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Lecture
Chapter 4 - Human Digestion and Absorption
(On the Syllabus this is listed as Week 6)

The digestive system provides two major functions: digestion and absorption.

To be used by the body, food must be ingested and digested, and the nutrients must be absorbed and transported to the cells of the body. Carbohydrates, fats, and proteins are digested and absorbed as sugars, fatty acids, and amino acids. Some substances, such as water, can be absorbed without digestion, and others, such as dietary fiber, cannot be digested by humans and therefore cannot be absorbed.  Substances that cannot be absorbed pass through the digestive tract and are excreted in the feces.

Digestion and absorption occur as food is moved through the digestive system. The amount of time it takes for food to move through the digestive system is referred to as transit time. In a healthy adult, transit time is about 24-72 hours. It is affected by the composition of the diet, physical activity, emotions, medications, and illnesses. To measure transit time, researchers add a non-absorbable dye to a meal and measure the time between consumption of the dye and its appearance in the feces. The shorter the transit time, the more rapid the passage through the digestive tract.

The main part of the digestive system is the gastrointestinal tract. It is also referred to as the GI tract, gut, digestive tract, intestinal tract, or alimentary canal. The organs of the GI tract include the mouth, esophagus, stomach, small intestine, large intestine, rectum, and anus.  The auxillary organs include the liver, the pancreas, and the gallbladder.

Sphincters (circular muscles) located along the length of the GI tract  (see pg. 123)regulate the flow of foodstuff. There are 5 sphincters worth noting:
1) Lower esophageal sphincter: sits between the esophagus and the stomach. It prevents the backflow of the stomach contents up into the esophagus. The stomach contents are highly acidic. Heartburn is the pain felt when the acidic stomach content enters the esophagus. Coffee, nicotine, alcohol, pregnancy, etc. can trigger heartburn. Figure 4-11 on page 128 shows the pH Scale and gives a comparison of the relative acidity/alkalinity of various substances.

2)  Pyloric sphincter: sits between the stomach and the small intestine. It controls the movement of acidic stomach contents into the small intestine. ~ 1 teaspoon of stomach contents squirts into the small intestine at a time. This slow rate allows for bicarbonate ions from the pancreas to neutralize the acid from the stomach. This is important. Why? It reduces the risk of acid erosion of the small intestine and possibly the formation of an ulcer.

3)  Sphincter of Oddi: sits at the end of the gallbladder. This controls the release of bile into the small intestine when fat from the diet is present there. Bile is an emulsifier. An emulsifier promotes the mixing of fats/oils in a watery solution. An emulsifier breaks fat globules into tiny particles and keeps them suspended in a watery medium. An example would be an egg yolk. It has ~ 6 grams of fat, but you never see a huge fat globule. Egg yolks contain lecithin (an emulsifier) which causes the fat to be permanently suspended as tiny particles. When you make an oil and vinegar salad dressing at home, the oil does not stay evenly distributed in the vinegar. Why? An emulsifier was not added. So the oil and vinegar separate.

4)  Ileocecal valve: sits between the small intestine and the large intestine (colon).  It prevents the contents of the large intestine from backing up into the small intestine. This prevents the bacteria from the colon from invading and colonizing the small intestine. It's important for the small intestine to have a relatively low concentration of bacteria because bacteria can compete for nutrients, thus absorbing nutrients before the body can. The bacteria would be healthy, you would be malnourished.

5) 2 anal sphincters: sit at the end of the colon. One is under voluntary control. They allow us to control the process of defecation.

Without sphincters along the GI tract, we would suffer more heartburn, ulcers, and diarrhea. 

Enzymes In Digestion
Enzymes speed up digestion by facilitating chemical reactions. Almost every reaction in the body requires an enzyme to hasten its occurrence.

Enzymes work like a lock and key mechanism. Individual enzymes usually act only on a specific substance. Example: the enzyme that recognizes table sugar will ignore milk sugar.

Enzyme activation in the GI tract is very dependent on pH, as well as the presence of key vitamins and minerals. Example: digestive enzymes that work in the acid environment of the stomach will not work well in the alkaline/neutral environment of the small intestine.

Most digestive enzymes are made by the pancreas and small intestine. A few are made by the mouth and stomach. The Summary on page 80 gives an overview of the digestive secretions.

Peristalsis
Peristalsis refers to how food is propelled through the GI tract. In the esophagus there is coordinated squeezing and shortening wave-like muscle action once the bolus is swallowed. In the stomach, peristaltic waves create a mixing and grinding action ~ 3X/minute during digestion. The stomach wall has 3 opposing muscle layers-circular, diagonal, and longitudinal-which allows the stomach to contract in various directions to churn and mix the meal with gastric juices (see figure 4-12 on page 129).
In the small intestine, peristaltic contractions occur ~ every 4-5 seconds. In the colon, sluggish peristalsis occurs. Here mass movements help eliminate the stool.

The Small Intestine
Approximately 95% of absorption of nutrients occurs in the small intestine. The wall of the small intestine is folded; within the folds are fingerlike villi projections; the "fingers" trap foodstuffs to increase absorption. Each villi finger is made of absorptive cells; each of these cells has a microvilli cap. The intestinal enzymes are found on the hair-like projections that cover the surface of the microvilli (see Figure 4-14 on page 131).

Celiac disease is an inherited immune disorder which causes the villi of the small intestine to lay flat; therefore, nutrients can't be absorbed. Wheat gluten (the protein in wheat) or similar proteins in barley and rye trigger events that can cause bloating, diarrhea and malnutrition. Without eliminating the offending proteins, the condition can lead to fatigue, migraines, dermatitis, anemia, and osteoporosis. Celiac disease affects ~ 2 million people in the USA. Gluten is often added to processed foods so it's quite common in the foods we buy. Scientists are looking for therapeutic alternatives to help people with this condition.

Where do the nutrients go once they're absorbed by the villi of the small intestine?
There are 2 circulatory pathways that can be taken:
1)  Portal vein pathway
2)  Lymphatic system pathway

Which pathway the nutrients travel depends on whether they are soluble in water or fat and/or their size.

Nutrients which are water soluble and/or are small in size travel via the Portal vein path: proteins, CHOs, short-chain fatty acids, B-Vitamins, Vitamin C.
These nutrients leave the villi of the small intestine and travel to the portal vein which takes them to the liver and then to the bloodstream.
Small intestine---villi---portal vein---liver---bloodstream; from the bloodstream the nutrients make their way to the cells of the body.

Nutrients which are fat soluble or are large in size travel via the lymphatic system pathway: long-chain fatty acids, fat-soluble Vitamins: A, D, E, K, and large proteins.
These nutrients leave the villi of the small intestine and travel through the thoracic duct to the subclavian vein to the bloodstream.
Small intestine---villi---thoracic duct---subclavian vein---bloodstream; from the bloodstream the nutrients make their way to the cells of the body.

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Study Guide 
(This is for your use. This will not be submitted to the instructor.)

The following points are designed to help you get the most from your reading.

1. What is the GI tract? Where does the process of digestion begin? What is the pharynx?
    What is the function of the epiglottis? What is a bolus? What is the function of the
    esophagus? What is the function of the esophageal sphincter? What is chyme? What is
    the function of the pyloric sphincter? What is the function of the ileocecal valve?
    What is the function of the rectum?

2. What is peristalsis and where does it begin? What is segmentation? What is the
    function of a sphincter muscle?

3. What glands begin the process of digestion? What nutrient is partially digested by
    salivary enzymes? What is the action of hydrochloric acid? What is the function of
    mucus? What is the pH of the stomach? What does pancreatic juice contain? What
    organ produces bile? Where is bile stored? What is the action of bile? What is the pH
    of the intestines? What is intestinal flora and what is its function? What is the function
    of fiber in digestion?

4. Where does the primary absorption of nutrients take place in the body? What are villi?
   What are microvilli and how do they function? What is the function of the crypts?

5. Which nutrients are transported directly through the vascular (bloodstream) system?
   Which nutrients are transported through the lymphatic system, and how do they enter
   the blood circulatory system? What are the primary differences between the two
   transport systems in the body?

6. What are the signs of choking? What technique could you apply to assist a choking
    person?

7. What is the function of belching, and what foods may cause an increase in belching?
    What causes flatus, and what distinguishes it from bloating?

8. What is heartburn/acid indigestion? What could help alleviate the symptoms of
    heartburn/acid indigestion? What is the danger associated with vomiting?

9. What is constipation? How can constipation be prevented or treated?

10. What is the cause of peptic ulcers?

The GI Tract:
1. The digestive process begins in the  ______.
2. The enzyme, ____, partially digests starch.
3. The ____ prevents food from entering the trachea.
4. A portion of chewed, swallowed food is known as a  _____.
5. The  ______ conducts food past the diaphragm.
6. The sphincter muscle that keeps the bolus and acid in the stomach is the  ____.
7. the ____ adds acidic juices to the bolus and grinds it.
8. the partially digested, semi liquid mass is  known as ____.
9. The sphincter muscle that keeps chyme in the small intestine is the ____.
10. Digestion and absorption of nutrients occurs primarily in the  _____.
11. The ___ stores and drips bile into the small intestine.
12. The ____ stores and drips digestive juices into the small intestine.
13. The sphincter muscle that keeps chyme in the large intestine is the ____.
14. The colon or bowel is also known as the ____.
15. The muscle that holds waste in the large intestine is the ____.
16. The last sphincter in the GI tract that allows voluntary passage of waste is the ____.

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