Birth Control: Current Research in Birth Control

29 Jul 2010 | By Ramitan

Researchers are currently developing a number of birth control options for men, including hormonal contraceptive pills and implants. An injection for men under investigation contains a hormone that appears to interfere with the production of sperm. Reversible methods of vasectomy are also being explored.

Drugs known as gonadotropin-releasing hormone (GnRH) agonists are being investigated as birth control options for men and women. These drugs prevent the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland, which, in turn, blocks ovulation in women and spermatogenesis (the development of sperm) in men.

Researchers are also developing improved versions of existing birth control options. New types of diaphragms may one day include a one-size-fits-all device and a disposable, spermicide-releasing diaphragm. Other research focuses on biodegradable hormonal implants designed to dissolve in the body and new spermicidal preparations that would better protect against STIs.

A variety of birth control methods have been used throughout history and across cultures. In ancient Egypt women used dried crocodile dung and honey as vaginal suppositories to prevent pregnancy. One of the earliest mentions of contraceptive vaginal suppositories appears in the Ebers Medical Papyrus, a medical guide written between 1550 and 1500 bc. The guide suggests that a fiber tampon moistened with an herbal mixture of acacia, dates, colocynth, and honey would prevent pregnancy. The fermentation of this mixture can result in the production of lactic acid, which today is recognized as a spermicide.

Before the introduction of the modern birth control pill, women ate or drank various substances to prevent pregnancy or induce miscarriage. The seeds of Queen Anne’s lace, pennyroyal, giant fennel, and many other concoctions of plants and herbs were used as oral contraceptives. However, such folk remedies can be dangerous or even fatal.

The concept of the IUD was developed by ancient Turks and Arabs who inserted smooth pebbles into the uterus of a camel to prevent it from getting pregnant during treks across the desert. The use of colorful penis coverings can be traced back to ancient Egypt, but it is likely that their function was more decorative than contraceptive. In the 16th century the Italian anatomist Gabriello Fallopio (for whom the fallopian tubes that carry the eggs from the ovary to the uterus were named) described linen sheaths to be used to protect against syphilis. In the 17th century a physician in the court of King Charles II of England created a condom made of sheep intestines. Italian adventurer Giacomo Casanova is said to have referred to the device as an “English riding coat.” It was not until after the vulcanization of rubber in 1839 that the condom was widely used as a birth control device.

German physician Wilhelm Mensinga invented the modern diaphragm in 1880. The cervical cap was invented in 1860, but it did not receive the approval of the Food and Drug Administration for use in the United States until the late 1980s, despite its widespread use in Europe.

Concerns about overpopulation have also existed since ancient times. The Greek philosophers Plato and Aristotle warned of its dangers. In his essay De Anima, Roman philosopher Tertullian commented on the blessing of catastrophes that help curb overpopulation. In the 18th century British economist Thomas Malthus made overpopulation a topic of scholarly discussion. He was one of the first to apply statistics to the analysis of population growth. This approach became the science of demography.

Malthus was concerned about the human potential to produce offspring in far greater numbers than the Earth’s ability to provide subsistence. In his “Essay on the Principle of Population,” published in 1798, Malthus advocated what he termed “moral restraint” in the form of strict premarital chastity and delayed marriage to curb population growth. Malthus’s views were attacked by many as pessimistic, unsympathetic to the poor, and unrealistic in terms of his proposed solution. The birth control movement grew out of Malthus’s concerns, and his successors advocated more practical methods of contraception.

Margaret Sanger, an American nurse, pioneered the modern birth control movement in the United States. In 1912 she began publishing information about women’s reproductive concerns through magazine articles, pamphlets, and several books. In 1914 Sanger was charged with violation of the Comstock Law, federal legislation passed in 1873 prohibiting the mailing of obscene material, including information about birth control and contraceptive devices. In defiance of the Comstock Law and despite being jailed for these activities, Sanger continued to publish and disseminate information about birth control. In 1916 Sanger and her sister Ethel Byrne opened the first of several birth control clinics in Brooklyn, New York.

Congress revised the Comstock Law in 1936 to exclude birth control information and devices. Many states had laws prohibiting distribution or use of birth control devices but the constitutionality of these laws was increasingly questioned. In 1965, in Griswold v. Connecticut, the Supreme Court of the United States ruled that married people have the right to practice birth control without government intervention. In 1972, in Eisenstadt v. Baird, the Court held that unmarried people have the same right.

Today there are more birth control options than ever before, but overpopulation and unwanted pregnancies remain worldwide problems. Having more children than one can support may lead to poverty, malnutrition, illness, and high mortality rates for infants, children, and women.

The problem of teenage pregnancy is considerably worse in the United States than in almost any other developed country. Among developed countries, the United States has one of the highest birth rates for women under 20. A detailed study comparing Canada, England and Wales, France, The Netherlands, Sweden, and the United States suggested that the problem of teen pregnancy in the United States may be related to less sex education in schools and lower availability of birth control services and supplies to adolescents. This study counters the view of some people in the United States who argue that sex education or making birth control devices such as condoms available to school-age children promotes sexual activity.

Why does the neighborhood buck?

Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.

Category: Health & Medicine, Human Body  5 Comments

Acquired Immunodeficiency Syndrome

29 Jul 2010 | By Ramitan

Human Immunodeficiency Virus
The human immunodeficiency virus (HIV), which causes acquired immunodeficiency syndrome (AIDS), principally attacks CD4 T-cells, a vital part of the human immune system. As a result, the body’s ability to resist opportunistic viral, bacterial, fungal, protozoal, and other infection is greatly weakened. Pneumocystis carinii pneumonia is the leading cause of death among people with HIV infection, but the incidence of certain types of cancers such as B-cell lymphomas and Kaposi’s sarcoma is also increased. Neurological complications and dramatic weight loss, or “wasting,” are characteristic of endstage HIV disease (AIDS). HIV can be transmitted sexually; through contact with contaminated blood, tissue, or needles; and from mother to child during birth or breastfeeding. Full-blown symptoms of AIDS may not develop for more than 10 years after infection.

Acquired Immunodeficiency Syndrome (AIDS), human viral disease that ravages the immune system, undermining the body’s ability to defend itself from infection and disease. Caused by the human immunodeficiency virus (HIV), AIDS leaves an infected person vulnerable to opportunistic infections—infection by microbes that take advantage of a weakened immune system. Such infections are usually harmless in healthy people but can prove life-threatening to people with AIDS. Although there is no cure for AIDS, new drugs are available that can prolong the life spans and improve the quality of life of infected people.

Transmission of HIV—the AIDS-causing virus—occurs most commonly as a result of sexual intercourse. HIV also can be transmitted through transfusions of HIV-contaminated blood or by using a contaminated needle or syringe to inject drugs into the bloodstream. Infection with HIV does not necessarily mean that a person has AIDS. Some people who have HIV infection may not develop any of the clinical illnesses that define the full-blown disease of AIDS for ten years or more. Physicians prefer to use the term AIDS for cases where a person has reached the final, life-threatening stage of HIV infection.

AIDS is one of the deadliest epidemics in human history. It was first identified in 1981 among homosexual men and intravenous drug users in New York and California. Shortly after its detection in the United States, evidence of AIDS epidemics grew among heterosexual men, women, and children in sub-Saharan Africa. AIDS quickly developed into a worldwide epidemic, affecting virtually every nation. The United Nations Program on HIV/AIDS (UNAIDS) estimates that the worldwide number of new cases of HIV infection peaked in the late 1990s with more than 3 million people newly infected each year. However, some regions of the world, especially Vietnam, Indonesia, and other countries in southeast Asia, continued to see an increase in the early 2000s. In addition, the number of people living with HIV or AIDS has continued to rise as the result of new drug treatments that lengthen life.

A Global Epidemic
More than 42 million people around the world are currently infected with human immunodeficiency virus (HIV), the virus that causes acquired immunodeficiency syndrome (AIDS). New HIV infections have leveled off or even declined in most developed countries, but the virus is spreading rapidly through much of the developing world. In some areas of sub-Saharan Africa, one in four adults is carrying the virus.

While cases of AIDS have been reported in every nation of the world, the disease affects some countries more than others. About 90 percent of all HIV-infected people live in the developing world. AIDS has struck sub-Saharan Africa particularly hard. Two-thirds of all people living with HIV infection reside in African countries south of the Sahara, where AIDS is the leading cause of death.

In countries hardest hit, AIDS has sapped the population of young men and women who form the foundation of the labor force. Most die while in the peak of their reproductive years. Moreover, the epidemic has overwhelmed health-care systems, increased the number of orphans, and caused life expectancy rates to plummet. These problems have reached crisis proportions in parts of the world already burdened by war, political upheaval, or unrelenting poverty.

AIDS is the final stage of a chronic infection with the human immunodeficiency virus. There are two types of this virus: HIV-1, which is the primary cause of AIDS worldwide, and HIV-2, found mostly in West Africa. Inside the body HIV enters cells of the immune system, especially white blood cells known as T cells. These cells orchestrate a wide variety of disease-fighting mechanisms. Particularly vulnerable to HIV attack are specialized “helper” T cells known as CD4 cells. When HIV infects a CD4 cell, it commandeers the genetic tools within the cell to manufacture new HIV virus. The newly formed HIV virus then leaves the cell, destroying the CD4 cell in the process. No existing medical treatment can completely eradicate HIV from the body once it has infected human cells.

The loss of CD4 cells endangers health because these cells help other types of immune cells respond to invading organisms. The average healthy person has over 1,000 CD4 cells per microliter of blood. In a person infected with HIV, the virus steadily destroys CD4 cells over a period of years, diminishing the cells’ protective ability and weakening the immune system. When the density of CD4 cells drops to 200 cells per microliter of blood, the infected person becomes vulnerable to AIDS-related opportunistic infections and rare cancers, which take advantage of the weakened immune defenses to cause disease.

Scientists have identified three ways that HIV infections spread: sexual intercourse with an infected person, contact with contaminated blood, and transmission from an infected mother to her child before or during birth or through breast-feeding.

HIV transmission occurs most commonly during intimate sexual contact with an infected person, including genital, anal, and oral sex. The virus is present in the infected person’s semen or vaginal fluids. During sexual intercourse, the virus gains access to the bloodstream of the uninfected person by passing through openings in the mucous membrane—the protective tissue layer that lines the mouth, vagina, and rectum—and through breaks in the skin of the penis. In the United States and Canada, HIV is most commonly transmitted during sex between men, but the incidence of HIV transmission between men and women has rapidly increased. In most other parts of the world, HIV is most commonly transmitted through heterosexual sex.

Direct contact with HIV-infected blood occurs when people who use heroin or other injected drugs share hypodermic needles or syringes contaminated with infected blood. Sharing of contaminated needles among intravenous drug users has been a primary cause of HIV infection in parts of eastern Europe and central Asia.

Less frequently, HIV infection results when health professionals accidentally stick themselves with needles containing HIV-infected blood or expose an open cut to contaminated blood. Some cases of HIV transmission from transfusions of infected blood, blood components, and organ donations were reported in the 1980s. Since 1985 government regulations in the United States and Canada have required that all donated blood and body tissues be screened for the presence of HIV before being used in medical procedures. As a result of these regulations, HIV transmission caused by contaminated blood transfusion or organ donations is rare in North America. However, the problem continues to concern health officials in sub-Saharan Africa.

HIV can be transmitted from an infected mother to her baby while the baby is still in the woman’s uterus or, more commonly, during childbirth. The virus can also be transmitted through the mother’s breast milk during breast-feeding. Mother-to-child transmission accounts for 90 percent of all cases of AIDS in children. Mother-to-child transmission is particularly prevalent in Africa.

The routes of HIV transmission are well documented by scientists, but health officials continually grapple with people’s unfounded fears concerning the potential for HIV transmission by other means. HIV differs from other infectious viruses in that it dies quickly if exposed to the environment. No evidence has linked HIV transmission to casual contact with an infected person, such as a handshake, hugging, or kissing, or even sharing dishes or bathroom facilities. Studies have been unable to identify HIV transmission from modes common to other infectious diseases, such as an insect bite or inhaling virus-infected droplets from an infected person’s sneeze or cough.

Without medical intervention, AIDS progresses along a typical course. Within one to three weeks after infection with HIV, most people experience flu-like symptoms, such as fever, sore throat, headache, skin rash, tender lymph nodes, and a vague feeling of discomfort. These symptoms last one to four weeks. During this phase, known as acute retroviral syndrome, HIV reproduces rapidly in the blood. The virus circulates in the blood throughout the body, particularly concentrating in organs of the lymphatic system.

The normal immune defenses against viral infections eventually activate to battle HIV in the body, reducing but not eliminating HIV in the blood. Infected individuals typically enter a prolonged asymptomatic phase, a symptom-free period that can last ten years or more. While persons who have HIV may remain in good health during this period, HIV continues to replicate, progressively destroying the immune system. Often an infected person remains unaware that he or she carries HIV and unknowingly transmits the virus to others during this phase of the infection.

When HIV infection reduces the number of CD4 cells from around 500 to 200 per microliter of blood, the infected individual enters an early symptomatic phase that may last a few months to several years. HIV-infected persons in this stage may experience a variety of symptoms that are not life-threatening but may be debilitating. These symptoms include extensive weight loss and fatigue (wasting syndrome), periodic fever, recurring diarrhea, and thrush, a fungal mouth infection. An early symptom of HIV infection in women is a recurring vaginal yeast infection. Unlike earlier stages of the disease, in this early symptomatic phase the symptoms that develop are severe enough to cause people to seek medical treatment. Many may first learn of their infection in this phase.

If CD4 cell levels drop below 200 cells per microliter of blood, the late symptomatic phase develops. This phase is characterized by the appearance of any of the opportunistic infections and rare cancers known as AIDS-defining conditions. The onset of these illnesses is a sign that an HIV-infected person has developed full-blown AIDS. Without medical treatment, this stage may last from several months to years. The cumulative effects of these illnesses usually cause death.

Often the first opportunistic infection to develop is pneumocystis pneumonia, a lung infection caused by the fungus Pneumocystis carinii. This fungus infects most people in childhood, settling harmlessly in the lungs where it is prevented from causing disease by the immune system. But once the immune system becomes weakened, the fungus can block the lungs from delivering sufficient oxygen to the blood. The lack of oxygen leads to severe shortness of breath accompanied by fever and a dry cough.

In addition to pneumocystis pneumonia, people with AIDS often develop other fungal infections. Up to 23 percent of people with AIDS become infected with fungi from the genus Cryptococcus, which cause meningitis, inflammation of the membranes that surround the brain. Infection by the fungus Histoplasma capsulatum affects up to 10 percent of people with AIDS, causing general weight loss, fever, and respiratory complications.

Tuberculosis, a severe lung infection caused by the bacterium Mycobacterium tuberculosis, typically becomes more severe in AIDS patients than in those with a healthy immune system. Between the 1950s and the late 1980s, tuberculosis was practically eradicated in North America. In the early 1990s, doctors became alarmed when incidence of the disease dramatically escalated. This resurgence was attributed to the increased susceptibility to tuberculosis of people infected with HIV. Infection by the bacterium Mycobacterium avium can cause fever, anemia, and diarrhea. Bacterial infections of the gastrointestinal tract contribute to wasting syndrome.

Opportunistic infections caused by viruses, especially members of the herpesvirus family, are common in people with AIDS. One of the herpesviruses, cytomegalovirus (CMV), infects the retina of the eye and can result in blindness. Another herpesvirus, Epstein-Barr virus (EBV), may cause certain types of blood cancers. Infections with herpes simplex virus (HSV) types 1 or 2 may result in sores around the mouth, genital area, or anus.

Many people with AIDS develop cancers. The destruction of CD4 cells impairs the immune functions that halt the development of cancer. Kaposi’s sarcoma is a cancer of blood vessels caused by a herpesvirus. This cancer produces purple lesions on the skin, which can spread to internal organs and cause death. B cell lymphoma affects certain cells of the lymphatic system that fight infection and perform other vital functions. Cervical cancer is more common in HIV-infected women than in women free from infection.

A variety of neurological disorders are common in the later stage of AIDS. Collectively called HIV-associated dementia, they develop when HIV or another microbial organism infects the brain. The infection produces degeneration of intellectual processes such as memory and, sometimes, problems with movement and coordination.

HIV infection in children progresses more rapidly than in adults, most likely because a child’s immune system has not yet built up immunity to many infectious agents. The disease is particularly aggressive in infants—more than half of infants born with an HIV infection die before age two. Once a child is infected, the child’s undeveloped immune system cannot prevent the virus from multiplying quickly in the blood, and the disease progresses rapidly. In contrast, when an adult becomes infected with HIV, the adult’s immune system generally fights the infection. Therefore, HIV levels in adults remain lower for an extended period, delaying the progression of the disease.

Children develop many of the opportunistic infections that befall adults but also exhibit symptoms not observed in older patients. Among infants and children, HIV infection produces wasting syndrome and slows growth (generally referred to as failure to thrive). HIV typically infects a child’s brain early in the course of the disease, impairing intellectual development and coordination skills. While HIV can infect the brains of adults, it usually does so toward the later stages of the disease and produces different symptoms.

Children show a susceptibility to more bacterial and viral infections than adults. More than 20 percent of HIV-infected children develop serious, recurring bacterial infections, including meningitis and pneumonia. Some HIV-infected children suffer from repeated bouts of viral infections, such as chicken pox. Healthy children generally develop immunity to these viral illnesses after an initial infection.

Since HIV was first identified as the cause of AIDS in 1983, a variety of tests have been developed that help diagnose HIV infection as well as determine how far the infection has progressed. Other tests can be used to screen donated blood, blood products, and body organs for the presence of HIV.

Doctors determine if HIV is present in the body by identifying HIV antibodies, specialized proteins created by the immune system to destroy HIV. The presence of these antibodies indicates HIV infection because they form in the body only when HIV is present. HIV antibodies form anywhere from five weeks to three months after HIV infection occurs, depending upon the individual’s immune system. The antibodies are produced continually throughout the course of the infection.

The standard test to detect HIV antibodies in the blood is the enzyme-linked immunosorbent assay (ELISA). In this test, a blood sample is mixed with proteins from HIV. If the blood contains HIV antibodies, they attach to the HIV proteins, producing a telltale color change in the mixture. This test is highly reliable when performed two to three months after infection with HIV. The test is less reliable when used in the very early stage of HIV infection, before detectable levels of antibodies have had a chance to form. Doctors routinely confirm a positive result from an ELISA test by using the Western Blot test, which can detect lower levels of HIV antibodies. In this test a blood sample is applied to a paper strip containing HIV proteins. If HIV antibodies are present in the blood, they bind to the HIV proteins, producing a color change on the paper. The combination of the ELISA and the Western Blot test is more than 99.9 percent accurate in detecting HIV infection within 12 weeks following exposure.

Once tests confirm an HIV infection, doctors monitor the health of the infected person’s immune system by periodically measuring CD4 cell counts in the blood. The progressive loss of CD4 cells corresponds to a worsening of the disease as the immune system becomes increasingly impaired. Doctors also measure the viral load—the amount of the virus in the blood—using polymerase chain reaction (PCR) technology. PCR tests measure the level of viral ribonucleic acid (RNA), or HIV particles, in an infected person’s blood to determine how actively the virus is replicating and how fast the disease is progressing. Knowing the viral load helps doctors make decisions about the treatment and its effectiveness.

A modified ELISA test that detects p24 antigen, a protein produced by HIV, can determine if specific drug treatments are having a positive effect on a patient. Blood banks, plasma centers, clinical laboratories, private clinics, and public health departments also use this p24 antigen test to screen for the presence of HIV in blood, blood components, and organs before they are used in medical procedures.

Physicians prefer to differentiate between people who have HIV infection and those who have AIDS. The Centers for Disease Control and Prevention (CDC), based in Atlanta, Georgia, recommends that physicians reserve the diagnosis of AIDS for HIV-infected individuals whose CD4 count falls below 200 cells per microliter of blood. A diagnosis of AIDS can also be made without confirmation of CD4 levels if someone who has no other reason for immune system damage develops an opportunistic disease.

Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.

Category: Health & Medicine, Human Body  9 Comments

Usage and side effects of flonase nasal spray

29 Jul 2010 | By Ramitan

It’s important that you use a medication properly and also know about its side effects. This will help you make the best use of that particular medication and also help you experience its maximum benefits.

Taking Flonase

Flonase is a nasal spray and should be administered though the nostrils. The prescribed dosage of this spray should be taken at regular intervals and before administering the spray, don’t forget to blow your nose. While spraying, tilt your head back, close one nostril and spray into the other nostril by inserting the tip of the bottle a short way into it. After spraying, breathe in deeply through that nostril and repeat the process with the other nostril. Read the instructions manual carefully before you use the spray. Consult a pharmacist or your doctor if you have any doubts about using this allergy relief spray.

Side effects of the spray

Taking this spray can make you vulnerable to some side effects. Typically, these are of a mild nature and include, headache, nosebleed, sore throat, blood in nasal mucus, dry mouth and nasal dryness. Some of the rarer Flonase side effects include fever, sinus infection, diarrhea, dizziness, infection of the lungs or bronchitis, muscle pain, joints, abdominal pain, weight gain and at times a worsening of your asthma problem. Generally, this spray is well tolerated by children and adults alike, but awareness of its side effects can help you make use of this spray even more effectively. Such side effects will go away after some time, but if they become bothersome you must seek immediate medical help.

Category: Others, Promotion  11 Comments

Regaining your virility with cialis

29 Jul 2010 | By Ramitan

Love making is an eternal as well as a divine feeling which every human being is entitled to. It is the eventual desire in a person from which contentment and satisfaction comes out. Imagine if any problems arise in your sexual life, it will surely show in everything. Especially in a relationship, things can get worse if your male partner is facing with any problem related to erectile dysfunction.
Solving this problem from an emotional level is justified but what your partner is required to do is to consult with a medical help who will help him to recover initially by prescribing some medicines. But in that case, such treatments can even go out of reach of one’s affordability. In such a situation, the desired decision will be to seek help from the online pharmacy where you will easily find useful drugs like Cialis in a much affordable price range.
Due to the hard economic patch as the whole world is experiencing, online cialis is probably the best way to get rid of this problem. Remember if you’re sexually active, you will feel much lighter emotionally as well as physically. Cialis is that one drug that can save your sex life from suffering from a rough patch. The negative feeling that you have been experiencing all this while can be shunned with this single drug as it works wonder.
Regain your lost virility in the most successful and safest way that is been approved by doctors all over the world. Viagra køb shows faster results and is known for its long-lasting effect. So, waste no time in correcting your impotency with the best medical approach- cialis.

Category: Others, Promotion  14 Comments

Digestive System

27 Jul 2010 | By Ramitan

Digestive System, series of connected organs whose purpose is to break down, or digest, the food we eat. Food is made up of large, complex molecules, which the digestive system breaks down into smaller, simple molecules that can be absorbed into the bloodstream. The simple molecules travel through the bloodstream to all of the body’s cells, which use them for growth, repair, and energy.

All animals have a digestive system, a feature that distinguishes them from plants. Plants produce their own food in a process called photosynthesis, during which they use sunlight to convert water and carbon dioxide into simple sugars. But animals, including humans, must take in food in the form of organic matter, such as plants or other animals.

Digestion generally involves two phases: a mechanical phase and a chemical phase. In the mechanical phase, teeth or other structures physically break down large pieces of food into smaller pieces. In the chemical phase, digestive chemicals called enzymes break apart individual molecules of food to yield molecules that can be absorbed and distributed throughout the body. These enzymes are secreted (produced and released) by glands in the body.

The digestive system of most animals consists mainly of a long, continuous tube called the alimentary canal, or digestive tract. This canal has a mouth at one end, through which food is taken in, and an anus at the other end, through which digestive wastes are excreted. Muscles in the walls of the alimentary canal move the food along. Most digestive organs are part of the alimentary canal. However, two accessory digestive organs, the liver and pancreas, are located outside the alimentary canal. These organs contribute to chemical digestion by releasing digestive juices into the canal through tubes called ducts.

The simplest invertebrates (animals without backbones) do not have specialized digestive organs. Single-celled organisms, such as amoebas, rely on intracellular digestion (digestion within the cell). Some many-celled organisms, such as the sponge, also use intracellular digestion. The sponge obtains the tiny organic particles that make up its diet from water passing through its body. Water enters through the sponge’s pores and leaves through an opening called the osculum. As water flows through the interior canals of the sponge, specialized cells that line these canals, called collar cells, catch and engulf organic matter. Inside the collar cells, sacs called vacuoles form around the food and enzymes digest it. The digested food then passes to other cells in the sponge’s body.

Intracellular digestion meets the needs of simple animals, but more complex organisms require systems that are more specialized. Animals such as jellyfish and nonparasitic flatworms combine the intracellular process with some specialized digestive organs. These animals have a definite mouth and a saclike cavity, which is lined with digestive cells that secrete enzymes. Digestion begins when the enzymes break down food inside the cavity in an extracellular (outside the cell) process. Cells then engulf the partly digested food, and an intracellular process similar to that of sponges completes digestion. Wastes are excreted through the mouth.

Most of the more complex invertebrates and all vertebrates (animals with a backbone) digest food entirely through extracellular processes. Food moves in one direction, from mouth to anus, through the series of organs that make up the alimentary canal. Specialization of various parts of the alimentary canal improves the body’s ability to break down food and absorb various kinds of nutrients. The mouth of many animals contains teeth or other structures to break up large lumps of food. Behind the mouth, the pharynx and esophagus swallow the food and move it to the stomach. The stomach temporarily stores the food, mixes it with digestive juices, and carries out some digestion.

Digestion is completed in the intestine. The liver and pancreas pour their digestive juices into the anterior (front) end of this organ. After the anterior intestine absorbs the usable products of digestion, the walls of the posterior (rear) intestine absorb leftover water. In vertebrates the anterior intestine is called the small intestine; the posterior intestine is the large intestine. Feces, composed of unabsorbed and indigestible food residues, form in the posterior intestine, where they are stored until they are excreted through the anus.

Within this basic plan, the specific components of the digestive system vary enormously from one animal to another. For example, a fish’s pharynx contains gill slits for breathing but has no digestive function. An earthworm’s stomach consists of two organs: a crop, in which food is stored, and a muscular gizzard, which carries out mechanical digestion by grinding food against particles of sand. The stomachs of ruminant mammals, such as cattle and deer, consist of three or four compartments, each performing a specific function. Amphibians, reptiles, and birds have an organ called a cloaca, which serves as an exit for both digestive wastes and sex cells.

Four-Chambered Stomach
Ruminant animals, including sheep, domestic cattle, goats, deer, and giraffes, have four-chambered stomachs.

If a human adult’s digestive tract were stretched out, it would be 6 to 9 m (20 to 30 ft) long. In humans, digestion begins in the mouth, where both mechanical and chemical digestion occur. The mouth quickly converts food into a soft, moist mass. The muscular tongue pushes the food against the teeth, which cut, chop, and grind the food. Glands in the cheek linings secrete mucus, which lubricates the food, making it easier to chew and swallow. Three pairs of glands empty saliva into the mouth through ducts to moisten the food. Saliva contains the enzyme ptyalin, which begins to hydrolyze (break down) starch—a carbohydrate manufactured by green plants.

Once food has been reduced to a soft mass, it is ready to be swallowed. The tongue pushes this mass—called a bolus—to the back of the mouth and into the pharynx. This cavity between the mouth and windpipe serves as a passageway both for food on its way down the alimentary canal and for air passing into the windpipe. The epiglottis, a flap of cartilage, covers the trachea (windpipe) when a person swallows. This action of the epiglottis prevents choking by directing food from the windpipe and toward the stomach.

The presence of food in the pharynx stimulates swallowing, which squeezes the food into the esophagus. The esophagus, a muscular tube about 25 cm (10 in) long, passes behind the trachea and heart and penetrates the diaphragm (muscular wall between the chest and abdomen) before reaching the stomach. Food advances through the alimentary canal by means of rhythmic muscle contractions (tightenings) known as peristalsis. The process begins when circular muscles in the esophagus wall contract and relax (widen) one after the other, squeezing food downward toward the stomach. Food travels the length of the esophagus in two to three seconds.

A circular muscle called the esophageal sphincter separates the esophagus and the stomach. As food is swallowed, this muscle relaxes, forming an opening through which the food can pass into the stomach. Then the muscle contracts, closing the opening to prevent food from moving back into the esophagus. The esophageal sphincter is the first of several such muscles along the alimentary canal. These muscles act as valves to regulate the passage of food and keep it from moving backward.

The stomach, located in the upper abdomen just below the diaphragm, is a saclike structure with strong, muscular walls. The stomach can expand significantly to store all the food from a meal for both mechanical and chemical processing. The stomach contracts about three times per minute, churning the food and mixing it with gastric juice. This fluid, secreted by thousands of gastric glands in the lining of the stomach, consists of water, hydrochloric acid, an enzyme called pepsin, and mucin (the main component of mucus). Hydrochloric acid creates the acidic environment that pepsin needs to begin breaking down proteins. It also kills microorganisms that may have been ingested in the food. Mucin coats the stomach, protecting it from the effects of the acid and pepsin. About four hours or less after a meal, food processed by the stomach, called chyme, begins passing a little at a time through the pyloric sphincter into the duodenum, the first portion of the small intestine.

Most digestion, as well as absorption of digested food, occurs in the small intestine. This narrow, twisting tube, about 2.5 cm (1 in) in diameter, fills most of the lower abdomen, extending about 6 m (20 ft) in length. Over a period of three to six hours, peristalsis moves chyme through the duodenum into the next portion of the small intestine, the jejunum, and finally into the ileum, the last section of the small intestine. During this time, the liver secretes bile into the small intestine through the bile duct. Bile breaks large fat globules into small droplets, which enzymes in the small intestine can act upon. Pancreatic juice, secreted by the pancreas, enters the small intestine through the pancreatic duct. Pancreatic juice contains enzymes that break down sugars and starches into simple sugars, fats into fatty acids and glycerol, and proteins into amino acids. Glands in the intestinal walls secrete additional enzymes that break down starches and complex sugars into nutrients that the intestine absorbs. Structures called Brunner’s glands secrete mucus to protect the intestinal walls from the acid effects of digestive juices.

The small intestine’s capacity for absorption is increased by millions of fingerlike projections called villi, which line the inner walls of the small intestine. Each villus is about 0.5 to 1.5 mm (0.02 to 0.06 in) long and covered with a single layer of cells. Even tinier fingerlike projections called microvilli cover the cell surfaces. This combination of villi and microvilli increases the surface area of the small intestine’s lining by about 150 times, multiplying its capacity for absorption. Beneath the villi’s single layer of cells are capillaries (tiny vessels) of the bloodstream and the lymphatic system. These capillaries allow nutrients produced by digestion to travel to the cells of the body. Simple sugars and amino acids pass through the capillaries to enter the bloodstream. Fatty acids and glycerol pass through to the lymphatic system.

A watery residue of indigestible food and digestive juices remains unabsorbed. This residue leaves the ileum of the small intestine and moves by peristalsis into the large intestine, where it spends 12 to 24 hours. The large intestine forms an inverted U over the coils of the small intestine. It starts on the lower right-hand side of the body and ends on the lower left-hand side. The large intestine is 1.5 to 1.8 m (5 to 6 ft) long and about 6 cm (2.5 in) in diameter.

The large intestine serves several important functions. It absorbs water—about 6 liters (1.6 gallons) daily—as well as dissolved salts from the residue passed on by the small intestine. In addition, bacteria in the large intestine promote the breakdown of undigested materials and make several vitamins, notably vitamin K, which the body needs for blood clotting. The large intestine moves its remaining contents toward the rectum, which makes up the final 15 to 20 cm (6 to 8 in) of the alimentary canal. The rectum stores the feces—waste material that consists largely of undigested food, digestive juices, bacteria, and mucus—until elimination. Then, muscle contractions in the walls of the rectum push the feces toward the anus. When sphincters between the rectum and anus relax, the feces pass out of the body.

The body coordinates the various steps of digestion so that the process proceeds smoothly and cells obtain a steady supply of nutrients and energy. The central nervous system and various glands control activities that regulate the digestive process, such as the secretion of enzymes and fluids. For example, the presence of food in the esophagus, stomach, or intestines triggers peristalsis. Food entering the stomach also stimulates the central nervous system to initiate the release of gastric juice. And as hydrochloric acid passes from the stomach, the small intestine produces secretin, a substance that simulates secretion of pancreatic juice.

Infection of or damage to any part of the digestive system may affect digestion as well as other bodily functions. Common infectious agents that attack digestive organs include the mumps virus, which often infects the salivary glands; the bacterium Helicobacter pylori, which causes most stomach and duodenal ulcers; and viruses and bacteria that cause various forms of gastroenteritis, often called stomach flu or traveler’s diarrhea. Appendicitis is an inflammation of the appendix, a tube-like pouch about 9 cm (3.5 in) long that branches off the large intestine. It occurs most commonly among children and young adults. Diarrhea—frequent elimination of loose, watery feces—is a symptom of many disorders that occurs when the large intestine is irritated or inflamed. As a result, food residues move through it too quickly for it to absorb the excess water. The opposite condition, constipation, occurs when the large intestine absorbs too much water because food residues are moving slowly. As a result, the feces become hard and dry, which may make elimination difficult.

Cancerous tumors may develop in any part of the digestive system, though they most commonly occur in the large intestine, rectum, and anus (see Colorectal Cancer). Colitis, which has various causes, is a potentially life-threatening inflammation of the large intestine (see Ulcerative Colitis). Chronic conditions that cause at least intermittent distress include irritable bowel syndrome, caused by spasms of muscles in the lower intestine, and Crohn’s disease, an inflammation of the intestines. Abnormal sensitivity to proteins called glutens can damage the lining of the small intestine and hinder absorption of nutrients, leading to malnutrition and other problems. The eating disorders anorexia nervosa and bulimia disrupt the normal functioning of the digestive system and are potentially fatal.

Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.

Category: Anatomy & Physiology, Human Body  11 Comments
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