Absorption Disorders Overview, Types

Digestive System Anatomy

Small Intestine Function, Location, Parts, Diseases & Facts
The salt and water come from the food and liquid we swallow and the juices secreted by the many digestive glands. Foods to enjoy during phase 1: Much of the controversy has been because of application of inappropriate methods to estimate the expected glycemic responses for mixed meals. Another function of this hormone is to suppress hunger. Axe on Youtube 1.

Digestive System Physiology

Gastrointestinal tract

The first step in digestion of a fat is to dissolve it into the watery content of the intestinal cavity. The bile acids produced by the liver act as natural detergents to dissolve fat in water and allow the enzymes to break the large fat molecules into smaller molecules, some of which are fatty acids and cholesterol. The bile acids combine with the fatty acids and cholesterol and help these molecules to move into the cells of the mucosa.

In these cells the small molecules are formed back into large molecules, most of which pass into vessels called lymphatics near the intestine. These small vessels carry the reformed fat to the veins of the chest, and the blood carries the fat to storage depots in different parts of the body. Another important part of our food that is absorbed from the small intestine is the class of chemicals we call vitamins. There are two different types of vitamins, classified by the fluid in which they can be dissolved: Most of the material absorbed from the cavity of the small intestine is water in which salt is dissolved.

The salt and water come from the food and liquid we swallow and the juices secreted by the many digestive glands. In a healthy adult, more than a gallon of water containing over an ounce of salt is absorbed from the intestine every 24 hours. Why Is Digestion Important? When we eat such things as bread, meat, and vegetables, they are not in a form that the body can use as nourishment.

Our food and drink must be changed into smaller molecules of nutrients before they can be absorbed into the blood and carried to cells throughout the body. Digestion is the process by which food and drink are broken down into their smallest parts so that the body can use them to build and nourish cells and to provide energy.

The digestive system is a series of hollow organs joined in a long, twisting tube from the mouth to the anus. Inside this tube is a lining called the mucosa.

In the mouth, stomach, and small intestine, the mucosa contains tiny glands that produce juices to help digest food. Digestion involves the mixing of food, its movement through the digestive tract, and chemical breakdown of the large molecules of food into smaller molecules. Digestion begins in the mouth, when we chew and swallow, and is completed in the small intestine.

The chemical process varies somewhat for different kinds of food. Movement of Food Through the System. The large, hollow organs of the digestive system contain muscle that enables their walls to move. The movement of organ walls can propel food and liquid and also can mix the contents within each organ.

Typical movement of the esophagus, stomach, and intestine is called peristalsis. The action of peristalsis looks like an ocean wave moving through the muscle. The muscle of the organ produces a narrowing and then propels the narrowed portion slowly down the length of the organ.

These waves of narrowing push the food and fluid in front of them through each hollow organ. The first major muscle movement occurs when food or liquid is swallowed. Although we are able to start swallowing by choice, once the swallow begins, it becomes involuntary and proceeds under the control of the nerves. The esophagus is the organ into which the swallowed food is pushed. It connects the throat above with the stomach below. At the junction of the esophagus and stomach, there is a ringlike valve closing the passage between the two organs.

However, as the food approaches the closed ring, the surrounding muscles relax and allow the food to pass. The food then enters the stomach, which has three mechanical tasks to do.

First, the stomach must store the swallowed food and liquid. This requires the muscle of the upper part of the stomach to relax and accept large volumes of swallowed material. The second job is to mix up the food, liquid, and digestive juice produced by the stomach. The lower part of the stomach mixes these materials by its muscle action. The third task of the stomach is to empty its contents slowly into the small intestine. Several factors affect emptying of the stomach, including the nature of the food mainly its fat and protein content and the degree of muscle action of the emptying stomach and the next organ to receive the stomach contents the small intestine.

As the food is digested in the small intestine and dissolved into the juices from the pancreas, liver, and intestine, the contents of the intestine are mixed and pushed forward to allow further digestion.

Glands of the digestive system are crucial to the process of digestion. They produce both the juices that break down the food and the hormones that help to control the process. The glands that act first are in the mouth--the salivary glands.

Saliva produced by these glands contains an enzyme that begins to digest the starch from food into smaller molecules. The next set of digestive glands is in the stomach lining. They produce stomach acid and an enzyme that digests protein. One of the unsolved puzzles of the digestive system is why the acid juice of the stomach does not dissolve the tissue of the stomach itself.

In most people, the stomach mucosa is able to resist the juice, although food and other tissues of the body cannot. After the stomach empties the food and its juice into the small intestine, the juices of two other digestive organs mix with the food to continue the process of digestion. One of these organs is the pancreas. Click on a thumbnail image for an annotated enlargement.

The most common types of epithelial tissue are regularly associated with particular functions and locations. Stratified squamous epithelium consists of flattened squamous cells on the surface overlying multiple layers of cells that usually are more cuboidal toward the base of the epithelium.

Stratified squamous epithelium is usually protective. The multiple layers are too thick for efficient transport of materials neither secretory or absorptive. The innermost layer continually produces cells via mitosis to replace those lost from the outer surface. Simple columnar epithelium lines the digestive tract and the female reproductive tract as well as numerous other surfaces. How many cell layers appear in a section depends on the angle between the section plane and the surface of the epithelium.

A single surface is usually not lined by several types of epithelia, so the number of epithelial cell layers will normally be the smallest number of layers visible anywhere along the surface lined by the epithelium.

A pseudostratified columnar epithelium appears stratified, typically with nuclei located in at least two more-or-less distinct levels. But in fact every cell rests on the basement membrane, so the epithelium is technically "simple", in spite of appearances.

Cuboidal epithelium consists of boxy cuboidal cells on the surface. If stratified, the deeper layers are usually also cuboidal.

Cuboidal epithelium is commonly encountered in glandular ducts. Cuboidal epithelial cells may be active pumping material into or out of the lumen or passive, depending on location and cellular specialization. Small ducts typically have a simple cuboidal epithelium. Larger ducts may have a stratified cuboidal epithelium. Simple squamous epithelium consists of a single, very thin layer flattened squamous cells.

Simple squamous epithelium may be located at sites of rapid diffusion, such as the lining of lung alveoli , the lining of blood vessels called endothelium , and at sites where very little activity is occuring, such as Bowman's capsule in the kidney and the lining of major body cavities called mesothelium. Transitional epithelium , also called urothelium , is a stratified epithelium lining the distensible walls of the urinary tract.

The name "transitional" derives from this tissue's ability to change its shape from cuboidal to squamous when stretched. Endothelium and mesothelium are special names given to the lining of certain internal surfaces. The entire circulatory system heart, arteries, veins, capillaries, sinusoids and lymphatics is lined by a simple squamous epithelium that is called endothelium.

The inner lining of the cornea is also called "endothelium". The major body cavities peritoneal, pleural, pericardial are lined by a mesodermally derived simple squamous epithelium that is called mesothelium. Glandular epithelium may be differentiated into secretory units which can be specialized for various products and ducts which typically appear less specialized, although ducts may function actively to reabsorb water and thus concentrate the secretory product.

Although large exocrine glands like the liver and pancreas may appear solid, each secretory cell has an apical surface exposed to a lumen. This lumenal space, in turn, leads through ducts to the outside of the body. Certain epithelia, notably epidermis and intestinal epithelium , are continually recycled, with with new cells being created by mitotic activity while old cells are sloughed off from the surface of the epidermis or the tips of intestinal villi. Many additional epithelial cells not just those of skin and intestine have the ability to respond to the stimulus of injury with mitotic activity and cell migration, to regenerate tissue following damage.

This confers on most epithelia an "automatic" ability to deal effectively with injury by replacing lost tissue with new growth from undamaged edges. The surface location of many epithelial tissues exposes them to a variety of insults, ranging from mechanical damage cuts, scrapes and active penetration mosquitoes, parasites, hypodermics to bacterial and fungal attack and poisoning by toxic chemicals.

In simple clean wounds of the skin, one of the earliest healing accomplishments may be proliferation and spread of epithelial keratinocytes, re-establishing epidermal continuity in as little as 24 hours. Deep third degree burns are so serious largely because they destroy the many hair follicles and sweat glands that invaginate deep into the dermis and serve as efficient sources of epithelial regrowth after more superficial injury.

The importance of epithelial cell regeneration is dramatically illustrated by recovery from cholera. A toxin from the cholera vibrio kills the intestinal epithelium. Resulting loss of bodily fluid from the uncovered mucosa leads to copious diarrhea, massive dehydration, and death within a few days.

However, if patients can be kept hydrated for those few days, epithelial replacement by stem cell division will restore normal function. When an epithelial cells' ability to divide is stimulated inappropriately, it can result in the formation of a tumor.

Cells in epithelial tumors often retain their basic epithelial character, remaining attached to one another and differentiating to form layered structures.

As long as the neoplastic cells respect the basement membrane, the tumor will remain localized. But once cells break through this boundary they can enter circulation and metastasize. The name carcinoma is applied to any cancer malignant neoplasm of epithelial origin; adenocarcinoma names a cancer of glandular origin. Cancers of mesenchymal origin are called sarcomas.

How to use this resource. Epithelial tissue comprises an uninterrupted layer of cells.

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