Like the respiratory system, the digestive system is the point of entry into the body for needed nutrients.
         Review Chapter 3 and 17 - nutrients, nutrition, metabolism.
            What are the nutrients?
                 Compare and contrast the two general classes?
                    List the nutrients in each class.
                    Review the role of each nutrient.

        Keep this Table handy - you will refer to it again.

Since you know that the large surface area of the respiratory membrane is the functional unit of the respiratory system - what would you predict to be the functional unit of the digestive system?

The primary function is Absorption of Nutrients
    The primary functional unit of the Digestive System is the absorptive surface of the small
      and large intestines.

Nutrients must be:

Ingested – taken into the mouth
Mechanically processed
Secretion - Mixed with digestive enzymes secreted by the digestive organs
Digested
Absorbed and
Elimination of wastes - indigestible materials

Therefore, these last are a list of the specific functions that support the main function of the digestive system.

These support functions place nutrients within close proximity to the absorptive surface area and break the nutrients down into pieces that are small enough to be absorbed via diffusion and active transport.

DEFENSE - the mucous lining of the digestive tract is a prime route for foreign particles to enter the body, therefore, there are Lymphatic nodule tissues integrated into the wall of the digestive system.  In addition, Lacteals, specialized lymphatic capillaries are found in the microvilli of the intestines.

Absorption is the movement of molecules, atoms and ions across the membranes of the digestive tract. The large food particles we eat must be broken apart to the component molecules, atoms, and ions.
      Remember - the FORCES that move these particles operate on an atomic and molecular
      scale - the particles must be small enough that these forces can efficiently move the particles.

Mechanical processing breaks the LARGE particles down into very small pieces that can be chemically digested.  These particles are still too big to be effectively moved by diffusion, active transport, or even the specialized transport mechanisms.

Chemical digestion uses digestive enzymes and HCl (hydrochloric acid) to break the small pieces produced by mechanical processing down to molecular or atomic size.

Both processes work together to decompose the food to it’s component molecules, atoms and ions.

Review Chapter 2 and 3 – cellular transport mechanisms.
   What are the ‘forces’ that move molecules, atoms and ions in the body?
     Why do solutes move?
     Why does water move?
     Describe the characteristics of water that support the function of the digestive system.

List the important elements found in the body.
  How do these elements enter the body?
List the 6 types of nutrients (review Chapter 17 - Metabolism)
  The organic molecules are broken down into their repeating units -
         because the organic molecules are often LARGE molecules, and our body needs the repeating
         units to use as building blocks for molecules that our body will synthesize.
  Minerals are elements - absorbed as atoms or ions
  Vitamins are small molecules that are absorbed as they exist - we use em at they exist
  Water is a small molecule and is absorbed as water

Now - let's get started with the Digestive System.

Describe the functions of the Oral Cavity

Analysis of materials - sweet - carbohydrate, temperature, toxic - bad tasting, etc
Lubrication of materials
Mechanical processing of materials

Tongue
Teeth
Saliva

Describe the structure and function of the teeth and tongue

Teeth – mechanical processing
       3 main shapes –

incisors – blade shape; cut or clip foods
canines – pointed; slash or tear foods
molars – flat surface with rounded crowns;  mash or grind foods.

Teeth differ from 'bone' in that the external surface is covered by enamel, a very hard, durable mixture of calcium and flourine.

Tongue - mechanical processing
                    - Compression, abrasion, distortion
                    - Manipulation to assist chewing prepare for swallowing
                    - Sensory analysis by touch, temperature, taste receptors

What are the functions of teeth and the tongue?

Explain the function of saliva

Saliva - a thick mucous that:
    - coats food in a slippery mucous.   
    - mixes with food to make it wet so that it can more easily change shape to slide down the
          esophagus.   
    - contains antibodies against pathogens.   
    - contains digestive enzymes, IgA (antibody).

Salivary amylase is a digestive enzyme that breaks down starches.

What are the functions of saliva?
  What is the main component of saliva?

What is the function of salivary amylase?
  Why is it useful to have an enzyme that breaks down carbohydrates present in the saliva?
Glucose - the repeating unit of starch - is the most readily used source of energy in the cellular respiration.  Therefore, carbohydrate digestion begins in the mouth.

The Oral cavity passes food into the pharynx.

The pharynx has 3 parts:

Nasopharynx which contacts only air and opens into the oropharynx.
Oropharynx opens into the laryngopharynx.
Laryngopharynx opens into both the larynx (through the glottis) and into the esophagus.

Air, dry food and wet food pass through the oropharynx and laryngopharynx. The epithelium must be able to protect against abrasion from dry foods, chemical abrasion from hot, cold or acidic, basic foods. Etc.

The esophagus is a muscular tube about 30 cm long, which transports food to the stomach.  The esophagus is lined with mucous membrane made of stratified squamous epithelium.

Bolus – the mass of food mixed with saliva in the mouth, pharynx and esophagus

Once food enters the stomach it is mixed with gastric juices: water, HCl, and enzymes.

Chyme – the bolus mixed with the gastric juices is called Chyme in the stomach and small intestine.
      - a viscous, soupy mixture of ingested materials, gastric juices, and water.

What is the function of the soft palate and the uvula?

Define Alimentary canal and GI tract.
Compare and contrast the alimentary canal with the GI tract and the digestive tract. Make a table.

The wall of the organs of the digestive tract generally has 4 layers.

Mucosa – a mucous membrane lining of the lumen, contacts the food.  It is therefore, and
    epithelium.

Submucosa – a connective tissue layer containing vascular capillary beds, blood vessels, lymphatics and nerves.
     The capillaries
             - supply nutrients to the cells of the digestive tract
             - receive absorbed nutrients from the food.
     The lymphatics include LACTEALs which are lymphatic capillaries with specialized
          transport mechanisms for absorption of large molecules - especially, large lipid
          droplets.

Muscularis – a smooth muscle layer with circular smooth muscles running around the
    tube and longtitudinal muscles running the length of the tube. Responsible for peristalsis
    and segmentation.

Serosa – the capsule that surrounds the tube – this is the layer that is INTERNAL toward the interior of the body. Remember – food inside the intestines IS STILL OUTSIDE BODY.
   Remember the donut analogy?
     How is food in the lumen of the intestines still outside the body?

What kind of connective tissue (CT) is the serosa?
  What are the characteristics of the CT that makes up a capsule or sheath?
    What is the orientation of the fibers?
      What are the fibers found in this CT?
How many layers are in the wall of the hollow organs of the digestive tract?
   List each and give the function.

List the 4 types of membranes found in the body. Review Chapter 4, Tissues
   Which are 'dry'?
   Which are 'wet'?
   Which are exposed only to the internal environment?
   Which are exposed to the external environment?
Why are lymphatic nodules found in the mucosa of the wall of the alimentary canal?
    Review Chapter 14, lymphatic nodules.
    What function of the digestive system, does the mucosa and the lymphatic tissues provide?

The mucous lining of the digestive tract is an epithelium.
   How do you know it is an epithelium? Reveiw Chapter 4, Epithelial Tissues.
     List the two types of epithelium.
     List the three shapes of cells on the epithelial surfaces.
        What is the major function of each?

   What is the function of the digestive system?
     Which type of cell would you expect to find in the mucosa?
       You must know the function of the specific organ to answer this question correctly.
         What are the characteristics of the 'food' that passes through the oral cavity, oropharynx,
         laryngopharynx and esophagus?

Liquid, solid, dry, sharp edges, acidic, basic, hot, cold, etc -
Which type of epithelium would best protect the underlying tissues?
What is the major function of these tissues?

What is the function of the Small and Large intestines?
  What are the characteristics of the 'food' in those structures?
Which type of epithelium would best perform that function while acting as a nonspecific defense physical barrier.

Stratified squamous epithelia can be found lining the oral cavity, oro and laryngo pharynx, esophagus, and rectum.
Simple epithelia - mostly columnar is found lining the stomach, small and large intestines -due to the specific functions of each organ.

Peristalsis - Smooth muscles contract in a coordinated fashion, one section of muscle after another and moves

              - food along the alimentary canal.
              - bolus down the esophagus,
              - chyme through thestomach and small intestines
              - feces through the large intestines, rectum and anus
              - urine through the ureters,
              - sperm along the vas deferens,
              - eggs along the fallopian tubes,
              - etc.

Reverse peristalsis - vomiting, nausea.  Coordinated muscle contractions eject
   food from the stomach and first part of the small intestines through the
   esophagus and oral cavity.
  Function - to eject substances from the digestive tract that the body recognizes
    as 'lethal' to the body.

Segmentation – random smooth muscle contractions that MIX food in a segment of the alimentary canal with digestive enzymes. This more efficiently digests the food for absorption.

Where are the muscles located?

Stomach – functions

- Temporary storage of food
- Mechanical processing is finished in the stomach
- Chemical digestion gets well underway
- Produce Intrinsic factor

Gastro - refers to 'stomach'
Gastric secretions:
    The Parietal Cells in the lining of the stomach secrete
               HCl - hydrochloric acid - pH = 1.  It is VERY STRONG acid. 
                    acid breakdown of foods
                    kills bacteria
                    denatures proteins
               Intrinsic factor - responsible for uptake of vitamin B12 from dietary foods.
    The Chief cells in the lining of the stomach secrete
              Pepsinogen - a proprotein that becomes active when exposed to the HCl (pH = 1)
                 in the gastric juices.
              Pepsin - the active form of pepsinogen - digests Proteins.

What is a 'proprotein' or 'proenzyme'?
  When do these inactive proteins become active?

 The stomach dumps chyme through the pyloric sphincter into the small intestine

What is Chyme?
   What is the physical state of chyme? (hint: what are the 3 physical states of matter?)
      How does this physical state aid in digestion and absorption of nutrients?
        What is the component of chyme that gives it this physical state?

NOTE: absorption is NOT a function of the stomach. 
     - The food in the stomach is still too LARGE for absorption (chemical digestion has
          not yet finished),
     - the lining of the stomach is protected by a thick layer of alkaline mucous from the
         acidity of the chyme

    Since absorption is NOT a function,
     - the cells of the stomach wall do not have
         specialized transport mechanisms
     - and the lining of the stomach is WATERPROOF.  This is why you can make a canteen
         from an animal stomach.

 

The Greater Omentum is a folded sheet of irregular dense connective tissue that comes off the bottom of the stomach and hangs down on the ventral side of the abdomen like an apron.  It often is filled with fat -and forms the 'abdominal fat' so roundly maligned by the popular medicine. The Lesser Omentun is a sheet of irregular dense connective tissue that comes off the bottom of the liver and attaches to the top of the stomach.  It holds the stomach in place, relative to the other organs. The Mesentaries - sheets of irregular dense connective tissues that connect to all the intestines and hold many of the organs in place relative to each other. The abdominal organs MUST remain in place, relative to each other, so that food can pass through the tube.  If the organs get 'twisted' or out of place, the GI tract can become blocked, which can be lethal.

 

The Small intestine is where most of the absorption of all the nutrients except water occurs
         - it has an absorptive surface area of around 2000 ft2.

It is about 6m long and has three sections:

Duodenum – receives chyme from the stomach, and buffers the pH of the chyme and mixes it with digestive enzymes from the Liver and Pancreas. Most of the chemical digestion is completed here.

Jejunum – receives chyme from duodenum. Function is to absorb nutrients: fats, proteins, carbohydrates, minerals, and vitamins.

Ileum – completes the absorption of these nutrients and some reabsorption of water.

    Absorption is accomplished via:

Diffusion
Facilitated diffusion
Active transport - Specialized transport mechanisms for large molecules assist absorption.
Special transport mechanisms -  Fats especially are absorbed via special transport
     mechanisms into the lacteals - specialized lymphatic capillaries found in the microvilli.

Fat is a NONpolar molecule - therefore it does NOT readily dissolve in aqueous
   solutions (chyme is mostly water).

The absorbed substances go into the capillary beds in the submucosa - the blood is therefore very 'nutrient-rich'.

What is absorbed in the small intestine?
  What transport processes move nutrients from the chyme into the body?
     Explain why chyme is actually EXTERNAL to the body.
        What is the function of lymphatic NODULES and where are they found?

List the parts of the small intestine.
  What is the function of each part?
 

Ileocecal sphincter controls the passage of chyme from the ileum into the cecum of the colon.

Colon –AKA Large Intestine;
      Function: reabsorb water and compact feces.

   has 5 sections:

Ascending colon – MOST water reabsorption occurs here. Usually on the right side of the
      abdominal cavity.

Cecum – a 6 cm X 6 cm chamber at the beginning of the ascending colon.
     Receives chyme from the ileum.
     Point of attachment of the vermiform appendix.

Transports chyme upward, toward the head and into the transverse colon. 
Since chyme must move upward, against gravity, movement is slow, therefore
   water has opportunity to osmose from the chyme. Reabsorption of water
   changes the chyme to Feces.

Feces – the undigested materials in the large intestine from which water has been
      reabsorbed. When water is removed from the feces, the feces becomes more
      compact and 'solid'.

Transverse colon – transports feces across the top of the abdominal cavity to the
      descending colon

Descending colon – transports feces caudally along the left side of the abdominal cavity
      to the sigmoid colon

Sigmoid colon (what does sigmoid mean? What is the ‘shape’ of this section?) transports
      feces to the Rectum

List the parts of the colon.
  Where is the appendix?
    Where is most water reabsorbed?
      What is feces?

Rectum - the last 15 cm or so of the large intestine is usually empty until the colon produces a bowel
    movement. Stretch receptors stimulate a defecation reflex.

Diarrhea: movement of feces is faster than colon’s water absorption/adjustment capacity resulting in excessive loss of water and electrolytes.  
Often accompanies intestinal infections and toxins.  Diarrhea is one of the body's methods of getting rid of pathogens, toxins, etc - things in the digestive tract that the body recognizes as 'lethal'. 
Dehydration - loss of so much water that normal metabolism cannot continue.
 

Constipation: feces moves so slowly that too much water is reabsorbed.   So much water is reabsorbed, that the mass of feces is too 'dry' and cannot easily change shape to move along the colon and through the anal opening. 
Impaction - a hard fecal mass that blocks the intestines stopping movement through the tract and therefore absorption of nutrients.
Prevention -
     Dietary fiber provides bulk and water holding capacity;
     Exercise - muscular contractions put pressure on the feces and help it move along.

Compare and contrast: diarrhea and 'reverse peristalsis'.

List the characteristics of water.
  Consider how each characteristic supports the functions of the various systems that
          we've studied.
      How would dehydration affect these functions? 

List the electrolytes. 
  What is the function of electrolytes?
    How would 'loss of electrolytes' affect metabolism?

 

Anal sphincters – two (2) of them.
          The Internal anal sphincter is under autonomic control. The defecation reflex relaxes the
               internal anal sphincter.

The external anal sphincter is under voluntary control. Relaxes and permits passage of feces through the anus.

Anus –the waste opening for the alimentary canal.

Compare and contrast: Food, Bolus, Chyme, and Feces.
Why is water mixed into the bolus to form chyme?
  What happens to the water after nutrient absorption has been accomplished?
     Why do we reabsorb the water?
       If we did not reabsorb the water, where would we need to live?

Compare and contrast the
      oral sphincter,
      hiatal sphincter,
      pyloric sphincter,
      ileocecal sphincter, and
      anal sphincters.
What substance passes through each?
Name the structures on either side of each.

Small intestine - Most absorption of the energy nutrients: the Organic molecules, minerals and vitamins occurs in the jejunum and ileum.

Large intestine – most absorption of water occurs in the ascending colon.

Salivary glands -covered above, with the oral cavity

Pancreas – a GLAND with both
        - endocrine functions - insulin and glucagon
        - exocrine functions - pancreatic enzymes or digestive enzymes

The pancreas is the only gland that produces enzymes that break down all three major types of organic nutrients.

What are the three types of organic nutrients?

The three types of digestive enzymes produced by the pancreas are:
        Carbohydrase
        Protease
        Lipase

What is the function of each of these?
How is the pancreas unique amongst the digestive system glands?

What is amylase?

Hepatic portal system –
    Function: Transport nutrient rich blood from the digestive capillaries to the sinusoids of the liver.

A portal system carries blood from one capillary bed to another, without passing through the heart.

The capillaries in the lining of the digestive tract get loaded with nutrients directly from the gut. Many absorbed molecules must be chemically altered before the body can use them. Some of the absorbed molecules are ‘toxic’ in the absorbed form. The nutrient rich blood from the digestive capillaries goes into the hepatic portal vein, which goes to the central vein of the lobule of the liver and into sinusoids.

Liver – is the chemical processing plant for the body and has over 200 vital functions.

Makes bile and cholesterol.
Removes molecules from the blood and chemically alters those molecules for our use.

Lobule - The functional unit of the liver is made up a central vein that feeds nutrient rich blood into the sinusoids and hepatocytes that absorb molecules from the blood and chemically alter those molecules.

What is the function of the hepatic portal system?
Where did the molecules in this blood come from?
What are hepatocytes?
  What is their function?

Gall bladder – stores bile.

Bile helps absorb fats and fat soluble vitamins from the chyme.

What are the fat soluble vitamins? hint: Chapter 17, Metabolism
    What is a major risk associated with LOW fat diets?

Why do we need a special substance to help absorb fats?
The fat molecules are NONpolar and clump together in aqueous solutions into large clumps that are not easily absorbed.  Bile salts emulsify the fat to form tiny lipid droplets helping them become suspended in the chyme in droplets that are small enough to be absorbed.  These tiny fat droplets are 'picked up' by special transport mechanisms and passed into the lacteals.

What would be the results of a decreased level of bile salts in the bile?

There are over 400 species of microorganisms that live in and on the human body at all times.

They are often called probiotics because we live in a mutualism type of symbiosis –

We provide these Probiotics with
     environment that meets their metabolic needs
         food
         water
         temperature
         pH
         etc.

Probiotics help us:
    occupy surface area so that pathogens cannot establish a colony,
    digest foods that we cannot, and
    produce vitamins that we need
           Vitamin K: fat-soluble used by liver – 4 clotting factors; deficiency due to inadequate
                dietary fats, metabolic problems, chronic diarrhea.
           Biotin: water soluble- glucose metabolism; few deficiencies.
           Vitamin B5, water soluble, manufacture of steroid hormones/neurotransmitters;
                few deficiencies
    create an environment in which pathogens cannot survive (pH, natural antibiotics, etc).

They also produce flatus (gas) and H2S gas, which gives flatus its odor.

Symbiosis is two or more organisms living in close proximity (usually in or on the other).
       Mutualism - a type of symbiosis in which all that live together get benefits from living together.
              The living arrangement has 'mutual' benefits.
       Parasitism - a type of symbiosis in which one organism benefits, the others are harmed
            Pathogens are parasites.

Probiotics and multicellular organisms have a mutualistic living arrangement.

How many different probiotics are there?
How do probiotics help us?
  List the ways they help us.
    How do probiotics benefit from this arrangement?
      What are the two types of symbiosis discussed here?
        Compare and contrast these symbiotic relationships.

Physical movement slows - constipation increases.
Cardiovascular functioning slows - absorption slows.
Liver functions decrease.
Pancreatic functions decrease.
Gall bladder and bile functions change.
Sensory inputs decrease therefore the Nervous System resposnces

Remember – all the systems have to work together to maintain overall homeostasis.

Define homeostasis.

How does the digestive system function support the function of the other systems?
How do the functions of the other systems support the function of the digestive system?

How does the digestive system function support overall homeostasis?

NOTE: this is a REVIEW - we covered this with Chapter 3, too.

Describe aerobic cellular respiration.
  Where does each of the three phases of aerobic CR take place?
    What is the simple formula for aerobic CR?
Go to the Class Notes for Chapter 17, and review them.