Digestive System

alimentary canal and accessory organs

Basic Functions

Ingestion, mastication, deglutition - eating

Movement of food along the GI - peristalsis

Digestion - chemical and physical

breakdown of food

Absorption of food breakdown products

into circulatory and lymphatic systems

Elimination - excretion of waste products

and toxins

The Digestive System

Composed of a two parts

*A long tube - termed the alimentary canal. The alimentary canal extends from mouth to anus and is about 30 ft in length

*Accessory structures -these structures aid in digestion (salivary glands, teeth, liver, GB, pancreas)

Digestion Begins in the Mouth

Oral or buccal cavity - moistens and aids

mechanical digestion of food (minor amount of saliva secreted here)

Tongue - helps to manipulate food, moves food to the back of the mouth for swallowing, contains taste buds, glands located here secrete lingual lipase

allows for speech

Salivary Glands

The majority of saliva comes from

three paired structures

Parotid, submandibular, and

sublingual glands

Saliva pH is 6-7 and composed of

99.5% H2O.

1000 to 1500 ml secreted daily

under nervous system control (salivary nuclei triggered by smell, sight, thought or presence of food in mouth)

These glands secrete Salivary Amylase which breaks polysaccharides into disaccharides

Teeth

Mastication - chewing

1st step of digestion

Two dentitions

1. Deciduous teeth

-appear in 6 months

-20 teeth

-formula 2 incisors,

1 canine, 2 molars

2. Permanent Teeth

-appear 6-12 years

-32 in all

-formula 2 incisors,

1 canine (cuspids), 2 bicuspid

(premolars) and 3 molars

Schimmels Dental Chart

Digestion Begins With

Mastication - chewing/teeth 20-30 bites

Moistening - saliva

Chemical -

Salivary Amylase - polysaccharides (starch) into disaccharides

Lingual Lipase - triglycerides to fatty acids and monoglycerides

Deglutition - Swallowing

Voluntary stage - tongue moves food back into the oropharynx

Pharyngeal stage - bolus stimulates receptors in oropharynx causing a

cessation of respiration and epiglottis covers airway

Esophageal stage - dilation of upper/lower esophageal sphincters,

food moves into esophagus and down to the stomach via peristalsis

Esophageal bolus moves behind heart!

Lower esophageal sphincter must now open

4 layers of the GI tract

Mucosa - mucous membrane, first layer,

composed of epithelium (stratified. sqamous/simple columnar), and is in contact with food

Submucosa - composed of loose areolar connective tissue and binds mucosa to muscle layer, contains immune system of the gut

Muscularis - skeletal muscle in mouth, pharynx and esophagus. The rest of the tract is smooth muscle. This layer usually contains 2 layers (3 in stomach) - inner circular/outer longitudinal

Serosa - serous membrane, made up of connective tissue and epithelium. Below the diapham this layer is termed the visceral peritoneum.

4 layers of the GI tract

Peritoneum

The Peritoneum is a serous membrane that lines

the walls of the abdomen and organs

The parietal layer lines the walls of the abdomen

The visceral layer covers the the internal organs

The space between these layers is called the

peritoneal cavity and contains serous fluid

Organs behind this cavity (kidneys/pancreas) are

termed retroperitoneal

Infection in this area is called peritonitis

Stomach

Stomach

The mucosa of the stomach consists of gastric pits lined with 4

types of cells

1. Chief cells - pepsinogen/gastric lipase

2. Parietal cells - HCL/Intrinsic factor

3. Mucous cells - mucous barrier

4. G cells - gastrin

All secretions equal gastric juice (2000 to 3000 ml per day)

Gastric Pits

Mechanical digestion

Peristaltic mixing waves q 15-25 secs

Food mixed with HCL and reduced to chyme

Food is stored in the fundus then ground down in the body

and pylorus

Each wave ejects chyme through the pyloric sphincter into

the duodenum

H. pylori-bacteria associated with Ulceration

Reflux -smoking, coffee, alcohol, and vitamin deficiency ?

Chemical Digestion in the Stomach

1. HCL denatures protein - destroys three dimensional structure of protein

2. HCL converts pepsinogen (zymogen) into pepsin (active protease)

3. Pepsin breaks large polypeptides into peptides

4. Gastric Lipase digests buttermilk in infants

Alcohol, ASA and water are absorbed in stomach

Mucus prevents auto-digestion of this layer

Nerve/hormonal interactions

1. HCL denatures protein - destroys three

dimensional structure of protein

2. HCL converts pepsinogen (zymogen)

into pepsin (active protease)

3. Pepsin breaks large polypeptides into

peptides

4. Gastric Lipase digests buttermilk in

infants

Alcohol, ASA and water are absorbed in

stomach

Mucus prevents auto digestion of this layer

The hormones of digestion

Acetylcholine - parasymp nervous output

stimulates parietal HCL production, peptic pepsinogen release, and mucous secretion

Histamine - amino acid derivative that stimulates HCL production via H2 parietal cell activation. This compound is required for gastrin and acetylcholine activation of parietal cells. Blocked by tagamet-cimetidine

Gastrin - secreted by G-cells in response to the presence of peptides/amino acids in the stomach. This hormone increases HCL production, contracts LES, increases stomach motility, and relaxes the pyloric sphincter leading to the SI

Alcohol, caffeine, and amino acids stimulate gastric production as well

More hormones

Secretin - secreted by endocrine cells located in

the duodenum in response to the presence of acidic chyme. This hormone triggers the release of bicarbonate from the pancreas to neutralize acidic chyme and create the proper pH for pancreatic enzyme activity

CCK(cholecystokinin) - fat entering the small intestine triggers the release of CCK from local endocrine cells. CCK enhances the action of secretin, stimulates pancreatic enzyme secretion and causes the GB to contract releasing bile into the SI.

Both Secretin/CCK slow stomach emptying

CCKs more interesting aspect

CCK in the brain is related to anxiety

CCK injected into humans creates feeling of

panic and intense anxiety

Drugs that Block CCK in the brain reduce

anxiety

GIP

Glucose insulinotropic peptide is released in

response to glucose and fat in the small intestine

It slows gastric gland secretion and emptying

GIP also stimulates the release of insulin which allows for cellular uptake of glucose/glycogen storage

The Liver, GB , Pancreatic interface

Right and Left Hepatic ducts

Common Hepatic duct

Cystic duct

Pancreatic duct and Accessory duct

Hepatopancreatic ampulla

The Liver

Generation of bile salts

from cholesterol via HMG - CoA reductase

Lipid metabolism

Protein synthesis and

recycling

Phase1/2 detox

reactions

Hormone metabolism

Storage of glycogen,

vitamins (A,B,D,E,K) and iron

Activation of multiple vitamin prestages

Phagocytosis of worn out blood cells

Gall Bladder

Stores and concentrates bile by reabsorbing water

Bile salts and lecithin combine with cholesterol to

form micelles that are soluble and keep cholesterol in solution preventing stone formation

Ejection of bile primed by CCK

Cholelithiasis - cholecystitis 1. Too much H2O

absorbed from bile 2. High fat high cholesterol diet 3. Too much cholesterol in bile - chenodeoxycholic bile acid therapy for the dissolution of stones

Pancreas

Exocrine function

Digestive enzyme release

(CCK)

Bicarbonate - primed by

secretin

Endocrine function

Glucagon

Insulin

Somatostatin

What does insulin do to blood sugar?

Netters

Pancreatic Enzymes

Pancreatic amylase -

starches to disaccharides

Trypsin, chymotrypsin and carboxypeptidase -

proteins into peptides

Pancreatic lipase

triglycerides into fatty acids and monosaccharides

Pancreatic nucleases -

RNA/DNA into nucleotides

Small Intestine

Duodenum, Jejunum and Ileum

Consists of 4 layers with

modifications to the mucosa and submucosa to increase surface area

Villi-absorptive finger has a LP core that contains blood vessels+lymphatics (lacteal)

Microvilli-threadlike projection of the epithelial cell. Each cell has about 1,700 - this creates the brush border and secretes digestive enzymes

Plica circularis - The submucosa is thrown into large folds

Villi and Plica Circularis

Glandular types of the small intestine

Intestinal Glands - Crypts of leiberkuhn

these glands secrete intestinal juice (combined with the brush border secretions)

Duodenal glands - Brunners glands - secrete an alkaline mucous that combines with bicarbonate ions from the pancreas to neutralize acidic chyme from the stomach - inhibited by symp. Activity or chronic stress leading to ulcer formation

Intestinal Juice

1 to 2 liters produced per day

Derived from intestinal glands and the surface of

enterocytes (brush border)

Brush border enzymes include;

*maltase, sucrase, lactase, alpha destrinase - disaccharides into monosaccharides

*peptidases

peptides into amino acids

*nucleosidases and phosphatases - nucleotides into N bases, pentose and P

Chemical Digestion in the Small Intestine revisited

Pancreatic enzymes

Pancreatic amylase -

starches to disaccharides

Trypsin, chymotrypsin

and carboxypeptidase -

proteins into peptides

Pancreatic lipase

triglycerides into fatty acids and monosaccharides

Pancreatic nucleases -

RNA/DNA into

nucleotides

Brush border enzymes

Maltase, sucrase, lactase,

alpha destrinase disaccharides into monosaccharides

Peptidases

peptides into amino acids

Nucleosidases and phosphatases

nucleotides into N bases, pentose and P

Enzymes reviewed

Mechanical Digestion

Inner circular and outer longitudinal muscle

muscle layers allow for segmentation

Chyme is confined to one region of the small

intestine

It washes back and forth to make contact with the

mucosa

Peristalsis then moves the the chyme to the next part

of the small intestine where segmentation occurs again

Regulation of secretion in the small intestine

The intestine has a localized nervous system called the Enteric nervous system

*it responds to the presence of chyme and performs visceral reflexes

This division is influenced heavily by both sympathetic and parasympathetic branches of the ANS.

Absorption in the Small Intestine

To be absorbed

Carbohydrates must be converted onto

monosaccharides

Proteins must be converted to amino acids

Lipids must be converted to fatty acids

and monoglycerides

90% of absorption occurs in the small

intestine, 10% in the rest of the tract

Absorption

Vitamin Absorption

Fat soluble vitamins ADEK pass into

lacteals with triglycerides

Water soluble vitamins pass by diffusion

Vitamin B12 absorption requires intrinsic

factor from the parietal cells - no intrinsic factor = Pernicious anemia secondary to a lack of B12 absorption

Gastroileal reflex

Ileocecal valve opens

Chyme passes into the cecum via peristalsis

This initiated by the hormone gastrin

Fecal material moves from one haustra to the next

(haustral churning)

Haustral churning continues until the second half

of the transverve colon where mass peristalsis moves it into the retum

Fiber

Water Soluble Fiber -1. slows uptake of sugar

from the intestine thus preventing large spikes in blood sugar and the resulting insulin resistance 2. Serves as a food source for GI flora (FOS-fructo oligosaccharides) 3. Short chain fatty acids production as a results of bacterial mediated breakdown supplying the preferred energy supply for the cells lining the colon

Water Insoluble Fiber - 1. Bulking agent that improves bowel motility 2. Absorbs toxins in the GI tract improving excretion

In the Colon

Water is reabsorbed

Bacteria break down whats left of the chyme

E coli; synthesis of vitamins B1, B2, B6, B5, B12,

folate, Biotin, Vitamin K

Deranged flora no longer produce these vitamins.

Instead they scavenge vitamins polyamine formation, scatole, indole

Fungal infections as an indicator

Lactobacillus acidophilus/bacterium bifidum/coli

culture preps

MALT

Physiology of Defecation

Mass peristalsis of fecal material from

sigmoid colon into the rectum

Rectal wall stretching initiates defecation

reflex

Parasympathetic nervous system (sacral

portion)

Diarrhea

Constipation