Digestive System

Function:

Prepares food to be used by the cells by breaking it down

Passage of food from the mouth to the anus takes about 24-33 hours

Steps of breaking down food for the cells:

• Ingestion- taking in food, chewing, swallowing• Digestion- mechanical grinding of food to break

it down• Absorption- if nutrients are small enough, they

pass through the intestinal wall to the blood• Transportation- blood carries nutrients to the

cells and stores the excess in the liver• Utilization- cells use the nutrients from the blood• Elimination- undigested and unabsorbed

materials are removed from the body by defecation

Parts of the digestive system:

• Mouth• Esophagus• Stomach• Small intestines• Large intestines

Accessory Structures:

• Liver• Gall bladder• Pancreas• Salivary glands• Teeth• Tongue

Lining of the digestive system(mouth to large intestines):

4 layers (Tunics):• Mucosa- innermost lining

-Composed of mucous membrane-Allows food to slide easy

• Submucosa- 2nd layer (below mucosa)-Has a lot of nerves and blood vessels

• Muscularis- has muscles that aid in movement of food through the system

• Serosa- outermost layer-Coats the entire digestive tract

Objectives:

• Explain how digestion if accomplished by the mouth.

• Describe the function(s) of the esophagus and explain how these functions are accomplished.

Mouth:

• Where food enters the digestive system

• Function: teeth, tongue, and saliva mechanically begin breakdown of food

• It takes about 20 seconds to bite and chew food enough to swallow

3 ways the mouth breaks down food:

• Chewing (called mastication) physical breakdown of food

• Saliva- releases chemicals that aid in food breakdown

• Salivary amylase- an enzyme released in saliva that breaks down starch (carbohydrates)

• Other foods aren’t broken down by enzymes here

• Swallowing- food is formed in a ball when chewed and then swallowed

• The ball of food is called a bolus• When food is swallowed, it is still too large to be absorbed to

the cells

Esophagus:

• Muscular tube that connects the mouth to the stomach

• It takes 5-8 seconds for food to pass through it

How does food go thru the esophagus?

• Food is pushed down the esophagus by gravity and the waves of the esophagus called peristaltic waves (peristalsis)

• In peristaltic waves:– the muscular wall of the

esophagus contracts pushing food down the esophagus

Why does food not enter the windpipe when swallowing?

• The epiglottis closes the windpipe off when swallowing to keep food out of the trachea (windpipe)

• The epiglottis stays open at all times unless swallowing so air can go to the lungs

Stomach:• Football shaped organ

when full• Food stays here 2-3 hours• It has 3 layers of muscle

that contract to churn (mix)

food so that it further breaks

it down

Functions of stomach:• storage of food • mixing of the food with gastric

secretions to form chyme • emptying of the contents into the

small intestine at a rate that is proper for digestion and absorption

Heartburn:

• The sphincters of the stomach close it off by squeezing the tubes closed

• Sometimes the sphincter does not close completely

• This allows the acids of the stomach to go up into the esophagus causing a burning sensation

• It is called heartburn because it feels like a burn that is at the heart

Stomach openings:

The stomach has 2 openings:Esophageal sphincter

(also called cardiac sphincter)-

where esophagus attaches to

stomach (food enters)

Pyloric sphincter- where it

attaches to the small intestine

(food exits)

Other parts of the stomach:

-Fundus

-Body

-Rugae (antrum)-

expands when

the stomach is

filled with food

How does the stomach chemically break down food?

• The walls of the stomach have millions of gastric glands that secrete gastric juices

• These juices have enzymes called pepsin and hydrochloric acid (very acidic)

• Pepsin- breaks down proteins (protein breakdown does not start until it gets to the stomach)

• Hydrochloric acid- drops the pH of the stomach to 2 so it dissolves good and kills bacteria

Stomach acid:• The pH of stomach juices are very

acidic• If the acids were to contact your

skin, it would result in a major burnWhy does the stomach not burn? It is lined with a layer of mucous and mucous glands which work as a barrier against the acids

Ulcer- hole in the lining of the stomach where acid exits

          Ulcers

Small intestines:

• About 6 meters long (18 feet)

• Narrow in width (small)• Food is here 2-3 hours• When food enters the

small intestine from the stomach, it is a soupy mixture called chyme

Parts of the small intestine:

3 segments:• Duodenum- 1st

segment-joins stomach

-About 22 cm long• Jejunum- 2nd

segment- the next 6 ft

-Curves downward• Ileum- last segment-

joins large intestine

Small intestine functions:

• When food enters the intestines from the stomach, it is still too large to be absorbed into the blood

2 processes (functions) occur in the small intestine:

1. Digestion:

-The small intestine finishes digestion

2. Absorption:

-when molecules are broken down small enough, molecules move across the lining of the intestines

to the blood

Digestion in the small intestine:

• Completed in small intestine

How?• The pancreas secretes enzymes which a

released in the small intestine that finish the breakdown of carbohydrates and proteins.

• It also releases the enzyme lipase that breaks down fat (lipids)

• Also, the liver releases bile that breaks down fats

Absorption process:• When molecules are digestion small enough, they go to the

blood through the lining of the small intestine

How?

• The walls of the intestines are

lined with fingerlike projections

called villi.

• The villi have blood vessels

within them.

• They increase the surface area

allowing the digested particles to

contact the intestines, soaking

through to the blood (absorption)

Absorption through villi:food molecules inside small intestines

to cells

outside intestines (blood vessels carry absorbed nutrients to cells of the body)

Exiting the small intestine:

• Once food reaches the end of the small intestine, all food that can be digested is broken down

• Only food that we cannot breakdown such as cellulose remains

• The mixture is still soupy (chyme) when it reaches the large intestine

Large intestine:

• Also called the colon• It is about 5 -6 ft long

(it is wide)• Undigested food

stays here about 18 hours

Parts of the large intestine:3 main segments

• Ascending colon- 1st segmentconnects to small intestine

• Transverse colon- 2nd segment

• Descending colon- last segmentconnects to rectum

Cecum- where small and large intestines join

3 Functions of large intestine:

• Absorb sodium and water-Undigested material enters the large intestine (chyme) and water is removed from it

• Incubate bacteria-Bacteria grow and undigested material and convert some of it to vitamins (the bacteria is what produces the smell)

• Eliminate waste -Once water is removed, the undigested material (feces) is eliminated out of the rectum called defecation

How is material moved thru the large intestines?

• Mass movement- the colon contracts in sections forcing feces to the next section until it exits the body

• Mass movement is triggered by food ingestion and stomach expansion

The rectum has 2 sphincters that control release of feces

• Internal sphincter- involuntary-can’t control it

• External sphincter- voluntary- can control it

Appendix

• Worm-like extension of the large intestine

• It is at the top of the colon where it bends

• Purpose:-Unknown-Thought to be a reservoir to incubate bacteria

• Appendicitis- inflammation of appendix- dangerous because if it ruptures it will release bacteria

Large intestine disorders:

• Diarrhea- occurs when the material moves too fast through the large intestine-Unable to remove the water-It is the body’s natural response to expel foreign matter (like viruses) so they won’t make you sicker

• Constipation- occurs when the material passes to slow absorbing too much water-Can be triggered by not eating enough fiber, not taking in enough water, digestive disorders

Accessory Structures:

• Liver• Gall bladder• pancreas• Salivary glands• Teeth• tongue

Liver

• Weighs around 3 lbs.• Has multiple functions:

– Secretes bile– Removes nutrients from blood (storage)– Converts excess glucose to glycogen when needed– Stores iron and vitamins– Converts protein to fatty acids and urea– Metabolizes fats, proteins, carbohydrates– Makes plasma proteins– Detoxifies drugs– Removes bacteria and worn out red blood cells

Bile:• Secreted by liver to the digestive tract

Components:• 97% water• Salts, pigments, cholesterol

– The pigments give it the green color– Some of the pigments help breakdown food

Jaundice- condition where

the skin turns yellow due

to the build up of bile in the liver

How does bile get to the intestines?

• Common bile duct- released bile to the duodenum (small intestine)

• Common hepatic duct- takes bile to gall bladder to store

• Gall bladder- where excess bile is stored

Pancreas:

• Releases enzymes that aid in digestion and absorption of food

• Releases these juices to the duodenum (small intestine)

• Pancreatic duct- takes the pancreatic juices to the intestine

Salivary glands:• Secrete saliva (about 1 liter/day).

Very little is secreted while we sleep, this is why you awaken with a dry mouth

3 main pairs of salivary glands:1. Parotid- largest pair-in front of and

below ears-Secretes to the cheek area-Mumps- swelling of these glands

2. Submandibular- at an angle to the jaw-Secretes under the tongue

3. Sublingual- under the tongue and secretes under the tongue

Saliva:

• Secreted in mouth

• Smell, taste, and sight of food stimulates the secretion of saliva to prepare the mouth for ingestion of food (“makes my mouth water”)

• Sour pickles and lemonade are said to be the strongest stimulates of saliva secretion

Components of Saliva:

• 1. serous- thin watery secretion– Contains digestive enzymes– Secreted by the parotid and submandibular

glands

• 2. mucous- lubricates– Secreted by the sublingual and

submandibular gland

• 3. salts, antibodies, and substances that kill bacteria

5 Functions of saliva

• Lubricate• Moisten food to make

easier to swallow (forms bolus)

• Aids in taste• Washes away

particles and bacteria• Releases salivary

amylase (breaks down carbohydrates)

Teeth:• Aids in digestion by

mechanically grinding food to break it down

• Teeth are rooted in sockets on bony ridges of the jaw bones

• Gingivae- gums- cover the projections that extend from the socket

• Periodontal ligaments- attach teeth to the sockets

Tooth structures:• Crown- part above the gums• Roots- part beneath the gum• Dentin- substance that teeth

are made of -gives them shape and rigidity

• Enamel- tough protective covering of the crown-hardest substance in body-prevents wear

• Cementum- covers dentin part of root

• Pulp cavity- inside the dentin-filled with pulp

• Pulp- area that contains the blood vessels and nerves-called the nerve of the tooth-very sensitive

• Root canal- the part of the pulp that extends to the roots

Types of teeth:

• 1. deciduous- called milk teeth or baby teeth

-Start to come in around 6 months and have a full set by 2 years

• 2. permanent- replaces deciduous by pushing them from underneath out of socket

-Adults have a max. of 32

Kinds of teeth based on function:

• Incisors- used to bite and tear- front teeth-8 of these

• Canines- used to tear- pointed-4 of these

• Premolars- used to grind and crush-Behind canines-8 of the these

• Molars- used to grind and crush-12 of these-The last molars are the wisdom teeth which usually come in at around age 18

Teeth disorders:• Impacted teeth- often

wisdom teeth are too large to fit in the area at the back of the mouth– They cannot come thru so they

press against other teeth causing pain

• Dental plaque- light colored film formed by bacteria and saliva near the gum line

• Caries- progressive decay that produces cavities– Sugar on teeth is decomposed

by bacteria which sometimes damages the enamel

Tongue

• Moves food around to aid in digestion and taste

• Movement of the tongue is limited by a layer of mucous lining called the lingual frenunculum

• Papillae- projections on the top of the tongue that hold the taste buds

Taste buds:• Taste buds are located all

over the surface of the tongue

• However, taste buds for certain taste are found more concentrated in specific areas4 main taste regions:

• 1. sweet (tip)

• 2. salty (sides)

• 3. bitter (back center)

• 4. sour (mid area, behind sweet)

Enzymes that breakdown nutrients (proteins, fats, carbohydrates):

• Salivary amylase- breaks down carbohydrates in the mouth to monosaccharides

• Pepsin- breaks down proteins in the stomach to amino acids

• Lipase- breaks down fats to glycogen in the small intestine

*Bile- (not an enzyme) breaks down fats

Form that nutrients must be in to be in to be small enough to be

absorbed to the cells:

• Proteins- broken down to amino acids

• Fats- broken down to glycogen

• Carbohydrates- broken down to monosaccharides (glucose)