THE PHYSIOLOGY OF DIGESTIVE SYSTEM

INTEGRATIVE TEACHING BLOC 13

Dr.Swanny, MSc

Learning Objectives

1. Understand the function of digestive system.

2. Correlate the anatomical and histological structure with physiologic function.

3. Understand the process in digestive system

4. Apply the mechanism to describe the pathophysiology of digestive system disorders.

An illustrative problem

A 19 year-old boy was brought to hospital because of severe vomiting and diarrhea.

Explanation

From clinical point:Severe vomiting and diarrhea disease in

GI tract Dehydration.From physiology point:Vomiting and diarrhea disturbances in GI

tract function motility, absorption, secretion Homeostasis imbalance

Why is GI tract so interesting?

GIT diseases today account for nearly one-tenth of the money spent on health care.

Heart burn, indigestion, gas and constipation are troublesome rather than major health risks, but their significance should not be underestimated.

What is the function of GI Tract?

Gastrointestinal tract is a long tube whose function is to move nutrients, water and electrolytes from the external into internal environments.

Processes of Digestive Function

• Ingestion• Mechanical processing• Digestion• Secretion• Absorption• Excretion

Layers of the digestive tract

Layers of the digestive tract

Mucosa

Submucosa

Muscularis externa

Regulation of GI function

Movement of Digestive Materials

• Smooth muscle tissue found in almost every organ

• Pacesetter cells– Trigger waves of contraction– Vital role in moving materials along tract

• Peristalsis• segmentation

The Oral Cavity• Analyzes material before swallowing• Mechanically processes material through action

of: – Teeth– Tongue– Surfaces of palate

• Lubricates material by lubricating it with mucus and salivary secretions

• Begins digestion of carbohydrates and lipids with help of salivary enzymes

The Tongue

• Manipulates material inside mouth• Base of tongue extends to pharynx• Primary functions:

– Mechanical processing by compression, abrasion, distortion

– Manipulation to assist in chewing and to prepare material for swallowing

– Sensory analysis by touch, temperature, taste receptors

Salivary Secretions• Each produce slightly different type of saliva• Parotid glands

– Salivary amylase• Breaks down starch so it can be absorbed by digestive tract

• Submandibular and sublingual glands– Secretion contains less enzymes but more buffers

and mucus• All 3 release saliva during eating

– Saliva increases pH • Goes from 6.7 to 7.5

• What division of the nervous system controls secretion

Esophagus

The Esophagus: Swallowing

• Also called degluition• Voluntary and involuntary actions• Before it can occur, material must have

proper texture and consistency• Material shredded moistened with

saliva approved by taste receptors tongue compacts debris into bolus

Stages of Swallowing• Oral phase

– Compression of bolus against hard palate– Tongue retracts

• Forces bolus into pharynx• Elevates hard palate

– Prevents bolus from entering nasopharynx– Only phase that can be consciously controlled

• Pharyngeal phase– Bolus in contact with sensory receptors

• Initiates swallowing reflex• Larynx elevates, epiglottis folds (protects glottis)• Contraction of pharyngeal muscles forces into esophagus

• Esophageal phase– Bolus enters esophagus – Peristalsis– Approach to stomach opens lower esophageal sphincter

• 9 seconds from oral cavity to stomach

The Stomach• 4 primary functions

– Temporary storage of ingested food– Mechanical breakdown of resistant materials– Breaking chemical bonds in food materials

• Acids and enzymes– Production of intrinsic factor

• Necessary for absorption of vitamin B12 • Ingested materials mix with secretions of glands

– Product is highly acidic soupy mixture (partially digested)

• Product called chyme (pyloric sphincter regulates flow into small intestines)

Stomach

Gastric glands

Gastric glands

Parietal cells

Acid production

Functions of gastric juice

Reflux

Reflux

Secretory Cells• Parietal Cells

– Secretes intrinsic factor • Allows absorption of vitamin B12

– Secretes HCl• Lowers pH of gastric juice (1.5-2.0)

– Kills microorganisms– Breaks down cell walls and connective tissue in food– Activates enzymatic secretions of chief cells

• Chief Cells– Pepsinogen (inactive enzyme)– HCl stimulates pepsinogen to convert to pepsin– Pepsin: protein-splitting enzyme

Digestion in the Stomach• Performs preliminary digestion of proteins

(pepsin) and carbohydrates (salivary amylase)• Pepsin activity increases when pH 2.0

– Does not complete protein digestion• No nutrient absorption because:

– Epithelial cells covered by alkaline mucus and not directly exposed to chyme

– Epithelial cells lack specialized transport mechanisms– Gastric lining impermeable to water– Digestion has not proceeded to completion by time

chyme leaves stomach• Carbohydrates, lipids, proteins only partially broken down

Small intestine

Intestinal Movements

• Segmentation – Mix with mucous secretions and enzymes

• Slow peristaltic contractions as absorption occurs

• Local contractions (no CNS involvement)• Takes 5 hours for ingested food to go from

duodenum to ileum

Small intestine

Intestinal Secretions

• Roughly 1.8 L secreted every day• Functions

– Moistens intestinal contents– Buffers acids– Dissolves digestive enzymes from pancreas

• Under hormonal and CNS controls – Particularly in the duodenum

• 1st region to receive chyme– Acid content must be neutralized

Controls of Intestinal Secretions

• Submucosal glands protect duodenal epithelium from gastric acids and enzymes– increase secretion in response to:

• Local reflexes• Parasympathetic NS through vagus nerve

– Stimulate, glands begin secreting before chyme reaches pyloric sphincter

• Sympathetic inactivates

Intestinal Hormones• Secretin

– Released when pH falls in duodenum• Occurs when chyme enters duodenum

– Primary effect is to increase secretion of bile and buffers by liver and pancreas

• Cholecystokinin (CCK)– Secreted when chyme enters duodenum– Targets:

• Pancreas: accelerates production and secretion of all types digestive enzymes

• Gallbladder: causes ejection of bile into duodenum• Gastric Inhibitory Peptide (GIP)

– Secreted when fats and carbohydrates enter small intestine– Inhibits gastric activity and causes release of insulin from

pancreas (islets of Langerhans)

Pancreatic juice

Enzyme Zymogen ActivatorAction

Trypsin Trypsinogen EnterokinaseCleaves internal

peptide bondsChymotrypsin Chymotrypsinogen Trypsin

Cleaves internal peptide bonds

Elastase Proelastase TrypsinCleaves internal

peptide bondsCarboxypeptidase Procarboxypeptidase Trypsin

Cleaves last amino acid from carboxyl-terminal end of polypeptide

Phospholipase Prophospholipase TrypsinCleaves fatty

acids from phospholipids such as lecithin

Lipase None NoneCleaves fatty

acids from glycerolAmylase None None

Digests starch to maltose and short chains of glucose molecules

Cholesterolesterase None NoneReleases

cholesterol from its bonds with other molecules

Ribonuclease None NoneCleaves RNA to

form short chainsDeoxyribonuclease None None

Cleaves DNA to form short chains

Pancreatic juice

Control of Pancreatic Secretion• Acid chyme in duodenum secretin released

triggers pancreas to secrete alkaline fluid (pH 7.5 – 8.8)– Alkaline fluid contains buffers (sodium bicarbonate)

• Increase pH of chyme• CCK controls production and secretion of pancreatic

enzymes– Pancreatic amylase– Pancreatic lipase– Nucleases– Proteases (70% total enzyme secretion)

• Trypsin, chymotrypsin, peptidase– Catabolize into short peptide chains and amino acids

• Powerful so pancreas secretes inactive forms– Activated by other enzymes in intestinal tract

Liver

Enterohepatic circulation

Liver Functions

• Metabolic Regulation• Hematological Regulation• Production and Role of Bile

Liver functionsFunctional Category ActionsDetoxication of Blood Phagocytosis by Kupffer cells

Chemical alteration of biologically active molecules (hormones and drugs)Production of urea, uric acid, and other molecules that are less toxic than parent compoundsExcretion of molecules in bile

Carbohydrate Metabolism Conversion of blood glucose to glycogen and fatProduction of glucose from liver glycogen and from other molecules (amino acids, lactic acid) by gluconeogenesisSecretion of glucose into the blood

Lipid Metabolism Synthesis of triglyceride and cholesterolExcretion of cholesterol in bileProduction of ketone bodies from fatty acids

Protein Synthesis Production of albuminProduction of plasma transport proteinsProduction of clotting factors (fibrinogen, prothrombin, and others)

Secretion of Bile Synthesis of bile saltsConjugation and excretion of bile pigment (bilirubin)

Bile production

Bile salts

Blood detoxification

Glucose, triglycerides, and ketones

Metabolic Regulation• Primary organ for regulating composition of

circulating blood• Liver cells can:

– Extract absorbed nutrients or toxins from blood before reach rest of body

– Monitor and adjust circulating levels of organic materials

• Excesses removed or stored• Deficiencies corrected by mobilizing stored reserves or

synthesizing compound– Circulating toxins and metabolic wastes removed for

later inactivation or excretion– Fat soluble vitamins (A, D, K, E) absorbed and stored

Hematological Regulation • Largest blood reservoir in body• Receives 25% of CO• Kupffer cells remove aged or damaged RBCs,

cell debris and pathogens from circulation– Antigen-presenting cells

• Can stimulate immune response• Hepatocytes synthesize plasma proteins

– Determines osmotic concentration of blood (aren’t they smart!)

– Transports nutrients– Establish clotting and complement system

Production and Role of Bile

• Synthesized in liver, excreted into duodenum• Consists of water, ions, bilirubin, cholesterol,

and bile salts (lipids)– Water and ions dilute and buffer acids in chyme as

enters duodenum– Bile salts required for normal digestion and absorption

of fats• Break drops of lipids by emulsification

– Creates tiny droplets coated with bile salts» Increase surface area for enzymatic attack

The Gallbladder

• Major functions– Bile storage

• Contains 40-70 mL when full– Bile modification

• Bile secretion continuous– Bile release into duodenum occurs only under

stimulation of CCK• Relaxes hepatopancreatic sphincter• Amount secreted depends on the amount of fat

present in the chyme

Processing and Absorption of Nutrients

• Digestive system must first break down physical structure– Disassembles component molecules into

smaller fragments• Produces small organic molecules that can be

released into blood stream– Used to generate ATP or to synthesize complex

carbohydrates, proteins, lipids

Absorption

• Bile salts– Reabsorbed; transported to liver

• Vitamins– Bacteria in colon generate 3 vitamins that supplement

our dietary supply• Vitamin K: fat soluble; needed by liver to synthesize 4

clotting factors• Biotin: water soluble; important for glucose metabolism• Vitamin B5: water soluble; required to manufacture steroid

hormones and some NTs

Water and Electrolyte Absorption

• Water conservation occurs passively– Follows osmotic gradient

• Low to high concentration of solutes

• Epithelial cells continually absorbing dissolved nutrients and ions– Gradually lowers solute concentration in intestine

• Water moves into surrounding tissue (“follows” solutes)

• Absorption of sodium and chloride most important factor – Promotes water movement

• Other ions:– Calcium, potassium, magnesium, iodine, bicarbonate, iron

Absorption of Vitamins• Organic compounds related to lipids and carbohydrates

– Required in small quantities• Fat-soluble vitamins

– A, D, E, K– Enter duodenum in fat droplets mixed with dietary lipids– Absorbed from micelles– K also produced from bacteria

• Water-soluble vitamins– Participants in enzyme action– All but one easily absorbed by digestive epithelium

• B12 cannot be absorbed until bound to intrinsic factor

Neuronal regulation

Gastric phase

Gastric phase

Intestinal phase

Enteric NS

Paracrine regulation

Deuodenal hormones

Large Intestine

• Main functions:– Reabsorption of water and compaction of feces– Absorption of important vitamins liberated by bacterial

action– Storage of fecal material prior to defecation

• Divided into 3 parts– Cecum– Colon– Rectum

Large intestine

Physiology of the Large Intestine: Absorption

• Reabsorption of water– 1500 mL/day in colon; 1300 mL reabsorbed

• Feces composed of:– 75% water– 5% bacteria– 20% undigested material, inorganic material,

remains of epithelial cells• Also reabsorb bile salts, vitamins, bilirubin

products, toxins

Absorption• Bilirubin products

– Product of heme breakdown– Bacteria convert bilirubin into other products

• Some absorbed and excreted in urine (yellow color)• Others when exposed to oxygen further modified into brown

pigment in feces• Toxins

– Bacterial action breaks down peptides and generates:• Ammonia (taken to liver and made into untoxic material then to

kidneys)• Nitrogen-containing compounds• Hydrogen sulfide

– Indigestible carbohydrates• Not altered by intestinal enzymes• Nutrient source for resident bacteria

Movements of the Large Intestine

• Movement from cecum to transverse colon slow– Allows time for water resorption

• Mass movements move from transverse to rest of colon– Powerful peristaltic contractions– Force fecal matter into rectum and causes

urge to defecate

Defecation

• Rectum usually empty– Mass movement forces fecal matter in

• Distention of rectal wall triggers defecation reflex– Stretch receptors in rectal walls stimulate a series of

local peristaltic contractions in colon and rectum• Moves feces towards anus

– Parasympathetic neurons in sacral region activated by stretch receptors

• Stimulate increased peristalsis throughout large intestine

Defecation

• Internal anal sphincter– Must relax so feces can move into anus

• External anal sphincter clamps shut– Therefore release is conscious

THE END