The Digestive System

IntroductionThe digestive system consists of the

muscular digestive tract and various accessory organs.

Digestive functions include ingestion, mechanical processing, digestion, secretion, absorption, compaction, and excretion.

Overview of the digestive TractThe digestive tract includes the oral

cavity, pharynx, esophagus, stomach, small intestine, large intestine, rectum and anus.

Histological Organization The epithelium and underlying connective

tissue,the lamina propria, form the mucosa of the digestive tract.

Next, outward, are the submucosa, the muscularis externa, and the adventitia, a layer of loose connective tissue.

In the peritoneal cavity, the muscularis externa is covered by the serosa, a serous membrane

Double sheets of peritoneal membrane called mesenteries suspend the digestive tract.

Movement of Digestive Materials The neurons that innervate the smooth

muscle of the muscularis external are not under voluntary control.

The muscularis externa propels materials through the digestive tract by means of the contractions of peristalsis.

Segmentation movements in areas of the small intestine churn digestive materials.

Peristalsis

The Oral CavityThe functions of the oral cavity are:

Analysis of potential foods Mechanical processing using the teeth,

tongue and palatal surfaces Lubrication by mixing with mucus and

salivary secretions Digestion by salivary enzymes

The oral cavity, or buccal cavity, is lined by oral mucosa

The hard palate and soft palate form its roof and the tongue forms its floor.

The TongueThe primary functions of the tongue

include: Mechanical processing Manipulation to assist in chewing and

swallowing Sensory analysis.

The Salivary GlandsThe salivary glands discharge their

secretions into the oral cavitySaliva lubricates the mouth, dissolves

chemicals, flushes the oral surfaces and helps to control bacteria.

Salivation is usually controlled by the ANS.

TeethMastication occurs through the contact

of the opposing surfaces of the teethThe periodontal ligament anchors the

tooth ins a bony socket.Dentin forms the basic structure of a

toothThe crown is coated with enamel, and

the root is covered with cementum

The 20 primary teeth or deciduous teeth are replaced the the 32 teeth of the secondary dentition during development.

The Pharynx and EsophagusThe pharynx is the passageway between

the oral cavity and the esophagusThe esophagus is a muscular tube about

10 in. long and ¾ inches in diam. It is lined with stratified squamous epithelium

that resists abrasion, hot or cold temp. and chemical attack. Mucus glands lubricate surface and keep material from sticking.

Swallowing

Called deglutition

The mass of food being swallowed is called a bolus

The StomachThe stomach has four major functions:

Temporary storage of ingested food The mechanical breakdown of food. The disruption of chemical bonds by acids

enzymes The production of intrinsic factor a

compound necessary for the absorption of vitamin B12

Chyme forms in the stomach as gastric and salivary secretions are mixed with food.

The four regions of the stomach are the cardia, fundus, body and pylorus

The pyloric sphincter guards the exit from the stomach.

In a relaxed state the stomach lining contains numerous rugae (ridges and folds)

The Gastric WallWithin the gastric glands, parietal cells

secrete intrinsic factor and hydrochloric acid.

Chief cells secrete pepsinogen which acids in the gastric lumen convert to the enzyme pepsin (digests proteins)

Gastric gland endocrine cells secrete the hormone gastrin.

The Regulation of Gastric Activity

Gastric secretion includes: The cephalic phase which prepares the

stomach to receive ingested materials The gastric phase which begins with the

arrival of food in the stomach The intestinal phase which controls the

rate of gastric emptying

The Small IntestineThe small intestine includes the

duodenum, the jejunum, and the ileum.The ileocecal valve, a sphincter, marks

the transition between the small and large intestines.

The Intestinal WallThe intestinal mucosa bears transverse

folds called plicae, or plicae circulares,and small projections called intestinal

villi.They increase the surface area for

absorption. Each villus contains a terminal lymphatic

called a lacteal which absorbs lipids.

Intestinal Secretions Intestinal glands secrete intestinal juice,

mucus,and hormones. Intestinal juice moistens the chyme, helps

buffer acids,and dissolves digestive enzymes and the products of digestion.

Intestinal hormones include secretin, cholecystokinin(CCK) and gastric inhibitory peptide.

Digestion and the Small intestine

The most important digestive and absorptive functions occur in the small intestine.

Digestive enzymes and buffers are provided by the pancreas, liver and gallbladder.

The PancreasThe pancreatic duct penetrates the wall

of the duodenum, where it delivers the secretions of the pancreas.

The pancreas has two functions Endocrine (secreting insulin and glucagon

into the blood) Exocrine (secreting water, ions, and

digestive enzymes into the small intestine) Pancreatic enzymes include lipases,

carbohydreases and proteases and nucleases.

The Control of Pancreataic Secretion

When chyme arrives in the small intestine secretin triggers the pancreatic production of a fluid containing buffers, primarily sodium bicarbonate, what help bring the pH of the chyme under control.

Also CCK is released which stimulates the pancreas to secrete pancreatic amylase, pancreatic lipase, nucleases and protease enzymes.

The LiverLargest visceral organ in the body, with

over 200 known functions.

Liver FunctionsMetabolic regulation: controls the

composition of circulating blood. Filter out toxins.

Hematological regulation: liver is the largest blood reservoir in the body. Removes aged or damaged red blood cells, debris, and pathogens.

The gallbladder stores and concentrates bile for release into the duodenum.

Bile salts emulsify lipids for digestion.The liver produces about 1 liter of

bile/day but it is only released into the intestine when CCK is released.

The large IntestineThe main functions of the large intestine

are: Reabsorb water and compact the feces Absorb vitamins liberated by bacteria Store fecal material prior to defecation.

Three Parts of Large IntestineCecum: collects and stores material

from the ileum. Vermiform appendix is attached to the cecum.

Colon: larger diameter and thinner wall than sm. Intestine/

Rectum: terminates in the anal canal leading to the anus.

Physiology of the Large Intestine

The large intestine reabsorbs water and other substances such as vitamins, bilirubin, bile salts, and toxins;

Bacteria residing in the large intestine are responsible for intestinal gas or flatus. They also produce vitamin K (needed for clotting), Biotin (glucose metabolism), and vitamin B5(steroid hormones and neurotransmitters)

Digestion and AbsorptionThe digestive system breaks down the

physical structure of the ingested material and then disassembles the component molecules into smaller fragments through hydrolysis.

Amylase breaks down complex carbohydrates. These are broken down into monosaccharides by other enzymes and are absorbed by the intestinal epithelium through facilitated diffusion and co-transport.

Triglycerides are emulsified into large lipid drops. The results interact with bile salts to form micelles which diffuse across the intestinal epitheliums.

Protein digestion involves the gastric enzyme pepsin and various pancreatic proteases.

Water and Electrolyte Absorption

About 2-2.5 liters of water are ingested each day and digestive secretion provide 6-7 liters

Nearly all is reabsorbed by osmosis.

Fat-soluble vitamins are enclosed within fat droplets are are absorbed with the products of lipid digestion.

The nine water-soluble vitamins are important as cofactors and coenzymes in enzymatic reactions..

Aging and the Digestive System

Age –related changes include a thinner and more fragile epithelium a reduction in epithelial stem cell

division and weaker peristaltic contraction as smooth

muscle tone decreases.