Gastrointestinal / Digestive System

Gastrointestinal System (Digestive System)

The human digestive system is a complex process that consists of breaking down large organic masses into smaller particles that the body can use as fuel. The breakdown of the nutrients requires the coordination of several enzymes secreted from specialized cells within the mouth, stomach, intestines, and liver. The digestive system comprises of two main components:

The digestive tract includes the mouth, esophagus, stomach, small and large intestine and anus.

Associated glands include salivary glands, liver and pancreas.
 

Mouth

In the human body, the mouth (oral cavity) is a specialized organ for receiving food and breaking up large organic masses. In the mouth, food is changed mechanically by biting and chewing. Humans have four kinds of teeth: incisors are chisel-shaped teeth in the front of the mouth for biting; canines are pointed teeth for tearing; and premolars and molars are flattened, ridged teeth for grinding, pounding, and crushing food.

In the mouth, food is moistened by saliva, a sticky fluid that binds food particles together into a soft mass. Three pairs of salivary glands—the parotid glands, the submaxillary glands, and the sublingual glands—secrete saliva into the mouth. The saliva contains an enzyme called amylase, which digests starch molecules into smaller molecules of the disaccharide maltose.

During chewing, the tongue moves food about and manipulates it into a mass called a bolus. The bolus is pushed back into the pharynx (throat) and is forced through the opening to the esophagus.

Parts of the Tooth:

Each tooth has four main parts, including the following:

• Enamel - the outer layer of the tooth.
• Dentin - the inner layer and the main part of the tooth.
• Pulp - part of the inside of the tooth that contains the nerve.
• Root - the part of the tooth that secures it into the jaw

Teeth: The teeth are the hardest substances in the human body. Besides being essential for chewing, the teeth play an important role in speech. Parts of the teeth include:

• Enamel: The hardest, white outer part of the tooth. Enamel is mostly made of calcium phosphate, a rock-hard mineral.
• Dentin: A layer underlying the enamel. Dentin is made of living cells, which secrete a hard mineral substance.
• Pulp: The softer, living inner structure of teeth. Blood vessels and nerves run through the pulp of the teeth.
• Cementum: A layer of connective tissue that binds the roots of the teeth firmly to the gums and jawbone.
• Periodontal Ligament: Tissue that helps hold the teeth tightly against the jaw.
• A normal adult mouth has 32 teeth, which (except for wisdom teeth) have erupted by about age 13:
• Incisors (8 total): The middlemost four teeth on the upper and lower jaws.
• Canines (4 total): The pointed teeth just outside the incisors.
• Premolars (8 total): Teeth between the canines and molars.
• Molars (8 total): Flat teeth in the rear of the mouth, best at grinding food.
• Wisdom teeth or third molars (4 total): These teeth erupt at around age 18, but are often surgically removed to prevent displacement of other teeth.

The crown of each tooth projects into the mouth. The root of each tooth descends below the gum line, into the jaw.

Esophagus

After being chewed and swallowed, the food enters the esophagus. The esophagus is a long tube that runs from the mouth to the stomach. It uses rhythmic, wave-like muscle movements (called peristalsis) to force food from the throat into the stomach. This muscle movement gives us the ability to eat or drink even when we're upside-down.

In the stomach The esophagus joins the stomach at a point just below the diaphragm. A valvelike ring of muscle called the cardiac sphincter surrounds the opening to the stomach. The sphincter relaxes as the bolus passes through and then quickly closes.

Stomach

The stomach is a large, sack-like organ that churns the food and bathes it in a very strong acid (gastric acid). Food in the stomach that is partly digested andThe stomach stores food and prepares it for further digestion. In addition, the stomach plays a role in protein digestion. Gastric glands called chief cells secrete pepsinogen. Pepsinogen is converted to the enzyme pepsin in the presence of hydrochloric acid. Hydrochloric acid is secreted by parietal cells in the stomach lining. The pepsin then digests large proteins into smaller proteins called peptides mixed with stomach acids is called chyme.

The Small Intestine

After being in the stomach, food enters the duodenum, the first part of the small intestine. It then enters the jejunum and then the ileum (the final part of the small intestine). In the small intestine, bile (produced in the liver and stored in the gall bladder), pancreatic enzymes, and other digestive enzymes produced by the inner wall of the small intestine help in the breakdown of food.

Large Intestine

The small intestine joins the large intestine in the lower right abdomen of the body. The two organs meet at a blind sac called the cecum and a small fingerlike process called the appendix. Evolutionary biologists believe the cecum and appendix are vestiges of larger organs that may have been functional in human ancestors.

The large intestine is also known as the colon. It is divided into ascending, transverse, and descending portions, each about one foot in length. The colon's chief functions are to absorb water and to store, process, and eliminate the residue following digestion and absorption. The intestinal matter remaining after water has been reclaimed is known as feces. Feces consist of nondigested food (such as cellulose), billions of mostly harmless bacteria, bile pigments, and other materials. The feces are stored in the rectum and passed out through the anus to complete the digestion process.

The Rectum

Rectum is an eight-inch chamber that connects the colon to the anus. 

The Rectum:

    Receives stool from the colon.
    Lets the person know there is stool to be evacuated.
    Holds the stool until evacuation happens.

When anything (gas or stool) comes into the rectum, sensors send a message to the brain. The brain then decides if the rectal contents can be released or not. If they can, the sphincters relax and the rectum contracts, expelling its contents. If the contents cannot be expelled, the sphincters contract and the rectum accommodates so that the sensation temporarily goes away.

The anus

It is the last part of the digestive tract. It consists of the muscles that line the pelvis (pelvic floor muscles) and two other muscles called anal sphincters (internal and external).

Stool, or waste left over from the digestive process, passes through the colon by means of peristalsis, first in a liquid state and ultimately in solid form. As stool passes through the colon, any remaining water is absorbed. Stool is stored in the sigmoid (S-shaped) colon until a "mass movement" empties it into the rectum, usually once or twice a day.

It normally takes about 36 hours for stool to get through the colon. The stool itself is mostly food debris and bacteria. These bacteria perform several useful functions, such as synthesizing various vitamins, processing waste products and food particles, and protecting against harmful bacteria. When the descending colon becomes full of stool it empties its contents into the rectum to begin the process of elimination.

Liver

The liver has an important function in processing the products of human digestion. For example, cells of the liver remove excess glucose from the bloodstream and convert the glucose to a polymer called glycogen for storage.

Thirty per cent of the blood pumped through the heart in one minute passes through the body's chemical factory, which is called the liver. The liver cleanses the blood and processes nutritional molecules, which are distributed to the tissues. The liver also receives bright red blood from the lungs, filled with vital oxygen to be delivered to the heart. The only part of the body which receives more blood than the liver is the brain. The liver is located at the top of the abdomen, just below the diaphragm and has two main lobes. It is the largest gland in the body, weighing 2.5 to 3.3 pounds. When we eat, more blood is diverted to the intestines to deal with digestive processes; when not eating, three-fourths of the blood supply to the liver comes from the intestines. It also produces about two and one-half pints of bile in its ducts, which is delivered to the gallbladder through a small tube called the "cystic duct" for storage.

Gallbladder 

The gallbladder is a storage sac for excess bile. Bile made in the liver travels to the small intestine via the bile ducts. If the intestine doesn't need it, the bile travels into the gallbladder where it awaits the signal from the intestines that food is present. Bile serves two main purposes. First, it helps absorb fats in the diet and Accessory Digestive Organs.

Pancreas 

The pancreas is located deep in the abdomen, partially behind the stomach. The other part is nestled in the curve of the duodenum (small intestine). Among other functions, the pancreas is the chief factory for digestive enzymes that are secreted into the duodenum.  These enzymes break down protein, fats, and carbohydrate.

Salivary Glands

The glands are found in and around the mouth. Three major salivary glands are the parotid, submandibular, and sublingual glands.

They all secrete saliva into mouth. Besides these glands, there are many tiny glands called minor salivary glands located in the lips, inner cheek area (buccal mucosa), and extensively in other linings of the mouth and throat. Salivary glands produce the saliva used to moisten the mouth, initiate digestion, and help protect teeth from decay.