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Digestive System
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Digestive System

Single-celled organisms can directly take in nutrients from their outside environment. Multicellular animals, with most of their cells removed from contact directly with the outside environment, have developed specialized structures for obtaining and breaking down their food. Animals depend on two processes: feeding and digestion.

Animals are heterotrophs, they must absorb nutrients or ingest food sources. Ingestive eaters, the majority of animals, use a mouth to ingest food. Absorptive feeders, such as tapeworms, live in a digestive system of another animal and absorb nutrients from that animal directly through their body wall. Filter feeders, such as oysters and mussels, collect small organisms and particles from the surrounding water. Substrate feeders, such as earthworms and termites, eat the material (dirt or wood) they burrow through. Fluid feeders, such as aphids, pierce the body of a plant or animal and withdraw fluids.

The digestive systems of representative animals

Intracellular digestion: food is taken into cells by phagocytosis with digestive enzymes being secreted into the phagocytic vesicles; occurs in sponges, coelenterates and most protozoans.

Extracellular digestion: digestion occurs in the lumen (opening) of the digestive system, with the nutrient molecules being transferred to the blood or body fluid; occurs in chordates, annelids, and crustaceans.

Stages in the Digestive Process

  1. movement: propels food through the digestive system
  2. secretion: release of digestive juices in response to a specific stimulus
  3. digestion: breakdown of food into molecular components small enough to cross the plasma membrane
  4. absorption: passage of the molecules into the body's interior and their passage throughout the body
  5. elimination: removal of undigested food and wastes

Three phases occur during what we loosely refer to as "digestion". Digestion proper, which is the mechanical and chemical breakdown of food into particles/molecules small enough to pass into the blood stream. Absorption into the blood stream. Assimilation, the passage of the food molecules into body cells.

Components of the Digestive System

The human digestive system is a coiled, muscular tube (6-9 meters long when fully extended) extending from the mouth to the anus. Several specialized compartments occur along this length: mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus. Accessory digestive organs are connected to the main system by a series of ducts: salivary glands, parts of the pancreas, and the liver and gall bladder (bilary system).

Microbial Life in Digestive Tract

The gastrointestinal tract contains an immensely complex ecology of microorganisms. A typical person harbors more than 500 distinct species of bacteria, representing dozens of different lifestyles and capabilities. The composition and distribution of this menagerie varies with age, state of health and diet.

The number and type of bacteria in the gastrointestinal tract vary dramatically by region. In healthy individuals the stomach and proximal small intestine contain few microorganisms, largely a result of the bacteriocidal activity of gastric acid; those that are present are aerobes and facultative anaerobes. One interesting testimony to the ability of gastric acid to suppress bacterial populations is seen in patients with achlorhydria, a genetic condition which prevents secretion of gastric acid. Such patients, which are otherwise healthy, may have as many as 10,000 to 100,000,000 microorganisms per ml of stomach contents.

In sharp contrast to the stomach and small intestine, the contents of the colon literally teem with bacteria, predominantly strict anaerobes (bacteria that survive only in environments virtually devoid of oxygen). Between these two extremes is a transitional zone, usually in the ileum, where moderate numbers of both aerobic and anaerobic bacteria are found.



Peptic ulcers result when these protective mechanisms fail. Bleeding ulcers result when tissue damage is so severe that bleeding occurs into the stomach. Perforated ulcers are life-threatening situations where a hole has formed in the stomach wall. At least 90% of all peptic ulcers are caused by Helicobacter pylori. Other factors, including stress and aspirin, can also produce ulcers.

Lactose Intolerance

Alternative names Lactase deficiency; Milk intolerance; Disaccharidase deficiency; Dairy product intolerance


Lactose intolerance is the inability to digest lactose, a type of sugar found in milk and other dairy products. It is caused by a deficiency of the enzyme lactase.

Causes, incidence, and risk factors

Lactose intolerance occurs when the small intestine does not produce enough of the enzyme lactase. Babies' bodies produce this enzyme so they can digest milk, including breast milk. Before humans became dairy farmers, most people did not continue to drink milk, so their bodies did not produce lactase after early childhood.

People from cultures in which adult consumption of milk and milk products occurred earliest are less likely to suffer from lactose intolerance than those from areas where dairy farming began more recently. As a result, lactose intolerance is more common in Asian, African, African-American, Native American, and Mediterranean populations than it is among northern and western Europeans.

Lactose intolerance can begin at various times in life. In Caucasians, it usually starts to affect children older than age 5. In African-Americans, lactose intolerance often occurs as early as age 2.

When people with lactose intolerance consume milk products, they may have symptoms such as abdominal bloating, excessive intestinal gas, nausea, diarrhea, and abdominal cramping.

Lactose intolerance is very common in adults and is not dangerous. Many adults have some degree of lactose intolerance by age 20 (approximately 30 million Americans).

Lactose intolerance is sometimes seen in premature babies. Full-term babies generally do not show signs of lactose intolerance until they are at least 3 years old.

Eliminating milk from the diet can result in a deficiency of calcium, vitamin D, riboflavin, and protein. Therefore, a milk substitute is a necessity. For infants younger than 2 years, soy formulas are adequate substitutes. Good alternatives for toddlers are soy or rice milk. Older children may also use lactase-treated cow's milk. Goat's milk is low in lactose, but should only be used in children if properly fortified with essential amino acids and vitamins.