THE DIGESTIVE SYSTEM

Lecture Notes

I. Digestive system functions
A. Ingestion

B. Movement of food through GI tract (propulsive movements)

1. Voluntary (skeletal muscles in oral cavity and pharnyx) =swallowing and defecation
2. Involuntary (smooth muscle tissue in esophagus to large intestine )= peristalsis, migrating motility complexes (intestinal peristalsis) , mass (large intestinal) peristalsisC. Digestion (breakdown of food that is too large to be absorbed into blood or lymph) 1. Mechanical digestive movements
chewing
churning in stomach
segmentation in small intestine
haustral churning in large intestine

2. Chemical digestion - mostly enzyme-mediated hydrolysis of food molecules. Enzymes secreted by organs of gastrointestinal (GI) tract and accessory glands

D. Absorption - transport of digested end products and water from GI tract to blood or lymph

E. Defecation - elimination of indigestible substances

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II. Anatomy of digestive system

A. General

1. Gastrointestinal organs form tube (GI tract) and accessory organs secreting into tube.

2. Most organs in abdominopelvic cavity

3. Most organs lined by peritoneum (serous membrane) to reduce friction 4. Blood supply

a. Arterial from abdominal Aorta

b. Venous = hepatic portal vein - takes nutrient-rich (sugars and amino acids) blood to liver from stomach/spleen/intestines

5. Lymph vessels/nodules
lymph vessels common in GI tract (lacteals), Peyer's patches lymphatic nodules common to remove infectious microbesB. Histology of GI tract (general) 1. Mouth and most of  esophagus
Stratified unkeratinized squamous epithelial tissue lines the lumen so resistant to physical damage due to dry hard food. Skeletal muscle around oral cavity provides conscious swallowing movements. Smooth muscle provide propulsive force to move food down esophagus.

2. Stomach though large intestine.

a. Mucosa 1) Simple columnar epithelium with goblet cells

2) Functions
secretion of mucus, enzymes & hormones
absorption of digested end products

b. Submucosa 1) Fibrous connective tissue with lots of blood/lymph vessels & nerves

2) Functions
connects mucosa to next layer
blood & nerve supply
 
 

c. Muscularis 1)Smooth muscle layers (2 or 3)

2) Function
move of food down tract or propulsive movements
provide mechanical digestive movements
inner circular layer forms sphincters (valves) that prevent back flow andcontrol forward rate of food travel

d. Serosa 1) Serous membrane (epithelium over connective tissue)

2) Function
reduce friction during movements.

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Diagram the wall of a generic gastrointestinal organ. Identify tissue making up each layer. Provide functions for each layer. C. Structure/function of digestive system organs 1. Oral cavity a. teeth, mandible, maxilla, muscles, lips
mastication and voluntary swallowing with skeletal muscles

b. lining of oral cavity is non-keratinized stratified squamous epithelium to resist abrasion damage.

c. Salivary glands (accessory glands) produce saliva which is mostly H2O to soften food to allow easier swallowing
(salivary amylase enzyme)
 

2. Pharynx - voluntary and involuntary swallowing of food

3. Esophagus
non-keratinized stratified squamous epithelium to resist abrasion damage plus mucus glands to soften food
wave-like peristalic contractions move food past lower esophageal sphincter into stomach.
lower esophageal sphincter prevents gastric acid from exiting stomach back up into esophagus

4. Stomach

a. Gross anatomy
expansion of GI tract with three muscular layers
mucosal folded (rugae) for expanding volume of stomach
pyloric sphincter controls gastric emptying

b. Histology of stomach

1) Columnar epithelium with very many goblet cells (really a sheet of mucus cells) in mucosa layer makes mucus to protect mucosal cells from gastric acid. Gastric glands or pits extend through mucosal layer.
Gastric gland cells
a) Zymogenic (chief) cells produce pepsin (from inactive pepsinogen)

b) Parietal cells secrete HCl
activates pepsinogen, kills bacteria (low pH) and deactivates amylases

c) Enteroendocrine cells (G cells) secrete hormone, gastrin

2) smooth muscle in muscularis layer has three sublayers that can not be distinguished (=muscularis externa)
Funtions to churn (physcially digest) food and move food (peristalsis) toward small intestine.

 

5. Small intestine
a. Gross anatomy
pyloric sphincter regulates emptying of acidic chyme (partially digested food) produced by stomach
folds of the mucosa and submucosa (circular folds) increase mixing action
duodenum - recieve bile salts from liver and pancreatic secretions (enzymes and bicarbonate)
jejunum
ileum -- ileocecal valve regulates emptying of small instestine into larger intestine

b. Histology

1)Columnar epithelium with few, but distinct, goblet cells in mucosa layer makes alkaline mucus to protect mucosal cells from digestion and acidic stomach chyme.intestinal glands (crypts of Lieberkühn) extend through mucosa. mucosal absorptive epithelial cells have membrane extensions (microvilli or brush border) the have embedded digestive enzymes. (brush border disaccaridases, brush border peptidases, brush border nucleotidases)

finger-like projections of mucosal layer (villi) increase surface area for absorption of nutrients (monosaccarides, amino acids, water, monoglycerides, fatty acids, nucleotides, vitamins, minerals)

cells in intestinal glands secrete lysozymes (disease resistance), alkaline mucus (protect against gastric acid and provide water for hydrolysis).

Enteroendocrine cells produce hormones (S-cells, Secretin and CCK cells, Cholecystokinin)


2) submucosa has Peyer's patches - aggregate lymph nodules to resist disease
lacteals promote fat absorption.

3) Muscularis has two distinct layers
inner circular layer forms valves and sphincters and promotes mixing movements
outer longitudinal layer promotes longitudinal movements
 segmentation movements mix food, migrating motility complexes (a form of peristalsis) moves food

Compare and contrast the histology and gross anatomy of the small intestine and the stomach. Identify important cellular functions within those organs. 

6. Large intestine
a) gross anatomy
ileocecal valve regulates movement of material into large intestine.
mass peristalsis moves feces towards anal canal.
haustra sacs fill and contract (churning) to promote water reabsorption
Anal sphincters allow defecation.

b) histology
1) Columnar epithelium with many goblet cells in mucosa layer makes mucus to package feces, and promote absorption of remaining water.
bacteria living here produce vitamin K & some B vitamins 2) smooth muscle layers promote mass peristalsis to move feces as well as provide haustral churning.
Skeletal muscle provides voluntary control over defecation.

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7. Liver (accessory gland)a. Gross anatomy
receives blood from hepatic artery, carrying oxygenated blood, and hepatic portal vein, carrying nutrients recently absorbed from small intestines.

b. Histology
blood flows through enlarged capillaries (sinusoids) made up of hepatocytes which:
1) convert absorbed carbohydrate nutrients (glucose to glycogen and back);
2) use absorbed amino acids to produce plasma proteins;
3) detoxify other chemicals absorbed by digestive system;
4) phagocytize pathogens from spleen, stomach and small intestine;
5) store absorbed vitamins and minerals
6) produce bile

b. Bile composition function
bile is mostly cholesterol derivatives used to emulsify fats (separate large fat globules into small ones).

8. Gall bladder -(accessory gland)
stores stores bile and releases bile.
Hepatic duct transports bile to gall bladder from liver, and cystic duct transprots bile to common bile duct and then to small intestine.

9. Pancreas -(accessory gland)
Duct carries pancreatic secretions to duodenum of small intestine

acini cells in pancreas produce:
a) digestive enzymes for all types of nutrients, including
lipase
amylase
ribonuclease, deoxyribonuclease
Trypsin, chymotrypsin, carboxypeptidase

b) bicarbonate to neutralize gastric acid in small intestine.

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Give the functions of all digestive organs.

III . Mechanical digestion (D) and propulsive (P) movements
A. Voluntary movements like Chewing (D), swallowing (P) and defecation (P) are under conscious control of CNS.

B. Involuntary movements (peristalsis (P), stomach churning (D) , migrating motility complexes (intestinal peristalsis) (P), segmentation (D) , mass peristalis (P), haustral churning (D) are controlled by ANS and enteric plexuses.

Name and describe the digestive and propulsive movements in all organs, from mouth to large intestine.


IV. Hormonal regulation of gastric secretions and mobility

A. Cephalic stage
Increased parasympathetic stimulations via vagus nerve stimulate production of:
pepsinogen (pepsin), HCl, mucus and hormone gastrin

B. Gastric stage
1 Nervous negative feedback loop-stimulus of distension of stretch receptors and rising pH (both resulting from addition of food- proteins buffer or reduce acidity) triggers parasympathetic and enteric neural stimulation of muscularis and parietal cells, respectively. Muscularis increases gastric mobility and parietal cells secrete more acid and greater gastric mixing/emptying. Response is decreased gastric pH (back to normal acidic levels) and less food in stomach.

2 Hormonal negative feedback loop. stimulus of distension of stretch receptors and increase of pH (both resulting from addition of food, particularly proteins) triggers parasympathetic neural stimulation (neurotransmiter ACh) of G cells to produce more gastrin. Gastrins promote acid production from parietal cells, increased gastric mixing by muscularis. Gastrin also relaxes pyloric and ileocecal sphincters to promote passage of chyme out of stomach and small intestine, respectively.

C. Intestinal stage
1 Nervous negative feedback loop-stimulus of distension of duodenal stretch receptors (resulting from addition of food) inhibits neural stimulation of gastric muscularis to reduce gastric mobility and emptying (enterogastric reflex). Response is increased less food in intestine (less acid to damage less protected intestinal mucosa).

2 Hormonal negative feedback loop. stimulus of distension of duodenal stretch receptors as well as partially digested proteins and fatty acids in duodenum promotes S cells to produce more secretin and CCK cells to produce CCK. Secretin targets parietal cells in stomach mucosa to reduce gastric acid and CCK targets stomach muscularis to reduce gastric mixing. Response is less acidic chyme entering intestine.

Secretin and CCK have further roles in digestive system.

V. Summary of digestive hormones

A. Gastrin
Stimulated by rising ph (less acid), produced by G cells of stomach, targets parietal cells to increase acidity and muscularis to promote gastric mixing and emptying.

B. Secretin.

Stimulated by distended duodenum and injection of acidic chyme into duodenum, produced by S cells of intestine. targets parietal cells to inhibit gastric secretion of acid. Increased secretin also increases release of more
bicarbonate in pancreatic juice making intestine more alkaline. Increased secretin also contributes to increased bile production. Response is less acidic chyme into duodenum.

C. CCK = cholecystokinin

Stimulated by increased fatty chyme in the intestine, produced by CCK cells of intestine, targets gallbladder to promote release of bile into duodenum via common bile duct. Increased CCK also increases release of more pancreatic enzymes (particularly lipases). CCK also targets pyloric sphincter smooth muscle to decrease gastric emptying. Response is less fatty chyme into duodenum. Increased fat digestion and absorption occurs as a result (maximum caloric intake).

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Describe the specific effects of Gastrin, Secretin and CCK on digestive secretions and motility.

VI. Chemical digestion and absorption
chemical digestion is result of series of enzyme mediated hydrolytic reactions where larger food molecules (substrates) are broken into smaller (product) molecules. Those products then become the substrates for next enzyme in the series and so.
Substrate--(Enzyme)--->Intermediate product also, intermediate sustrate---(Enzyme)---->End or final or absorbable product.
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A. Carbohydrates
1) digestion of starch and glycogen----(salivary amylase) (pancreatic amylase)--->di-and tri-saccharides like maltose, sucrose and lactose---(intestinal brush border disaccharidases like maltase, sucrase, lactase)---->monosaccharides like glucose, fructose, galactose

2) monosaccharides are absorbed by active transport into blood capillaries
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B. Proteins
1) digestion of proteins and polypeptices--(gastric pepsin) (pancreatic proteinases like trypsin, chymotrypsin)--->medium sized peptides as well as di- and tri-peptides----(carboxypeptidase, brush border dipeptidases) ---> amino acids

2) amino acids actively transported into blood capillaries

C. Lipids -
big gobs of neutral fats (triglycerides) are emulsified by bile salts into small droplets of fats.

1) Triglycerides---- (pancreatic lipases) ----> fatty acids & monoglycerides

2) fatty acids & monoglycerides diffuse into more permeable lacteals

D. Nucleic acids
1) digestion of nucleic acids-----  (pancreatic nucleases) ---> nucleotides

2) phosphate ions, sugars and nitrogen bases are actively transported to blood capillaries

E. Water - lots secreted in small intestine
Most passively absorbed via osmosis into blood capillaries

Describe the enzymatic pathways that result in endproducts for Carbohydrates, proteins, lipids and nucleic acids.

Professor Thomas M. Lancraft

Human Anatomy and Physiology Courses 
at St. Petersburg College
St. Petersburg/Gibbs Campus

5/2008