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Digestive System

Nov 23, 2014

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Digestive System

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

Digestive System ProcessesFive 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

mouth /oral /buccal cavity

Its functions include: Analysis of material before swallowing Mechanical processing by the teeth, tongue, and palatal surfaces Lubrication Limited digestion

Types of teethThere are different types of teeth for different functions:Sharp pointed teeth for cutting and tearing food

Grinding and mashing food Crushing and grinding food

Cutting and chopping food

DentitionThe 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 swallowing reflex

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).

Figure 24.4 Peristalsis

120 grams of glucose / day = 480 calories Propulsion, deglutition (swallowing), is involuntary and uses cranial nerves IX and X.

OrganSphincters Salivary glands Pancreas Blood vessels

SympatheticConstricts Produces mucus No supply Constricts

ParasympatheticRelaxes Produces saliva Produces juice No supply

Gastrointestinal (GI) Tract LayersStructure: 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 Wall

The Stomach

Functions of the stomach

Bulk storage of undigested food Mechanical breakdown of food Disruption of chemical bonds via acids and enzymes Production of intrinsic factor

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 pepsin

MM- muscularis mucosae SM- submucosaThe 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

Histology of the small intestine

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

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.

Intestinal movements 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)

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

Small Intestine

Duodenal glands (Brunners glands) and Paneth cells at the base of the Crypt of Lieberkuhn produce mucus, buffers

Ileum aggregated lymphoid nodules (Peyers patches) which play a role in the bodys immune system. They contain high concentrations of white blood cells (or lymphocytes) that help protect the body from infection and disease.

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

Functions of the large intestine Reabsorb water and compact material into feces Absorb vitamins produced by bacteria Store fecal matter prior to defecation

PROPULSION 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.

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.

The rectum

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

ABSORPTION

The Gastrointestinal Tract 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

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 FACTSFat (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 fatt

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