Digestive System

Digestive System

What is Digestion?? Processes that changes food into simpler

forms that can be absorbed thru cell membrane (mechanical means and chemical)

What are these processes?1. Ingestion2. Peristalsis3. Digestion4. Absorption5. Defecation

Two Categories of Organs

Alimentary canal Mouth Pharynx Esophagus Stomach Small Large intestines

Accessory Organs Salivary glands Liver Gallbladder Pancreas

Alimentary Canal

9 meters long!! Large intestines: 1.5 m (~5 feet) Small intestines: 6 m (~20 feet)

Originates from the endoderm of the embryo

FXN: moves food. HOW?

1. Mixing movements Smooth muscle contract rhythmically (every 20

sec or so) in the circular muscle layer Food + digestive juices + mucus

2. peristalsis Wavelike movements propelling movements A ring of contractions begin when food expands the

tube in the longitudinal layer

Where does digestion begin?

Mastication Mechanical digestion mixing with

saliva

Teeth Incisors: 8

bicuspids (premolars): 8

Cuspids (canines): 4

Molars: 12

Pharynx

Passageway of food into esophagus and air into larynx/trachea

swallowing mechanism (deglutition) InvoluntaryMade of muscles called constrictor muscles

(pull walls inward during swallowing)

Swallowing Steps1. Chew food and mix with saliva into a

bolus: voluntary2. Food reaches the pharynx and stimulates

sensory receptors: involuntary reflex3. Soft palate rises inhibits food from

entering…4. Epiglottis closes over larynx (no breathing)5. Muscles in lower pharynx relax6. The constrictor muscles contract and

stimulates peristaltic waves7. Esophagus opens

Esophagus

Esophagus

Straight tube: 25 cm long Penetrates through an opening: esophageal

hiatus (continuous with stomach) Passageway for food from pharynx to

stomach Contains many mucus glands Movement of food:

Gravity Peristaltic waves meet the esophageal

sphincter which regulates food into the stomach

Barrette’s esophagus the abnormal growth of intestinal-type cells

from the stomach border, into the esophagus.

How do we know physiology of the Stomach?

1822: Alexis Martin Shot himself in the stomach Left a fistula Army surgeon studied 8 years

1984: Barry Marshall from Australia Hypothesized that bacteria caused gastritis and ulcers Drank swamp water Heliobacter pylori

IN conclusion: Discoveries are made through observations and experimentation of bizarre phenomenon

Stomach characteristics J shaped, 25-30 cm long Under diaphragm on left side Hold up to 1 liter of contents Internal Characteristics:

Rugae: mucosal folds What is the benefit of these folds?

Gastric villi aid in secretion and absorption of gastric juices

Parts of Stomach Cardiac region

Around esophagus Fundic

Large ballooned area Pyloric

Near duodenum Contains pyloric sphincter muscle

Contents of Gastric Juices Mucous

Lubricates and protects stomach wall Prevents pepsin from digesting the protein in

stomach wall Pepsin-most active in acidic environment

Digests proteins into peptides

Contents of Gastric Juices HCl

Denatures proteins and kills microbes Intrinsic factors

Aids in absorption of Vit B 12 Gastrin

Regulatory hormone: regulates contents of stomach to the small intestines

Gastric Cells

Mucus cells: secrete mucus Chief cells: secrete pepsinogen

Pepsinogen vs. pepsin Parietal cells: secrete HCl and

intrinsic factor G-cells: secrete gastrin

Regulation of Gastric Secretions

Gastric juice produced continuously

Rate controlled Neurally- parasympathetic

Acetylcholine- inhibits stomatostatin Hormonally

Stomatostatin- inhibits acid secretion Gastrin- increases gastric gland secretion

Phases of Gastric Secretion Cephalic Phase

Sight, taste, smell, or thought of food Gastric juice secreted

Gastric Phase Food enters stomach Stimulates release of gastrin

Intestinal Phase Food enters small intestine Intestinal gastrin released

Gastric Absorption

Stomach Some water, certain salts, lipid-

soluble drugs Small Intestine

Most nutrients absorbed Large Intestine

Water, salt

Mixing and Emptying Actions

Chyme Semifluid paste of food particles and

gastric juice Peristaltic waves- mixes food Pyloric Sphincter relaxes

Mixing and Emptying Actions

Liquids- rapidly Solids- remain until well mixed Fatty foods- 3-6 hours Proteins- move quickly Carbohydrates- more rapidly than

proteins or fats

Digestion Animation http://highered.mcgraw-hill.com/sites/00

72495855/student_view0/chapter26/animation__organs_of_digestion.html

http://health.howstuffworks.com/adam-200142.htm

http://www.constipationadvice.co.uk/constipation/constipated-digestive-system.html

Gastric Bypass