1. Digestive System

2. By the end of this class you should understand:
The processes of physical and chemical digestion
The route of food, nutrients and waste through the body
The major nutrients the body requires for life
The major risks associated with excess and deficiencies of food

3. Digestive Organs
The digestive system is divided into two types of organs:
The alimentary organs that make up the alimentary canal
Accessory organs that do not make up the alimentary canal
The alimentary canal is the tube running through your body

4. Alimentary Organs Mouth Pharynx (throat) Esophagus Stomach
Small intestine
Large intestine
Anus

5. Movement
Organs are moved from one to the next through a process called peristalsis (the pushing of food through the alimentary canal by smooth muscle)
In the esophagus, swallowing is initiated by skeletal muscle but completed by smooth muscle and qualifies as peristalsis
The stomach, small and large intestine all have peristalsis but it is much slower

6. Digestive Function
The digestive system has one goal only: to put nutrients into the bloodstream so that all our cells can access those nutrients
Many excess nutrients can be stored
The digestive system has many “preparation steps” to prepare food for this process

7. The Most Vital Organ
Only one organ performs almost all the absorption of nutrients (excepting water): the small intestine
So called because it has a small diameter, though it is several meters long
If we drank liquids containing only single molecules of nutrients, we would need no other digestive organ!
This is what they inject into your blood when you’re in a coma

8. Two Types of Digestion
Digestion is the process of breaking down nutrients so they can be absorbed
Undigested nutrients cannot be absorbed and pass through the small intestine to the large intestine
For more information, ask someone who is lactose-intolerant about drinking milk
Mechanical (physical) digestion is the chewing and churning of food
Chemical digestion is the breaking of chemical bonds in food

9. Mechanical Digestion
The mouth (chewing) and stomach (churning) perform mechanical digestion of food
The mouth accomplishes this with hard calcium structures called teeth and heavy muscles attached to the mandible chewing
The stomach has layers of smooth muscles that churn the food before it moves to the small intestine

10. Chemical Digestion
The mouth and stomach also initiate the process of chemical digestion
The mouth has salivary glands that produce saliva, which begins chemical digestion
The stomach has pepsin, an enzyme that denatures and breaks down protein
Note chemical digestion of fats does not begin until the small intestine

11. The Stomach
The stomach produces gastric juice, which contains pepsin as well as other chemicals
This includes hydrochloric acid, which makes the stomach environment very acid
The acid itself is not for digesting the food
Kills bacteria
Enables pepsin to work

12. Stomach Ulcers
Stomach ulcers were once thought to be caused by stress but are now known to be a bacterial infection (but stress does block the immune system)
The scientist who discovered the bacterium that causes ulcers was eventually awarded the Nobel Prize
No one had believed him initially so he drank a flask of the bacterium to gain ulcers

13. Gastric Juice Gastric juice is the secretion of the stomach
Gastric juice is very acidic and can damage tissues other than the stomach lining
Repeated vomiting can cause gastric juice to erode the enamel of the teeth
If gastric juice is produced in excess it can overflow into the esophagus, causing “heartburn”
Once gastric juice mixes with and digests food it is called chyme

14. Processing of Chyme
Chyme is the partially digested food of the stomach mixed with acid
Chyme is delivered into the small intestine very slowly
Excess acid could damage the small intestine
Helps the intestine absorb all the nutrients
The small intestine has many digestive enzymes of its own that complete digestion

15. Accessory Organs
The small intestine has enzymes on its absorptive cells but also receives digestive juices from two accessory organs
The pancreas produces a massive battery of digestive enzymes to finish all digestion of food
Most prominent enzyme is trypsin, a protease that breaks down protein into amino acids
The liver produces bile, which is stored in the gallbladder until chyme enters the small intestine

16. Bile
Bile does not contain digestive enzymes and does not directly perform chemical digestion
Bile is instead an emulsifying agent, much like a detergent
To emulsify is to allow oil and water to mix
Bile is therefore vital for digesting and absorbing fats from food

17. Small Intestine Absorption
Since nutrients can only be absorbed through cell membrane, the small intestine epithelial tissue has adaptations for increasing surface area
The tissue is arranged into fingerlike villi (singular villus) which create more surface area
The individual cells have a brush border, or microvilli (singular microvillus)
These cells expend ATP to actively transport nutrients into the blood

18. Liver Function
Blood flows directly from the small intestine to the liver
This is called hepatic portal circulation
Protects the rest of the body from whatever you just ate
The liver has metabolic enzymes to process many nutrients after they are absorbed
Production of fat and glycogen
Storage of vitamins
Breakdown of toxins such as alcohol

19. Large Intestine
Once all nutrients are absorbed into the small intestine, the large intestine (which is mostly composed of a tube called the colon) carries remaining indigestible materials to the anus
The large intestine is filled with colonies of bacteria that feast on our leftovers (often producing gas as a waste product)
The large intestine is capable of absorbing water and vitamins, including those produced by bacteria

20. Vital Nutrients
The small intestine absorbs all the nutrients required for life
Some nutrients are required for energy and for building blocks for more cells and cell parts
Carbohydrates
Proteins
Lipids
Other nutrients are required for homeostasis but not energy
Water
Vitamins
Minerals

21. Carbohydrates
A single carbohydrate molecule is called a simple carbohydrate (such as glucose)
Starches are made of many bound together and are called complex carbohydrates
Some are more complex than others, and the more complex the branching, the longer the digestion takes
“White” starches such as white rice and pasta contain simpler chains
Complex carbs, since they break down more slowly, are less likely to cause blood sugar to spike

22. Proteins Proteins must be broken down to the individual amino acids
The human body can synthesize many of the amino acids from other amino acids (so they are nonessential), but there are certain ones that cannot be synthesized
These are called essential amino acids
If even one of the essential amino acids is missing, the diet is incomplete and no new protein can be made

23. Lipids There are many classes of lipids
Some vitamins and cholesterol are both lipids, but cannot be burned for energy
The energy-storing lipids are fatty acids, and these fats can be saturated or unsaturated
Saturated fats have no double bonds and so pack together more tightly than unsaturated fats
Treating unsaturated fats with hydrogenation can create trans fats, which are very difficult to metabolize and therefore very unhealthy

24. Vitamins and Minerals
Vitamins are organic compounds that serve key purposes in cell mechanisms but only in small amounts
Example: vitamin A used to make retinal
Minerals are inorganic elements and compounds, such as sodium, potassium, calcium, iron, and iodine
Also necessary for life

25. Hopefully that wasn’t too much to digest…
Enjoy your Thanksgiving weekend!
See you next week for the last week of new lessons!
Tuesday: Urinary system
Thursday: Reproductive system