Anatomy and Physiology

Gastrointestinal Tract

The Digestive System or Gastrointestinal Tract is basically a tube running

through the body from mouth to any. The organs of the digestive system include

the oral cavity, esophagus, stomach, small and large intestine, and rectum. Their

role is to break down food and deliver the products to the blood for dispersal to

the body cells. The undigested food that remains in the tract leaves the body

through the anus as feces. The breakdown activities that begin in the mouth are

completed in the small intestine. From that point on, the major function of the

digestive system is to reclaim water. The liver is considered to be a digestive

organ, because the bile it produces helps to break down fats. The pancreas,

which deliver digestive enzymes to the small intestine, also is functionally in

digestive organ.

Digestion is the process by which food is broken down into smaller pieces

so that the body can use them to build and nourish cells and to provide energy.

Digestion involves the mixing of food, its movement through the digestive tract

(also known as the alimentary canal), and the chemical breakdown of larger

molecules into smaller molecules. Every piece of food we eat has to be broken

down into smaller nutrients that the body can absorb, which is why it takes hours

to fully digest food.

The digestive system is made up of the digestive tract. This consists of a

long tube of organs that runs from the mouth to the anus and includes the

esophagus, stomach, small intestine, and large intestine, together with the liver,

gall bladder, and pancreas, which produce important secretions for digestion that

drain into the small intestine. The digestive tract in an adult is about 30 feet long.

DIGESTIVE SYSTEM

Mouth and Salivary Glands

Digestion begins in the mouth, where chemical and mechanical digestion occurs. Saliva or spit, produced by the salivary glands (located under the tongue and near the lower jaw), is released into the mouth. Saliva begins to break down the food, moistening it and making it easier to swallow. A digestive enzyme (called amylase) in the saliva begins to break down the carbohydrates (starches and sugars). One of the most important functions of the mouth is chewing. Chewing allows food to be mashed into a soft mass that is easier to swallow and digest later.

Movements by the tongue and the mouth push the food to the back of the throat for it to be swallowed. A flexible flap called the epiglottis closes over the trachea (windpipe) to ensure that food enters the esophagus and not the windpipe to prevent choking.

Esophagus

Once food is swallowed, it enters the esophagus, a muscular tube that is about 10 inches long. The esophagus is located between the throat and the stomach. Muscular wavelike contractions known as peristalsis push the food down through the esophagus to the stomach. A muscular ring (called the cardiac sphincter) at the end of the esophagus allows food to enter the stomach, and, then, it squeezes shut to prevent food and fluid from going back up the esophagus.

Stomach

The stomach is a J-shaped organ that lies between the esophagus and the small intestine in the upper abdomen. The stomach has 3 main functions: to store the swallowed food and liquid; to mix up the food, liquid, and digestive juices produced by the stomach; and to slowly empty its contents into the small intestine.

Only a few substances, such as water and alcohol, can be absorbed directly from the stomach. Any other food substances must undergo the digestive processes of the stomach. The stomach's strong muscular walls mix and churn the food with acids and enzymes (gastric juice), breaking it into smaller pieces. About 3 quarts of the gastric juice is produced by glands in the stomach every day.

The food is processed into a semiliquid form called chyme. About 4 hours or so after eating a meal, the chyme is slowly released a little at a time through

the pyloric sphincter, a thickened muscular ring between the stomach and the first part of the small intestine called the duodenum.

Small Intestine

Most digestion and absorption of food occurs in the small intestine. The small intestine is a narrow, twisting tube that occupies most of the lower abdomen between the stomach and the beginning of the large intestine. It extends about 20 feet in length. The small intestine consists of 3 parts: the duodenum (the C-shaped part), the jejunum (the coiled midsection), and the ileum (the last section).

The small intestine has 2 important functions. First, the digestive process is completed here by enzymes and other substances made by intestinal cells, the pancreas, and the liver. Glands in the intestine walls secrete enzymes that breakdown starches and sugars. The pancreas secretes enzymes into the small intestine that help breakdown carbohydrates, fats, and proteins. The liver produces bile, which is stored in the gallbladder. Bile helps to make fat molecules (which otherwise are not soluble in water) soluble, so they can be absorbed by the body. Second, the small intestine absorbs the nutrients from the digestive process. The inner wall of the small intestine is covered by millions of tiny fingerlike projections called villi. The villi are covered with even tinier projections called microvilli. The combination of villi and microvilli increase the surface area of the small intestine greatly, allowing absorption of nutrients to occur. Undigested material travels next to the large intestine.

Large Intestine

The large intestine forms an upside down U over the coiled small intestine. It begins at the lower right-hand side of the body and ends on the lower left-hand side. The large intestine is about 5-6 feet long. It has 3 parts: the cecum, the colon, and the rectum. The cecum is a pouch at the beginning of the large intestine. This area allows food to pass from the small intestine to the large intestine. The colon is where fluids and salts are absorbed and extends from the cecum to the rectum. The last part of the large intestine is the rectum, which is where feces (waste material) is stored before leaving the body through the anus.

The main job of the large intestine is to remove water and salts (electrolytes) from the undigested material and to form solid waste that can be excreted. Bacteria in the large intestine help to break down the undigested materials. The remaining contents of the large intestine are moved toward the rectum, where feces are stored until they leave the body through the anus as a bowel movement.

BILIARY SYSTEM

BILE

Bile is a yellow-green fluid that is made by the liver, stored in the gallbladder and passes through the common bile duct into the duodenum where it helps digest fat. The principal components of bile are cholesterol, bile salts, and the pigment bilirubin.

An imbalance between these components of bile -- cholesterol, bile salts, and bilirubin -- leads to the formation of gallstones. Cholesterol is normally kept in liquid form by the dissolving action of the bile salts; an increased amount of cholesterol in the bile overwhelms the dissolving capacity of the bile salts and leads to the formation of cholesterol gallstones. Similarly, a deficiency of bile salts promotes cholesterol gallstone formation.

The biliary system consists of the organs and ducts (bile ducts, gallbladder, and associated structures) that are involved in the production and transportation of bile. The transportation of bile follows this sequence:

► When the liver cells secrete bile, it is collected by a system of ducts that flow from the liver through the right and left hepatic ducts.

► These ducts ultimately drain into the common hepatic duct.► The common hepatic duct then joins with the cystic duct from the gallbladder

to form the common bile duct, which runs from the liver to the duodenum (the first section of the small intestine).

► However, not all bile runs directly into the duodenum. About 50 percent of the bile produced by the liver is first stored in the gallbladder, a pear-shaped organ located directly below the liver.

► Then, when food is eaten, the gallbladder contracts and releases stored bile into the duodenum to help break down the fats.

Functions of the biliary system:

The biliary system's main function includes the following:

► to drain waste products from the liver into the duodenum► to help in digestion with the controlled release of bile

Bile is the greenish-yellow fluid (consisting of waste products, cholesterol, and bile salts) that is secreted by the liver cells to perform two primary functions, including the following:

► to carry away waste► to break down fats during digestion

Bile salt is the actual component which helps break down and absorb fats. Bile, which is excreted from the body in the form of feces, is what gives feces its dark brown color.

Accessory Organs

Salivary Glands  

There are three pairs of salivary glands: the parotid, submandibular, and sublingual glands. Each gland releases salivary amylase that begins digestion by degrading starch in the mouth. The glands are composed of specialized cells that make enzymes. They are connected to the mouth by a duct. These ducts are lined with epithelium, which serves as a protective barrier from autodigestion. They also have some muscle in them that allows the ducts to contract. This enables the one-way flow of enzymes from the glands into the mouth.

 Pancreas  

The pancreas has both an endocrine and an exocrine function. The endocrine function allows the production of insulin, which aids in the regulation of sugar but is not involved in digestion. The exocrine function, on the other hand, is responsible for digestion. It is in this portion of the pancreas where enzymes of digestion are made. A neutral buffer, bicarbonate, is also produced here. Bicarbonate keeps the intestinal environment at a neutral pH. The enzymes and the bicarbonate are then released into the lumen of the intestine through the pancreatic duct.  

Figure 2.5: The Liver and Biliary Tract

Bile is made in the liver. It is then carried to the gall bladder via the bile ducts. The gall bladder stores the bile until it is ready to be released into the duodenum. Through molecular signalling from the small intestine, the gall bladder will contract and empty its contents into a larger bile duct that will end in the lumen of the small intestine.