Copyright © 2001 Benjamin Cummings, an imprint of Addison Wesley Longman, Inc.

Digestive System

Five Processes

• Mechanical processing and movement: chewing, mixing

• Secretion: fluid, digestive enzymes and hormones, bile, acid, alkali, mucus

• Digestion: breaking down food to smallest absorbable units

• Absorption: through mucosa, into blood or lymph vessels

• Elimination: undigested material eliminated

Digestive System Processes

• Its functions include:– Analysis of material before swallowing– Mechanical processing by the teeth, tongue, and palatal

surfaces– Lubrication– Limited digestion

mouth /oral /buccal cavity

Types of teethThere are different types of teeth for different functions:

Cutting and chopping food

Sharp pointed teeth for cutting and tearing food

Crushing and grinding food

Grinding and mashing food

Dentition

The crown is the part of the tooth above the gum line

The root is the part of the tooth below the gum line

•The enamel is the white part you can see covering the crown.•Enamel is the hardest substance in the human body•It covers the dentine and stops bacteria getting inside the tooth

The dentine is the major component within the tooth. It is made from softer material than enamel so is easier to decay.The pulp contains the nerves and blood vessels of the tooth. It is underneath the dentine. This is the bit which hurts when you have toothache!

The purpose of the lateral pterygoids is to work together to pull the mandible forward.

Dissection of the oral cavity of the pig

•The tongue pushes the bolus to the pharynx.

•Nerves send signals to lift the roof of the mouth (the soft palate) to keep food from entering the nasal cavity.

•Simultaneously, the larynx moves upward, tipping the epiglottis downward.

•Base of the tongue forces the epiglottis to fully cover the windpipe.

•Oesophageal sphincter opens as muscles of the pharynx relax.

•The bolus is swallowed by rhythmic contractions (peristalsis).

The swallowing reflex

Figure 24.4 Peristalsis

120 grams of glucose / day = 480 calories

Propulsion, deglutition (swallowing), is

involuntary and uses cranial nerves IX and X.

Organ Sympathetic Parasympathetic

Sphincters Constricts Relaxes

Salivary glands Produces mucus Produces saliva

Pancreas No supply Produces juice

Blood vessels Constricts No supply

Structure: common layers throughout the system• Mucosa: innermost layer, nutrients pass through• Muscularis mucosa• Submucosa: connective tissue, lymph and blood

vessels, nerves• Muscularis externa: two or three layers of smooth

muscle, responsible for motility in GI tract• Serosa: outermost layer, connective tissue sheath

Gastrointestinal (GI) Tract Layers

Gastrointestinal (GI) Tract Wall

The Stomach

• Bulk storage of undigested food• Mechanical breakdown of food• Disruption of chemical bonds via acids and

enzymes• Production of intrinsic factor

Functions of the stomach

This is the normal appearance of the stomach, which has been opened along the greater curvature. The oesophagus is at the left.

The Stomach Lining

•G- mucosa containing glandular tissue; different areas of the stomach contain different types of cells which secrete compounds to aid digestion. The main types involved are: parietal cells which secrete hydrochloric acid, chief cells which secrete pepsinMM- muscularis mucosae •SM- submucosa •The stomach contains three layers of involuntary smooth muscle which aid digestion by physically breaking up the food particles; •OM- inner oblique muscle •CM- circular muscle •LM- outer longitudinal muscle

The Stomach Lining

The Secretions of Hydrochloric Acid

The Phases of Gastric Secretion

The Phases of Gastric Secretion

The Phases of Gastric Secretion

Regions of the Small Intestine

The Structure of the Digestive Tract

• Plicae – Transverse folds of the intestinal lining

• Villi – Fingerlike projections of the mucosa

• Lacteals– Terminal lymphatic in villus

• Intestinal glands– Lined by enteroendocrine, goblet and stem cells

Histology of the small intestine

The Intestinal Wall

The Wall of the Small Intestine

Neutralising stomach acids, adds bile and pancreatic juice. Breakdown of proteins carbohydrates and lipids for absorption.

• Peristalsis• Segmentation• Reflexes that are initiated by stretch receptors in

the stomach• Reflex that triggers relaxation of the sphincter

between the small and large intestines (ileocaecal valve)

Intestinal movements

The double muscle layer (M) moves food through the intestine by peristalsis. The epithelial surface of the plicae (P) is further folded to form villi(V). Each villus has its own blood supply- the vessels can be seen in the submucosa (SM)- and blood containing digestive products from the small intestine is taken to the liver via the hepatic portal system.

The Intestinal Wall

Small Intestine

• Functions:– Digestion: neutralise acid from stomach, add digestive

enzymes and bile, break proteins, carbohydrates and lipids to absorbable materials

– Absorption: 95% of food absorbed here

• Structure– Regions: duodenum, jejunum, ileum– Mucosa adaptations: villi containing blood and lacteal

capillaries

Accessory Organs: Aid Digestion and Absorption

• Pancreas: exocrine functions– Secretes digestive enzymes and sodium bicarbonate

• Liver– Produces bile (acts as emulsifier – begins fat

breakdown– Hepatic portal system: drains blood from digestive tract– Metabolic functions: storage, synthesis, chemical

processing• Gallbladder: stores bile

• Duodenal glands (Brunner’s glands) and Paneth cells at the base of the Crypt of Lieberkuhn– produce mucus, buffers

• Ileum– aggregated lymphoid nodules (Peyer’s patches) which

play a role in the body’s immune system. They contain high concentrations of white blood cells (or lymphocytes) that help protect the body from infection and disease.

Small Intestine

In the depths of the crypts, you will find clusters of 3 to 5 cells with coarse red staining granules at the apical end. They are Paneth cells,named for  Josef Paneth (1857-1890), an Austrian physician who discovered them more than a century ago. They are believed to produce bactericidal materials and release them into the lumen of the gut. Paneth cells are found in the small intestine but not in the large intestine in the mucosa.

Goblet cells aren't found in the stomach but are numerous in the intestine.

The empty space in the villus is a lacteal (lymphatic capillary).

Note: the simple columnar epithelium (with the microvilli or brush border on the surface of the epithelium in the section) covering the surface of the villus with goblet cells (arrowed), and the endothelium lining the lacteal.

The ileum, the last section of the small intestine, is different from the duodenum and the jejunum by having more goblet cells in the mucosa and lymph nodules called Peyer's patches

Note the many goblet cells on the villi surface

• Pancreatic duct penetrates duodenal wall• Endocrine functions

– Insulin and glucagon• Exocrine functions

– Majority of pancreatic secretions– Pancreatic juice secreted into small intestine

• Carbohydrases• Lipases• Nucleases• Proteolytic enzymes

The pancreas

The Pancreas

Large IntestineAbsorbs nutrients and water and eliminates waste.

LARGE INTESTINES

•Mass movements occur within the large intestines.

•They are long slow-moving contractile waves that occur 3 or 4 times per day.

•Typically mass movements occur during or after eating.

The Large Intestine

• Reabsorb water and compact material into feces

• Absorb vitamins produced by bacteria• Store fecal matter prior to defecation

Functions of the large intestine

• Peristalsis occurs only after most nutrients have been absorbed.

• Peristaltic waves initiated in the duodenum begin to sweep slowly along the small intestines, moving 10-70 cm before dying out.

• Each successive wave is initiated a bit more distally, and this pattern of peristaltic activity, migrating mobility complex, continues until the undigested food is moved to the ileum (a 2 hour trip).

• The gastroileal reflex causes the ileocecal sphincter to relax.

PROPULSION

The large intestine primarily absorbs water, and compacts and dries out the fecal bolus. There are numerous goblet cells whose secretions act as lubrication for the moving material but no numerous villi for absorption.

• Last portion of the digestive tract• Terminates at the anal canal• Internal and external anal sphincters

The rectum

ABSORPTION

• The GI tract digests food, absorbs nutrients and water into the blood, and eliminates waste

• Components of the gastrointestinal tract– Mouth-site where food is moistened and chewed– Oesophagus-tube leading to the stomach– Stomach-secretes chemicals that work to digest foods– Small intestine-responsible for the majority of

digestion and absorption of nutrients– Large intestine (colon)-completes absorption of

nutrients and water– Rectum and anus-store and eliminate waste

The Gastrointestinal Tract

Accessory Organs that Aid Digestion and Absorption

• Pancreas: exocrine functions– Secretes digestive enzymes and sodium bicarbonate

• Liver– Produces bile (acts as emulsifer – begins fat breakdown– Hepatic portal system: drains blood from digestive tract– Metabolic functions: storage, synthesis, chemical

processing• Gallbladder: stores bile

Fat (lipid) makes up 37% of the calories in the diet

Fat is energy rich and provides 9 kcal/gm

Dietary lipids are 90% triacylglycerols; also include cholesterol esters, phospholipids, essential unsaturated fatty acids; fat soluble vitamins (A,D,E,K)

Usually all (98%) of the fat consumed is absorbed, and most is transported to adipose tissue for storage.

FAT FACTS

• Lipid digestion utilizes lingual and pancreatic lipases– Bile salts improve chemical digestion by emulsifying lipid

drops– Lipid-bile salt complexes called micelles are formed– Micelles diffuse into intestinal epithelia which releases

lipids into the blood as chylomicrons

Lipid digestion and absorption

Absorption of Fats

Fat Digestion & Absorption

Hydrolysis of Fats

• Hydrolysis of triglyceride into monoglyceride and free fatty acids is accomplished predominantly by pancreatic lipase.

• Lipase is a water-soluble enzyme, and with a little imagination, it's easy to understand why emulsification is a necessary prelude to its efficient activity.

Absorption of hydrolysed lipids

• The major products of lipid digestion - fatty acids and monoglycerides - enter the epithelium by simple diffusion across the plasma membrane– Fatty acids and monoglyceride are transported into the

endoplasmic reticulum, where they are used to synthesise triglyeride.

– The vesicles formed are then transported (via exocytosis) to the lacteal in the villus, which ends up being transported via the lymphatic vessel

Small intestine

Portal for transport of virtually all nutrients

Water and electrolyte balance

Enzymes associated with intestinal surface membranesi. Sucraseii. dextrinaseiii.Glucoamylase (maltase)iv.Lactasev. peptidases

• Water – Nearly all that is ingested is

reabsorbed via osmosis• Ions

– Absorbed via diffusion, cotransport, and active transport

• Vitamins – Water soluble vitamins are

absorbed by diffusion– Fat soluble vitamins are

absorbed as part of micelles• Vitamin B12 requires

intrinsic factor

Absorption

LARGE INTESTINES

• ABSORPTION

• VITAMINS, ELECTROLYTES,• AND WATER ARE• ABSORBED IN LARGE • INTESTINES.

• Preliminary digestion of proteins– Pepsin

• Permits digestion of carbohydrates• Very little absorption of nutrients

– Some drugs, however, are absorbed – Mucous secretion containing several hormones

• Enteroendocrine cells – G cells secrete gastrin– D cells secrete somatostatin

Digestion and absorption in the stomach

Stomach

• Not much carbohydrate digestion• Acid and pepsin to unfold proteins• Ruminants have forestomachs with

extensive microbial populations to breakdown and anaerobically ferment feed

STOMACH

• ABSORPTION

• 1. ASPIRIN• 2. ALCOHOL• 3. DRUGS THAT ARE FAT SOLUBLE

Absorption of Proteins and Carbohydrates

• Low pH destroys tertiary and quaternary structure

• Enzymes used include pepsin, trypsin, chymotrypsin, and elastase– Liberated amino acids are absorbed

Protein digestion and absorption

Absorption of Amino Acids

• Similar to that of glucose (active transport)– However sodium-dependent amino acid

transporters are required• Absorption of amino acids is dependent on

the gradient of the sodium ions

• Begins in the mouth – Salivary and pancreatic enzymes

• Disaccharides and trisaccharides– Brush border enzymes

• Monosaccharides• Absorption of monosaccharides occurs across the intestinal

epithelia

Carbohydrate digestion and absorption

Role of Endocrine and Nervous Systems in the Regulation of Digestion

• Regulation dependent on volume and content of food– Nervous system: stretch receptors in stomach– Hormones:

• Gastrin: stimulates release of gastric juice• Secretin: stimulates pancreas to secrete water and

bicarbonate• Cholecystokinin (CCK): signals pancreas to secrete

digestive enzymes

Disaccharides

Simple Sugars -

Carbohydrate absorption

apical basolateral

Absorption of Glucose

• Glucose is absorbed from the lumen by active transport– This requires specific carrier molecules (sodium dependent

glucose transporters) in the plasma membrane of epithelial.– These proteins transport both sodium and glucose into the cell

•Once inside the epithelium, glucose and sodium must be exported from the cell into the blood.

•Glucose diffuses down the concentration gradient (facilitated diffusion) into the capillary blood in the villus•Na+ diffuses into the blood via sodium-potassium pumps (this requires ATP)

•Other monosaccharides such as fructose are not absorbed by active transport.This only requires facilitated diffusion

Endocrine and Nervous Systems Regulation of Digestion

• Regulation dependent on volume and content of food– Nervous system: stretch receptors in stomach– Hormones:

• Gastrin: stimulates release of gastric juice• Secretin: stimulates pancreas to secrete water and

bicarbonate• Cholecystokinin (CCK): signals pancreas to secrete

digestive enzymes

Neural Control of small intestines

• Persistalsis is regulated extrinsically by the autonomic nervous system.

• The parasympathetic division is excitatory and the sympathetic is inhibitory.

Nutrients may beUtilized or Stored until needed

• Oesophagus, stomach, duodenum– These regions are almost sterile– Peristalsis and the rapid transport of food helps

prevent colonization by microbes• Tongue, teeth, jejunum, ileum, colon, rectum

– Tongue and teeth• Viridans streptococci bacteria are most prevalent in this

region– Lower small intestine and colon

• Microbiota here are microbial antagonists• Mucous membrane prevents microbes entering the

bloodstream

Normal Microbiota of the Digestive System

• Bacteria can infect the digestive system and cause disease ranging from mild to fatal

• Examples of bacterial digestive system infections– Dental caries, gingivitis, and periodontal disease– Peptic ulcers– Bacterial gastroenteritis– Bacterial food poisoning (intoxication)

Bacterial Diseases of the Digestive System

• Cause: Helicobacter pylori• Virulence factor: Presence of flagella, adhesins, urease,

and other enzymes• Portal of entry: Fecal-oral transmission likely• Signs/Symptoms: Primarily abdominal pain, although

nausea, vomiting, and weight loss may occur• Incubation period: Varies• Susceptibility: Those colonized by H. pylori• Treatment: Antimicrobial and acid-blocking drugs• Prevention: Lifestyle changes to reduce risk

Peptic Ulcers

Peptic Ulcers cont.

Major Digestive Enzymes

Word Bank

• Descending• Esophogus• Liver• Small intestine• Assimilate• Gall bladder• Villi• Gastric juices

•Cardiac•Transverse•Tongue•Pancreas•Pyloric•Hepatic duct•Ascending•Saliva•Mouth

•Swallow•Four•Two•Peristaltic•Appendix

Digestion Worksheet

1. Food goes through the ________ from the mouth to the stomach.

2. The stomach makes churning movements and produces __________.

3. The __________ ___________ assimilates dissolved food.

4. The __________ and __________ send digestive juices into the ____________.

5. The two openings at either end of the stomach are ____________ and _____________.

6. The _________ carries substances from the liver.

7. Villi in the small intestine _________ the dissolved food.

8. The colon has three main divisions: __________, __________, and __________.

9. Enzymes in _________ help begin digestion in the mouth.

10. The tongue helps to _________ food.11. The liver has _________ lobes.12. __________ movements help in digestion.13. The __________ serves no known useful

purpose.