The Human Digestive System

The Digestive SystemThe digestive system: is a group of organs that

take in food and change it into a form the body can use. Digestion: is the changing large molecules into

smaller ones capable of being used by the cells in the body.

Cells need food for energy, growth and repair.Can be compared to a factory – materials are

transported to different locations and are changed at every stop.

The final product is quite different than the original.

Nutritional RequirementsHumans need to take in food for:

fuel = chemical energy raw materials = carbon source essential nutrients = animals cannot make

elements (N, P, K, Fe, Na, K, Ca...), NAD, FAD, etc.

Getting & Using Food Ingest

taking in foodDigest

Physical digestionbreaking up food into smaller pieces

chemical digestionbreaking down food into molecules small enough

to be absorbed into cellsenzymes

Absorbabsorb across cell membrane

diffusionactive transport

Eliminate undigested extracellular material passes out of

digestive system

Physical vs. Chemical Change

Food gets broken down as it passes through the digestive system in 2 ways1. Physical Change:

Large pieces are broken down into smaller ones. The food is still in the same form (only the size and

shape of the food particles change).Occurs through chewing, grinding and mixing.

2. Chemical Change:This occurs when food changes form so the body may

use it.Body chemicals are added to food to make this happen.These chemicals are called enzymes.

Digestion

Digestive SystemWater, vitamins, and minerals can be

used by cells so do not need to be changed

Fats, proteins, and carbohydrates cannot be used so they need to be broken down by enzymesEnzymes: are chemicals that speed up

the rate of changeThere are different kinds of enzymes for

different nutrients.

Human digestive systemAlimentary Canal

Digestive SystemMain organ passageway:

1. Mouth (includes salivary glands)2. Esophagus 3. Stomach4. Small intestine5. Large intestine6. Anus

Additional organs necessary: Pancreas, liver, and gallbladder all secrete enzymes into the small intestine.

Common processes & structures

Movement & Control peristalsis

push food along by rhythmic waves of smooth muscle contraction in walls of digestive system

sphinctersmuscular ring-like valves, regulate the passage of material between sections of digestive system

Accessory glandssalivary glands, pancreas, liver & gall bladdersecrete digestive juices (enzymes & fluid)

Swallowing (& not choking)

Epiglottis problem: breathe & swallow through same orificeflap of cartilagecloses trachea (windpipe) when swallowingfood travels down esophagus

Esophagus move food along to stomach by peristalsis (pushing of food

along by waves of smooth muscle contractions)

IngestionMouth

physical digestionteeth

breaking up foodchemical digestion

salivaamylase

o enzyme digests starchmucin

o slippery protein (mucus)o protects soft lining of digestive systemo lubricates food for easier swallowing

buffers o neutralizes acid to prevent tooth decay

anti-bacterial chemicals o kill bacteria that enter mouth with food

mouthbreak up foodmoisten food digest starchkill germs

StomachFunctions

food storagecan stretch to fit ~2L food

disinfect foodHCl = pH 2 so kills bacteriaHas a mucus layer to protect itself from enzymes and HCl

chemical digestionpepsin

enzyme breaks down proteinsAfter the stomach completes its

part in digestion the food is now in a liquid form called chyme.

But the stomach is made out of protein!What stops the stomach from digesting itself?

mucus secreted by stomach cells protects stomach lining

StomachA large muscular bag that has gastric juice (juices

given off by the lining of the stomach) that mixes and digests protein. The gastric juice is made of an emzyme (pepsin) and HCl that digests protein.

The walls are muscular and thick that churns and grinds up the food. Once the food is mixed with the digestive juices, it starts to look like a milk shake.

The partially digested food is squeezed out of the stomach in spurts and moves to the small intestine.

Food usually stays in the stomach for 4 – 7 hours.

stomachkills germs store food break up fooddigest proteins

cardiacsphincter

pyloricsphincter

mouthbreak up foodmoisten food digest starchkill germs

Small IntestineFunction

major organ of digestion & absorption chemical digestion

digestive enzymesabsorption through lining by diffusion

over 6 meters in length! small intestine has huge surface area = 300m2

(~size of tennis court)Structure

A hollow tube with finger-like projections called villi. Each villus contains blood vessels that carry digested

food.From the blood vessels the food is carried to body

cells.

Duodenum 1st section of small intestines

acid food from stomach mixes with digestive juices from accessory glands:

pancreas liver gall

bladder

PancreasMakes enzymes to digest:

FatsProteinsCarbohydrates

These enzymes pass from the pancreas to the small intestine through a small tube.

Pancreas also produces insulin to regulate sugars.

stomachkills germs break up fooddigest proteinsstore food

pancreasproduces enzymes to digest proteins, starch &

fat

mouthbreak up foodmoisten food digest starchkill germs

Liver Digestive System Functions

produces bilestored in gallbladder until neededbreaks up fats

Stores unused sugar

Circulatory System ConnectionCirculatory System Connection

bile contains colors from old red blood cells collected in liver =iron in RBC rusts & makes feces brown

bile contains colors from old red blood cells collected in liver =iron in RBC rusts & makes feces brown

pancreasproduces enzymes to digest proteins, starch &

fats

stomachkills germs break up fooddigest proteinsstore food

liverproduces bile

- stored in gall bladderbreak up fats

mouthbreak up foodmoisten food digest starchkill germs

Digestive EnzymesCarbohydrate Digestion:

Mouth Salivary glands

Small IntestinePancreas

Protein Digestion:Stomach

Enzymes and HClSmall IntestinePancreas

Fat Digestion:Small Intestine PancreasLiver

Bile

Digestive enzymes

Absorption by Small Intestines

Absorption through villi & microvillifinger-like projectionsincrease surface area for absorption

Absorption of Nutrients

Occurs through diffusion.Final forms of nutrients:

Carbohydrates (starches and sugars) = glucose

Protein = amino acidsFats = glycerine and fatty acids

small intestinesbreakdown all foods

- proteins- starch- fats- nucleic acids

absorb nutrients

stomachkills germs break up fooddigest proteinsstore food

pancreasproduces enzymes to digest proteins & starch

liverproduces bile

- stored in gall bladderbreak up fats

mouthbreak up foodmoisten food digest starchkill germs

Large intestines (colon)Function

re-absorb wateruse ~9 liters of water every

day in digestive juices> 90% of water reabsorbed

not enough water absorbed back to body odiarrhea

too much water absorbed back to bodyoConstipation

No nutrients are absorbed here

Rectum Last section of colon (large intestines)eliminate fecesundigested materials

extracellular wasteomainly cellulose from plantsoroughage or fiber

saltsmasses of bacteria

stomachkills germs break up fooddigest proteinsstore food

small intestinesbreakdown food

- proteins- starch- fats

absorb nutrients

pancreasproduces enzymes to digest proteins & carbs

liverproduces bile

- stored in gall bladderbreak up fats

large intestinesabsorb water

mouthbreak up foodmoisten food digest starchkill germs

AppendixVestigial organVestigial organ

Problems with the Digestive System

Ulcers A sore or hole inside either the stomach or small intestine.Caused by the stomach lining being digested or “eaten away”

by enzymes and stomach acids.The mucus layer helps prevent this but this can be damaged by

a bacterial infection called Helicobacter pylori thus resulting in an ulcer.

HeartburnCaused by stomach acids moving into the esophagus.This causes a burning feeling.The esophagus is lies behind the heart so it feels like the heart

is burning.Eating too much at once or too much acidic or rich foods can

cause acid to back up.

Digestive System - GlandsSalivary glands – secrete amylase for preliminary

starch digestion.Glands in the stomach lining – secrete mucus and

gastric juice (contains pepsin and hydrochloric acid) for initial protein digestion.

Liver – secretes bile to emulsify fats.Pancreas – secretes sodium bicarbonate to neutralize

stomach acids and enzymes to act on proteins (proteases), carbohydrates (amylase) and fats (lipases).

Walls of the small intestine – secrete juices that complete the digestion of carbohydrates, fats and proteins.

Lab ActivityStations

Activity 1: How long is the Digestive System?

Materials: 4 different balls of yarn

Description: Students are to cut pieces of different coloured yarn for each of the following measurements. Then they are to tie them together to show how long each piece of the digestive system is in reference to each other.

Esophagus 25 cmStomach 20 cmSmall Intestine 700 cmLarge Intestine 150 cm

Activity 2: DigestionMaterials:

Sugar CubesGranulated SugarClear cups filled with water

Description:Place a sugar cube in a cup of

water. Place about a teaspoon full of granulated sugar in the other cup of water. Observe.

Activity 3: Carbohydrate Digestion

Materials: Unsalted crackers

Description: Students chew 2 unsalted crackers for two minutes without swallowing.

Activity 4: Hands on Digestion

Materials: Meatball size of hamburgerOne plastic bag1 M HCLDigestive juice A (pepsin, trypsin, and water)Digestive juice B (bile salts, pancreatin

enzyme and water)

Description: Place hamburger, 3 eyedroppers full of HCL, 1 tbsp of Digestive juice A and 2 Tbsp of Digestive juice B in the plastic bag. Knead with your hands for 10 minutes (stimulates the stomach) and it will have been reduced to liquid with a definite odor.

Activity 5: How do villi aid the Small Intestine in absorption?

Materials:Paper towels4 cups of equal amounts of waterGraduated cylinder

Description: Compare how 1,2,3, then 4 paper towels absorb. Dip each paper towel into a cup of water (same amount of water in each cup). Record the volume of water left in each cup (using the graduated cylinder).

Activity 6: The Digestive System Story

Description: Assign each student as a part of the digestive system and one student as food. As the food student moves past each digestive system part, the student labelled as that part must describe what they are doing to the food.

Questions:What system in your body is the same length

as the completed piece of yarn?In Activity 2, what can you conclude must be

done to food before digestion occurs?What physical and chemical changes occurred

to the soda crackers?What caused the physical and chemical

changes to the crackers?Did you notice a taste change in the crackers?How is physical digestion simulated in Activity

4?

Questions continuedWhat evidence was there that chemical

digestion occurred in the hamburger?Which paper towel had the largest surface

area?What relationship can you see between the

largest surface area and how villi aid in absorption?

Using Activity 6, create a diagram to demonstrate the passing of food through the digestive system.