Chapter 3:Digestion

The Digestive System (also known as the Gastrointestinal System)• A collection of organs whose purpose is to break down foods into

compounds that are small enough to be taken up into the body

Gastrointestinal (GI) Tract Organs• Food and nutrients pass through these organs

Accessory Organs that AID in Digestion• Chemicals necessary for digestion are supplied by these

organsMouth Salivary glands

Esophagus LiverStomach Gallbladder

Small Intestine Pancreas

Large Intestine

“Involuntary” photo

Sphincters!

• Sphincter – a ring-like muscle that relaxes or contracts to open or close bodily passageways• Regulate the rate at which food and nutrients pass from one organ to the next

• Four important sphincters in the digestive process• Upperesophageal sphincter

• Loweresophageal sphincter

• Pyloric sphincter

• Ileocecal valve

The Digestive Process

The Digestive Process has 4 basic steps:

1. Food is ingested – taken into the gastrointestinal (GI) tract via the mouth

2. Food is digested – broken down into smaller units

3. Small units of nutrients are absorbed – they pass from the GI tract into the blood stream

4. Anything leftover is excreted – eliminated through the anus

The Gastrointestinal Tract• A tubular channel extending from the mouth

to the anus; digestion and absorption occur here.• Includes what 5 organs?• Intestinal Lumen - the inner space of the GI

tract. This space is lined with mucosal cells, which produce mucus, a viscous solution that lubricates and protects the GI tract

Digestion

• Mechanical Digestion – Physical breakdown of food into small particles• Mastication and mixing

• Chemical Digestion – breaks chemical bonds to turn large molecules into smaller ones• Involves chemicals from the salivary glands, stomach, pancreas, liver, and

gallbladder

Mechanical Digestion• Peristalsis – rhythmic,

wavelike contractions of smooth muscle to propel food through the GI tract• Gastric Peristalsis

Mechanical Digestion

• Segmentation – Circular muscles in the small & large intestine contract so that fluid contents slosh back and forth between the segments. • Helps mix intestinal

contents

Chemical Digestion• Enzymes – Protein molecules that

catalyze (speed up) chemical reactions.• Without enzymes, digestion could not

occur• Produced by the salivary glands, stomach,

pancreas, and small intestine. • Only work for specific molecules

• Hormones – Chemicals that serve as messengers and help to control and regulate body processes• Regulate appetite, secretions, and physical

digestion

The Path of Digestion: The MouthThe mouth is the beginning of both physical and chemical digestion• Saliva – fluid secreted from the

salivary glands to moisten the food and provide lubrication• Salivary Amylase – Enzyme that

starts the digestion of carbohydrates• Lingual Lipase – Enzyme that starts

the digestion of lipids• The tongue helps mix the saliva with

food

The Path of Digestion: The Esophagus• After being swallowed, food is pushed through

(1)_______________________ into the esophagus and then through the (2)________________________ into the stomach.

A. Loweresophageal sphincterB. Ileocecal valveC. Upperesophageal sphincterD. Pyloric sphincterE. None of the above

The Path of Digestion: The Stomach• The stomach churns food to help break it

down. • The stomach cells release chemicals to break

bonds between nutrients• GASTRIC JUICE

• Hydrochloric acid (HCl) – dentatures (unravels) protein

• Protease – enzyme that helps digest protein• Gastric Lipase – enzyme that helps digest lipids• Mucus – protects lining of the stomach from HCl

• Chyme – partially digested, liquefied food that leaves the stomach through the ___________?

The Path of Digestion: The Small Intestine

• Main site of digestion & absorption of nutrients• Specialized structures to increase

surface area and increase absorption• Carbohydrates, protein, fats, and

other nutrients (like what??) are digested & absorbed in the small intestine

Digestion in the Small Intestine

• Mechanical Digestion – does it happen here?

• Chemical Digestion• Three organs provide enzymes needed (Hint: these are the accessory

organs that aid in digestion)

Pancreas

• Pancreas releases a liquid solution called Pancreatic Juice• Pancreatic Juice moves from the pancreas into the small intestine

where it digests nutrients by breaking down chemical bonds• Amylase – digests carbohydrates • Protease – digests proteins • Pancreatic Lipase – digests lipids • Bicarbonate – neutralizes HCl

Liver and Gallbladder

• Liver produces Bile• Bile – a substance necessary for digestion of lipids. It works with the

enzyme ____________ to maximize lipid digestion.• Gallbladder – storage facility for bile

In what organ along the gastrointestinal tract does bile interact with lipids to assist in digestion?

Absorption

• After food and nutrients are small enough, nutrients move out of the lumen of the small intestine into either the blood vessels OR the lymph vessels• Blood vessels: proteins (amino acids), carbohydrates, minerals, water-soluble

vitamins• Lymph vessels: lipids, fat-soluble vitamins

• The blood and lymph vessels are like highways that transport nutrients to the organs and tissues in the body

• Simple Diffusion – movement of a substance down a concentration gradient• Facilitated Diffusion –

movement of a substance down a concentration gradient with the help of a transport protein

Diffusion in the Small Intestine

• Active Transport – movement of a substance against a concentration gradient; requires both a transport protein and energy

Diffusion in the Small Intestine

The Path of Digestion: The Large Intestine• Waste products move from the small intestine through the

__________________ into the large intestine• Remaining nutrients are absorbed• Water is absorbed into the body• Over 1000 different species of bacteria feed on undigested fiber

• Produces gas!• Produces short chain fatty acids, which either feed the bacteria or are absorbed into the

body

The final step of the Digestive Process• Excretion• Waste products are excreted via the rectum and anus

• Takes 24-72 hours for foods to pass through the digestive system