digestive system introduction organs general structure structures and functions digestion absorption

Digestive SystemIntroductionOrgansGeneral StructureStructures and functionsDigestionAbsorption

Digestive system

Nutrients required by body cells have to be in simple and soluble form.

Ingested food is in complex form which has to be broken into simpler form.

The conversion of complex form of food into simple soluble form brought by digestive system.

Different organs that makes up digestive system are involved in digestion.

Main Organs (in order)

Mouth and Oral CavityPharynxEsophagusStomachSmall intestineLarge intestine

Accessory OrgansTeeth and tongueSalivary glandsLiverGallbladderPancreas

General StructureWalls of the digestive tract consist of four layers

throughout its length.From lumen to outward:

Mucosa/mucous membrane – epithelial layer coated with mucous

Submucosa – loose connective tissue: contain blood vessels, lymphoid, nerves and glands

Muscularis/musculosa–smooth muscle that produces wave-like contractions (peristalsis) to move food, consists of two layers, outer-longitudinal, inner-circular

Serosa/adventitia – connective tissue; outermost covering.

Baisc structure of Alimentary canal

Structures and functions

Oral cavity Hollow chamber with a roof, floor and walls. Food enters

through the mouth.Hard palate – bony structure in the anterior or front

portion of the mouth.Soft Palate – located above the posterior or rear portion

of the mouth; muscle tissue.Uvula – hangs down from the soft palate; helps prevent

food or liquid from entering the nasal cavities above the mouth.

Tongue – skeletal muscle covered with mucous membrane. Involved in mixing, swallowing and tasting.

Structures and functions

Teeth – 4 major types Incisors – cutting function during mastication.Canines (cuspids) – pierce or tear food.Premolars (bicuspids) – large flat surfaces with 2 or 3

grinding or crushing cusps on their surface. Grind food into a bolus so it can be swallowed.

Molars (tricuspids) – same as premolars.Deciduous teeth = baby teeth, 20 in number; adults

have 32 teeth

Structures and functions

Teeth cont’dParts of a tooth

Crown – portion that is exposed and visible in the mouth; covered with enamel.

Neck – narrow portion surrounded by the pink gingiva or gum tissue.

Root – fits into the socket (joint) of the upper or lower jaw.

Structures and functions

Structures and functions

Salivary Glands(3 pairs)Parotids – largest, lie just below and in front of each

ear at the angle of the jaw (mumps).Submandibular – open into the mouth on either side of

the lingual frenulum.Sublingual – open into the floor of the mouth.Saliva – contains mucus and a digestive enzyme called

salivary amylase which begins the process of chemical digestion of carbohydrates.

Location of salivary glands

Structures and functions

EsophagusMuscular, mucus lined tube that connects the pharynx

with the stomach (about 25 cm or 10 inches long).Peristalsis moves food from the mouth to the

stomach.

Structures and functionsStomachLies in the upper part of the abdominal cavity just under

the diaphragm.

Serves as a pouch that food enters after it has been chewed, swallowed and passed through the esophagus.

At the junction of the esophagus and the stomach is the cardiac sphincter (muscle) that keeps food from reentering the esophagus

Divided into three regions:Fundus – enlarged portion to the left of and below the

cardiac sphincter.Body – central part of the stomachPylorus – lower, narrow section which joins the first

part of the small intestine.

Structure of stomach

Structures and functionsThe lining of the stomach has numerous folds when empty

called rugae; they allow for expansion when we eat.Has numerous gastric glands that secrete “gastric juice”

into the stomach. Most common cells found in the glands are:

Parietal cells - produce HCl acid that denatures proteins.

Chief cells - produce pepsin that begins protein digestion.

Mucus cells - produce mucus the protects from HCl action.

Gastric pits and gland

Structures and functions

The stomach wall has 3 layers of muscle that allow mixing of the food with gastric juice, breaking it down into a semisolid mixture called chyme.

Pyloric sphincter (muscle) holds food in the stomach or allows for emptying of chyme into the small intestine. Controlled by the small intestine.

LS of stomach

Structures and functionsSmall intestineRoughly 7 meters in length.Smaller in diameter compared to the

large intestine.Divided into three sections:

Duodenum Jejunum Ileum

Structures and functions

Small intestineMucous lining, contains thousands of

microscopic glands which secrete intestinal digestive juices.

Intestinal lining has multiple circular folds, plicae, that increase the surface area available for absorption of nutrients.

The surface is covered with tiny finger-like projections called villi.

LS of small intestine

Structures and functions

Liver Fills the entire upper right section of the abdominal cavity

and extends part way into the left side. The largest gland in the body secretes bile

into hepatic ducts.Functions:Produces bileStores vitamins and mineralsPlays a part in maintaining blood sugar (glycogen) and

blood pressure (blood proteins).

Structures and functionsLiver cont’d

Bile breaks up (emulsifies) large fat globules into smaller particles to increase the surface area for digestion.

Bile gives the feces its color (no bile - stools become clay colored). Bile absorbed into the blood gives the skin a yellowish color = jaundice or icterus.

Liver – anterior view

Structures and functionsGall Bladder

Stores bileCystic duct arising from it joins with hepatic

duct.When fat enters the duodenum, the mucosal

cells release a hormone called cholecystokinin (CCK).

CCK stimulates the contraction of the gallbladder and forces bile into the duodenum via the common bile duct.

Gall bladder

Structures and functionsPancreasLies behind the stomach in the C-shaped

curve of the duodenum.Endocrine portion produces insulin and

glucagon.Exocrine portion (ascini) secretes pancreatic

juice into ducts that connect to the duodenum at the same area as the bile duct.

Pancreatic juice contains enzymes such amylase, trypsin, chymotrypsin and lipase and also bicarbonate that neutralizes the HCl in the chyme.

Pancreas

Structures and functionsLarge Intestine

About 5 feet in length, forms the lower portion of the digestive tract.

Undigested and unabsorbed food material enter the large intestine after passing through a sphincter-like structure called the ileocecal valve.

L/S of Large intestine

Structures and functionsLarge Intestine

One primary function of the large intestine is to remove water from the feces.

A few vitamins and minerals are also absorbed.Bacteria living in the large intestine break

down some of the remaining material. Bacteria also produce some of the B-complex

vitamins and vitamin K.

Structures and functionsLarge Intestine

Inner surface has no folds or villi – not well suited for absorption.

Large Intestine Cecum – ileocecal valve opens into this pouchlike

area.Ascending colon – right side of bodyTransverse colon – extends across the front of

the abdomenDescending colon – left side of the bodySigmoid colon – leads to the rectum

Large intestine

DigestionComplex process that occurs in the alimentary canal,

consists of mechanical and chemical changes that prepare food for absorption.

Mechanical digestion Breaks food into tiny particles, mixes them with digestive

juices, moves them along the alimentary canal and finally eliminates the digestive wastes from the body. Includes mastication, deglutition, peristalsis and defecation.

Digestion

Chemical digestion

Breaks down large, non-absorbable food molecules into

smaller, absorbable molecules that can easily pass through

intestine into blood and lymph.

Process includes numerous chemical reactions catalyzed by

specific enzymes in saliva, gastric, pancreatic, and

intestinal juices.

DigestionCarbohydrates Digestion begins in mouth (salivary amylase)Salivary and pancreatic amylase converts starches into

disaccharides (double sugars). Intestinal enzymes (sucrase, maltase, lactase) break down

disaccharides into monosaccharides.

Polysaccharide salivary amylase maltose &small polysaccharide

Undigested polysaccharides pancreatic amylase maltose & diaaccharides maltse,sucrase, lactase monosaccharides

Digestion

Proteins Starts in the stomach and ends in the small intestine. Pepsin in stomach,trypsin and chymotrypsin in pancreatic

juice digest protein. Aminopeptidase present in intestinal secretion finally digest

protein into amino acids.

Protein pepsin short polypeptides

Short polypeptides trypepsin, chymotrypsin small polypeptides & peptides carboxypeptidase, peptidases,dipeptidases amino acids

Digestion

Lipids (fat) Bile produced by liver is poured into the duodenum.It brings about emulsification.Pancreatic lipase splits the lipid molecules into fatty acids

and glycerol.

Fat bile salts emulsified fat droplets pancreatic lipase fatty acids & glycerol

AbsorptionStomach absorbs only a few substancesMain absorption occurs in small intestineAbsorption occurs by a combination of simple diffusion,

facilitated diffusion and active transportAmino acids and glucose are directly transported to

liver by hepatic portal veinFatty acids and glycerol are absorbed by intestinal cellsResembled as triglycerides in ERPacked into protein covered fat droplets-chylomicronsChylomicrons enter the lacteals and enter into blood

circulation

Hormonal controlGastrin(stomach) causes gastric glands in

stomach tosecrete pepsinogen.Stimulated by distention of stomach by food,

partiallydigested proteins and caffine

Secretin(duodenum) causes the pancreas and liver to secrete sodium bicarbonate and bile respectively.

Stimulated by the presence of acidic chyme in theDuodenum.

CCK/cholecytokinin(duodenum)Stimulates pancreas to release digestive

enzymes and gall bladder to empty bileStimulated by the presence of fatty acids and

partially digested protein in duodenum.

Gastric inhibitory peptide/GIP(duodenum) Decrease stomach churning, thus slowing

emptying. Stimulated by the presence of fatty acids or

glucose in duodenum