, the anesthetic machines in operating rooms; these allow for continuous monitoring of a patient, with medical staff being continuously informed of the changes in general condition of a patient. Some monitors can warn of pending fatal cardiac conditions before visible signs are noticeable to clinical staff, such as atrial fibrillation or premature ventricular contraction. A medical monitor or physiological monitor is a medical device used for monitoring, it can consist of one or more sensors, processing components, display devices, as well as communication links for displaying or recording the results elsewhere through a monitoring network. Sensors of medical monitors include mechanical sensors.
The translating component of medical monitors is responsible for converting the signals from the sensors to a format that can be shown on the display device or transferred to an external display or recording device. Physiological data are displayed continuously on a CRT, LED or LCD screen as data channels along the time axis, They may be accompanied by numerical readouts of computed parameters on the original data, such as maximum and average values and respiratory frequencies, so on. Besides the tracings of physiological parameters along time, digital medical displays have automated numeric readouts of the peak and/or average parameters displayed on the screen. Modern medical display devices use digital signal processing, which has the advantages of miniaturization and multi-parameter displays that can track many different vital signs at once. Old analog patient displays, in contrast, were based on oscilloscopes, had one channel only reserved for electrocardiographic monitoring. Therefore, medical monitors tended to be specialized.
One monitor would track a patient's blood pressure, while another would measure pulse oximetry, another the ECG. Analog models had a second or third channel displayed in the same screen to monitor respiration movements and blood pressure; these machines were used and saved many lives, but they had several restrictions, including sensitivity to electrical interference, base level fluctuations and absence of numeric readouts and alarms. Several models of multi-parameter monitors are networkable, i.e. they can send their output to a central ICU monitoring station, where a single staff member can observe and respond to several bedside monitors simultaneously. Ambulatory telemetry can be achieved by portable, battery-operated models which are carried by the patient and which transmit their data via a wireless data connection. Digital monitoring has created the possibility, being developed, of integrating the physiological data from the patient monitoring networks into the emerging hospital electronic health record and digital charting systems, using appropriate health care standards which have been developed for this purpose by organizations such as IEEE and HL7.
This newer method of charting patient data reduces the likelihood of human documentation error and will reduce overall paper consumption. In addition, automated ECG interpretation incorporates diagnostic codes automatically into
X-rays make up X-radiation, a form of electromagnetic radiation. Most X-rays have a wavelength ranging from 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 100 eV to 100 keV. X-ray wavelengths are shorter than those of UV rays and longer than those of gamma rays. In many languages, X-radiation is referred to with terms meaning Röntgen radiation, after the German scientist Wilhelm Röntgen who discovered these on November 8, 1895, credited as its discoverer, who named it X-radiation to signify an unknown type of radiation. Spelling of X-ray in the English language includes the variants x-ray, X ray. Before their discovery in 1895 X-rays were just a type of unidentified radiation emanating from experimental discharge tubes, they were noticed by scientists investigating cathode rays produced by such tubes, which are energetic electron beams that were first observed in 1869. Many of the early Crookes tubes undoubtedly radiated X-rays, because early researchers noticed effects that were attributable to them, as detailed below.
Crookes tubes created free electrons by ionization of the residual air in the tube by a high DC voltage of anywhere between a few kilovolts and 100 kV. This voltage accelerated the electrons coming from the cathode to a high enough velocity that they created X-rays when they struck the anode or the glass wall of the tube; the earliest experimenter thought to have produced. In 1785 he presented a paper to the Royal Society of London describing the effects of passing electrical currents through a evacuated glass tube, producing a glow created by X-rays; this work was further explored by his assistant Michael Faraday. When Stanford University physics professor Fernando Sanford created his "electric photography" he unknowingly generated and detected X-rays. From 1886 to 1888 he had studied in the Hermann Helmholtz laboratory in Berlin, where he became familiar with the cathode rays generated in vacuum tubes when a voltage was applied across separate electrodes, as studied by Heinrich Hertz and Philipp Lenard.
His letter of January 6, 1893 to The Physical Review was duly published and an article entitled Without Lens or Light, Photographs Taken With Plate and Object in Darkness appeared in the San Francisco Examiner. Starting in 1888, Philipp Lenard, a student of Heinrich Hertz, conducted experiments to see whether cathode rays could pass out of the Crookes tube into the air, he built a Crookes tube with a "window" in the end made of thin aluminum, facing the cathode so the cathode rays would strike it. He found that something came through, that would cause fluorescence, he measured the penetrating power of these rays through various materials. It has been suggested that at least some of these "Lenard rays" were X-rays. In 1889 Ukrainian-born Ivan Pulyui, a lecturer in experimental physics at the Prague Polytechnic who since 1877 had been constructing various designs of gas-filled tubes to investigate their properties, published a paper on how sealed photographic plates became dark when exposed to the emanations from the tubes.
Hermann von Helmholtz formulated mathematical equations for X-rays. He postulated a dispersion theory before Röntgen made his announcement, it was formed on the basis of the electromagnetic theory of light. However, he did not work with actual X-rays. In 1894 Nikola Tesla noticed damaged film in his lab that seemed to be associated with Crookes tube experiments and began investigating this radiant energy of "invisible" kinds. After Röntgen identified the X-ray, Tesla began making X-ray images of his own using high voltages and tubes of his own design, as well as Crookes tubes. On November 8, 1895, German physics professor Wilhelm Röntgen stumbled on X-rays while experimenting with Lenard tubes and Crookes tubes and began studying them, he wrote an initial report "On a new kind of ray: A preliminary communication" and on December 28, 1895 submitted it to Würzburg's Physical-Medical Society journal. This was the first paper written on X-rays. Röntgen referred to the radiation as "X"; the name stuck.
They are still referred to as such in many languages, including German, Danish, Swedish, Estonian, Japanese, Georgian and Norwegian. Röntgen received the first Nobel Prize in Physics for his discovery. There are conflicting accounts of his discovery because Röntgen had his lab notes burned after his death, but this is a reconstruction by his biographers: Röntgen was investigating cathode rays from a Crookes tube which he had wrapped in black cardboard so that the visible light from the tube would not interfere, using a fluorescent screen painted with barium platinocyanide, he noticed a faint green glow from the screen, about 1 meter away. Röntgen realized some invisible rays coming from the tube were passing through the cardboard to make the screen glow, he found they could pass through books and papers on his desk. Röntgen threw himself into investigating these unknown rays systematically. Two months after his initial discovery, he published his paper. Röntgen discovered their medical use when he made a picture of his wife's hand on a photographic plate formed due to X-rays.
The photograph of his wife's hand was the first photograph of a human body part using X-rays. When she saw the picture, she said "I have seen my death."The discovery of X-rays stimul
Clinical chemistry is the area of chemistry, concerned with analysis of bodily fluids for diagnostic and therapeutic purposes. It is an applied form of biochemistry; the discipline originated in the late 19th century with the use of simple chemical reaction tests for various components of blood and urine. In the many decades since, other techniques have been applied as science and technology have advanced, including the use and measurement of enzyme activities, spectrophotometry and immunoassay. There are now many blood tests and clinical urine tests with extensive diagnostic capabilities. Most current laboratories are now automated to accommodate the high workload typical of a hospital laboratory. Tests performed are monitored and quality controlled. All biochemical tests come under chemical pathology; these are performed on any kind of body fluid, but on serum or plasma. Serum is the yellow watery part of blood, left after blood has been allowed to clot and all blood cells have been removed; this is most done by centrifugation, which packs the denser blood cells and platelets to the bottom of the centrifuge tube, leaving the liquid serum fraction resting above the packed cells.
This initial step before analysis has been included in instruments that operate on the "integrated system" principle. Plasma is obtained by centrifuging the blood without clotting. Plasma is obtained by centrifugation; the type of test required dictates. A large medical laboratory will accept samples for up to about 700 different kinds of tests; the largest of laboratories do all these tests themselves, some must be referred to other labs. This large array of tests can be categorised into sub-specialities of: General or routine chemistry – ordered blood chemistries. Special chemistry - elaborate techniques such as electrophoresis, manual testing methods. Clinical endocrinology – the study of hormones, diagnosis of endocrine disorders. Toxicology – the study of drugs of abuse and other chemicals. Therapeutic Drug Monitoring – measurement of therapeutic medication levels to optimize dosage. Urinalysis – chemical analysis of urine for a wide array of diseases, along with other fluids such as CSF and effusions Fecal analysis – for detection of gastrointestinal disorders.
Common clinical chemistry tests include: A set of ordered tests are combined into a panel: Basic metabolic panel - 8 tests - sodium, chloride, blood urea nitrogen, glucose, calcium Comprehensive metabolic panel - 14 tests - above BMP plus total protein, alkaline phosphatase, alanine amino transferase, aspartate amino transferase, bilirubin Reference ranges for common blood tests Medical technologist Clinical Biochemistry Burtis, Carl A.. Tietz textbook of clinical chemistry. Saunders. P. 2448. ISBN 978-0-7216-0189-2. American Association of Clinical Chemistry Association for Mass Spectrometry: Applications to the Clinical Lab
Phlegm is a liquid secreted by the mucous membranes of mammals. Its definition is limited to the mucus produced by the respiratory system, excluding that from the nasal passages, that, expelled by coughing. Phlegm is in essence a water-based gel consisting of glycoproteins, immunoglobulins and other substances, its composition varies depending on climate and state of the immune system. Its color can vary from transparent to pale or dark yellow and green, from light to dark brown, to dark grey depending on the constituents. Contrary to popular misconception and misuse and phlegm are not always the same. Mucus is a normal protective layering around the airway, nasal turbinate, urogenital tract. Mucus is an adhesive viscoelastic gel produced in the airway by submucosal glands and goblet cells and is principally water, it contains high-molecular weight mucous glycoproteins that form linear polymers. Phlegm is more related to disease than is mucus and can be troublesome for the individual to excrete from the body.
Phlegm is a juicy secretion in the airway during inflammation. Phlegm contains mucus with virus, other debris, sloughed-off inflammatory cells. Once phlegm has been expectorated by a cough it becomes sputum. There are multiple factors that can contribute to an excess of phlegm in the larynx. Vocal abuse: Vocal abuse is the misuse or overuse of the voice in an unhealthy fashion such as clearing the throat, screaming, talking loudly, or singing incorrectly. Clearing the throat: Clearing the throat removes or loosens phlegm but the vocal cords hit together causing inflammation and therefore more phlegm. Yelling/screaming: Yelling and screaming both cause the vocal cords to hit against each other causing inflammation and phlegm. Nodules: Excessive yelling and incorrect singing as well as other vocal abusive habits can cause vocal nodules. See vocal fold nodule for more information on nodules. Smoking: Smoke is hot, polluted air which dries out the vocal cords. With each breath in of smoke, the larynx is polluted with toxins that inhibit it from rehydrating for about 3 hours.
The vocal cords need a fair amount of lubrication and swell from inflammation when they do not have enough of it. When the vocal folds swell and are inflamed, phlegm is created to attempt to ease the dryness. Experiment on smoking correlations: In 2002, an experiment was done and published by the American College of Chest Physicians to find if there was a correlation of smokers with coughing and phlegm. In the study, 117 participants were studied, a mix of current smokers, ex-smokers, non-smokers, a positive control of participants with a disease, COPD At the end of the experiment, experimenters found that there was a high correlation between phlegm and cough with smoking of 0.49 Illness: During illness like the flu and pneumonia, phlegm becomes more excessive as an attempt to get rid of the bacteria or viral particles within the body. A major illness associated with excess phlegm is acute bronchitis. A major symptom of acute bronchitis is an excess amount of phlegm and is caused by a viral infection, only bacterial infections, which are rare, are to be treated with an antibiotic.
Hay fever, asthma: In hay fever and asthma, inner lining in bronchioles become inflamed and create an excess amount of phlegm that can clog up air pathways. Air pollution: In studies of children, air pollutants have been found to increase phlegm by drying out and irritating parts of the throat. Humourism is an ancient theory that the human body is filled with four basic substances, called the four humours, which are held in balance when a person is healthy, it is related to the ancient theory of the four elements and states that all diseases and disabilities result from an excess or deficit in black bile, yellow bile and blood. Hippocrates, an ancient Greek medical doctor, is credited for this theory, about 400 BC, it influenced medical thinking for more than 2,000 years, until discredited in the 1800s. Phlegm was thought to be associated with apathetic behaviour; this adjective always refers to behaviour, is pronounced differently, giving full weight to the "g": not /ˈflɛmatɪk/ but /flɛgˈmatɪk/.
To have "phlegm" traditionally meant to have stamina and to be unswayed by emotion. Sir William Osler’s 1889 Aequanimitas discusses the imperturbability or calmness in a storm required of physicians. "'Imperturbability means coolness and presence of mind under all circumstances, calmness amid storm, clearness of judgment in moments of grave peril, impassiveness, or, to use an old and expressive word, phlegm." This was his farewell speech at the University of Pennsylvania in 1889 before becoming Physician-in-Chief at the founded Johns Hopkins Hospital in Baltimore, Maryland. This is from "Celebrating the Contributions of William Osler" in the Alan Mason Chesney Medical Archives of the Johns Hopkins Medical Institutions." The phlegm of Humourism is far from the same thing as phlegm. Nobel laureate Charles Richet MD, when describing humorism's "phlegm or pituitary secretion" in 1910 asked rhetorically, "this strange liquid, the cause of tumours, of chlorosis, of rheumatism, cacochymia - where is it?
Who will see it? Who has seen it? What can we say of this fanciful classification of humours into four groups, of which two are imaginary?" Phlegm may be a carrier of larvae of intestinal parasites. Bloody sputum can be a symptom of serious disease, but can be a rela
Passive smoking is the inhalation of smoke, called second-hand smoke, or environmental tobacco smoke, by persons other than the intended "active" smoker. It occurs when tobacco smoke permeates any environment, causing its inhalation by people within that environment. Exposure to second-hand tobacco smoke causes disease and death; the health risks of second-hand smoke are a matter of scientific consensus. These risks have been a major motivation for smoke-free laws in workplaces and indoor public places, including restaurants and night clubs, as well as some open public spaces. Concerns around second-hand smoke have played a central role in the debate over the harms and regulation of tobacco products. Since the early 1970s, the tobacco industry has viewed public concern over second-hand smoke as a serious threat to its business interests. Harm to bystanders was perceived as a motivator for stricter regulation of tobacco products. Despite the industry's awareness of the harms of second-hand smoke as early as the 1980s, the tobacco industry coordinated a scientific controversy with the purpose of stopping regulation of their products.
Second-hand smoke causes many of the same diseases as direct smoking, including cardiovascular diseases, lung cancer, respiratory diseases. These diseases include: Cancer: General: overall increased risk. Lung cancer: passive smoking is a risk factor for lung cancer. In the United States passive smoke is estimated to cause more than 7,000 deaths from lung cancer a year among non-smokers. Breast cancer: The California Environmental Protection Agency concluded in 2005 that passive smoking increases the risk of breast cancer in younger premenopausal females by 70% and the US Surgeon General has concluded that the evidence is "suggestive," but still insufficient to assert such a causal relationship. In contrast, the International Agency for Research on Cancer concluded in 2004 that there was "no support for a causal relation between involuntary exposure to tobacco smoke and breast cancer in never-smokers." A 2015 meta-analysis found that the evidence that passive smoking moderately increased the risk of breast cancer had become "more substantial than a few years ago."
Pancreatic cancer: A 2012 meta-analysis found no evidence that passive smoking was associated with an increased risk of pancreatic cancer. Cervical cancer: A 2015 overview of systematic reviews found that exposure to second-hand smoke increased the risk of cervical cancer. Bladder cancer: A 2016 systematic review and meta-analysis found that secondhand smoke exposure was associated with a significant increase in the risk of bladder cancer. Circulatory system: risk of heart disease, reduced heart rate variability. Epidemiological studies have shown that both active and passive cigarette smoking increase the risk of atherosclerosis. Passive smoking is associated with an increased risk of stroke, this increased risk is disproportionately high at low levels of exposure. Lung problems: Risk of asthma. Risk of chronic obstructive pulmonary disease According to a 2015 review, passive smoking may increase the risk of tuberculosis infection and accelerate the progression of the disease, but the evidence remains weak.
The majority of studies on the association between secondhand smoke exposure and sinusitis have found a significant association between the two. Cognitive impairment and dementia: Exposure to secondhand smoke may increase the risk of cognitive impairment and dementia in adults 50 and over. Children exposed to second-hand smoke show reduced vocabulary and reasoning skills when compared with non-exposed children as well as more general cognitive and intellectual deficits. Mental health: Exposure to secondhand smoke is associated with an increased risk of depressive symptoms. During pregnancy: Low birth weight, part B, ch. 3. Premature birth, part B, ch. 3 Laws limiting smoking decrease premature births. Stillbirth and congenital malformations in children Recent studies comparing females exposed to Environmental Tobacco Smoke and non-exposed females, demonstrate that females exposed while pregnant have higher risks of delivering a child with congenital abnormalities, longer lengths, smaller head circumferences, low birth weight.
General: Worsening of asthma and other conditions. A 2014 systematic review and meta-analysis found that passive smoking was associated with a increased risk of allergic diseases among children and adolescents. Type 2 diabetes, it remains unclear whether the association between passive diabetes is causal. Risk of carrying Neisseria meningitidis or Streptococcus pneumoniae. A possible increased risk of periodontitis. Overall increased risk of death in both adults, where it is estimated to kill 53,000 nonsmokers per year, making it the 3rd leading cause of preventable death in the U. S, in children; the World Health Organization states that passive smoking causes about 600,000 deaths a year, about 1% of the global burden of disease. As of 2017, passive smoking causes about 900,000 deaths a year, about 1/8 of all deaths caused by smoking. Skin conditions: A 2016 systematic review and meta-analysis found that passive smoking was associated with a higher rate of atopic dermatitis. Sudden infant death syndrome.
In his 2006 report, the US Surgeon General concludes: "The evidence is sufficient to infer a causal relationship between exposure to secondhand smok
Radiology is the medical specialty that uses medical imaging to diagnose and treat diseases within the human body. A variety of imaging techniques such as X-ray radiography, computed tomography, nuclear medicine including positron emission tomography, magnetic resonance imaging are used to diagnose or treat diseases. Interventional radiology is the performance of minimally invasive medical procedures with the guidance of imaging technologies such as X-ray radiography, computed tomography, nuclear medicine including positron emission tomography, magnetic resonance imaging; the modern practice of radiology involves several different healthcare professions working as a team. The radiologist is a medical doctor who has completed the appropriate post-graduate training and interprets medical images, communicates these findings to other physicians by means of a report or verbally, uses imaging to perform minimally invasive medical procedures; the nurse is involved in the care of patients before and after imaging or procedures, including administration of medications, monitoring of vital signs and monitoring of sedated patients.
The radiographer known as a "radiologic technologist" in some countries such as the United States, is a specially trained healthcare professional that uses sophisticated technology and positioning techniques to produce medical images for the radiologist and nurse to interpret. Depending on the individual's training and country of practice, the radiographer may specialize in one of the above-mentioned imaging modalities or have expanded roles in image reporting. Radiographs are produced by transmitting X-rays through a patient; the X-rays are projected through the body onto a detector. Röntgen discovered X-rays on November 8, 1895 and received the first Nobel Prize in Physics for their discovery in 1901. In film-screen radiography, an X-ray tube generates a beam of X-rays, aimed at the patient; the X-rays that pass through the patient are filtered through a device called an grid or X-ray filter, to reduce scatter, strike an undeveloped film, held to a screen of light-emitting phosphors in a light-tight cassette.
The film is developed chemically and an image appears on the film. Film-screen radiography is being replaced by phosphor plate radiography but more by digital radiography and the EOS imaging. In the two latest systems, the X-rays strike sensors that converts the signals generated into digital information, transmitted and converted into an image displayed on a computer screen. In digital radiography the sensors shape a plate, but in the EOS system, a slot-scanning system, a linear sensor vertically scans the patient. Plain radiography was the only imaging modality available during the first 50 years of radiology. Due to its availability and lower costs compared to other modalities, radiography is the first-line test of choice in radiologic diagnosis. Despite the large amount of data in CT scans, MR scans and other digital-based imaging, there are many disease entities in which the classic diagnosis is obtained by plain radiographs. Examples include various types of arthritis and pneumonia, bone tumors, congenital skeletal anomalies, etc.
Mammography and DXA are two applications of low energy projectional radiography, used for the evaluation for breast cancer and osteoporosis, respectively. Fluoroscopy and angiography are special applications of X-ray imaging, in which a fluorescent screen and image intensifier tube is connected to a closed-circuit television system; this augmented with a radiocontrast agent. Radiocontrast agents are administered by swallowing or injecting into the body of the patient to delineate anatomy and functioning of the blood vessels, the genitourinary system, or the gastrointestinal tract. Two radiocontrast agents are presently in common use. Barium sulfate is given rectally for evaluation of the GI tract. Iodine, in multiple proprietary forms, is given by oral, vaginal, intra-arterial or intravenous routes; these radiocontrast agents absorb or scatter X-rays, in conjunction with the real-time imaging, allow demonstration of dynamic processes, such as peristalsis in the digestive tract or blood flow in arteries and veins.
Iodine contrast may be concentrated in abnormal areas more or less than in normal tissues and make abnormalities more conspicuous. Additionally, in specific circumstances, air can be used as a contrast agent for the gastrointestinal system and carbon dioxide can be used as a contrast agent in the venous system. CT imaging uses X-rays in conjunction with computing algorithms to image the body. In CT, an X-ray tube opposite an X-ray detector in a ring-shaped apparatus rotate around a patient, producing a computer-generated cross-sectional image. CT is acquired in the axial plane, with coronal and sagittal images produced by computer reconstruction. Radiocontrast agents are used with CT for enhanced delineation of anatomy. Although radiographs provide higher spatial resolution, CT can detect more subtle variations in attenuation of X-rays. CT exposes the patient to more ionizing radiation than a radiograph. Spiral multidetector CT uses 16, 64, 254 o
Calcium is a chemical element with symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air, its physical and chemical properties are most similar to its heavier homologues strontium and barium. It is the fifth most abundant element in Earth's crust and the third most abundant metal, after iron and aluminium; the most common calcium compound on Earth is calcium carbonate, found in limestone and the fossilised remnants of early sea life. The name derives from Latin calx "lime", obtained from heating limestone; some calcium compounds were known to the ancients, though their chemistry was unknown until the seventeenth century. Pure calcium was isolated in 1808 via electrolysis of its oxide by Humphry Davy, who named the element. Calcium compounds are used in many industries: in foods and pharmaceuticals for calcium supplementation, in the paper industry as bleaches, as components in cement and electrical insulators, in the manufacture of soaps.
On the other hand, the metal in pure form has few applications due to its high reactivity. Calcium is the fifth-most abundant element in the human body; as electrolytes, calcium ions play a vital role in the physiological and biochemical processes of organisms and cells: in signal transduction pathways where they act as a second messenger. Calcium ions outside cells are important for maintaining the potential difference across excitable cell membranes as well as proper bone formation. Calcium is a ductile silvery metal whose properties are similar to the heavier elements in its group, strontium and radium. A calcium atom has twenty electrons, arranged in the electron configuration 4s2. Like the other elements placed in group 2 of the periodic table, calcium has two valence electrons in the outermost s-orbital, which are easily lost in chemical reactions to form a dipositive ion with the stable electron configuration of a noble gas, in this case argon. Hence, calcium is always divalent in its compounds, which are ionic.
Hypothetical univalent salts of calcium would be stable with respect to their elements, but not to disproportionation to the divalent salts and calcium metal, because the enthalpy of formation of MX2 is much higher than those of the hypothetical MX. This occurs because of the much greater lattice energy afforded by the more charged Ca2+ cation compared to the hypothetical Ca+ cation. Calcium, strontium and radium are always considered to be alkaline earth metals. Beryllium and magnesium are different from the other members of the group in their physical and chemical behaviour: they behave more like aluminium and zinc and have some of the weaker metallic character of the post-transition metals, why the traditional definition of the term "alkaline earth metal" excludes them; this classification is obsolete in English-language sources, but is still used in other countries such as Japan. As a result, comparisons with strontium and barium are more germane to calcium chemistry than comparisons with magnesium.
Calcium metal melts at 842 °C and boils at 1494 °C. It crystallises in the face-centered cubic arrangement like strontium, its density of 1.55 g/cm3 is the lowest in its group. Calcium can be cut with a knife with effort. While calcium is a poorer conductor of electricity than copper or aluminium by volume, it is a better conductor by mass than both due to its low density. While calcium is infeasible as a conductor for most terrestrial applications as it reacts with atmospheric oxygen, its use as such in space has been considered; the chemistry of calcium is that of a typical heavy alkaline earth metal. For example, calcium spontaneously reacts with water more than magnesium and less than strontium to produce calcium hydroxide and hydrogen gas, it reacts with the oxygen and nitrogen in the air to form a mixture of calcium oxide and calcium nitride. When finely divided, it spontaneously burns in air to produce the nitride. In bulk, calcium is less reactive: it forms a hydration coating in moist air, but below 30% relative humidity it may be stored indefinitely at room temperature.
Besides the simple oxide CaO, the peroxide CaO2 can be made by direct oxidation of calcium metal under a high pressure of oxygen, there is some evidence for a yellow superoxide Ca2. Calcium hydroxide, Ca2, is a strong base, though it is not as strong as the hydroxides of strontium, barium or the alkali metals. All four dihalides of calcium are known. Calcium carbonate and calcium sulfate are abundant minerals. Like strontium and barium, as well as the alkali metals and the divalent lanthanides europium and ytterbium, calcium metal dissolves directly in liquid ammonia to give a dark blue solution. Due to the large size of the Ca2+ ion, high coordination numbers are common, up to 24 in some intermetallic compounds such as CaZn13. Calcium is complexed by oxygen chelates such as EDTA and polyphosphates, which are useful in an