The Digestive System

The Digestive system plays a very important role in the functioning of the human body in that it regulates the energy break down for the cells of the body. Without the digestive system, the cells of the body would not be able to obtain the energy needed for proper functioning.

A basic definition of digestion is the process by which food substances are changed into forms that can be absorb through the cell membranes.

The digestive system is composed of 10 main organs:

small intestine

large intestine

The process of digestion begins in the mouth. The mouth has the ability to mechanically reduce the size of the food that is placed in it. While the dimensions of the food are being reduced in the mouth, Salivary glands secrete saliva. The saliva secreted by these glands is mixed with the food. When the saliva mixews with the food, the food particles bind. The combination of food dimension reduction and saliva is the beginning of the digestion process.

The mouth uses the 32 teeth within as tools to break down the food within.

Types of Teeth:

Incisors - their chisel-shape and sharp edges are used to bite off large pieces of food.

Cuspid - are useful in grasping and/or tearing food.

Molar - their flattened surfaces aid in the grinding of food.

Teeth are classified mainly by their shape and location. The reduction of the size of the food particles increases the overall surface area of the food particles. A more effective food particle/digestive enzyme interaction is the result of the increase in the overall surface area of the food particles.

The pharynx is a cavity that leads to the stomach. The cavity combines with the esophagus to create the passage that food takes to get to the stomach. The pharynx has no active role in digestion, but its muscular walls attribute to the swallowing of food.

The pharynx is a cavity formed form three other cavities:

The esophagus is the major passageway from the oral cavity to the stomach. It is a straight collapsible tube whose length is approximately .25 meters. To be more exact, the esophagus begins at the base of the pharynx, moves down behind the trachea, passes through the mediastinum, enters the diaphragm and is continuous with the stomach.

Functions of the stomach:

receiving food from the esophagus

mixing food with gastric juice

initiating the digestion of proteins

carrying on a limited amount of absorption

moving food into the small intestine

The stomach has a distinct J-shape. Structurally, the stomach is a pouch.

The stomach is composed of multiple regions:

One of the stomach's most useful attributes is its ability to secrete gastric juices. The inner lining of the stomach is a thick mucus membrane that has many small openings. The small openings are called gastric pits. The location of the gastric pits is at the end of the gastric glands.

Within these gstric glands lies multiple secretory cells:

Mucous cells can be found in the necks of the glands near the openings of the gstric pits, but both cheif cells and parietal cells are located in deeper parts of the glands. Each of these cells secretes a substance that aids in the digestive process. The chief cells secrete digestive enzymes, the parietal cells release hydrochloric acid, and mucous cells secrete large amounts of mucous which act as a protective coating against pepsin. When combined, these substances are called gastric juices.

Pepsin is a substance that is secreted by chief cells as an inactive enzyme called pepsinogen. When pepsinogen interacts with the hydrochloric acid produced by parietal cells, it changes into pepsin.

Pepsin functions as a protein-splitting enzyme. It is able to initiate the digestion of almost all types of dietary protein. Pepsin is most effective in an acid-filled environment. The high concentration of hydrochloric acid provides an excellent environment for pepsin to begin the digestion of proteins.

The secretion of gastric juices is regulated by the neural and hormonal systems. An enzyme called gastrin effects the gastric glands in such a way that their secretory activity increases.

Gastric secretions play a major role in the digestion of food, but it is not the only tool that the body has that plays a part in the digestion of food. Muscles of the stomach give the stomach the ability to mix the food and gastric juices within. The semi-fluid paste, which is a combination of food particles and gastric juices, is called chyme. The chyme is moved towards the pyloric region of the stomach by way of peristaltic waves. The chyme than moves into the small intestine.

Small Intestine

Structurally, the small intestine is a tubular organ. It consists of a complex system of loops and coils, which fills a great deal of the abdominal cavity. The small intestine receives secretions from the liver and the pancreas, finalizes the digestion of the nutrients in chyme, absorbs the different products of digestion, and moves the remaining residues to the large intestine.

The small intestine is made of three major pieces:

The duodenum, ileum, and jejunum are connected by a double-layered fold of peritoneum called mesentery. The mesentery can be seen in the picture above.

Intestinal villi play a major role in the in the absorption of digestive products. The villi line the small intestinal wall. Villi increase the surface area of the intestinal lining. The base of these glands secrete large amounts of a watery fluid. The watery fluid secreted by the base of the villi acts as a medium in which digestive products can move. Once the digestive products are in the watery fluid, they can than be moved back into the villi. Basically, the watery fluids are a vehicle that allow villi to acquire the needed digestive products.

The intestinal glands respond to multiple stimuli. They are stimulated by physical contact with chyme ,which is actually a combination of physical and chemical stimulation, and by reflexes activated by dissension of the intestinal wall.

The increased surface area caused by the presence of the villi gives the small intestine the ability to absorb the greatest amount of digestive products. The majority of digestive products never makes it to the distal end of the small intestine because of its effectiveness in absorbing.

The propulsion of chyme in the small intestine is a result of peristaltic waves. This method of propulsion is weak and because of its weakness, it takes three to ten hours for food to travel through the small intestine. The digestion process in this part of the alimentary canal is quite lengthy time wise.

Large Intestine

The Large Intestine has four basic functions:

the reabsorbtion of water

the reabsorbtion of electrolytes

the formation of feces

the storage of feces

The Large Intestine has four main parts:

ascending colon

transverse colon

desending colon

When compared to the small intestine, the large intestine plays a very little part in the digestive process. While the glands of the small intestine secrete a variety of substances, the only significant secretion of the large intestine is mucus. The mucus secreted by the walls of the large intestine functions only to protect the walls of the large intestine from materials passing through. The mucus also aids in the holding together of fecal matter. The large intestine takes in chyme that has not been freed by the small intestine of all digestible materials. Water and electrolytes which are stored inside of the chyme are absorbed by the large intestine. The substances that remain after the water and electrolytes have been absorbed become feces.

Major functions of Liver:

The conversion of glucose to glycogen

The conversion of glycogen to glucose

The conversion of noncarbohydrates to glucose

Oxidation of fatty acids

The synthesis of lipoproteins, phospholipids, and cholesterol.

The conversion of carbohydrates and proteins into fats.

Deamination of amino acids

The synthesis of urea

The synthesis of blood proteins

The interconversion of amino acids

Stores glycogen, vitamins A, D, and B12, and iron

Removes damaged red blood cells and foreign substances b phagocytosis

Alters the compostion of toxic substances

The gallbladder stores bile, which is a yellowish green liquid that is continuously secreted by the hepatic cells, between meals, increases the concentration of bile by reabsorbing water, and regulates the movement of bile into the small intestine. The bile stored in the gallbladder is composed of bile salts, bile pigments, cholesterol, and various electrolytes.

Bile goes into the duodenum only when the gallbladder is stimulated to contract by the hormone cholecystokinin.

One of the most important parts of bile is the bile salts. The bile salts do not function as digestive enzymes, but they do aid in digesting process. Bile salts enhance the absorption properties of the fatty acids and certain fat-soluble vitamins.

Fat-soluble vitamins that bile salts affect:

Fat molecules naturally clump together. When they do, the form fat globules. The masses of fat formed are difficult to digest, and this is where bile salts come in. Bile salts cause fat globules to break up. This makes it easier for fat-splitting enzymes to digest the fat molecules. The process of breaking up fat globules is called emulsification.

A majority of the pancreas is made up of the cells that produce pancreatic juice. The enzymes present in pancreatic juice are able to digest a variety of substances:

Pancreatic amylase is the enzyme that digests carbohydrates.

Pancreatic lipase is the enzyme that digests fat.

Carboxypeptidase, chymotrypsin, and trypsin areprotein-splitting

Nucleases are enzymes that break down nucleic acid molecules.

Nerve actions and hormones regulate the release of pancreatic juice.

Secretin is a hormone that stimulates the secretion of pancreatic juice. Secretin is secreted when the chyme enters the duodenum. The high concentration of bicarbonate ions neutralize the acidity of the chyme and makes a more suitable environment for the digestive enzymes in the intestine.

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