Gastrointestinal System Functions

• The gastrointestinal system ingests raw materials.
• The gastrointestinal system breaks down raw materials to usable elements, both physically and chemically.
• The gastrointestinal system absorbs elements.
• The gastrointestinal system eliminates what is not usable.

System Overview

• The digestive tract, also known as the alimentary tract or canal, is a muscular tube containing the organs of digestion.
• The digestive tube begins at the mouth and ends at the anus.
• Organs between the mouth and anus include the pharynx, esophagus, stomach, small intestine, and large intestine.
• Accessory organs of digestion include teeth, salivary glands, liver, pancreas, and gallbladder.

Functions of the Gastrointestinal Tract

• The gastrointestinal tract is responsible for ingestion.
• Mastication, or chewing, is also a function of the gastrointestinal tract.
• Digestion is a function of this tract.
• Secretion is a function of this tract.
• Absorption is a function of this tract.
• Excretion or defecation is a function of this tract.

Oral Cavity

• The opening behind the mouth is the oral cavity.
• The hard and soft palate form the roof of the oral cavity.
• The tongue is the floor of the oral cavity.
• The cheeks are the walls of the oral cavity.
• The mouth receives and tastes food, mechanically breaks it down, and begins starch digestion.

Tongue

• The tongue's base (area of attachment) and the uvula create a barrier to the pharynx.
• The tongue provides taste stimuli to the brain, determines temperature, and manipulates food.
• The lingual frenulum, a membrane under the tongue, prevents swallowing the tongue and helps with speaking.

Swallowing

• The tongue pushes food into a ball-like mass called a bolus, which is then swallowed and passed to the pharynx.
• The uvula aids in swallowing by directing food toward the pharynx and blocking food from entering the nose.

Salivary Glands

• There are three pairs of salivary glands controlled by the autonomic nervous system: parotid, sublingual, and submandibular.
• The parotid salivary glands are found slightly inferior and anterior to each ear, and swell when you get mumps.
• The sublingual salivary glands are found under the tongue.
• The submandibular salivary glands are located along both sides of the inner surface of the mandible.
• The ducts from the submandibular glands empty into the upper portion of the oral cavity.

Pharynx

• The pharynx has three parts: the nasopharynx, oropharynx, and laryngopharynx.
• The nasopharynx is primarily part of the respiratory system and blocked by the soft palate.
• The oropharynx and laryngopharynx act as passageways for food, water, and air.
• The epiglottis covers the trachea to prevent food from entering the lungs, forcing food into the opening for the esophagus.

Esophagus

• The esophagus extends from the pharynx, through the thoracic cavity and diaphragm, connecting to the stomach in the peritoneal cavity.
• Peristalsis, rhythmic contractions, pushes food down the esophagus.
• The esophagus has no role in digestion and absorption.

Stomach

• The stomach is located on the left side of the abdominal cavity, under the diaphragm, and covered by the liver.
• The stomach is approximately 10 inches long with a variable diameter depending on the amount of food eaten.
• Rugae, or folds, help the stomach expand and contract.

Stomach Functions

• The stomach temporarily holds food.
• The stomach secretes gastric acids and enzymes that mix with food, performing chemical digestion.
• The stomach regulates the rate the partially digested food, known as chyme, enters the intestine.
• The stomach absorbs small amounts of water and substances like alcohol.

Gastric Juice

• Gastric juice contains hydrochloric acid (HCl), pepsinogen, and mucus.
• Pepsin breaks down protein, and HCl breaks down the connective tissue.
• Mucous cells generate a thick layer of mucus that shields the stomach from the effects of stomach acids.
• The bolus of food becomes chyme once it enters the stomach.

Action of the Stomach

• It takes about 4 hours for the stomach to empty following a meal.
• Liquids and carbohydrates pass through quickly.
• Proteins take more time to pass through.
• Fats take the longest, approximately 4-6 hours.

Control of Stomach Action

• The rate of chyme movement is important.
• A slow chyme movement decreases nutrient digestion and absorption and may cause erosion of the stomach lining.
• A fast chyme movement may cause food particles to be insufficiently mixed with gastric juices, leading to poor digestion.
• Chyme that isn't given time to neutralize can cause erosion of the intestinal lining.

Small Intestine

• The small intestine is the major organ of digestion because most food is digested there.
• The small intestine is small in diameter but long, with an average length of 6-20 feet.
• The small intestine has three parts: the duodenum, jejunum, and ileum.

Small Intestine Function

• The walls of the small intestine secrete digestive enzymes, which are important for the final stages of chemical digestion
• The walls of the small intestine secrete hormones that stimulate the pancreas and gallbladder to act, and control stomach activity
• Most absorption of usable nutrients occurs when chyme comes in contact with the mucosal walls of the small intestine.
• Amino acids, fatty acids, simple sugars, vitamins, and water are all absorbed in the small intestine.

Duodenal Function

• The pyloric valve allows small portions of chyme to enter the duodenum, because the small intestine can only process small amounts of food at a time.
• In the small intestine, segmentation occurs, mixing chyme and digestive juices.
• In the small intestine, peristalsis moves food toward the large intestine.

Large Intestine

• The large intestine starts at the junction of the small intestine, at the ileocecal orifice, and extends to the anus.
• The large intestine borders the small intestine.
• It takes the large intestine about sixteen hours to finish digestion of food.

Functions of the Large Intestine

• The large intestine is responsible for water absorption.
• The large intestine allows for absorption of vitamins produced by normal bacteria.
• The large intestine is responsible for packaging and compacting waste products for elimination from the body.
• There are no villi, so little nutrient absorption occurs.

Regions of the Large Intestine

• The large intestine is approximately 5 feet long and 2.5 inches in diameter.
• The large intestine can be divided into three main regions: the cecum, colon, and rectum.

Clinical Application - Appendix

• The appendix is attached to the cecum.
• It is uncertain what the function of the appendix is, but it may be part of the lymphatic system and a reservoir for necessary bowel flora in case of severe diarrhea.
• If the appendix becomes blocked or inflamed, it causes appendicitis and must be treated with antibiotics or surgical removal.

Regions of the Large Intestine - The Colon

• The ascending colon travels up the right side to the level of the liver and absorbs some water.
• The transverse colon travels across the abdomen just below the liver and the stomach.
• The descending colon bends downward near the spleen and travels to the left side, becoming the sigmoid colon.
• The sigmoid colon extends to the rectum.

Regions of the Large Intestine - The Rectum

• The rectum opens to the anal canal that leads to the anus.
• The anal sphincter opens and closes to allow the passage of solid waste (feces).

Defecation

• As the rectum fills with feces, a defecation reflex occurs, causing rectal muscles to contract and the anal sphincter to relax.
• There are two sphincters surrounding the anal opening, which are the internal (involuntary) and external (voluntary) anal sphincters.
• You have control, within limits, of when and how often you defecate.

Accessory Organs

• Accessory organs needed for digestion, besides salivary glands, are the liver, gallbladder, and pancreas.
• These organs are indirectly part of the gastrointestinal system, but are needed for proper and efficient functioning of the digestive process.

The Liver

• The liver weighs 1.5 kg, is located inferior to the diaphragm, and is the largest glandular organ in the body.
• The liver is also the largest organ in the abdominopelvic cavity, performing many functions vital to life.
• It is divided into the large right lobe and a smaller left lobe, where the right lobe has two smaller inferior lobes.

Functions of the Liver

• The liver detoxifies the body of harmful substances, such as certain drugs and alcohols.
• The liver creates body heat.
• The liver destroys old blood cells and recycles their usable parts, eliminating unneeded parts.
• The liver eliminates pigment bilirubin in bile, giving feces their distinctive color.
• The liver produces the factors needed for blood clotting.
• The liver manufactures bile, needed for the digestion of fats.
• The liver synthesizes urea, a by-product of protein metabolism that can be eliminated by the body.
• The liver stores the simple sugar, glucose, as glycogen.
• The liver stores vitamins A, B12, D, E, and K.
• The liver makes cholesterol.

Bile

• Bile helps absorb fat from the small intestine and transports bilirubin and excess cholesterol to the intestine for elimination.
• The salts found in bile act like a detergent, breaking fat up into tiny droplets through emulsification.

Gallbladder

• The gallbladder is a sac-shaped organ, 3–4 inches long, and located under the liver’s right lobe.
• The function of the gallbladder is to store bile from the liver until it is needed.

Pancreas

• The pancreas secretes buffers and digestive enzymes through the pancreatic duct to the duodenum.
• Buffers are needed to neutralize the acidity of the chyme, to a pH ranging from 7.5–8.8, saving the intestinal walls from damage.
• The secretory action of the pancreas is activated by the release of hormones from the duodenum.

Common Disorders of the Digestive System

• Symptoms of digestive disorders: vomiting, diarrhea, constipation, abdominal pain

Vomiting

• Vomiting is a protective mechanism that rids the digestive tract of an irritant or overload of food that stimulates sensory fibers.
• Motor impulses are sent to the diaphragm and abdominal muscles to contract, squeezing the stomach muscles and opening the cardiac sphincter at the esophageal opening, and regurgitating contents.

Diarrhea

• Diarrhea results when fluid contents in the small intestine are rushed through the large intestine without reabsorbing enough water.
• The rapid movement reduces nutrient and electrolyte absorption and can cause serious problems.

Constipation

• Constipation is the opposite of diarrhea.
• Feces pass too slowly through the colon, and too much water is absorbed.
• The stool becomes hard and dry and difficult to pass.
• Constipation does not refer to the frequency of stooling, but instead to the hard consistency of the stool.