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GI tract

- aka alimentary tube
- Muscular tube that extends from mouth to anus
• Mouth, pharynx, esophagus, stomach, small and large intestines, rectum, anus


Extent to which digestive tract absorbs nutrients and how well body uses them

accessory organs

They secrete digestive chemicals
• Salivary glands, liver, gall bladder, pancreas

human digestive system

• A flexible, muscular tube that digests food and
absorbs its nutrients and some nonnutrients.
- A complete system with many specialized organs - About 6.5 to 9 meters long if extended
• 26 feet long!!
• Lined with mucus-secreting epithelium
• Movement is one way, from mouth to anus
• Digestion is virtually continuous
- Sleep and exercise
• Within24to48hours
- 90% of carbs, fat, and protein are digested and absorbed

4 stages of food processing

1. •Ingestion = Is another word for eating
2. •Digestion = Is the breakdown of food to small molecules
3. •Absorption = Is the uptake of the small nutrient molecules by the body's cells
•Elimination= Is the disposal of undigested materials from the food we eat


Digestion begins in the mouth.
• Mechanical digestion - Biting and grinding actions break and mash food into smaller pieces.
• Chemical digestion
- Saliva mixes and lubricates food.
- Salivary amylase and lipase begin breakdown of starch and fat, respectively.

taste buds & smell

Taste Buds
- Specialized sensory structures on the tongue and in the lining of mouth and throat
Five Tastes: Sweet, sour, salty, bitter, and umami
- Chemicals in food become airborne and stimulate nasal passages, contributing to sense of taste

four (5) basic chemical tastes

Four (5) basic chemical tastes
- Sweet, sour, bitter, and salty
- Savory or umami

mouth indigestion

Three major pairs of salivary glands.
• Saliva contains salivary amylase. - Begins starch digestion.
• Tongue is composed of striated muscle. -Mixes chewed food with saliva.
»Forms mixture into bolus

Pharynx and Esophagus

• Digestive and respiratory passages come together in the pharynx.
• Esophagus takes food to stomach by ...
- Peristalsis - Rhythmical contraction to move contents in tubular organs.


- Is a muscular tube
- Connects the pharynx to the stomach
- Moves food down by peristalsis
• Wave-like contraction and relaxation of muscle


flap that folds down over trachea (windpipe) when you swallow


Muscular contractions that move small amounts of
food and beverages through intestinal tract

Stomach: Gastroesophageal sphincter

- Located at the end of the esophagus; controls opening and closing of stomach.

Stomach: chyme

- Mixture of gastric juice and partially digested food

Stomach: Mucus

- Lubricates and protects stomach walls

Stomach: Pyloric sphincter

- Controls rate of chyme release into small intestine

Stomach: mechanical digestion

Mechanical Digestion
- Begins in the mouth with chewing.
• Saliva moistens food for easier swallowing.
Continues in Stomach and intestines
- liquefy foods by mashing and squeezing.
• Peristalsis - wavelike muscular squeezing of the digestive tract that pushes their contents along.


- Can store food for several hours
- Churns food
- Mixes food with gastric juices, which are acidic

stomach: gastric juice

Gastric Juice is secretion of -> HCl
-Stomach has a pH of 2
•Enzyme for chemical digestion of food (remember hydrolysis)
-Example: Pepsin is a hydrolytic enzyme that acts on protein to produce peptides.

stomach: gastric glands

-Another section of gland secretes mucus
• Protects the wall of the stomach from digestive enzymes
• Ulcer is open sore in stomach wall from enzymatic digestion
- Usually associated with a bacterial infection - Heliobacter pylori

mechanical aspect of digestion

-The stomach's pyloric valve at its lower end controls the exit of the chyme
- Liquefied, partially digested food
- Mixed with saliva and gastric juices
-Chyme is squirted into small intestine after a few hours in the stomach.
-Small intestine contracts to move contents to large intestine (colon).

small intestine

Where most nutrients are digested & absorbed
• Duodenum • Jejunum • Ileum
- Is the largest part of the alimentary canal
- Function = Is the major organ for chemical digestion and absorption
• Chyme from stomach enters the duodenum
- First part of the small intestine
- Further chemical digestion of food
- Liver and pancreas send their secretions to duodenum.
-tennis court surface area

mixing chyme

• A thick mixture of food and gastric fluid
• High acidity kills many pathogens
• Mixed and moved by waves of stomach contractions (peristalsis)
• Further mixed in small intestine with back and forth movement (segmentation)


Ringlike muscular contractions followed by relaxation of small region of small intestine

Accessory organs: Liver

- processes and stores many nutrients
• Makes cholesterol and bile
-Liver produces bile which is stored in GB.
• Bile contains bile salts which break up fat into fat droplets via emulsification
- acts like a detergent to allow fats/oils to disperse in water.

Accessory organs: gallbladder

stores bile that aids in fat digestion

Accessory organs: pancreas digestion

-produces and secretes many digestive enzymes and bicarbonate ions
-Pancreas produces pancreatic juice and digestive enzymes.
• Examples:
-Pancreatic amylase digests starch to maltose.
- Trypsin digests protein to peptides.
-Lipase digests fat droplets to glycerol and fatty acids.
• Also secretes sodium bicarbonate
-Neutralizes stomach acid
-pH of small intestine slightly basic

Accessory organs: pancreas hormones

- Excretes juice that
• Neutralizes stomach acids
• Contains digestive enzymes
- This is an exocrine function - excretion to an opening through a duct
-Also secretes hormones into the blood that regulate sugar uptake and release
• Insulin and glucagon
-Endocrine function - secretion to the blood stream

small intestine: duodenum

- Is the first part of the small intestine
- Receives digestive agents from several accessory organs and acid chyme from the stomach
- Continues the chemical digestion of food

small intestine: jejunum & ileum

- Are the next sections of the small intestine
- Are specialized for absorption

absorption by villi

Mucus membrane layer of small intestine has ridges and furrows that give it a corrugated surface.Tiny projections that line the small intestine
-Villi are ridges on the surface, which contain even smaller ridges, microvilli.
• Greatly increase absorptive area.
• Each villus contains blood capillaries and a lymphatic capillary (lacteal).

absorptive cells

Remove nutrients from chyme and transfer them into intestinal blood or lymph

absorption of nutrients

• Passage of molecules into internal environment
Absorption of Nutrients
• Occurs mainly in jejunum and ileum of small intestine
• Segmentation mixes the lumen contents against wall and enhances absorption
- although food has been ingested It is not technically "in" the body yet
• Body and alimentary canal can be though of as a tube, or torus
- It must be absorbed into the body through the circulatory system

absorption of nutrients: Carbs

- Begin in the mouth
- Acids of stomach deactivate salivary amylase
- Pancreatic enzymes continue breakdown of carbs (in sm. intestine)
- Enzymes on VILLI cells finish digestion to simple sugars which are absorbed

absorption of nutrients: proteins

- stomach acids unravel proteins
- Gastric juice protein digestion begin protein digestion (in stomach but stop working in sm. intest)
- Pancreatic enzymes continue breakdown into smaller fragments
- Enzymes on villi cells finish digestion to amino acids which are absorbed

absorption of nutrients: fat

- Most fat is first emulsified by bile (made in liver, stored in gall bladder) in the small intestine
• A small amount of fat is digested by salivary enzymes
- Pancreatic enzymes digest fats to their components
- These are mostly absorbed by the lymphatic vessels of villi, eventually draining into blood vessels
- Fats and lipids combine with bile salts
- enter epithelial cells by diffusion
- leave epithelial cells by exocytosis and enter internal environment through a lacteal of the lymphatic system

absorptions mechanisms: monosaccharides & AA

cross epithelial cells, then from cell into internal environment

absorptions mechanisms: glucose & AA

enter blood vessels directly

absorption by villi

-Sugars and amino acids
• enter the capillaries of a villus
-Fat components (Glycerol and fatty acids) • Enter the small intestine epithelial cells
-Become packaged as lipoproteins droplets
» Enter the lacteal, not directly into blood capillaries

lymphatic vessels

initially transport most of the products of fat digestion and a few fat soluble vitamins, later delivering them to the bloodstream.

blood vessels

carry the products of carbohydrate and protein digestion, most vitamins and the minerals from the digestive tract to the liver

large intestines (1/2)

Two major sections: -Colon and rectum
• Absorption of water and minerals occur
- Feces - form as chyme becomes semisolid (all nutrients have been absorbed out)
- Rectum - lower part of large intestine where feces are stored

large intestines description & consists of:

-Is shorter, but wider, than the small intestine
-Consist of the cecum, Colon (and appendix )
• Rectum

large intestine: colon

• Concentrates and stores feces
• Reabsorbs water, minerals & some vitamins
-Water in the digestive tract
» 1.5 L water from food and drink
» 8.5 L water from secretion of various digestive
» 95% absorbed by small intestine, most of
remainder by large intestine
• Stores ingestible material until elimination
- Sodium ions are actively transported out of lumen and water follows by osmosis
• Lining secretes
- Mucus - protection
- Bicarbonate - buffering

cystic fibrosis

- An inherited, incurable disease
• Thick sticky mucus blocks passageways
• Mucus blocks pancreatic ducts, interfering
with nutrient digestion, especially fat

inflammatory bowel disease

- Group of diseases that cause inflammation and swelling of the intestines
• Includes Crohn's disease & ulcerative colitis


-Produces feces, the waste product of food
• Remains from the small intestine after most water has been reabsorbed
-Indigestible food
-Half of bulk is normal gut bacteria

microbes in your digestive tract

The large intestine contains vast numbers of various types of bacteria, that can:
-metabolize undigested food material -make vitamin K and biotin
-produce gas
Bacteria eventually excreted in feces (50%).
• Slow movement of material through colon allows growth of bacteria
- Harmless - unless they escape into abdominal cavity

colon: absorption of nutrients

• Indigestible carbohydrates (dietary fiber) and unabsorbed fats are metabolized by colon bacteria
- Some of the products can be absorbed as nutrients
• small fat fragments released from the fiber provide a tiny bit of energy
- gas is also produced


• Last 20cm of large intestine
• Stores indigestible material, for elimination through..
-Opening to anal canal - Anus.


• Distension of the last part of the colon,the rectum,
triggers a reflex action
• Smooth muscle of anal sphincter relaxes
- Followed by defecation
- Feces= fiber, undigested material, sloughed intestinal cells & bacteria
• Voluntary contraction of external sphincter can prevent defecation


if appendix blocked, bacterial growth causes inflammation

colon cancer

- Symptoms include blood in feces
- Can be caused by a genetic defect
- Low-fiber diet is a predisposing factor
• Bulk waste spends more time in colon
-Cells exposed to natural and artificial carcinogens for a longer time

gut reaction: constipation

Difficult & infrequent bowel movements
- Lack of fiber in diet
- Inadequate water intake - Anxiety or depression
- Changes in daily routine - Ignoring need to defecate
• May lead to hemorrhoids or diverticulitis
• First line of treatment is increasing dietary fiber

gut reaction: diarrhea

Frequent, loose bowel movements
• Causes: generally bacterial or viral infections
• Treatment:
- OTC medication
- Replacing fluid, sodium, and potassium to prevent dehydration
- Immediate treatment of severe
diarrhea in infants or the elderly is crucial

gut reaction: vomiting

Contraction of abdominal muscles to expel toxic stomach and duodenal contents
•Causes: Ingestion of toxic substances,intense pain, head injury, motion sickness, pregnancy, or touching back of throat
• Treatment: Avoid solid food until resolved; sip water or non-carbonated beverages to prevent dehydration.
Contact physician if signs of dehydration are present and vomiting lasts longer than a few hours (children) or a day (adults).

gut reaction: heartburn

Gnawing pain or burning in upper chest from stomach acid entering esophagus
- Occasional heartburn affects about 50% of U.S. adults
• Causes: Pregnancy, smoking, excess body fat, alcohol, caffeine, citrus juice, onions, chocolate, mints, fatty foods, spices, and eating too much

gut reaction: peptic ulcer

Sore in lining of stomach or duodenum
• Symptoms: Deep, dull upper abdominal pain ~2 hrs after eating
- Often caused by Helicobacter pylori infection
• Treatment:
- Antibiotics to eliminate Helicobacter pylori
- Identification and avoidance of aggravating foods

gut reaction: irritable bowel syndrome

Intestinal cramps and abnormal bowel function (ranges from diarrhea to constipation)
- Affects ~10 to 30% of adults
• Unknown causes — linked to stress
• Treatment includes:
- Stress management
- Possible elimination of problematic foods, such as
dairy products, legumes, and certain vegetables & fruits.

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