Digestive SystemDr Kamelia

Digestive Functions & Processes

• Functions– ingestion = intake of food– digestion = breakdown of molecules– absorption = uptake of nutrients into blood or lymph– defecation = elimination of undigested material

• Processes– motility = muscular contractions that break up food,

mix it with enzymes & move it along– secretion = digestive enzymes & hormones– membrane transport = absorption of nutrients

Stages of Digestion

• Mechanical digestion is physical breakdown of food into smaller particles– teeth & churning action of stomach & intestines

• Chemical digestion is series of hydrolysis reactions that break macromolecules into their monomers– enzymes from saliva, stomach, pancreas & intestines– results

• polysaccharides into monosaccharides• proteins into amino acids• fats into glycerol and fatty acids

Saliva

• Functions of saliva– moisten, begin starch & fat digestion, cleanse teeth, inhibit

bacteria, bind food together into bolus• Hypotonic solutions of 99.5% water and solutes:

– amylase = begins starch digestion– lingual lipase = digests fat after reaches the stomach– mucus = aids in swallowing– lysozyme = enzyme that kills bacteria– immunoglobulin A = inhibits bacterial growth– electrolytes = Na+, K+, Cl-, phosphate & bicarbonate

• pH of 6.8 to 7.0

Swallowing

The esophagusSaliva contains salivary amylase. It hydrolyzes the glucose polymers starch and glycogen. The epiglottis blocks the top of the windpipe when we swallow. Peristalsis moves the bolus along the epiglottis.

The stomachThe stomach can hold 2L of food and water. The lining of the stomach secretes gastric

acid. Gastric acid contains HCl and pepsin. Pepsin is an enzyme that hydrolyzes proteins. But it breaks proteins into smaller polypeptides.

Functions of Hydrochloric Acid• Activates enzymes pepsin & lingual lipase• Breaks up connective tissues & plant cell

walls– liquifying food to form chyme

• Converts ingested ferric ions (Fe+3) to ferrous ions (Fe+2) that can be absorbed & utilized for hemoglobin synthesis

• Destroys ingested bacteria & pathogens

Small Intestine

• Nearly all chemical digestion and nutrient absorption occurs in the small intestine

  Oral Cavity StomachSmall Intestine

 Lumen  Brush border

Carbohydrates

Salivary AmylasePolysaccharides     ==>    smaller Polysaccharides(starch,glycogen) (maltose)

 Pancreatic AmylasePolysaccharides  ==>   disaccharides (maltose)

Disaccharidases ==>   monosaccharides

Proteins  

   PepsinProteins    ==>    small polypeptide

Trypin, ChymotrypinPolypeptides ==>   smaller polypeptides ==>aminopeptidase, carboxypeptidase ==>   Amino Acids

Dipeptidasessmall polypeptides ==>   Amino Acids

Nucleic Acids

   

NucleasesDNA, RNA           ==>      Nucleotides    ==>

Nucelotidases ==>   nuclesides ==>Nucelosidases ==>   Nitrogenous bases, sugars, phosphates

Carbohydrates

Proteins

Fats

The digestive enzymes in the table below are summarized according to type of food that they digest.

FOOD TYPEENZYME SOURCE PRODUCTS

CARBOHYDRATES Salivary amylase Pancreatic amylaseMaltase

Salivary glands PancreasSmall intestine

Maltose MaltoseGlucose

PROTEINS Pepsin TrypsinPeptidases

Stomach mucosa Pancreas Intestinal mucosa

Peptides PeptidesAmino acids

FATS Lipase Pancreas Fatty acidsand glycerol

The table below shows digestive enzymes grouped by source of the enzyme.

SOURCEENZYME FOOD PRODUCT

MOUTH (salivary glands)

Salivary amylase Polysaccharides Maltose

STOMACH Pepsin Proteins Peptides

PANCREAS Pancreatic amylase TrypsinLipase

Polysaccharides ProteinsFats

Maltose PeptidesFatty acidsand glycerol

SMALL INTESTINE Maltase Peptidases

Maltose Peptides

Glucose Amino acids

• Digestive function is affected by hormones produced in many endocrine glands, but the most profound control is exerted by hormones produced within the gastrointestinal tract. The gastrointestinal tract is the largest endocrine organ in the body and the endocrine cells within it are referred to collectively as the enteric endocrine system. Three of the best-studied enteric hormones are:

• Gastrin: Secreted from the stomach and plays an important role in control of gastric acid secretion.

• Cholecystokinin: A small intestinal hormone that stimulates secretion of pancreatic enzymes and bile.

• Secretin: Another hormone secreted from small intestinal epithelial cells; stimulates secretion of a bicarbonate-rich fluids from the pancreas and liver.

HORMONES

Major Activities Stimuli for Release

GastrinStimulates gastric acid secretion and proliferation of gastric

epitheliumPresence of peptides and amino acids in gastric lumen

Cholecystokinin

Stimulates secretion of pancreatic enzymes, and contraction and emptying of the gall bladder

Presence of fatty acids and amino acids in the small intestine

SecretinStimulates secretion of water and bicarbonate from the pancreas

and bile ductsAcidic pH in the lumen of the small intestine

GhrelinAppears to be a strong stimulant for appetite and feeding; also a

potent stimulator of growth hormone secretion.Not clear, but secretion peaks prior to feeding and

diminishes with gastric filling

MotilinApparently involved in stimulating housekeeping patterns of motility

in the stomach and small intestineNot clear, but secretion is associated with fasting

Gastric inhibitory

polypeptide

Inhibits gastric secretion and motility and potentiates release of insulin from beta cells in response to elevated blood glucose concentration

Presence of fat and glucose in the small intestine

Hormones and Digestion:Feedback loops.The hormone Gastrin stimulates the production of gastric acid.The pyloric sphincter regulates the release of chyme (nutrient broth) into the small intestine.Bile contains bile salts that aid digestion and absorption of fats.

• Secretin hormone production is stimulated by acid chyme entering the duodenum. This hormone stimulates the pancreas to release bicarbonate to neutralize the acid.

• CCK stimulates the gall bladder to release bile and the panceas to release pancreatic juices.

• If the chyme is rich in fats causes the release of enterogastrone. This hormone inhibits peristalsis and the release of secretion of acid.

The small intestine

Essential Amino Acids• The human liver can synthesize 9 of the 20 amino acids used

in proteins. Those that cannot be synthesized are called the

essential amino acids. They must be supplied by diet.

Bile Ducts and Gallbladder

Definition:Bile is produced by the liver cells and collects in the small bile ducts between the microscopic liver lobules which lead to the larger ducts. Outside the liver, two outflowing hepatic ducts unite to form the common hepatic duct. From here the bile enters the cystic duct and the common bile duct . The common bile duct joins the pancreatic duct to enter the duodenum. The opening is controlled by a valve which regulates the flow of gastric juices (bile, pancreatic enzymes) into the duodenum.

• Function:

Bile is produced in the liver and aids the digestion of fatty food substances. Between 250 and 1000 millilitres of bile are produced each day. The most important constituents of bile are: water, bile salts, cholesterol , phospholipids . Between meals, the bile is stored in the gallbladder, which contracts once or twice as food is taken. This causes the bile to enter the duodenum via the common bile duct. 80 to 90% of the bile acids are later reabsorbed by the small intestine and re-enter the liver via the bloodstream (portal system).

Fat Digestion & Absorption

Absorption and Motility

• Transit time is 12 to 24 hours– reabsorbs water and electrolytes

• Feces consist of water & solids (bacteria, mucus, undigested fiber, fat & sloughed epithelial cells

• Haustral contractions occur every 30 minutes– distension of a haustrum stimulates it to contract

• Mass movements occur 1 to 3 times a day– triggered by gastrocolic and duodenocolic reflexes

• filling of the stomach & duodenum stimulates motility• moves residue for several centimeters with each contraction

Defecation

• Stretching of the rectum stimulates defecation– intrinsic defecation reflex via the myenteric plexus

• causes muscularis to contract & internal sphincter to relax– relatively weak contractions

• defecation occurs only if external anal sphincter is voluntarily relaxed– parasympathetic defecation reflex involves spinal cord

• stretching of rectum sends sensory signals to spinal cord• splanchnic nerves return signals intensifying peristalsis

• Abdominal contractions increase abdominal pressure as levator ani lifts anal canal upwards – feces will fall away