Digestive Physiology - the “How” component By Karla Nix and Donna Mook.

Digestive Physiology - the Digestive Physiology - the “How” component“How” component

By Karla Nix and Donna MookBy Karla Nix and Donna Mook

What is Digestive What is Digestive Physiology?Physiology?

The Physiology of digestion is how the anatomical components of the digestive system actually “digest” intaken food. The anatomical components complete digestion by breaking down complex, large food substances into smaller, simpler substances. These simpler

substances are then absorbed by the system and any wastes are excreted

from the system.

DigestionDigestion

Digestion is accomplished by:Digestion is accomplished by:

Ingestion – taking in food substancesIngestion – taking in food substances Propulsion – the movement of the food bolusPropulsion – the movement of the food bolus Mechanical/Chemical Digestion & Absorption Mechanical/Chemical Digestion & Absorption

of nutrients – usually in small intestineof nutrients – usually in small intestine Defecation – how solid wastes are excreted; Defecation – how solid wastes are excreted;

accomplished by the large intestineaccomplished by the large intestine

Mechanical DigestionMechanical Digestion

Occurs in area from mouth to small Occurs in area from mouth to small intestine and involves:intestine and involves:

Mastication – chewingMastication – chewing Degultition – both voluntary and Degultition – both voluntary and

involuntaryinvoluntary Churning – in stomach segmentation Churning – in stomach segmentation

in small intestinein small intestine

Chemical DigestionChemical Digestion

Occurs in area from mouth to small Occurs in area from mouth to small intestine and involves:intestine and involves:

Major portion in stomach and upper Major portion in stomach and upper small intestinesmall intestine

Saliva components and many gastric Saliva components and many gastric juices, which are acidic (low pH), and juices, which are acidic (low pH), and break down food particlesbreak down food particles

Digestive Activity ControlDigestive Activity Control

By hormone producting cells and By hormone producting cells and local nerve plexuseslocal nerve plexuses

By CNS – autonomic nervesBy CNS – autonomic nerves

Physiological “Travel” through Physiological “Travel” through the Digestive Systemthe Digestive System

MouthMouth StomachStomach Small IntestineSmall Intestine Large IntestineLarge Intestine

The Mouth (Oral Cavity)The Mouth (Oral Cavity)

Digestion begins here, both Digestion begins here, both mechanical and chemicalmechanical and chemical

Salivary glands found here produce Salivary glands found here produce saliva, an aqueous substancesaliva, an aqueous substance

SalivaSaliva

Consists mainly of waterConsists mainly of water Slightly acidicSlightly acidic Salivation controlled by PNS through 7Salivation controlled by PNS through 7thth and 9 and 9thth cranial cranial

nervesnervesContains:Contains:

Defensin – produced when trauma occursDefensin – produced when trauma occurs Salivary Amylase – enzyme; polysaccharides – Salivary Amylase – enzyme; polysaccharides –

monosaccharidesmonosaccharides ElecrolytesElecrolytes Protein mucin – to lubricate oral cavityProtein mucin – to lubricate oral cavity IgA antibodiesIgA antibodies Nitric Oxide – bacteriostatic enzymeNitric Oxide – bacteriostatic enzyme Lysozyme – bacteriostatic enzymeLysozyme – bacteriostatic enzyme

StomachStomach

The stomach is a major site of chemical digestion, as The stomach is a major site of chemical digestion, as one would imagine. The digestion of proteins here, one would imagine. The digestion of proteins here,

leads to Chyme production. Parietal cells of the leads to Chyme production. Parietal cells of the stomach produce Intrinsic factor, which is the stomach produce Intrinsic factor, which is the

stomach’s most important function. Without intrinsic stomach’s most important function. Without intrinsic factor, Vitamin B12 would not be absorbed and this factor, Vitamin B12 would not be absorbed and this would lead to immature RBC’s resulting in a form of would lead to immature RBC’s resulting in a form of

anemia known as Pernicious anemia. The PNS is chiefly anemia known as Pernicious anemia. The PNS is chiefly involved here, and stretch receptors of the stomach act involved here, and stretch receptors of the stomach act on the PNS, causing gastrin production from chief and on the PNS, causing gastrin production from chief and

parietal cells, once they are activated by a “full parietal cells, once they are activated by a “full stomach”. The PNS is also responsible for churning in stomach”. The PNS is also responsible for churning in

the stomach, or what we refer to as “growling”.the stomach, or what we refer to as “growling”.

The Stomach The Stomach (continued)(continued)

““Basic”(high pH) foods, caffeine, and partially digested Basic”(high pH) foods, caffeine, and partially digested proteins lead to gastrin secretion. Gastrin leads to HCL proteins lead to gastrin secretion. Gastrin leads to HCL secretion from parietal cells and pepsinogen secretion secretion from parietal cells and pepsinogen secretion

from chief cells. HCL creates an acidic environment from chief cells. HCL creates an acidic environment necessary for protein digestion and pepsinogen necessary for protein digestion and pepsinogen activation. About 3mL of chime is ejected into activation. About 3mL of chime is ejected into

duodenum by each peristaltic wave, the remaining duodenum by each peristaltic wave, the remaining chyme is moved back and “churned” again by the chyme is moved back and “churned” again by the

stomach. This achieves a thorough mixing and also a stomach. This achieves a thorough mixing and also a breaking down of food particles. Stomach emptying breaking down of food particles. Stomach emptying

usually takes ~ 4 hours , with large amount of liquids usually takes ~ 4 hours , with large amount of liquids making emptying faster, and fatty meals making making emptying faster, and fatty meals making

emptying slower! emptying slower!

Gastric Secretion regulation in Gastric Secretion regulation in StomachStomach

Cephalic----an example is seeing cakes at the bakery---Cephalic----an example is seeing cakes at the bakery---the act of seeing or thinking of food stimulates vagus the act of seeing or thinking of food stimulates vagus nerve of PNS and causes gastric secretion. nerve of PNS and causes gastric secretion.

Gastric phase----gastrin and vagus nerve stimulate Gastric phase----gastrin and vagus nerve stimulate secretion after food has already entered stomach secretion after food has already entered stomach

Intestinal----increased secretion (food entering Intestinal----increased secretion (food entering duodenum) followed by inhibition of gastric secretions duodenum) followed by inhibition of gastric secretions and pyloric sphincter closure---allowing food to enter and pyloric sphincter closure---allowing food to enter intestine in small portions and at intervals intestine in small portions and at intervals

HCL secretion controlled by PNS nerve fibers, gastrin, HCL secretion controlled by PNS nerve fibers, gastrin, and histamine and histamine

Small IntestineSmall Intestine

Main function is absorptionMain function is absorption Food remains here ~ 4-6 hours Food remains here ~ 4-6 hours Intestinal juice (water and mucus—alkaline) Intestinal juice (water and mucus—alkaline)

produced by Brunner’s glands in duodenum and produced by Brunner’s glands in duodenum and intestinal goblet cells, when acidic food enters intestinal goblet cells, when acidic food enters duodenum duodenum

Segmentation ----mixing of foods accompanied by Segmentation ----mixing of foods accompanied by enzymatic action enzymatic action

Bile delivered to duodenum by way of bile duct----Bile delivered to duodenum by way of bile duct----hormone CCK is stimulus for bile entering hormone CCK is stimulus for bile entering duodenum, rather than its normal storage in the duodenum, rather than its normal storage in the gall bladder gall bladder

BileBile

Consists of bile salts, phospholipids, Consists of bile salts, phospholipids, bile pigments and cholesterol bile pigments and cholesterol

Functions to emulsify (mix) fats Functions to emulsify (mix) fats causing their breakdown causing their breakdown

Recyclable bile salts act to facilitate Recyclable bile salts act to facilitate cholesterol and fat absorption, and cholesterol and fat absorption, and also keep cholesterol dissolved in also keep cholesterol dissolved in bile bile

Pancreatic JuicePancreatic Juice

Aqueous in natureAqueous in nature Contains: protein digesting proteases Contains: protein digesting proteases

along with enzymes amylases , along with enzymes amylases , lipases, and nucleases lipases, and nucleases

Contains bicarbonate ions which Contains bicarbonate ions which provide a basic, high pH, provide a basic, high pH, environment for optimal pancreatic environment for optimal pancreatic enzyme function, and also neutralize enzyme function, and also neutralize the acidic chyme the acidic chyme

Absorption in Small Absorption in Small IntestineIntestine

Most foodstuffs, water, and electrolytes absorbed here Most foodstuffs, water, and electrolytes absorbed here Fat and water absorbed by passive transport--- glucose, Fat and water absorbed by passive transport--- glucose,

fructose, galactose and amino acids absorbed by active fructose, galactose and amino acids absorbed by active transport transport

Fat absorbed in mucosal surface as fatty acids and Fat absorbed in mucosal surface as fatty acids and monoglycerides---both of which have been broken down monoglycerides---both of which have been broken down from larger structures from larger structures

Chylomicrons—lipids combined with proteins in intestinal Chylomicrons—lipids combined with proteins in intestinal cells, they enter the lacteals and are carried to thoracic cells, they enter the lacteals and are carried to thoracic duct duct

Most vitamins diffuse easily into cells w/ exception of Most vitamins diffuse easily into cells w/ exception of vitamin B12 vitamin B12

Peristalsis---starts after most nutrients absorbed and Peristalsis---starts after most nutrients absorbed and allows the contents of the small intestine to enter cecum allows the contents of the small intestine to enter cecum by its waves of motionby its waves of motion

Large IntestineLarge Intestine

Functions to absorb water from undigested food and Functions to absorb water from undigested food and excretion of semi-solid and solid wastes excretion of semi-solid and solid wastes

Bacterial flora found here ferment carbohydrates Bacterial flora found here ferment carbohydrates (partially or undigested) resulting in flatus, and (partially or undigested) resulting in flatus, and synthesize Vitamins K and B synthesize Vitamins K and B

Nutrient-poor food usually spends ~ 24 hours here Nutrient-poor food usually spends ~ 24 hours here Very little digestion takes place Very little digestion takes place Water and few electrolytes reabsorbed into colon Water and few electrolytes reabsorbed into colon Propulsion of fecal matter is a main function and it Propulsion of fecal matter is a main function and it

is carried out by food residue-filled haustrums is carried out by food residue-filled haustrums contract and propel residue to another haustrum—contract and propel residue to another haustrum—this is a slow movement accompanied by further this is a slow movement accompanied by further water re-absorption water re-absorption

MetabolismMetabolism

Enzymatic action leads to food being absorbed into blood Enzymatic action leads to food being absorbed into blood stream, then the blood carries the nutrients to the cells. stream, then the blood carries the nutrients to the cells.

At the cellular level, nutrients are broken down by reactions or At the cellular level, nutrients are broken down by reactions or used to further synthesize carbohydrates, proteins and fats used to further synthesize carbohydrates, proteins and fats

Reactions in body:Reactions in body: Either---ANABOLIC……GENESIS------smaller substance put Either---ANABOLIC……GENESIS------smaller substance put

together to make larger, complex substances---requires together to make larger, complex substances---requires energy, or CATABOLIC----LYSIS---larger substances broken energy, or CATABOLIC----LYSIS---larger substances broken down to form smaller substances---usually by hydrolysis----down to form smaller substances---usually by hydrolysis----releases energy releases energy

Glucose is the most immediate form of energy (ATP) found in Glucose is the most immediate form of energy (ATP) found in the body the body

ATP is the ONLY usable form of energy in the body—every ATP is the ONLY usable form of energy in the body—every energy form must be broken down or converted into ATP in energy form must be broken down or converted into ATP in order for the body to take advantage of it order for the body to take advantage of it

How Glucose (Carbohydrate) is How Glucose (Carbohydrate) is converted to ATP and used by converted to ATP and used by

the Body the Body Glycolysis—the anaerobic pathway or Glycolysis—the anaerobic pathway or

breakdown of glucose occurring in cytoplasm breakdown of glucose occurring in cytoplasm of cellof cell

22Glycolysis: Glucose + 2NAD +2ATP +2P---- Glycolysis: Glucose + 2NAD +2ATP +2P---- pyruvic acid + 2NADH + 2ATP ( a net gain of pyruvic acid + 2NADH + 2ATP ( a net gain of 2 ATP molecules for every one glucose) 2 ATP molecules for every one glucose)

Glycolysis used by mitochondria-lacking Glycolysis used by mitochondria-lacking RBC’s and by skeletal muscles for short RBC’s and by skeletal muscles for short periods of activity periods of activity

Carbohydrate Metabolism Carbohydrate Metabolism continuedcontinued

Pyruvic acid produced by glycolysis rxn has a high bond-Pyruvic acid produced by glycolysis rxn has a high bond-energy energy

If O2 is available, pyruvic acid enters mitochondria, where If O2 is available, pyruvic acid enters mitochondria, where they are broken down and used in rxn to make Acetyl CoA they are broken down and used in rxn to make Acetyl CoA

Acetyl CoA helps in formation of citric acid---also 1st step of Acetyl CoA helps in formation of citric acid---also 1st step of Kreb’s Cycle Kreb’s Cycle

Kreb’s Cycle--- “aerobic metabolism” an oxidative rxn, occurs Kreb’s Cycle--- “aerobic metabolism” an oxidative rxn, occurs in mitochondria, and produces carbon dioxide, ATP and NADH in mitochondria, and produces carbon dioxide, ATP and NADH

Electron Transport Chain---aerobic rxn in mitochondria whose Electron Transport Chain---aerobic rxn in mitochondria whose products meet 90% of the body’s ATP needs------WOW !!!!products meet 90% of the body’s ATP needs------WOW !!!!

1 glucose molecule processed leaves the cell with 36 net ATP 1 glucose molecule processed leaves the cell with 36 net ATP molecules molecules

More ATP is produced in the mitochondria than the cytoplasmMore ATP is produced in the mitochondria than the cytoplasm

Protein MetabolismProtein Metabolism

Proteins broken down into their amino acid Proteins broken down into their amino acid building blocks which are then oxidized to building blocks which are then oxidized to form ATP. Ammonia produced is converted to form ATP. Ammonia produced is converted to urea by the liver and excreted in the urine urea by the liver and excreted in the urine

Synthesize new proteins, enzyme synthesis, Synthesize new proteins, enzyme synthesis, antibody production ----protein synthesization antibody production ----protein synthesization promoted by growth hormone, estrogen, promoted by growth hormone, estrogen, testosterone, and thyroid hormones testosterone, and thyroid hormones

Remaining amino acids used in Remaining amino acids used in GLUCONEOGENESIS (making glucose) or GLUCONEOGENESIS (making glucose) or LIPOGENESIS (making lipids) LIPOGENESIS (making lipids)

Lipid MetabolismLipid Metabolism

Non-soluble triglycerides are combined with proteins and Non-soluble triglycerides are combined with proteins and transported into bloodtransported into blood

LIPOPROTEINS---combination of fat and protein----LIPOPROTEINS---combination of fat and protein----chylomicrons, VLDL,LDL,HDL chylomicrons, VLDL,LDL,HDL

Chylomicrons transport lipids in diet, VLDLs transport Chylomicrons transport lipids in diet, VLDLs transport lipids produced by the body to adipose tissue, LDLs carry lipids produced by the body to adipose tissue, LDLs carry the majority of blood cholesterol (High LDL can form the majority of blood cholesterol (High LDL can form plaques), and HDLs remove excess cholesterol from plaques), and HDLs remove excess cholesterol from blood and cells which is removed after transport to the blood and cells which is removed after transport to the liverliver

Fats can be oxidized to produce ATP---triglycerides Fats can be oxidized to produce ATP---triglycerides broken into fatty acids and glycerol broken into fatty acids and glycerol

Glycerol can be used to produce ATP also, but it is Glycerol can be used to produce ATP also, but it is converted to glucose if the body does not need ATP converted to glucose if the body does not need ATP

Clinical ApplicationsClinical Applications

Pernicious Anemia—mentioned previously---a type Pernicious Anemia—mentioned previously---a type of anemia that occurs when there is an absence of of anemia that occurs when there is an absence of intrinsic factor (by parietal cells) in the stomach intrinsic factor (by parietal cells) in the stomach

Gastric ulcers---condition where mucosal barrier of Gastric ulcers---condition where mucosal barrier of stomach broken which results in an erosion of the stomach broken which results in an erosion of the surface epithelium----can cause bleeding, surface epithelium----can cause bleeding, perforation and possibly peritonitis perforation and possibly peritonitis

Gall Stones----due to a decrease in bile salts, Gall Stones----due to a decrease in bile salts, which leads to crystallization and ultimately gall which leads to crystallization and ultimately gall stone formation stone formation

Hirschsprung’s disease----condition where large Hirschsprung’s disease----condition where large intestine lacks parasympathetic ganglia---intestine lacks parasympathetic ganglia---especially in rectal area----leads to chronic especially in rectal area----leads to chronic constipation, and must be treated surgically constipation, and must be treated surgically