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Digestive System consists of two parts:

1. Gastrointestinal tract – a tube from mouth to anus

2. Accessory organs of digestion – teeth, tongue, salivary glands, liver, gall bladder, pancreas

Functions of digestive system-

1. Ingestion – Taking in food and liquids

2. Secretion – Water, enzymes, acid or buffers secreted into the gastrointestinal tract

3. Mixing and propulsion – Food is churned and moved along the GIT by peristalsis.

4. Digestion – Food is mechanically and chemically broken into smaller particles

5. Absorption – Passage of digested substances into GIT wall and then into blood & lymph

6. Defecation – Elimination of undigested or unabsorbed waste in the feces.

Layers of the Gastro-intestinal tract –

Gastrointestinal tract consists of 4 layers (from inside to outside):

1. Mucosa – Mucous membrane lining made of epithelium, connective tissue & smooth muscle.

-It also contains Mucosa-associated lymphatic tissue (MALT) that protects GIT from pathogens.

2. Submucosa – Consists of connective tissue, blood vessels, lymphatic vessels & neurons.

-The network of neurons controls motility and secretions (from mucosal & submucosal glands).

3. Muscularis – It consists of skeletal muscle in the mouth, pharynx, esophagus & anal sphincter.

-It voluntarily controls swallowing and defecation.

Rest of the GIT has smooth muscle (inner – circular fibers; outer – longitudinal fibers)

-It involuntarily controls food breakdown, mixing and propulsion.

4. Serosa & Peritoneum– It consists of simple squamous epithelium and connective tissue.

-It is also called visceral peritoneum in the abdomen (secretes peritoneal fluid for lubrication).

-Peritoneal folds contain blood vessels, lymphatics and nerves.

Greater omentum - a fatty serosal layer - drapes transverse colon and small intestine.

-It stores fat and has lymph nodes that help fight pathogens.

A sickle-shaped falciform ligament attaches liver to the diaphragm & anterior abdominal wall.

Mesentry – a peritoneal flap – attaches small intestine to the posterior abdominal wall.

MOUTH –

Also called oral cavity, it consists of cheeks, hard & soft palate, tongue & lips.

Uvula - a fold hanging posteriorly from the soft palate – helps prevent food entering nasal cavity.

Palatine tonsils are situated posterior to fauces (mucosal folds between mouth and oropharynx).

TONGUE –

Composed of skeletal muscle covered by mucosa, it forms the floor of oral cavity.

It places food on the teeth for chewing and helps with swallowing and speech.

Lingual frenum is a mucosal fold on the inferior aspect of the tongue (tongue tie)

Lingual tonsils lie posteriorly at the base of the tongue. They help protect oral cavity from pathogens.

Papillae are projections on the tongue that contain taste buds.

SALIVARY GLANDS –

Three pairs of salivary glands are situated outside mouth.

1. Parotid glands are situated inferio-posterior to the pinna of the ears between skin and masseter.

2. Submandibular glands are situated below the posterior tongue

3. Sublingual glands are situated below the middle of tongue.

Mumps is the inflammation & enlargement of parotid glands.

-They produce saliva containing:
• Water (lubricates mouth for speech and helps taste food)
• Mucus (lubricates food for easy swallowing)
• Amylase (digests carbohydrates) and
• Lysozyme (destroys bacteria)

Salivation is regulated by parasympathetic nervous system (via VII and IX nerves)

Sympathetic stimulation (due to stress) & dehydration lead to dry mouth
TEETH –

They are located in the bony sockets of maxillae and mandible.

Gums (gingivae) cover these sockets.

Periodontal ligament (dense fibrous connective tissue) anchors tooth to the bony socket.

A tooth consists of 3 parts:

• Crown –portion of tooth exposed above the gums
• Root – embeds teeth in the socket
• Neck – where root meets crown (at the gum line)

Dentin is the calcified connective tissue that gives teeth its’ shape and hardness.

Enamel, made of hard calcium salts, covers dentin over the crown.

-It protects tooth from wear and acids.

Bony cementum covers dentin over the root attaching it to the periodontal ligament.

Pulp cavity under dentin is filled with pulp (contains blood vessels, lymphatics & nerves)

Root canals are extensions of pulp cavity in the root of the tooth.

-Blood vessels, lymphatics and nerves enter the root canal though it’s base.

Endodontics deals with alveolar bone, root, pulp and periodontal ligament problems.

Orthodontics deals with teeth alignment problems (braces).

Periodontics deals with problems of structures surrounding teeth (gums)

20 deciduous (milk) teeth appear at around 6 months and shed by 6-12 years of age.

32 permanent teeth appear when deciduous teeth are shed.

Types of teeth:

• Incisors – 8 midline teeth, used to cut into food.
• Canines (cuspids) – 4 pointed teeth next to incisors, used to tear & shred food
• Premolars – 8 teeth next to canines, used to crush & grind food
• Molars – 12 teeth at the back of the mouth, used to crush & grind food

Mastication - chewing of food.

Bolus - food softened by chewing and mixing with saliva, which is ready to be swallowed.

PHARYNX –

-Muscular tube covered by mucous membrane - extending from internal nares to larynx & esophagus.

Divided into nasopharynx, oropharynx & laryngopharynx.

Muscular contractions of oropharynx & laryngopharynx propel food into the esophagus.

ESOPHAGUS –

It is a 10” long muscular tube posterior to trachea through which food passes to the stomach.

Food is swallowed in three stages:

1. Voluntary stage – tongue pushes bolus back into oropharynx

2. Pharyngeal stage – food passes through pharynx into esophagus (breathing stops as soft palate
moves up to close nasopharynx & epiglottis closes larynx).

3. Esophageal stage – food is pushed through esophagus to the stomach by peristalsis

Cardiac sphincter at the lower end of esophagus prevents regurgitation of food.

Gastro-esophageal reflux disease (GERD) - food regurgitates into esophagus causing heartburn.

STOMACH –

This J-shaped muscular sac connects esophagus to the duodenum.

It is divided into 4 parts (from superior to inferior):
• Cardia
• Fundus
• Body
• Pylorus

Pyloric sphincter (pyloric valve) prevents food from escaping into duodenum during churning.

Rugae are large folds in stomach mucosa when stomach is empty.

Stomach mucosa is made of non-ciliated simple columnar epithelial cells (mucous surface cells)

Epithelial cells go deep inside mucosal gastric pits to form gastric glands.

-Gastric glands contain 3 types of exocrine gland cells:

• Mucous neck cells – secrete mucus
• Chief cells – secrete inactive enzyme pepsinogen
• Parietal cells – secrete intrinsic factor & HCl (activates pepsinogen to pepsin)

-Gastric glands also contain endocrine cells (G cell) - produce hormone gastrin.

Gastric juice is the mixture of mucus, HCl and pepsinogen.

Gastrin stimulates gastric juice secretion, increase GIT motility & relaxes pyloric sphincter.

Intrinsic factor facilitates vitamin B12 absorption (preventing pernicious anemia)

Stomach has three smooth muscle layers:
• Oblique inner layer
• Circular middle layer
• Longitudinal outer layer

Stomach digestion & Absorption –

Peristaltic waves occur every 20 seconds when the stomach is full.

-They mix food and gastric juice in the stomach and change it to thin liquid called chyme.

Stomach pepsin activated by stomach acid (pH 2.0) breaks down proteins into peptides.

Mucus, secreted by mucous cells, prevents digestion of proteins in the stomach lining.

Epithelial cells in the stomach are impermeable to most substances, except:
• Water
• Ions
• Short-chain fatty acids
• Drugs (aspirin)
• Alcohol

Gastric secretion & Motility regulation –

Stomach is emptied in about 2-6 hours (Carbohydrates empty faster than proteins and fats)

Gastric secretion increases in response to:
• Parasympathetic stimulation conducted along the Vagus nerve
• Thought, sight, taste or smell of food (Pavlov’s reflex)
• Stomach stretching (due to food)
• Partially digested proteins
• Gastrin secretion
• Caffeine etc.

Gastric secretion decreases in response to:
• Sympathetic stimulation
• Fear, anxiety, anger

When chyme reaches small intestine, it releases hormones secretin and cholecystokinin.
-Secretin inhibits gastric secretion (stimulating bile and pancreatic juice secretion).

-Cholecystokinin inhibits gastric emptying (stimulating pancreatic juice secretion & bile ejection).

Vomiting is forced emptying of stomach and duodenal contents through mouth.

-It is controlled by the chemoreceptor- trigger-zone in the medulla oblongata.

-It may lead to dehydration and electrolyte & acid-base imbalance.

PANCREAS –

It is situated posterio-inferior to stomach.

Pancreatic juice reaches duodenum via the pancreatic duct (unites with the common bile duct).

99% of pancreatic cells are arranged in clusters called acini (secrete pancreatic juice).

-Islets of Langerhans, the other 1% of pancreatic cells, secrete hormones - insulin, glucagon,
somatostatin and pancreatic polypeptide.

Pancreatic juice (pH 7.1 – 8.2) consists of water, sodium bicarbonate, salts and enzymes

-It contains the enzymes:
• Pancreatic amylase – digests carbohydrates
• Pancreatic lipase – digests fats
• Trypsin, chymotrypsin, carboxypeptidase & elastase – digest proteins
• Ribonuclease & deoxyribonuclease – digest RNA & DNA

Enterokinase activates inactive enzyme trypsinogen to trypsin in the small intestine.

-This prevents auto-digestion of pancreas by trypsin.

Pancreas secretion is increased by parasympathetic stimulation, secretin & cholecystokinin.

LIVER & GALL BLADDER –

Situated in the RUQ below diaphragm, liver weighs about 3 pounds.

It is covered by a capsule and peritoneum.

It is divided by falciform ligament into 2 main lobes (right and left)

Liver consists of functional units called lobules.

In a lobule, hepatocytes (liver cells) are arranged radially around a central vein.

Hepatocytes produce bile (made of H2O, bile salts, bile pigments, cholesterol, lecithin & ions)

Sinusoids, large endothelium-lined spaces between hepatocytes, transport blood in the liver.

Kupffer’s cells in the sinusoids (star-shaped RE cells) destroy pathogens & worn out blood cells.

Gall bladder is located anterio-inferior to the liver. It stores bile made by the liver.

Right & Left hepatic ducts drain bile from the right & left lobes of liver

-Hepatic ducts join to form the common hepatic duct

-Common hepatic duct and cystic duct (from the gall bladder) join to form the common bile duct.

-Common bile duct and pancreatic duct join and drain bile into the duodenum.

Blood supply to the liver -

Hepatic artery supplies oxygenated blood to the liver.

Hepatic portal vein supplies nutrients absorbed in the small intestine.

-Blood then flows into liver sinusoids where:
• Hepatocytes take up oxygen, nutrients and toxins and remove wastes (CO2).
• Kupffer’s cells remove dead matter and pathogens.

Sinusoids drain blood into central vein ? hepatic vein ? inferior vena cava ? R. atrium

Bile –

Bile salts emulsify (breakdown large fat globules into smaller fat droplets) and help fat absorption.

-Emulsification facilitates digestion of fat by lipase.

Bile excretes bilirubin (bile pigment obtained from the breakdown of hemoglobin) in urine & stool.

Parasympathetic stimulation, secretin & cholecystokinin increase bile flow to the duodenum.

Gallstones in biliary tree may occlude bile flow.

-This is corrected by lithotripsy or cholecystectomy.

Functions of liver –

1. Carbohydrate metabolism – Stores glycogen.
- Converts glycogen, fructose, aminoacids & lactic acid into glucose

2. Lipid metabolism – Stores triglycerides and synthesizes lipoproteins, cholesterol & bile salts.

3. Protein metabolism – Synthesizes plasma proteins (albumin, globulin, prothrombin, fibrinogen)
- Converts amino acids to energy, glucose or fats.
4. Detoxification – Detoxifies drugs, alcohol & hormones (penicillin, sulfa, T3 and T4, estrogens)

5. Bilirubin excretion – Excretes bilirubin (a waste product of RBC destruction) in bile

6. Stores vitamins and minerals – Stores vitamins A, B12, D, E, K and minerals copper and iron.
- It also activates vitamin D (along with skin and kidneys)

7. Protection – Kupffer’s cells phagocytose bacteria, dead and foreign matter

SMALL INTESTINE –

This primary organ of digestion & absorption is 1” in diameter and about 10 ft long.

It extends from the pyloric sphincter to the ileocecal sphincter.

Food takes 3-5 hours to travel through the small intestine.

Duodenum is the C-shaped first part (10” long), followed by jejunum (3 ft long).

-Most absorption occurs in these two.

Ileum follows jejunum and is about 6 ft long.

Absorption and digestion in the small intestine is enhanced by:

• Circular folds - permanent ridges in mucosa that cause chyme to spiral.

• Villi - finger-like folds in mucosa that increase surface area (20 - 40 / mm¬2)
[contain connective tissue, arteriole, venule, capillaries and lacteal (lymphatic capillary)]

• Microvilli - microscopic projections on absorptive cells (200 million / mm2)

Intestinal epithelial cells consist of absorptive cells, goblet cells, hormone cells and Paneth cells.

Mucosal epithelium lining intestinal glands secretes intestinal juice (pH 7.6).

Paneth cells phagocytose and secrete lyzosyme (kills bacteria).

Hormone-producing cells produce secretin, cholecystokinin & GIP (stimulates insulin release)

Mucosa-associated lymphatic tissue (MALT) protects intestine from pathogens.

-It is assisted by solitary lymphatic nodules and Peyer’s patches (lymphatic follicles) in the ileum.

Duodenal glands produce alkaline mucus that neutralizes stomach acid.

Small intestine has a thin, outer longitudinal and thick, inner circular smooth muscle layers.

Digestion in the small intestine occurs by mechanical and chemical means.

Two types of movements aid mechanical digestion:
• Segmentations - localized contractions that mix food with chyme and aid absorption
• Peristalsis – Also called migratory motility complex (MMC) that propels food in intestine

Chyme (containing partially digested carbohydrates & proteins) enters small intestines from stomach.

Pancreatic juice, intestinal juice and bile chemically digest this food in the small intestine.

Carbohydates -

-Pancreatic amylase digests glycogen, starch & dextrin ? disaccharides (cellulose is not digested)

• Maltase - digests disaccharide maltose ? 2 molecules of glucose.
• Sucrase - digests disaccharide sucrose ? glucose and fructose.
• Lactase - digests disaccharide lactose ? glucose & galactose.

Proteins -

Pancreatic juice enzymes (trypsin, chymotrypsin, elastase and carboxypeptidase) digest proteins.

-Peptidases further digest protein into amino acids, dipeptides and tripeptides.

Lipids –

Bile salts first emulsify triglyceride globules into small droplets called micelles.

Pancreatic lipase then converts triglyceride ? monoglyceride and 2 fatty acid molecules.

Nucleic Acids –

Ribonuclease (pancreatic and intestinal juice) breaks RNA ? pentose and nitrogenous base.

Deoxyribonuclease (pancreatic and intestinal juice) breaks DNA ? pentose and nitrogenous base.

Regulation of intestinal secretions & motility occurs via enteric nervous system in response to chyme.

-Parasympatheic stimulation increases motility and secretions.

-Sympathetic stimulation decreases motility and secretions.

ABSORPTION of the following occurs in the epithelial cells of the small intestine:

• Carbohydrates – monosaccharides (glucose, galactose, fructose)
• Proteins - amino acids, dipeptides & tripeptides
• Fats – fatty acids & monoglycerides

About 90% absorption occurs in the small intestine. 10% occurs in the stomach & large intestine.
Carbohydrates and proteins reach liver via hepatic portal vein.

Lipids reach the liver as follows:

1. Short chain fatty acids (10-12 carbon) reach liver via hepatic portal vein.

2. Long chain fatty acids and monoglycerides are emulsified into micelles by bile salts.

-Micelles are absorbed into epithelial cells.

-Monoglycerides are broken by lipase into glycerol and fatty acids.

- Glycerol and fatty acids recombine to form triglycerides.

- Proteins coat triglycerides, phospholipids and cholesterol to form chylomicrons.

-Chylomicrons are large and can only enter lacteals (not capillaries)

- They reach heart via lymphatic vessels, thoracic duct and left subclavian vein.

- Liver and adipose tissue remove chylomicrons from the blood.

Water – About 9.3 liters of water is ingested and secreted in the GIT.

8.3 liters is absorbed in small intestine, 0.9 liters in large intestine and 100 ml is excreted in feces.

Electrolytes – They come from GIT secretions, food and water.

Na+, Cl-, I-, NO3¬- , Ca++, Fe++, K+, Mg++, PO4--- are reabsorbed in the intestines.

Vitamins -

Fat-soluble vitamins (A, D, E, K) are absorbed along with the fats.

Water-soluble vitamins (B, C) are absorbed by diffusion.

Vitamin B12 combines with intrinsic factor in the stomach and is absorbed in the intestine.

LARGE INTESTINE –

It extends from ileocecal sphincter to the anal sphincter. It is 2.5” in diameter and 5 ft long.

It functions to:

• Complete absorption
• Production of vitamins
• Formation and expulsion of feces

It is divided into cecum, colon, rectum and anal canal.
Cecum is the wide sac-like first part of the colon where appendix attaches.

-It continues upwards as colon.

Colon is divided into 4 parts – Ascending, transverse, descending and sigmoid colon.

Colon continues as rectum and anal canal (1’ long) and opens to the exterior as anus.

Anus has internal (involuntary) sphincter and external (voluntary) sphincter.

Walls of large intestine have simple columnar epithelium with:
• Absence of circular folds or villi
• Mostly absorptive cells (absorbs water) and goblet cells (produces mucus)

Its outer longitudinal muscle layer is bundled into 3 bands called tenia coli.

Contraction of tenia coli gathers colon into pouches called haustra.

Hemorrhoids are varicosities of rectal veins caused by pregnancy or constipation (1?, 2?, 3?)

Digestion & Absorption -

Movement of chyme in the large intestine occurs as follows:

1. Haustral churning - Haustra fills with chyme and then contracts to push chyme into next haustrum.

2. Peristalsis – Chyme passes through the ileocecal sphincter slowly except following meals.

3. Mass peristalsis – Food in stomach causes strong peristalsis, pushing colon contents into rectum.

Colon secretes mucus, but no enzymes.

Colon bacteria break down:

1. Carbohydrates into H2, CO2 and methane gas (causing flatulence)
2. Proteins into amino acids and then indole & skatole (detoxified in liver & excreted in urine)
3. Bilirubin into stercobilin and urobilinogen.

-Colon bacteria also produce vitamin B and K.

Peritonitis is peritoneal inflammation caused by leakage of bacteria into peritoneum (stab)

Colon absorbs:
• 800 ml of water daily (and excretes 100 ml of water in feces).
• Electrolytes (Na+ and Cl-)
• Vitamins

Feces contain water, salts, dead epithelial cells, indigestible food and unabsorbed food.

Defecation reflex is initiated by the stretching of rectum wall by feces.

-Internal anal sphincter opens due to increased rectal pressure and parasympathetic stimulation.

-Defecation is assisted by voluntary contraction of diaphragm and abdominal muscles.

Diarrhea is caused by:
• Increased intestinal motility (due to stress, infection)
• Decreased water absorption in the colon.

Constipation is caused by:
• Decreased intestinal motility (due to low fiber diet, lack of exercise, stress, drugs)
• Increased water absorption in the colon.

Regulation of food intake –

Primarily carried out by following centers in the hypothalamus:

• Feeding center – responsible for hunger sensations
• Satiety center – responsible for feeling of fullness and satisfaction.

Food regulation is also controlled by:

• Cerebral cortex (psychological factors)
• Brain stem
• Limbic system (fear, anxiety, rage)
• Stretching of stomach & duodenum (activates satiety center, inhibits feeding center)

Appetite is suppressed by the release of following hormones:

• Glucagon
• Cholecystokinin
• Epinephrine
• Leptin

Appetite is stimulated by the release of following hormones:

• Growth Hormone Releasing hormone (GHRH) – from hypothalamus
• Insulin
• Glucocorticoids
• Epinephrine
• Progesterone

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