Digestive System

Function

The function of the digestive system is to change complex nutrient molecules that can be absorbed into the blood or lymph to be transported to cells

Divisions of digestion

1.The alimentary tube extends from the mouth to the anus. It consist of the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.

2. The accessory organs of the digestion are the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Types of digestion

Food is broken down in two complementary processes

1. Mechanical digestion is the physical breaking up of food into smaller pieces

2. Chemical digestion uses enzymes to break up food particles, and turn them into something the body can utilize

Oral Cavity

Food enters the oral cavity by way of the mouth

The boundaries are the hard and soft palates, the cheeks, and the floor of the mouth

Within the oral cavity are the teeth and tongue

Teeth

The function of the teeth is to chewIndividuals develop two sets of teeth

1) deciduous and 2) permanentDeciduous teeth arrive around 6 months

of age, and are a set of 20 teethPermanent teeth begin to replace

deciduous teeth at the age of six, and consist of 32 teeth

Teeth

Enamel is the hard outer covering of the tooth, and is more resistant to decay than other parts of the tooth

Dentin is within the enamel, and is very similar to bone; dentin also forms the root of tooth

The innermost portion of a tooth is the pulp cavity, which contains the blood vessels and nerve endings

TongueThe tongue is made of skeletal muscle On the upper surface are small

projections called papillae, which contain taste buds

Sensory nerves for taste are the cranial nerves

Elevation of the tongue is the first step in swallowing, and is a voluntary action

The mass of food, called a bolus, is pushed backward toward the pharynx

Salivary glands

Saliva is the digestive secretion in the oral cavity produced by three pairs of glands

1. Parotid glands are just below the ears2. Submandibular are posterior to the

mandible3. Sublingual are on the floor of the mouthEach gland has at least one duct to take saliva

to the oral cavity

Cont.

Secretion of saliva is continuous, but varies

The presence, sight, or smell of food increases the amount saliva

Stressful situations decrease the amount of saliva

Salivary amylase is the digestive enzyme that breaks down starch molecules

Pharynx No digestion takes place in the

pharynx, its only related function is swallowing

The reflex center for swallowing is the medulla

The medulla coordinates:1. Constriction of the pharynx2. Closure of the epiglottis3. Peristalsis of the esophagus

Esophagus

The esophagus is a muscular tube that takes food from the pharynx to the stomach; no digestion takes place

Peristalsis of the esophagus propels food The lower esophageal sphincter relaxes to

permit food to enter the stomach, then contracts to prevent backup

If contraction is not complete gastric juices may splash up, I.e. heartburn

Layers of alimentary tube

4 layers1. Mucosa2. Submucosa3. External muscle layer4. serosa

Mucosa

The mucosa, or lining, is made of epithelial tissue and smooth muscle

The epithelium secretes mucus, which lubricates the passage of food, and secretes digestive enzymes

Submucosa

The submucosa is made of connective tissue

Meissner’s plexus is the nerve network that regulate secretions

Also known as the brain of the gut

External muscle layer

The external layer has two layers, inner and outer

Contractions of the external layer help break up food and mix it with digestive juices

Serosa

The outermost layer, is fibrous connective tissue

The serosa is the mesentary lining the abdominal cavity

Stomach The stomach is a sac that extends from the

esophgus to the small intestine Both mechanical and chemical digestion

take place in the stomach Parts of the stomach:1. Cardiac is the opening of the esophagus2. Fundus is the level above the opening3. Body is the large central portion4. Pylorus is where most digestion takes place

Cont.

The folds within the stomach called rugae allow the stomach to stretch without tearing

Parietal cells secrete hydrochloric acid, which begins the digestion of proteins

Mucus cells secrete mucus, which coats the stomach, and prevents the erosion of gastric juices

The pH of the stomach is around 1, which helps to kill microorganisms

Small intestine Extends from the stomach to the large

intestine The small intestine has 3 parts1. Duodenum2. Jejunum3. Ileum -Digestion is completed in the small

intestine -Peyer’s patches are lymph nodes that

destroy pathogens in the sm. intestine

Liver

Consist of two large lobes, right and left The capillaries of the liver are called

sinusoids The only digestive function of the liver is the

production of bile Bile carries bilirubin and excess cholesterol to

the intestines for elimination Bile salts emulsify fats in the small intestine;

this a mechanical process The hormone secretin, stimulates the

production of bile

Gallbladder

Is located on the underside of the liverBile from the liver is stored in the

gallbladder until neededCholecystokinin is the hormone that

stimulates the contraction of the smooth muscle in the gallbladder, which forces out bile

Gallstones

Are caused by cholesterol crystals, that if large enough cannot pass out of the gallbladder

The backing up of bile can cause juandice

Lithotripsy is the generation of shock waves that break down gallstones into smaller pieces

Pancreas

Pacreatic enzymes1. Amylase- digest starch 2. Lipase- converts fats to fatty acids3. Trypsin- digest polypeptides to amino acidsThe pancreas also produces bicarbonate juice,

which neutralizes the gastric acid that come from the stomach

Secretin stimulates the production of bicarbonate juice in the pancreas

Disorders of the stomach

Pyloric stenosis means that the opening of the pyloric sphincter is narrowed, blocking the emptying of the stomach

A gastric ulcer is an erosion of the mucosa of the stomach; it is the result of over secretion of HCl

The bacteria Helicobacter pylori causes most gastric ulcers

Large Intestine

Extends from the small intestines to the anus

The first portion is called the cecum; the ileocecal valve prevents fecal matter from back flowing into the small intestine

The appendix is attached to the cecumInfection of the appendix caused by the

fecal matter is called appendicitis

Cont.

The last portion of the large intestine is called the colon; no digestion occurs here

The only secretion of the colon is mucus that coats for the passage of feces

The functions of the colon are the absorption of minerals, water, and vitamins

About 80% of the water that enters the colon is absorbed

The flora of the colon prevents the growth of pathogens

Disorders of the Intestines

Duodenal ulcers are erosions of the intestinal wall, caused by gastric juices

Lactose intolerance is the inability to digest lacose; when lactose is not digested it undergoes fermentation within the intestine

Salmonella is caused by a bacteria found in meats and eggs, they cause the intestines to secrete large amounts of fluid

Cont.

Paralytic ileus is the ceasing of contraction of the smooth muscle layer of the intestine, causing intestinal blockage

Infant botulism is caused by the ingestion of bacterial spores; an infant’s stomach does not produce enough HCl to destroy spores

Diets high in fiber have been thought to reduce the frequency of certain diseases

Absorption

Most absorption takes place in the small intestine

Absorption needs a large surface area, projections called villi help increase surface area

Elimination of feces

Feces consist of cellulose and other undigestible material, and water

A spinal cord reflex called the defecation reflex controls the elimination of feces

The relaxation of the internal anal sphincter permits defecation to occur

The external anal sphincter makes it possible to delay defecation

Problems of the Liver

Hepatitis is the inflammation of the liver caused by viruses

Hepatitis A is an intestinal virus that is spread by the digestion of fecal matter

Hepatitis B is contracted by exposure to body fluids of an infected person (blood, semen)

Hepatitis C virus is spread through body fluids, blood, and mucus membranes

Other Functions of Liver1. Carbohydrate metabolism- excess glucose

is stored as glycogen2. Amino acid metabolism- amino acids are

changed to carbohydrates 3. Lipid metabolism- synthesis of cholesterol4. Formation of bilirubin- old RBC’s are

phagocytized and bilirubin is formed5. Phagocytosis- phagocytized old RBC’s and

bacteria are absorbed by the colon6. Detoxification- liver enzymes change

poisons to less harmful substances