Digestive system

INTRODUCTION TO THE DIGESTIVE SYSTEM

The digestive system includes the digestive tract and its accessory organs, which process food into molecules that can be absorbed and utilized by the cells of the body. Food is broken down, bit by bit, until the molecules are small enough to be absorbed and the waste products are eliminated. The digestive tract, also called the alimentary canal or gastrointestinal (GI) tract, consists of a long continuous tube that extends from the mouth to the anus. It includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The tongue and teeth are accessory structures located in the mouth. The salivary glands, liver, gallbladder, and pancreas are major accessory organs that have a role in digestion. These organs secrete fluids into the digestive tract.

Food undergoes three types of processes in the body:

Digestion
Absorption
Elimination

Digestion and absorption occur in the digestive tract. After the nutrients are absorbed, they are available to all cells in the body and are utilized by the body cells in metabolism.

To achieve the goal of providing energy and nutrients to the body, six major functions take place in the digestive system:

INJESTION

The first activity of the digestive system is to take in food through the mouth. This process, called ingestion, has to take place before anything else can happen.The mouth is responsible for this function, as it is the orifice through which all food enters the body. The mouth and stomach are also responsible for the storage of food as it is waiting to be digested. This storage capacity allows the body to eat only a few times each day and to ingest more food than it can process at one time.

SECRETION

In the course of a day, the digestive system secretes around 7 liters of fluids. These fluids include saliva, mucus, hydrochloric acid, enzymes, and bile. Saliva moistens dry food and contains salivary amylase, a digestive enzyme that begins the digestion of carbohydrates. Mucus serves as a protective barrier and lubricant inside of the GI tract. Hydrochloric acid helps to digest food chemically and protects the body by killing bacteria present in our food. Enzymes are like tiny biochemical machines that disassemble large macromolecules like proteins, carbohydrates, and lipids into their smaller components. Finally, bile is used to emulsify large masses of lipids into tiny globules for easy digestion.

MIXING AND MOVEMENT

The digestive system uses 3 main processes to move and mix food:

Swallowing. Swallowing is the process of using smooth and skeletal muscles in the mouth, tongue, and pharynx to push food out of the mouth, through the pharynx, and into the esophagus.

Peristalsis. Peristalsis is a muscular wave that travels the length of the GI tract, moving partially digested food a short distance down the tract. It takes many waves of peristalsis for food to travel from the esophagus, through the stomach and intestines, and reach the end of the GI tract.

Segmentation. Segmentation occurs only in the small intestine as short segments of intestine contract like hands squeezing a toothpaste tube. Segmentation helps to increase the absorption of nutrients by mixing food and increasing its contact with the walls of the intestine.

Digestion

Digestion is the process of turning large pieces of food into its component chemicals. 

DIGESTION

Digestion is the process of turning large pieces of food into its component chemicals. Mechanical digestion is the physical breakdown of large pieces of food into smaller pieces. This is done in two ways; MECHANICALLY AND CHEMICALLY

 This mode of digestion begins with the chewing of food by the teeth and is continued through the muscular mixing of food by the stomach and intestines. Bile produced by the liver is also used to mechanically break fats into smaller globules. While food is being mechanically digested it is also being chemically digested as larger and more complex molecules are being broken down into smaller molecules that are easier to absorb. Chemical digestion begins in the mouth with salivary amylase in saliva splitting complex carbohydrates into simple carbohydrates. The enzymes and acid in the stomach continue chemical digestion, but the bulk of chemical digestion takes place in the small intestine thanks to the action of the pancreas. The pancreas secretes an incredibly strong digestive cocktail known as pancreatic juice, which is capable of digesting lipids, carbohydrates, proteins and nucleic acids. By the time food has left the duodenum, it has been reduced to its chemical building blocks—fatty acids, amino acids, monosaccharides, and nucleotides.

ABSORPTION

Once food has been reduced to its building blocks, it is ready for the body to absorb. Absorption begins in the stomach with simple molecules like water and alcohol being absorbed directly into the bloodstream. Most absorption takes place in the walls of the small intestine, which are densely folded to maximize the surface area in contact with digested food. Small blood and lymphatic vessels in the intestinal wall pick up the molecules and carry them to the rest of the body. The large intestine is also involved in the absorption of water and vitamins B and K before feces leave the body.

EXCRETION

The final function of the digestive system is the excretion of waste in a process known as defecation. Defecation removes indigestible substances from the body so that they do not accumulate inside the gut. The timing of defecation is controlled voluntarily by the conscious part of the brain, but must be accomplished on a regular basis to prevent a backup of indigestible materials.

ORGANS OF THE DIGESTIVE SYSTEM

At its simplest, the digestive system is a tube running from mouth to anus. Its chief goal is to break down huge macromolecules (proteins, fats and starch), which cannot be absorbed intact, into smaller molecules (amino acids, fatty acids and glucose) that can be absorbed across the wall of the tube, and into the circulatory system for dissemination throughout the body.

Regions of the digestive system can be divided into two main parts: the alimentary tract and accessory organs. The alimentary tract of the digestive system is composed of the mouth, pharynx, esophagus, stomach, small and large intestines, rectum and anus. Associated with the alimentary tract are the following accessory organs: salivary glands, liver, gallbladder, and pancreas.

MOUTH

Food begins its journey through the digestive system in the mouth, also known as the oral cavity. Inside the mouth are many accessory organs that aid in the digestion of food—the tongue, teeth, and salivary glands. Teeth chop food into small pieces, which are moistened by saliva before the tongue and other muscles push the food into the pharynx. 

Teeth- The teeth are 32 small, hard organs found along the anterior and lateral edges of the mouth. Each tooth is made of a bone-like substance called dentin and covered in a layer of enamel—the hardest substance in the body. Teeth are living organs and contain blood vessels and nerves under the dentin in a soft region known as the pulp. The teeth are designed for cutting and grinding food into smaller pieces. 

Tongue-The tongue is located on the inferior portion of the mouth just posterior and medial to the teeth. It is a small organ made up of several pairs of muscles covered in a thin, bumpy, skin-like layer. The outside of the tongue contains many rough papillae for gripping food as it is moved by the tongue’s muscles. The taste buds on the surface of the tongue detect taste molecules in food and connect to nerves in the tongue to send taste information to the brain. The tongue also helps to push food toward the posterior part of the mouth for swallowing. 

Salivary Glands- Surrounding the mouth are 3 sets of salivary glands. The salivary glands are accessory organs that produce a watery secretion known as saliva. Saliva helps to moisten food and begins the digestion of carbohydrates. The body also uses saliva to lubricate food as it passes through the mouth, pharynx, and esophagus.

Pharynx

Food is forced into the pharynx by the tongue. When food reaches the opening, sensory receptors around the fauces respond and initiate an involuntary swallowing reflex. This reflex action has several parts. The uvula is elevated to prevent food from entering the nasopharynx. The epiglottis drops downward to prevent food from entering the larynx and trachea in order to direct the food into the esophagus. Peristaltic movements propel the food from the pharynx into the esophagus.

Esophagus

The esophagus is a collapsible muscular tube that serves as a passageway between the pharynx and stomach. As it descends, it is posterior to the trachea and anterior to the vertebral column. It passes through an opening in the diaphragm, called the esophageal hiatus, and then empties into the stomach. The mucosa has glands that secrete mucus to keep the lining moist and well lubricated to ease the passage of food. Upper and lower esophageal sphincters control the movement of food into and out of the esophagus. The lower esophageal sphincter is sometimes called the cardiac sphincter and resides at the esophagogastric junction.

Stomach

The stomach has both a mechanical and a chemical function in digestion. The upper part of the smooth (involuntary) stomach muscle relaxes to allow a large volume of food to be stored. The lower muscle then contracts in a rhythmical manner in order to churn the food inside and mix it together with the gastric acid (mainly hydrochloric acid) and digestive enzymes Pepsin, Gelatinase and Gastric Amylase and Lipase which break it down further. The stomach must then empty its contents into the small intestine.

Small Intestine

Whilst in the small intestine food is subjected to yet more enzymes, those from the Pancreas and from the glands within the intestine walls which break down carbohydrates and proteins. It is also mixed with a product of the liver which is stored and released into the intestine by the gall bladder. This is commonly known as bile. Bile works to dissolve fat so that it can be digested by the other enzymes. Rhythmic smooth muscle contraction continues within the small intestine and pushes the digesting food through its narrow tube.

Once the food is completely broken down into its individual components it is absorbed through the intestinal walls, into the blood flow of the capillaries which surround the intestine. To make this process faster and more efficient the intestinal walls contain numerous folds which are covered in finger-like projections called villi. This vastly increases the surface area of the intestine wall for molecules of digested food to pass through.

The small intestine is divided into the duodenum, jejunum, and ileum.

Large Intestine

The large intestine continues the foods journey and is the bodies last chance to absorb any water and minerals still remaining. The rest of the contents of the large intestine is waste such as undigested pieces of food and fiber. This is passed through to the rectum where it is stored until you go to the toilet.
In the large intestine,the passage of the digesting food in the colon is a lot slower, taking from 12 to 50 hours until it is removed by defecation. The colon mainly serves as a site for the fermentation of digestible matter by the gut flora. The time taken varies considerably between individuals. The remaining semi-solid waste is termed feces and is removed by the coordinated contractions of the intestinal walls, termed peristalsis, which propels the excreta forward to reach the rectum and exit via defecation from the anus.

ACCESSORY ORGANS

The salivary glands, liver, gallbladder, and pancreas are not part of the digestive tract, but they have a role in digestive activities and are considered accessory organs.

Salivary Glands

Three pairs of major salivary glands (parotid, submandibular, and sublingual glands) and numerous smaller ones secrete saliva into the oral cavity, where it is mixed with food during mastication. Saliva contains water, mucus, and enzyme amylase. Functions of saliva include the following:

It has a cleansing action on the teeth.
It moistens and lubricates food during mastication and swallowing.
It dissolves certain molecules so that food can be tasted.
It begins the chemical digestion of starches through the action of amylase, which breaks down polysaccharides into disaccharides.

Liver

The liver is located primarily in the right hypochondriac and epigastric regions of the abdomen, just beneath the diaphragm. It is the largest gland in the body. On the surface, the liver is divided into two major lobes and two smaller lobes. The functional units of the liver are lobules with sinusoids that carry blood from the periphery to the central vein of the lobule.

The liver receives blood from two sources. Freshly oxygenated blood is brought to the liver by the common hepatic artery, a branch of the celiac trunk from the abdominal aorta. Blood that is rich in nutrients from the digestive tract is carried to the liver by the hepatic portal vein.

The liver has a wide variety of functions and many of these are vital to life. Hepatocytes perform most of the functions attributed to the liver, but the phagocytic Kupffer cells that line the sinusoids are responsible for cleansing the blood.

Liver functions include the following:

secretion
synthesis of bile salts
synthesis of plasma protein
storage
detoxification
excretion
carbohyrate metabolism
lipid metabolism
protein metabolism
filtering
Gallbladder

The gallbladder is a pear-shaped sac that is attached to the visceral surface of the liver by the cystic duct. The principal function of the gallbladder is to serve as a storage reservoir for bile. Bile is a yellowish-green fluid produced by liver cells. The main components of bile are water, bile salts, bile pigments, and cholesterol.

Bile salts act as emulsifying agents in the digestion and absorption of fats. Cholesterol and bile pigments from the breakdown of hemoglobin are excreted from the body in the bile.

Pancreas

The pancreas has both endocrine and exocrine functions. The endocrine portion consists of the scattered islets of Langerhans, which secrete the hormones insulin and glucagon into the blood. The exocrine portion is the major part of the gland. It consists of pancreatic acinar cells that secrete digestive enzymes into tiny ducts interwoven between the cells. Pancreatic enzymes include anylase, trypsin, peptidase, and lipase. Pancreatic secretions are controlled by the hormones secretin and cholecystokinin.

DISEASES OF DIGESTIVE SYSTEM

ULCERS

Ulcers damage the lining of the stomach because of bacterial infections or adverse side effects of medications such as aspirin, ibuprofen or naporoxen, according to the National Digestive Diseases Information Clearinghouse. Spicy foods and stress may aggravate ulcers, but do not cause them. Antibiotics or antacids usually alleviate problems long enough for minor ulcers to heal.

ACID REFLUX

Heartburn or acid indigestion results in stomach acid backing up into the esophagus to create a burning sensation or sour taste in the mouth. Gastroesophageal reflux disease (GERD) affects people who suffer acid reflux symptoms more than twice a week. Acid reflux occurs because the lower esophageal sphincter does not close properly. Some people may get more bouts of heartburn from certain foods, but each individual has different reactions to particular foods. Cigarette smoking contributes to heartburn by causing the esophageal sphincter to relax.

CELIAC DISEASE

Celiac disease can affect children or adults. Symptoms include diarrhea, bloating, abdominal pain, skin rash or a thinning of bones. In children it can cause growth failure. People with the disease are advised to eliminate foods with gluten, a protein that is found in wheat, rye and barley. Gluten damages the lining of the small intestine for people with the disease.

IRRITABLE BOWEL SYNDROME

Irritable bowl syndrome affects the muscles in the intestines. It can cause gas, abdominal pain, diarrhea or constipation. The syndrome can be painful and uncomfortable. It usually does not lead to further diseases. Most people can control symptoms through diet, stress management and medication.

INFLAMMATORY BOWEL DISEASES

Inflammatory bowel diseases include Crohn’s disease and ulcerative colitis. It leads to severe bowel problems, abdominal pain and malnutrition. Crohn’s disease and ulcerative colitis can be painful and debilitating. Medication can eliminate symptoms and prevent flare-ups. Surgery may be needed in some cases to repair the colon.

DIVERTICULITIS

The disease occurs when pouches in the large intestine become infected and inflamed, according to the American Academy of Family Physicians. Symptoms may include severe abdominal pain often in the lower left side of the abdomen, constipation, diarrhea, nausea or fever. People with the disease are advised to eat more fiber to help with proper digestion.

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MR. SANNI HAMMED OLAYINKA