intro the digestive system prepares food molecules for use in the body. digestion: the process by...

Intro

• The digestive system prepares food molecules for use in the body.

• Digestion: the process by which food is changed into a form that can be absorbed through the cell membranes of the alimentary canal

• It accomplishes this through and

Digestion: 2 types

1. Mechanical digestion: physical digestion ( food is broken into smaller particles)

2. Chemical digestion: series of catabolic reactions that change the chemical makeup of the food molecules

The digestive organs are broken into 2 groups:

• 1 Gastrointestinal Tract (GI tract): essentially a tube running from mouth to anus

• 2. Accessory digestive organs: structures which contribute to digestion, but are not a part of the tube ( teeth, tongue, liver, pancreas, gall bladder)

Mucosa: inner most lining of lumen, has 3 layers

A. Epithelium: innermost sublayer generally composed of simple columnar epithelium and goblet cells. Functions in protection, secretion and absorption

B. Lamina propria: areolor connective tissue that holds the epithelium tissue to underlying layers, it is rich in blood and lymph

C. Muscularis mucosa: smooth muscle, keeps mucosa folded and increases surface area

Histology

• The wall of the GI tract is composed of 4 layers or tunics ( from inside hollow tube called lumen to outside:

1. Mucosa: inner most lining of lumen, has 3 layers

Histology

2. Submucosa: thick layer of areolar C.T. It is highly vascularized and has nerve endings

3. Muscularis: composed of 2 layers of muscle: an inner layer running in circular rings and an outer layer running longitudinally ( lengthwise)

4. Serosa: outer most layer composed of connective tissue and epithelium

Some terms

• Peritoneum: double layered serous membrane that lines the body cavity

• Parietal peritoneum: lines the wall of abdominal cavity

• Visceral peritoneum: covers digestive organs• The space between the two is the peritoneal cavity• Mesentery: binds small intestines to posterior body

wall• Peritonitis: inflammation of peritoneum, is

dangerous since it can easily spread to all abdominal organs

Mouth (BUCCAL CAVITY)

• Is formed by cheeks, lips, tongue and hard and soft palates

• Tongue: is necessary for speech and housing taste buds, also mixes food when chewing and shapes it into a rounded mass called a bolus

• It aso initiates swallowing

Salivary glands

• Are buccal glands in the mouth which secrete some saliva

• Most come from 3 pairs of glands located outside the buccal cavity

1. Paratid: under front of ears2. Submandibular: near base of tongue in back

of mouth3. Sublingual: beneath tongue• Mumps: a viral infection in one of these glands,

usually the paratid

Saliva

• Produced at a rate of 1 to 1 ½ liters per day

• Is 99.5% water and some amylase ( enzymes that break down starch)

• Functions to lubricate food allowing for tasting and swallowing

• Is under involuntary neural control

• Mastication: chewing

Teeth (dentes)

• A set of teeth is called dentition

• Humans have 2 sets of teeth:

1. Deciduous teeth: the 20 baby teeth or milk teeth

2. Permanent teeth: 32

Tooth Parts

• Crown: exposed portion of tooth• is covered with enamel• Gingiva: gum• Root: protein embedded in jaw• Dentin: bone-like tissue comprising bulk of tooth• Cementum: connective tissue which cements

tooth to periodontal ligament which anchors teeth to jaw and acts as a shock absorber

Deglutition ( swallowing)

• Involves coordinating the esophagus, tongue, soft palate, pharynx and 22 plus muscles

• Composed of 2 Phases:

1. Buccal ( voluntary) tongue pushes bolus into oropharynx

2. Pharyngeal ( involuntary) all routes except desired one are closed

• Pharyngeal continued: actions include

• Soft palate rises blocking nasopharynx

• Tongue blocks mouth

• Larynx rises contracting epiglottis which closes glottis

• Pharyngeal muscles propel bolus into esophagus

3. Esophageal ( involuntary): wave-like contractions called peristalysis alternate between the longitudinal and circular layers of the esophagus moving the bolus to the stomach

Stomach

• Has 4 regions: cardiac, pyloric, body and fundic ( see diagram)

• Folds within stomach that increase surface area for digestion are called rugae

• The stomach serves primarily as a mixing and holding chamber with both digestion and absorption being very limited

• The stomach emptying and inhibition of gastric secretions are controlled by the small intestines via the release of hormones

The stomach wall has the same 4 layers with 2 modifications

• 1. mucosa layer: when not stretched lies in folds called rugae

• This layer is dotted with millions of gastric pits leading to gastric glands containing several types of secretory cells

Types of secretory cells:

1. zymogenic ( chief) cells: secrete pepsinogen to digest pepsin

2. Parietal cells: secrete HCL3. Mucous cells: secrete a sticky mucous coat

that counter-acts stomach acid ( it’s a base) preventing stomach from digesting itself

4. Enteroendocrine cells: secrete diff hormones which stimulate parietal and chief cells to secrete

All above are known as Gastric juices:

2. submucosa: not modified3. Muscularis: this is the mixing churning

layer. The soupy liquid of food and gastric juices is now called chyme

4. Serosa: not modifiedGastric secretions is under the control of the

nervous system: when food is seen, smelled, tasted and when stomach Is expanded

Pancreas• Is approximately 7 inches long and 2 inches wide

and is located behind the stomach• The pancreas has both endocrine and exocrine

functions• The clusters of Acini cells secrete the pancreatic

juice into the duodenum (upper small intestines)• The secretion is controlled by the presence of the chyme

• Pancreatic juice consist of: water, pancreatic amylase ( digests carbohydrates) trypsin ( protein digesting) and lipase ( lid or fat digesting) thus breaking down all categories of food

Liver

• The largest glandular organ of the body, it weighs approximately 3 pounds

• It is divided into 4 loges: right, left , quadrate, and caudate lobes

Liver Functions:

• Carbohydrate metabolism• Lipid metabolism• Protein metabolism ex heparin• Storage of vitamines A, D, E, K, iron and copper• Filters blood• Detoxification• Secretes bile salts• Production of RBCs

Small intestines

• Portion of the GI tract between pyloric sphincter of stomach and ileocecal sphincter of small intestine

• Function of small intestines: major site of digestion and absorption (90% absorption in small intestines)

• Absorption is the passage of nutrients from the alimentary canal to blood or lymph

3 portions of small intestines

• Duodenum: 1 foot long

• Jejunum : 8 feet long

• Ilium: 12 feet long

Digestive modifications

• There are intestinal glands into the pits between villi which secrete intestinal juice ( watery/alkaline fluid which is reabsorbed)

• Duodenal ( Brunner’s glands): in submucosa secrete protective alkaline mucus

• Goblet cells: found between absorptive cells, secrete mucus

Large Intestines (colon)

• The terminal portion of the alimentary canal which is approximately 5 feet in length and connects to the ileum of the small intestines

• Functions:• Manufacture, vitamins B & K• Formation of feces: composed of 30% water,

30% bacteria, the remainder is dead cells, cellulose and undigestive food

• Absorption of water

Bile

• Contains bile salts and bile pigments : waste from breaking down of RBCs

• The color of feces is due to these pigments

• Failure to get aid to rid them results in jaundice ( yellow skin)

Physiology of Digestive system

• After nutrients are broken down by digestion and absorbed, their components may be:

1. Converted into a storage for starch or adipose

2. Used as building blocks to grow and repair

3. Be catabolized ( burned) for energy

• Digestion reduces carbohydrates down into various simple sugars which are then transported to the liver

• If not stored in the liver these sugars are converted into glucose

• All cellular respiration is centered on “burning” glucose

• The energy released in cellular respiration either escapes as heat or is trapped in the bonds of the ATP molecules

• The ATP is then shuttled around the cell providing the energy for all the various metabolic processes in life

• It takes about 4 hr for GI tract to complete absorption

• This time period during and after a meal is called the absorptive state

• The post-absorptive state is that period when the GI tract is empty

• During the absorptive state glucose is the major fuel, where as in the post-absorptive state, bodily reserves of glycogen and fat are catabolized

Metabolic Rate

• The energy used by the body per hour (kcal/m2/hr)• Factor which influence metabolic rate include:1. Exercise/activity2. Hormones3. Age4. Sex5. Body surface area compared to volume ( meaning

thinner people have a higher rate)6. Body temperature7. food

• Since many factors affect a persons metabolic rate, it is usually measured at a standard condition ( reclining 12 hr after eating, mentally and physically relaxed) this is called Basal Metabolic Rate

Digestive System

Duodenum

• Cardiac region of the stomach

• Note the gastic pits, glands mucosa and submucosa