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#901 2021-01-10 00:20:46

ganesh
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Re: Miscellany

879) Guinea pig

Guinea pig, (Cavia porcellus), a domesticated species of South American rodent belonging to the cavy family (Caviidae). It resembles other cavies in having a robust body with short limbs, large head and eyes, and short ears. The feet have hairless soles and short sharp claws. There are four toes on the forefeet and three on the hind feet. Several breeds of domesticated guinea pigs exist, which are sometimes grouped by coat texture and hair length. The term guinea pig is also used colloquially to refer to a person who serves as a test subject in an experiment.

Among rodents, domestic guinea pigs are fairly large, weighing 500 to 1,500 grams (roughly 1 to 3 pounds) and having a body 20 to 40 cm (8 to 16 inches) long. The tail is not visible externally. There is a crest of longer hairs at the neck, but length and texture of the fur vary from smooth (short or long) to coarse and short or long and silky. Coloration is extremely variable: the coat may be white, cream, tan, reddish or chocolate brown, black, or a combined pattern.

Guinea pigs eat vegetation and do not require water to drink if supplied with sufficiently moist food, but they must have water if fed dry commercial food. They breed all year in captivity. Females bear up to 13 young per litter (4 is average); gestation takes 68 days. Although the young can scamper about and eat solid food the day they are born, they are not fully weaned for about three weeks. Females mature in two months, males in three, and captive guinea pigs live up to eight years, although three to five is typical.

No natural population of this species exists in the wild. Guinea pigs were apparently domesticated more than 3,000 years ago in Peru, coinciding with humans’ transition from a nomadic to an agricultural lifestyle. The Incas kept guinea pigs, and the animals were bred during the same period by various people who lived along the Andes Mountains from northwestern Venezuela to central Chile. These rodents remain a sustainable food source for the native peoples of Ecuador, Peru, and Bolivia, who either keep them in their homes or allow them to scavenge freely both indoors and out. Guinea pigs were taken to Europe in the 16th century, and since the 1800s they have been popular as pets. They are also used internationally as laboratory animals for studies of anatomy, nutrition, genetics, toxicology, pathology, serum
development, and other research programs.

The origin of the colloquial name guinea pig is a subject of much debate. The first part of the name may have been derived from the price of the animal in 16th- and 17th-century England—that is, possibly one guinea—or it may have arisen from the animals’ being carried to European markets after first being transferred to ships in ports in Guinea. The moniker could also have originated with a mispronounced form of the word Guiana, the name of the region where some guinea pigs were collected. Another possible etymology is from the name of the class of ships—the Guineamen—that transported the animal. These were vessels that made port in West Africa as part of the transatlantic slave trade. The second part of the name also originated with Europeans, who compared the squealing sound the animal made (as well as the taste of its cooked flesh) to that of a pig.

There are five nondomesticated members of the genus Cavia that are also called guinea pigs: the Brazilian guinea pig (C. aperea) found from Colombia, Venezuela, and the Guianas south to northern Argentina; the shiny guinea pig (C. fulgida), inhabiting eastern Brazil; the montane guinea pig (C. tschudii), ranging from Peru to northern Chile and northwestern Argentina; the greater guinea pig (C. magna), occurring in southeastern Brazil and Uruguay; and the Moleques do Sul guinea pig (C. intermedia), which is limited to an island in the Moleques do Sul archipelago off the southern coast of Brazil. Breeding and molecular studies suggest that the domestic guinea pig was derived from one of the wild Brazilian, shiny, or montane species.

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#902 2021-01-11 00:16:44

ganesh
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Re: Miscellany

880) Sheep

Sheep, (Ovis aries), species of domesticated ruminant (cud-chewing) mammal, raised for its meat, milk, and wool. The sheep is usually stockier than its relative the goat (genus Capra); its horns, when present, are more divergent; it has scent glands in its face and hind feet; and the males lack the beards of goats. Sheep usually have short tails. In all wild species of sheep, the outer coat takes the form of hair, and beneath this lies a short undercoat of fine wool that has been developed into the fleece of domesticated sheep. Male sheep are called rams, the females ewes, and immature animals lambs. Mature sheep weigh from about 35 to as much as 180 kg (80 to 400 pounds).

A sheep regurgitates its food and chews the cud, thus enabling its four separate stomach compartments to thoroughly digest the grasses and other herbage that it eats. The animals prefer grazing on grass or legume vegetation that is short and fine, though they will also consume high, coarse, or brushy plants as well. They graze plants closer to the root than do cattle, and so care must be taken that sheep do not overgraze a particular range. Sheep are basically timid animals who tend to graze in flocks and are almost totally lacking in protection from predators. They mature at about one year of age, and many breed when they reach the age of about one and a half years. Most births are single, although sheep do have twins on occasion. The lambs stop suckling and begin to graze at about four or five months of age.

Sheep were first domesticated from wild species of sheep at least 5000 BCE, and their remains have been found at numerous sites of early human habitation in the Middle East, Europe, and Central Asia. Domesticated sheep are raised for their fleece (wool), for milk, and for meat. The flesh of mature sheep is called mutton; that of immature animals is called lamb. There were estimated to be more than one billion sheep in the world in the early 21st century. The major national producers are Australia, New Zealand, China, India, the United States, South Africa, Argentina, and Turkey. Countries that have large areas of grassland are the major producers.

Domestic sheep differ from their wild progenitors and among themselves in conformation, quantity and quality of fleece, colour, size, milk production, and other characteristics. Most breeds of domesticated sheep produce wool, while a few produce only hair, and wild sheep grow a combination of wool and hair. Several hundred different breeds of sheep have been developed to meet environmental conditions influenced by latitudes and altitudes and to satisfy human needs for clothing and food. Breeds of sheep having fine wool are generally raised for wool production alone, while breeds with medium or long wool or with only hair are generally raised for meat production. Several crossbreeds have been developed that yield both wool and meat of high quality, however. Of the more than 200 breeds of sheep in the world, the majority are of limited interest except in local areas.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#903 2021-01-12 00:44:16

ganesh
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Re: Miscellany

881) Marathon

Marathon, long-distance footrace first held at the revival of the Olympic Games in Athens in 1896. It commemorates the legendary feat of a Greek soldier who, in 490 BC, is supposed to have run from Marathon to Athens, a distance of about 40 km (25 miles), to bring news of the Athenian victory over the Persians and then expired. The story of this messenger from the Battle of Marathon was later conflated with the story of another Greek soldier, Pheidippides, who ran from Athens to Sparta in advance of the fighting. Appropriately, in 1896 the first modern marathon winner was a Greek, Spyridon Louis.

In 1924 the Olympic marathon distance was standardized at 42,195 metres (26 miles 385 yards). This was based on a decision of the British Olympic Committee to start the 1908 Olympic race from Windsor Castle and finish it in front of the royal box in the stadium at London. The marathon was added to the women’s Olympic program in 1984.

After the Olympic Games championship, one of the most coveted honours in marathon running is victory in the Boston Marathon, held annually since 1897. It draws athletes from all parts of the world and in 1972 became the first major marathon to officially allow women to compete. Other premiere marathons are held in London, Chicago, Berlin, New York City, Tokyo, and Amsterdam. Marathons are not held on the track but on roads, and, despite the fact that courses are not of equal difficulty, the International Association of Athletics Federations (IAAF) does list world records for the marathon and also for the half-marathon. World-record times in the marathon steadily declined over the course of the 20th century from slightly under three hours to slightly more than two hours.

It was long considered necessary for a runner to prepare for a marathon by training over that distance. At the 1952 Olympic Games, however, Czech Emil Zátopek set an Olympic record of 2 hours 23 minutes 3.2 seconds, even though he had never run the distance before. In the decades following, other first-time marathoners also won premiere events and set records at the distance. By the late 20th century, road racing, and marathon running in particular, had grown to become a recreational activity with broad appeal. Ultramarathons, which are neither Olympic nor IAAF events, are longer races based on a specific distance or an allotted time period for competition, such as a 12-hour race.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#904 2021-01-13 00:02:54

ganesh
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Re: Miscellany

882) Cornea

Cornea, dome-shaped transparent membrane about 12 mm (0.5 inch) in diameter that covers the front part of the eye. Except at its margins, the cornea contains no blood vessels, but it does contain many nerves and is very sensitive to pain or touch. It is nourished and provided with oxygen anteriorly by tears and is bathed posteriorly by aqueous humour. It protects the pupil, the iris, and the inside of the eye from penetration by foreign bodies and is the first and most powerful element in the eye’s focusing system. As light passes through the cornea, it is partially refracted before reaching the lens. The curvature of the cornea, which is spherical in infancy but changes with age, gives it its focusing power; when the curve becomes irregular, it causes a focusing defect called astigmatism, in which images appear elongated or distorted.

The cornea itself is composed of multiple layers, including a surface epithelium, a central, thicker stroma, and an inner endothelium. The epithelium (outer surface covering) of the cornea is an important barrier to infection. A corneal abrasion, or scratch, most often causes a sensation of something being on the eye and is accompanied by intense tearing, pain, and light sensitivity. Fortunately, the corneal epithelium is able to heal quickly in most situations.

The collagen fibres that make up the corneal stroma (middle layer) are arranged in a strictly regular, geometric fashion. This arrangement has been shown to be the essential factor resulting in the cornea’s transparency. When the cornea is damaged by infection or trauma, the collagen laid down in the repair processes is not regularly arranged, with the result that an opaque patch or scar may occur. If the clouded cornea is removed and replaced by a healthy one (i.e., by means of corneal transplant), usually taken from a deceased donor, normal vision can result.

The innermost layer of the cornea, the endothelium, plays a critical role in keeping the cornea from becoming swollen with excess fluid. As endothelial cells are lost, new cells are not produced; rather, existing cells expand to fill in the space left behind. Once loss of a critical number of endothelial cells has occurred, however, the cornea can swell, causing decreased vision and, in severe cases, surface changes and pain. Endothelial cell loss can be accelerated via mechanical trauma or abnormal age-related endothelial cell death (called Fuchs endothelial dystrophy). Treatment may ultimately require corneal transplant.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#905 2021-01-14 00:34:12

ganesh
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Re: Miscellany

883) Iris

Iris, in anatomy, the pigmented muscular curtain near the front of the eye, between the cornea and the lens, that is perforated by an opening called the pupil. The iris is located in front of the lens and ciliary body and behind the cornea. It is bathed in front and behind by a fluid known as the aqueous humour. The iris consists of two sheets of smooth muscle with contrary actions: dilation (expansion) and contraction (constriction). These muscles control the size of the pupil and thus determine how much light reaches the sensory tissue of the retina. The sphincter muscle of the iris is a circular muscle that constricts the pupil in bright light, whereas the dilator muscle of the iris expands the opening when it contracts. The amount of pigment contained in the iris determines eye colour. When there is very little pigment, the eye appears blue. With increased pigment, the shade becomes deep brown to black. Inflammation of the iris is termed iritis or anterior uveitis, a condition that commonly has no determinable cause. As a result of inflammation, the iris sticks to the lens or the cornea, blocking the normal flow of fluid in the eye. Complications of iritis include secondary glaucoma and blindness; treatment usually involves topical steroid eyedrops.

The-human-eye-with-the-cornea-lens-optic-nerve-retina-and-fovea-all-indicated.png


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#906 2021-01-15 00:45:47

ganesh
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Re: Miscellany

884) Placenta

Placenta, in zoology, the vascular (supplied with blood vessels) organ in most mammals that unites the fetus to the uterus of the mother. It mediates the metabolic exchanges of the developing individual through an intimate association of embryonic tissues and of certain uterine tissues, serving the functions of nutrition, respiration, and excretion.

All of the fetal membranes function by adapting the developing fetus to the uterine environment. Lying in the chorionic cavity (a thin liquid-filled space) between two membranous envelopes (chorion and amnion) is a small balloon-like sac, yolk sac, or vitelline sac, attached by a delicate strand of tissue to the region where the umbilical cord (the structure connecting the fetus with the placenta) leaves the amnion. Two large arteries in the umbilical cord radiate from the attachment of the cord on the inner surface of the placenta and divide into small arteries that penetrate outward into the depths of the placenta through hundreds of branching and interlacing strands of tissue known as villi. The chorionic villi cause the mother’s blood vessels in their vicinity to rupture, and the villi become bathed directly in maternal blood. The constant circulation of fetal and maternal blood and the very thin tissue separation of fetal blood in the capillaries from maternal blood bathing the villi provide a mechanism for efficient interchange of blood constituents between the maternal and fetal bloodstreams without (normally) allowing any opportunity for the blood of one to pour across into the blood vessels of the other.

Nutrients, oxygen, and antibodies (proteins formed in response to a foreign substance, or antigen), as well as other materials in the mother’s blood, diffuse into the fetal blood in the capillaries of the villi, and nitrogenous wastes and carbon dioxide diffuse out of these capillaries into the maternal blood circulation. The purified and enriched blood in the capillaries of the villi is collected into fetal veins, which carry it back to the inner surface of the placenta and collect at the attachment of the cord to form the umbilical vein. This vein enters the cord alongside the two arteries and carries the blood back to the fetus, thus completing the circuit to and from the placenta.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#907 2021-01-16 00:25:37

ganesh
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Re: Miscellany

885) Plasma

Plasma, also called blood plasma, the liquid portion of blood. Plasma serves as a transport medium for delivering nutrients to the cells of the various organs of the body and for transporting waste products derived from cellular metabolism to the kidneys, liver, and lungs for excretion. It is also a transport system for blood cells, and it plays a critical role in maintaining normal blood pressure. Plasma helps to distribute heat throughout the body and to maintain homeostasis, or biological stability, including acid-base balance in the blood and body.

Plasma is derived when all the blood cells—red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes)—are separated from whole blood. The remaining straw-coloured fluid is 90–92 percent water, but it contains critical solutes necessary for sustaining health and life. Important constituents include electrolytes such as sodium, potassium, chloride, bicarbonate, magnesium, and calcium. In addition, there are trace amounts of other substances, including amino acids, vitamins, organic acids, pigments, and enzymes. Hormones such as insulin, corticosteroids, and thyroxine are secreted into the blood by the endocrine system. Plasma concentrations of hormones must be carefully regulated for good health. Nitrogenous wastes (e.g., urea and creatinine) transported to the kidney for excretion increase markedly with renal failure.

Plasma contains 6–8 percent proteins. One critical group is the coagulation proteins and their inhibitors, synthesized primarily in the liver. When blood clotting is activated, fibrinogen circulating in the blood is converted to fibrin, which in turn helps to form a stable blood clot at the site of vascular disruption. Coagulation inhibitor proteins help to prevent abnormal coagulation (hypercoagulability) and to resolve clots after they are formed. When plasma is allowed to clot, fibrinogen converts to fibrin, trapping the cellular elements of blood. The resulting liquid, devoid of cells and fibrinogen, is called serum. Biochemical testing of plasma and serum is an important part of modern clinical diagnosis and treatment monitoring. High or low concentrations of glucose in the plasma or serum help to confirm serious disorders such as diabetes mellitus and hypoglycemia. Substances secreted into the plasma by cancers may indicate an occult malignancy; for instance, an increased concentration of prostate-specific antigen (PSA) in a middle-aged asymptomatic man may indicate undiagnosed prostate cancer.

Serum albumin, another protein synthesized by the liver, constitutes approximately 60 percent of all of the plasma proteins. It is very important in maintaining osmotic pressure in the blood vessels; it is also an important carrier protein for a number of substances, including hormones. Other proteins called alpha and beta globulins transport lipids such as cholesterol as well as steroid hormones, sugar, and iron.

The gamma globulins, or immunoglobulins, are an important class of proteins that are secreted by B lymphocytes of the immune system. They include most of the body’s supply of protective antibodies produced in response to specific viral or bacterial antigens. Cytokines are proteins synthesized by cells of various organs and by cells found in the immune system and bone marrow in order to maintain normal blood cell formation (hematopoiesis) and regulate inflammation. For example, one cytokine called erythropoietin, synthesized by specialized kidney cells, stimulates bone marrow blood progenitor cells to produce red blood cells. Other cytokines stimulate the production of white blood cells and platelets. Another protein system in the plasma, called complement, is important in mediating appropriate immune and inflammatory responses to a variety of infectious agents.

The electrolytes and acid-base system found in the plasma are finely regulated. For example, potassium is normally present in plasma in a concentration of only 4 milliequivalents per litre. A slight rise in plasma potassium (to 6–7 milliequivalents per litre) can result in death. Likewise, sodium, chloride, bicarbonate, calcium, and magnesium levels in the plasma must be precisely maintained within a narrow range. Smaller molecules such as sodium, potassium, glucose, and calcium are primarily responsible for the concentration of dissolved particles in the plasma. However, it is the concentration of much larger proteins (especially albumin) on either side of semipermeable membranes such as the endothelial cells lining the capillaries that creates crucial pressure gradients necessary to maintain the correct amount of water within the intravascular compartment and, therefore, to regulate the volume of circulating blood. So, for example, patients who have kidney dysfunction or low plasma protein concentrations (especially low albumin) may develop a migration of water from the vascular space into the tissue spaces, causing edema (swelling) and congestion in the extremities and vital organs, including the lungs.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#908 2021-01-17 00:33:29

ganesh
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Re: Miscellany

886) Eyeball

Eyeball, spheroidal structure containing sense receptors for vision, found in all vertebrates and constructed much like a simple camera. The eyeball houses the retina—an extremely metabolically active layer of nerve tissue made up of millions of light receptors (photoreceptors)—and all of the structures needed to focus light onto it. The sclera, the tough protective outer shell of the eyeball, is composed of dense fibrous tissue that covers four-fifths of the eyeball and provides attachments for the muscles that move the eye. The sclera is itself covered anteriorly by the conjunctiva, a transparent mucous membrane that prevents the eye from drying out. At the front of the eye, the tear film covers the transparent cornea, the “window” through which light passes into the eye. Working in concert with the aqueous humour behind it, the cornea provides the greatest focusing power of the eye. However, unlike the lens, the shape and focusing power of the cornea are not adjustable. Other important structures in the eyeball include the iris and the lens. Much of the eyeball is filled with a transparent gel-like material, called the vitreous humour, that helps to maintain the spheroidal shape.

Immediately beneath the sclera is an underlying vascular layer, called the uvea, that supplies nutrients to many parts of the eye. One component of the uvea is the ciliary body, a muscular structure located behind the iris that alters the shape of the lens during focusing and produces the aqueous humour that bathes the anterior chamber. The other components of the uvea are the iris and the choroid. The choroid is a highly vascular tissue layer that provides blood to the outer layers of the retina that lie over it.

The cornea, where the focusing process begins, is curved to a much greater extent than the rest of the eyeball. Defects in corneal curvature cause a distortion of vision known as astigmatism. Behind the cornea is the anterior chamber, which extends posteriorly to the plane of the iris and pupil. It is filled with a watery fluid called the aqueous humour. The iris is a doughnut-shaped, muscular curtain that opens and closes to regulate the amount of light entering the eye through the pupil, the opening at the iris’s centre. The aqueous humour flows through the pupil from the posterior chamber (a small space between the iris and the lens) to the anterior chamber and out of the eye through the trabecular meshwork and Schlemm’s canal, which encircles the peripheral iris. Some aqueous humour also exits the eye directly through the ciliary body. The ciliary muscle attachments and the lens separate the aqueous humour in front from the vitreous humour behind.

The shape of the lens is controlled by the action of the ciliary body, altering the focusing power of the lens as needed. The cornea and lens focus an image onto the retina at the back of the eye. If the image is projected too far in front of the retina, it causes the visual defect called myopia, or nearsightedness. If the image is theoretically focused “behind” the retina, the result is hyperopia, or farsightedness. If no deformation of the lens is present, the image is projected onto the fovea, a structure near the centre of the retina that contains a large number of cone photoreceptors and that provides the sharpest vision. When stimulated by light, retinal photoreceptor cells send signals to neighbouring cells in the retina that then relay the signals through the optic nerve to the visual centres of the brain.

eye-diagram-2.png


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

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#909 2021-01-18 00:23:13

ganesh
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Re: Miscellany

887) Blind spot

Blind spot, small portion of the visual field of each eye that corresponds to the position of the optic disk (also known as the optic nerve head) within the retina. There are no photoreceptors (i.e., rods or cones) in the optic disk, and, therefore, there is no image detection in this area. The blind spot of the right eye is located to the right of the centre of vision and vice versa in the left eye. With both eyes open, the blind spots are not perceived because the visual fields of the two eyes overlap. Indeed, even with one eye closed, the blind spot can be difficult to detect subjectively because of the ability of the brain to “fill in” or ignore the missing portion of the image.

The optic disk can be seen in the back of the eye with an ophthalmoscope. It is located on the nasal side of the macula lutea, is oval in shape, and is approximately 1.5 mm (0.06 inch) in diameter. It is also the entry point into the eye for major blood vessels that serve the retina. The optic disk represents the beginning of the optic nerve (second cranial nerve) and the point where axons from over one million retinal ganglion cells coalesce. Clinical evaluation of the optic nerve head is critical in the diagnosis and monitoring of glaucoma and other optic neuropathies that may lead to vision loss.

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It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#910 2021-01-19 00:03:28

ganesh
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Re: Miscellany

888) Retina

Retina, layer of nervous tissue that covers the inside of the back two-thirds of the eyeball, in which stimulation by light occurs, initiating the sensation of vision. The retina is actually an extension of the brain, formed embryonically from neural tissue and connected to the brain proper by the optic nerve.

The retina is a complex transparent tissue consisting of several layers, only one of which contains light-sensitive photoreceptor cells. Light must pass through the overlying layers to reach the photoreceptor cells, which are of two types, rods and cones, that are differentiated structurally by their distinctive shapes and functionally by their sensitivity to different kinds of light. Rods predominate in nocturnal animals and are most sensitive to reduced light intensities; in humans they provide night vision and aid in visual orientation. Cones are more prominent in humans and those animals that are active during the day and provide detailed vision (as for reading) and colour perception. In general, the more cones per unit area of retina, the finer the detail that can be discriminated by that area. Rods are fairly well distributed over the entire retina, but cones tend to concentrate at two sites: the fovea centralis, a pit at the rear of the retina, which contains no rods and has the densest concentration of cones in the eye, and the surrounding macula lutea, a circular patch of yellow-pigmented tissue about 5 to 6 mm (0.2 to 0.24 inch) in diameter.

When light enters the eye, it passes through the cornea and the lens and is refracted, focusing an image onto the retina. Light-sensitive molecules in the rods and cones react to specific wavelengths of light and trigger nerve impulses. Complex interconnections (synapses) between and within retinal cell layers assemble these impulses into a coherent pattern, which in turn is carried through the optic nerve to the visual centres of the brain, where they are further organized and interpreted.

eye-anatomy-retina.jpg


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#911 2021-01-20 00:11:05

ganesh
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Re: Miscellany

889) Embryo

Human And Animal

Embryo, the early developmental stage of an animal while it is in the egg or within the uterus of the mother. In humans the term is applied to the unborn child until the end of the seventh week following conception; from the eighth week the unborn child is called a fetus.

A brief treatment of embryonic development follows.

In organisms that reproduce mating, the union of a male gamete and female gamete results in a zygote, or fertilized egg, which undergoes a series of divisions called cleavages as it passes down the fallopian tube. After several cleavages have taken place, the cells form a hollow ball called a blastula. In most mammals the blastula attaches itself to the uterine lining, thus stimulating the formation of a placenta, which will transfer nutrients from the mother to the growing embryo. In lower animals the embryo is nourished by the yolk.

By the process of gastrulation, the embryo differentiates into three types of tissue: the ectoderm, producing the skin and nervous system; the mesoderm, from which develop connective tissues, the circulatory system, muscles, and bones; and the endoderm, which forms the digestive system, lungs, and urinary system. Mesodermal cells migrate from the surface of the embryo to fill the space between the other two tissues through an elongated depression known as the primitive streak. As the embryo develops, the cell layers fold over so that the endoderm forms a long tube surrounded by mesoderm, with an ectodermal layer around the whole.

Nutrients pass from the placenta through the umbilical cord, and the amnion, a fluid-filled membrane, surrounds and protects the embryo. The division of the body into head and trunk becomes apparent, and the brain, spinal cord, and internal organs begin to develop. All of these changes are completed early in embryonic development, by about the fourth week, in humans.

Between the head and the heart, a series of branchial arches, cartilaginous structures that support the gills of fishes and larval amphibians, begin to form. In higher vertebrates these structures form part of the jaw and ear. Limb buds also appear, and by the end of the embryonic stage, the embryo is distinguishable as a representative of its species.

2-Photographs-of-human-embryos-at-five-stages-of-gestation-reproduced-from-the.png


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#912 2021-01-21 00:53:31

ganesh
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Re: Miscellany

890) Serum albumin

Serum albumin, protein found in blood plasma that helps maintain the osmotic pressure between the blood vessels and tissues. Serum albumin accounts for 55 percent of the total protein in blood plasma. Circulating blood tends to force fluid out of the blood vessels and into the tissues, where it results in edema (swelling from excess fluid). The colloid nature of albumin—and, to a lesser extent, of other blood proteins called globulins—keeps the fluid within the blood vessels. Albumin also acts as a carrier for two materials necessary for the control of blood clotting: (1) antithrombin, which keeps the clotting enzyme thrombin from working unless needed, and (2) heparin cofactor, which is necessary for the anticlotting action of heparin. The serum albumin level falls and rises in such liver disorders as cirrhosis or hepatitis. Transfusions of serum albumin are used to combat shock and whenever it is necessary to remove excess fluid from the tissues. Similar albumin compounds with other functions occur in plants, animal tissues, egg whites, and milk.

Human serum albumin is the serum albumin found in human blood. It is the most abundant protein in human blood plasma; it constitutes about half of serum protein. It is produced in the liver. It is soluble in water, and it is monomeric.

Albumin transports hormones, fatty acids, and other compounds, buffers pH, and maintains oncotic pressure, among other functions.

Albumin is synthesized in the liver as preproalbumin, which has an N-terminal peptide that is removed before the nascent protein is released from the rough endoplasmic reticulum. The product, proalbumin, is in turn cleaved in the Golgi apparatus to produce the secreted albumin.

The reference range for albumin concentrations in serum is approximately 35–50 g/L (3.5–5.0 g/dL). It has a serum half-life of approximately 21 days. It has a molecular mass of 66.5 kDa.

The gene for albumin is located on chromosome 4 in locus 4q13.3 and mutations in this gene can result in anomalous proteins. The human albumin gene is 16,961 nucleotides long from the putative 'cap' site to the first poly(A) addition site. It is split into 15 exons that are symmetrically placed within the 3 domains thought to have arisen by triplication of a single primordial domain.

Prolonged release of drugs

Cancer is the uncontrolled growth of cells with loss of differentiation and commonly with metastasis. Anticancer drugs are used to control the growth of cancerous cells. Oxaliplatin is a third-generation platinum-derived antineoplastic agent that has been proved effective mainly against advanced colorectal cancer (CRC). Oxaliplatin administration has an acute excitatory and sensitizing effect, including unpleasant cold allodynia in the distal extremities, mouth, and throat and usually associated with muscle cramps. However, the major side effect, a dose-limiting toxicity of the peripheral nerves, affects the sustainability of the planned treatment. Although these acute symptoms resolve within 1 week, severe chronic sensory neuropathy develops with the increasing cumulative dose and is characterized by distal paresthesia and numbness. Drug  delivery  systems  aim to deliver the drugs over an extended duration or at a specific time during treatment. Therefore, the improvement of the efficiency and convenience of a drug release system is of paramount importance. In the past two decades, numerous nanoparticles (NPs) have been extensively explored in order to achieve precisely controlled drug delivery. However, the application and development of high-efficiency drug release system are still limited due to a lacking of simple, stable and efficient drug delivery vehicle. Human serum albumin (HSA) is a highly water-soluble globular monomeric plasma protein with a relative molecular weight of 67 KDa, consisting of 585 amino acid residues, one sulfhydryl group and 17 disulfide bridges. Among nanoparticulate carriers, HSA nanoparticles have long been the center of attention in the pharmaceutical industry due to their ability to bind to various drug molecules, great stability during storage and in vivo usage, no toxicity and antigenicity, biodegradability, reproducibility, scale up of the production process and a better control over release properties. In addition, significant amounts of drug can be incorporated into the particle matrix because of the large number of drug binding sites on the albumin molecule.

Function

i) Maintains oncotic pressure
ii) Transports thyroid hormones
iii) Transports other hormones, in particular, ones that are fat-soluble
iv) Transports fatty acids ("free" fatty acids) to the liver and to myocytes for utilization of energy
v) Transports unconjugated bilirubin
vi) Transports many drugs; serum albumin levels can affect the half-life of drugs. Competition between drugs for albumin binding sites may cause drug interaction by increasing the free fraction of one of the drugs, thereby affecting potency.
vii) Competitively binds calcium ions (Ca2+)
viii) Serum albumin, as a negative acute-phase protein, is down-regulated in inflammatory states. As such, it is not a valid marker of nutritional status; rather, it is a marker of an inflammatory state
ix) Prevents photodegradation of folic acid
x) Prevent pathogenic effects of Clostridium difficile toxins

Measurement

Serum albumin is commonly measured by recording the change in absorbance upon binding to a dye such as bromocresol green or bromocresol purple.

Reference ranges
The normal range of human serum albumin in adults (> 3 y.o.) is 3.5–5.0  g/dL (35–50 g/L). For children less than three years of age, the normal range is broader, 2.9–5.5 g/dL.

Low albumin (hypoalbuminemia) may be caused by liver disease, nephrotic syndrome, burns, protein-losing enteropathy, malabsorption, malnutrition, late pregnancy, artefact, genetic variations and malignancy.

High albumin (hyperalbuminemia) is almost always caused by dehydration. In some cases of retinol (Vitamin A) deficiency, the albumin level can be elevated to high-normal values (e.g., 4.9 g/dL). This is because retinol causes cells to swell with water (this is also the reason too much Vitamin A is toxic). This swelling also likely occurs during treatment with 13-cis retinoic acid (isotretnoin), a pharmaceutical for treating severe acne, amongst other conditions. In lab experiments it has been shown that all-trans retinoic acid down regulates human albumin production.

Pathology

i) Hypoalbuminemia

Hypoalbuminemia means low blood albumin levels. This can be caused by:

a) Liver disease; cirrhosis of the liver is most common
b) Excess excretion by the kidneys (as in nephrotic syndrome)
c) Excess loss in bowel (protein-losing enteropathy, e.g., Ménétrier's disease)
d) Burns (plasma loss in the absence of skin barrier)
e) Redistribution (hemodilution [as in pregnancy], increased vascular permeability or decreased lymphatic clearance)
f) Acute disease states (referred to as a negative acute-phase protein)
g) Malnutrition and wasting
i) Mutation causing analbuminemia (very rare)
j) Anorexia nervosa (most common cause in adolescents)

ii) Hyperalbuminemia

Hyperalbuminemia is an increased concentration of albumin in the blood. Typically, this condition is due to dehydration. Hyperalbuminemia has also been associated with high protein diets.

Medical use

Human albumin solution (HSA) is available for medical use, usually at concentrations of 5–25%.

Human albumin is often used to replace lost fluid and help restore blood volume in trauma, burns and surgery patients. There is no strong medical evidence that albumin administration (compared to saline) saves lives for people who have hypovolaemia or for those who are critically ill due to burns or hypoalbuminaemia. It is also not known if there are people who are critically ill that may benefit from albumin.[20] Therefore, the Cochrane Collaboration recommends that it should not be used, except in clinical trials.

In acoustic droplet vaporization (ADV), albumin is sometimes used as a surfactant. ADV has been proposed as a cancer treatment by means of occlusion therapy.

Human serum albumin may be used to potentially reverse drug/chemical toxicity by binding to free drug/agent.

Human albumin may also be used in treatment of decompensated cirrhosis.

Human serum albumin has been used as a component of a frailty index.

It has not been shown to give better results than other fluids when used simply to replace volume, but is frequently used in conditions where loss of albumin is a major problem, such as liver disease with ascites.

Glycation

It has been known for a long time that human blood proteins like hemoglobin and serum albumin may undergo a slow non-enzymatic glycation, mainly by formation of a Schiff base between ε-amino groups of lysine (and sometimes arginine) residues and glucose molecules in blood (Maillard reaction). This reaction can be inhibited in the presence of antioxidant agents. Although this reaction may happen normally, elevated glycoalbumin is observed in diabetes mellitus.

Glycation has the potential to alter the biological structure and function of the serum albumin protein.

Moreover, the glycation can result in the formation of Advanced Glycation End-Products (AGE), which result in abnormal biological effects. Accumulation of AGEs leads to tissue damage via alteration of the structures and functions of tissue proteins, stimulation of cellular responses, through receptors specific for AGE-proteins, and generation of reactive oxygen intermediates. AGEs also react with DNA, thus causing mutations and DNA transposition. Thermal processing of proteins and carbohydrates brings major changes in allergenicity. AGEs are antigenic and represent many of the important neoantigens found in cooked or stored foods. They also interfere with the normal product of nitric oxide in cells.

Although there are several lysine and arginine residues in the serum albumin structure, very few of them can take part in the glycation reaction.

Oxidation

The albumin is the predominant protein in most body fluids, its Cys34 represents the largest fraction of free thiols within body. The albumin Cys34 thiol exists in both reduced and oxidized forms. In plasma of healthy young adults, 70–80% of total HSA contains the free sulfhydryl group of Cys34 in a reduced form or mercaptoalbumin (HSA-SH). However, in pathological states characterized by oxidative stress and during the aging process, the oxidized form, or non-mercaptoalbumin (HNA), could predominate. The albumin thiol reacts with radical hydroxyl (.OH), hydrogen peroxide (H2O2) and the reactive nitrogen species as peroxynitrite (ONOO.), and have been shown to oxidize Cys34 to sulfenic acid derivate (HSA-SOH), it can be recycled to mercapto-albumin; however at high concentrations of reactive species leads to the irreversible oxidation to sulfinic (HSA-SO2H) or sulfonic acid (HSA-SO3H) affecting its structure. Presence of reactive oxygen species (ROS), can induce irreversible structural damage and alter protein activities.

Loss via kidneys

In the healthy kidney, albumin's size and negative electric charge exclude it from excretion in the glomerulus. This is not always the case, as in some diseases including diabetic nephropathy, which can sometimes be a complication of uncontrolled or of longer term diabetes in which proteins can cross the glomerulus. The lost albumin can be detected by a simple urine test. Depending on the amount of albumin lost, a patient may have normal renal function, microalbuminuria, or albuminuria.

Interactions

Human serum albumin has been shown to interact with FCGRT.

It might also interact with a yet-unidentified albondin (gp60), a certain pair of gp18/gp30, and some other proteins like osteonectin, hnRNPs, calreticulin, cubilin, and megalin.

Crystal-structure-of-human-serum-albumin-The-illustration-shows-the-tertiary-structure.png


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#913 2021-01-22 00:10:34

ganesh
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Registered: 2005-06-28
Posts: 32,080

Re: Miscellany

891) Nephron

Nephron, functional unit of the kidney, the structure that actually produces urine in the process of removing waste and excess substances from the blood. There are about 1,000,000 nephrons in each human kidney. The most primitive nephrons are found in the kidneys (pronephros) of primitive fish, amphibian larvae, and embryos of more advanced vertebrates. The nephrons found in the kidneys (mesonephros) of amphibians and most fish, and in the late embryonic development of more advanced vertebrates, are only slightly more advanced in structure. The most advanced nephrons occur in the adult kidneys, or metanephros, of land vertebrates, such as reptiles, birds, and mammals.

Each nephron in the mammalian kidney is a long tubule, or extremely fine tube, about 30–55 mm (1.2–2.2 inches) long. At one end this tube is closed, expanded, and folded into a double-walled cuplike structure. This structure, called the renal corpuscular capsule, or Bowman’s capsule, encloses a cluster of microscopic blood vessels—capillaries—called the glomerulus. The capsule and glomerulus together constitute the renal corpuscle. Blood flows into and away from the glomerulus through tiny arteries called arterioles, which reach and leave the glomerulus through the open end of the capsule. In the renal corpuscle, fluid filters out of the blood in the glomerulus through the inner wall of the capsule and into the nephron tubule. As this filtrate passes through the tubule, its composition is altered by the secretion of certain substances into it and by the selective reabsorption of water and other constituents from it. The final product is urine, which is conveyed through the collecting tubules into the renal pelvis.

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#914 Yesterday 00:43:34

ganesh
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Registered: 2005-06-28
Posts: 32,080

Re: Miscellany

892) Truss bridge

Truss bridge, bridge with its load-bearing structures composed of a series of wooden or metal triangles, known as trusses. Given that a triangle cannot be distorted by stress, a truss gives a stable form capable of supporting considerable external loads over a large span. Trusses are popular for bridge building because they use a relatively small amount of material for the amount of weight they can support. They commonly are used in covered bridges, railroad bridges, and military bridges.

Form And Mechanics

The individual pieces of a truss bridge intersect at truss joints, or panel points. The connected pieces forming the top and bottom of the truss are referred to respectively as the top and bottom chords. The sloping and vertical pieces connecting the chords are collectively referred to as the web of the truss.

The component parts of a truss bridge are stressed primarily in axial tension or compression. A single-span truss bridge is like a simply supported beam because it carries vertical loads by bending. Bending leads to compression, in the top chords (or horizontal members), tension in the bottom chords, and either tension or compression in the vertical and diagonal members, depending on their orientation.

Early trusses were built without precise knowledge of how the loads are carried by each part of the truss. The first engineer to analyze correctly the stresses in a truss was Squire Whipple, an American who designed hundreds of small truss bridges and published his theories in 1869. Understanding precisely how loads were carried led to a reduction in materials, which by then were shifting from wood and stone to iron and steel.

History And Uses

There is no evidence of truss bridges in the ancient world, but the 13th-century sketchbook of the French architect Villard de Honnecourt depicts a species of truss bridge, and the Italian Andrea Palladio’s “Treatise on Architecture” (1570) describes four designs. Several notable covered bridges, which are enclosed truss bridges, were constructed in Switzerland. The Kappel Bridge (1333) of Luzern has been decorated since 1599 with 112 paintings in the triangular spaces between the roof and the crossbeams, depicting the history of the town and the lives of its two patron saints.

In the 18th century, designs with timber trusses reached new span lengths. In 1755 a Swiss builder, Hans Grubenmann, used trusses to support a covered timber bridge with spans of 51 and 58 metres (171 and 193 feet) over the Rhine at Schaffhausen. He and his brother also built a notable arch-truss bridge over the Limmat River in Baden with a clear span of 61 metres (200 feet).

In North America the covered truss bridges underwent further evolution. From simple king-post trusses, in which the roadway was supported by a pair of heavy timber triangles, American carpenters in the 18th and 19th centuries developed bridges combining simplicity of construction with their other economic advantages. The first long covered bridge in America, with a 55-metre (180-foot) centre span, was built by Timothy Palmer, a Massachusetts millwright, over the Schuylkill River at Philadelphia in 1806. A New Haven architect named Ithiel Town patented the Town lattice, in which a number of relatively light pieces, diagonally crisscrossed, took the place of the heavy timbers of Palmer’s design and of the arch. Another highly successful type was designed by Theodore Burr, of Torrington, Connecticut, combining a Palladio truss with an arch. Burr’s McCall’s Ferry Bridge (1815; on the Susquehanna River near Lancaster, Pennsylvania) had a record-breaking span of 108 metres (360 feet). Numerous Town and Burr designs remained standing throughout North America into the early 21st century, some dating back to the early 19th century.

With the increasing importance of locomotive transportation in the 19th century, iron was adopted for covered bridges to carry the heavy loadings of the railroad. At first metal was used for only part of the truss, in either vertical or diagonal members, and later for the whole truss. Cast iron and wrought iron were soon replaced by steel, and a principal form of the modern railroad bridge rapidly evolved. The metal truss did not require protection from the weather and consequently was not covered. The two systems most commonly used are the Pratt and the Warren; in the former the sloping web members are parallel to each other, while in the latter they alternate in direction of slope.

Steel truss bridges have also been built as part of highway systems around the world. The longest continuous-truss bridge in the world is the Ikitsuki Bridge (1991) in Japan, with a main span of 400 metres (1,300 feet). The Astoria Bridge (1966), spanning the mouth of the Columbia River between the states of Oregon and Washington in the United States, consists of three spans reaching a total length of 6,545 metres (21,474 feet) and including a main span of 376 metres (1,232 fee); it is the second longest continuous-truss bridge.

Truss bridges have been used in military operations, particularly where riverbanks are steep or navigation must be kept open. They are usually made up in panels that can be readily transported and quickly bolted together. Such military truss bridges were pioneered in World War II by the highly successful British-invented Bailey bridge, which played an especially important role in the Allied campaign in Italy.

truss-bridge-designs.jpg


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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#915 Today 00:09:19

ganesh
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Registered: 2005-06-28
Posts: 32,080

Re: Miscellany

893) Suspension bridge

Suspension bridge, bridge with overhead cables supporting its roadway. One of the oldest of engineering forms, suspension bridges were constructed by primitive peoples using vines for cables and mounting the roadway directly on the cables. A much stronger type was introduced in India about the 4th century AD that used cables of plaited bamboo and later of iron chain, with the roadway suspended.

In modern times, the suspension bridge provided an economical solution to the problem of long spans over navigable streams or at other sites where it is difficult to found piers in the stream. British, French, American, and other engineers of the late 18th and early 19th centuries encountered serious problems of stability and strength against wind forces and heavy loads; failures resulted from storms, heavy snows, and droves of cattle. Credit for solving the problem belongs principally to John Augustus Roebling, a German-born American engineer who added a web truss to either side of his roadways and produced a structure so rigid that he successfully bridged the Niagara Gorge at Niagara Falls, New York, the Ohio River at Cincinnati, and, finally, in his masterpiece, the East River between Brooklyn and Manhattan at New York City.

The technique of cable spinning for suspension bridges was invented by the French engineer Louis Vicat, a contemporary of Roebling. Vicat’s method employed a traveling wheel to carry the continuous cable strand from the anchorage on one side up over the tower, down on a predetermined sag (catenary) to the midpoint of the bridge, up and over the tower on the farther side to the farther anchorage, where a crew received the wheel, anchored the strand, and returned the wheel, laying a fresh strand. From these successive parallel strands a cable was built up.

Another major development in the modern suspension bridge was the pneumatic caisson, which permitted pier foundation at great depths. It was used initially by French, British, and American engineers, including Washington Roebling, who completed his father’s Brooklyn Bridge.

For a time in the 1930s, American engineers experimented with a narrow solid girder in place of the web truss to stiffen the roadway, but the failure of the Tacoma Narrows Bridge in 1940 under aerodynamic forces instigated a return to the web truss. Later, aerodynamically stable box girders replaced the web truss.

By the late 1980s, three suspension bridges (the Golden Gate, in San Francisco, the Verrazano-Narrows, in New York City, and the Humber Bridge, near Hull, England) had main-span lengths of more than 4,000 feet (1,200 metres). Modern steel alloys are considered capable of much greater spans. Though suspension bridges can be made strong enough to support freight trains, they have nearly all been designed for automobile traffic.

A cable-braced bridge was developed by German engineers at Cologne, Düsseldorf, and elsewhere in the 1950s and ’60s; in this form a single tower at the midpoint supports the roadway by means of a number of cables. Another development of the 1960s, aimed at reducing time of construction, was cable fabricated in the shop.

how-bridges-work-6.jpg


It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi. 

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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