1. Topic 3: Digestion, Absorption, and Transport Chris Blanchard
BMS208 Human Nutrition
Topic 3: Digestion, Absorption, and Transport
Chris Blanchard

2. Objectives
List the segments of the digestive tract in order from the mouth to the colon.
Explain the mechanical processes of digestion in order of occurrence in the digestive tract.
List the five organs and their secretions which assist in the breakdown of food.
List the secretions that break down carbohydrates.
List the secretions that break down protein.
List the secretions that break down fat.
Describe the anatomy of the absorptive system.
Explain the role of the circulatory system and the lymphatic system in transport of nutrients.
Identify the hormones involved in digestive and absorptive processes.
Apply the concepts presented in the chapter to explain common digestive tract problems.

3. Digestion
The process of breaking down foods into nutrients to prepare for absorption
Need to overcome several challenges.

4. Digestion challenges Humans breathe, eat and drink though the mouth
Food must pass through the muscular diaphragm to reach the stomach
Material must be kept moving forward at an appropriate pace
The correct amount of fluid must be provided to allow lubrication but not too much that the material moves too quickly

5. Digestion challenges (cont’)
Particles must be finely divided for digestion but excreted with a low water content. Hence water needs to be removed before excretion
Digestive enzymes degrade carbohydrates, fats and protein, but the digestive tract is also composted of these cells
Once matter reaches the end of the digestive tract, it must be excreted in an appropriate manner

6. Anatomy of the Digestive Tract
The gastrointestinal (GI) tract is the flexible muscular tube from mouth to anus.
The lumen is the inner space of the tract.

7. Anatomy of the Digestive Tract (cont’)
Mouth
The beginning of the digestive system.
Involves mastication (chewing), the stimulation of taste buds, and swallowing.
The epiglottis closes to prevent food from entering the pharynx.
After swallowing the food is called a bolus.
Oesophagus
The tube that leads the bolus to the stomach.
There is a sphincter at the upper and lower ends of the oesophagus.

8. Anatomy of the Digestive Tract (cont’)
Stomach
Adds juices and grinds the bolus to a seimiliquid mass called chyme.
The pyloric sphincter regulates the flow of partially digested food into the small intestine.
Small intestine
Receives digestive juices from the gallbladder and the pancreas.
The three segments of the small intestine are the duodenum, the jejunum and the ileum.
Large intestine (colon)
Begins at the ileocecal valve and ends at the rectum and anus.
The chyme passes by the opening of the appendix.

9. Ileocecal valve (sphincter) Pharynx Pharynx Mouth Salivary glands
INGESTION
Appendix
Small intestine
Mouth
Salivary glands
Ileocecal valve (sphincter)
Pharynx
Pharynx
Mouth
Salivary glands
Epiglottis
Trachea (to lungs)
Upper esophageal sphincter
Pancreas
Epiglottis
Lower esophageal sphincter
Pancreatic duct
Trachea
Oesophagus
Oesophagus
Oesophageal sphincters
Stomach
Liver
Gallbladder
Pancreas
Pyloric sphincter
Pancreatic duct
Small intestine (duodenum, jejunum, ileum)
Bile duct
Stomach
Ileocecal valve
Pyloric sphincter
Appendix
Large intestine (colon)
Large intestine (colon)
Liver
Rectum
Rectum
Anus
Gallbladder
Anus
Bile duct
ELIMINATION
Fig. 3-1, p. 73

10. The Muscular Action of Digestion
Under autonomic control
Involves:
Peristalsis
Stomach action
Segmentation
Sphincter contractions

11. Peristalsis Pushes the digestive contents along. PERISTALSIS Chyme
The inner circular muscles contract, tightening the tube
and pushing the food forward in the intestine.
When the circular muscles relax, the outer longitudinal muscles contract, and the intestinal tube is loose.
As the circular and longitudinal muscles tighten and relax, the chyme moves ahead
of the constriction.

12. The Muscular Action of Digestion
Stomach action involves circular, longitudinal, and diagonal muscles.

13. Segmentation Promotes mixing with digestive juices. SEGMENTATION Chyme
Circular muscles contract, creating segments within the intestine.
As each set of circular muscles relaxes and contracts, the chyme is broken up and mixed with digestive juices.
These alternating contractions, occurring 12 to 16 times per minute, continue to mix the chyme and bring the nutrients into contact with the intestinal lining for absorption.

14. Sphincter contractions
Open and close passageways.
Prevents reflux and controls the passage of contents.

15. The Secretions of Digestion
Includes digestive enzymes that act as catalysts in hydrolysis reactions:
Saliva
Gastric Juice
Pancreatic juice and intestinal enzymes
Bile

16. Saliva Secreted by salivary glands
Contains: water, salts, mucus and enzymes

17. Gastric Juice Secreted by gastric glands in the stomach
Contains water, enzymes and hydrochloric acid
The goblet cells of the stomach wall secrete mucus to protect the walls of the stomach from the high acidity levels

18. Pancreatic juice Secreted by the pancreas
Contains intestinal enzymes (carbohydrase, lipase, protease) to digest food
Contains bicarbonate to neutralise the pH

19. Bile Produced by the liver Stored in the gall bladder
Acts as an emulsifier to suspend fat.

20. The Final Stage
Energy-yielding nutrients are disassembled for absorption
Vitamins, minerals and water can be absorbed
Some material is further digested through fermentation by bacteria
Undigested material continues through the digestive tract and forms the stool.

21. Absorption
The enormous surface area of the small intestine facilitates nutrient absorption.
Nutrients can be absorbed through simple diffusion, facilitated diffusion, or active transport.

22. FACILITATED DIFFUSION
Carrier loads nutrient on outside of cell . . .
Carrier loads nutrient on outside of cell . . .
Outside cell
Energy
Cell membrane
. . . and then releases it on inside of cell.
. . . and then releases it on inside of cell.
Inside cell
Some nutrients (such as water and small lipids) are absorbed by simple diffusion. They cross into intestinal cells freely.
SIMPLE DIFFUSION
FACILITATED DIFFUSION
Some nutrients (such as the water-soluble vitamins) are absorbed by facilitated diffusion. They need a specific carrier to transport them from one side of the cell membrane to the other.
ACTIVE TRANSPORT
Some nutrients (such as glucose and amino acids) must be absorbed actively. These nutrients move against a concentration gradient, which requires energy.
Figure 3.9: Absorption of Nutrients.
Absorption of nutrients into intestinal cells typically occurs by simple diffusion, facilitated diffusion, or active transport.

23. Anatomy of the Absorptive System
Villi
Micro villi
Crypts
Goblet cells

24. Anatomy of the Absorptive System (Cont’)
Villi are the fingerlike projections within the folds of the small intestine that move in a wave-like pattern to trap nutrients.

25. Anatomy of the Absorptive System (Cont’)
Microvilli are the microscopic hair-like projections on each villi.
Further increase the surface area for absorption

26. Crypts and Goblet cells
Crypts are the tubular glands that lie between the intestinal villi that secrete intestinal juices
Goblet cells are located between the villi and secrete a protective thick mucus.

27. Intestinal Cells
Cells of successive portions of the intestinal tract are specialized to absorb different nutrients
Digestive system can absorb all types of nutrients at once therefore “Food combining” which emphasizes separating food for digestive purposes is a myth.
Some combinations enhance absorption eg Vitamin C promotes absorptions of iron

28. Preparing Nutrients for Transport
Water-soluble nutrients and small products of fat digestion are released to the bloodstream.
Fat-soluble vitamins and larger fats form chylomicrons and are released to the lymphatic system.

29. The Circulatory System
The vascular system consists of arteries, capillaries and veins.
The hepatic portal vein directs blood from the GI tract to the liver.
The hepatic vein takes blood from the liver to the heart.
The liver protects against toxic substances.

32. The lymphatic system Consists of one-way vessels to transport fluid.
Lymph is a clear, yellowish fluid without red blood cells or platelets that moves through the body by muscle contractions.
The thoracic duct is the lymph’s route to the heart.
The subclavian vein provides a return of lymph to the vascular system.
Lacteals are the lymphatic vessels of the intestine that absorb nutrients and pass them to the lymphatic system.

33. The Health and Regulation of the GI Tract
The normal functions of the GI tract are influenced by:
Gastrointestinal bacteria
Hormonal and nervous control systems

34. Gastrointestinal Bacteria
A healthy GI tract has many different non-disease-causing bacteria known as flora or microflora.
Probiotics are bacteria found in the GI tract that can be beneficial to health. An example is the bacteria found in yogurt.
Prebiotics are foods that are used as food by intestinal bacteria.

35. Gastrointestinal Hormones and Nerve Pathways
Hormones act as messengers, and those involved in the GI tract are known as enterogastrones.
Gastrin: secreted by the stomach and stimulates HCl production
Pyloric sphincter: controlled by pH on the small intestine side
Secretin: is secreted by the duodenum into the bloodstream when it contains chyme. Secretin stimulates the pancreas to secrete bicarbonate-rich pancreatic juice

36. Gastrointestinal Hormones and Nerve Pathways (Cont’)
Pancreatic secretions change based on the content of the diet.
The pancreas is protected against enzymes by creating enzyme precursors called proenzymes or zymogen.
Cholecystokinin (CCK) is secreted into the bloodstream when there is fat in the intestine. This directs the gall bladder to contract and release bile.
CCK also slows motility for foods that take longer to be digested (fat and protein).

37. Common Digestive Problems

38. Choking Food becomes lodged in the trachea.
The larnyx cannot make sounds.

39. Choking
The Heimlich maneuver may need to be used.

40. Choking Strategies Small bites Chew thoroughly.
Don’t talk or laugh with food in the mouth.
Don’t eat when breathing hard.

41. Vomiting Body’s adaptive mechanism Dehydration is a concern.
May be self-induced as in eating disorders

42. Diarrhea Frequent, loose, watery stools
Irritable bowel syndrome or colitis is one of the common GI disorders.
Strategies
Rest
Drink fluids
Medical help is needed if it persists.

43. Constipation Defecation habits are different among people.
Many causes are possible.
Hemorrhoids may be a problem.

44. Constipation
Diverticulosis is a condition in which the intestinal walls weaken and bulge.
Bulging pockets are called diverticula.
Diverticulitis is a worsened condition and requires intervention.

45. Constipation
Use of laxatives, enemas and mineral oil may not be necessary with lifestyle changes.
Colonic irrigation is the internal washing of the large intestine and can be hazardous.
Strategies
High-fiber diet
Increased fluids
Exercise regularly.
Respond quickly to the urge to defecate.

46. Belching and Gas Strategies
Eat slowly.
Chew thoroughly.
Relax while eating.
Watch bothersome foods.
Hiccups are triggered by eating or drinking too fast.

47. Heartburn and “Acid Indigestion”
Gastroesophageal reflux is the backward flow of stomach contents into the oesophagus.

48. Heartburn and “Acid Indigestion”
Strategies
Small meals
Liquids between meals
Sit up while eating.
Wait 1 hour after eating before lying down.
Wait 2 hours after eating before exercising.
Refrain from tight-fitting clothing.
Avoid bothersome foods.
Refrain from tobacco use.
Lose weight if overweight.
Antacids and acid controllers

49. Ulcers Caused by Helicobacter pylori Strategies
Take prescribed medicine (antibiotics).
Avoid caffeine- and alcohol-containing foods.
Minimize aspirin and ibuprofen use.
No smoking.