1. Food As a Source of Nutrients
Humans obtain nutrients from food.
These nutrients are the source of raw materials that your body uses to build tissue and fuel cellular work.
There are six types of nutrients in food: carbohydrates, fats, proteins, vitamins, minerals, and water.

2. Four Stages of Food Processing
The process of how your body obtains raw materials from food is called nutrition.
This process involves the four stages.
First is eating.
Eating or drinking is called ingestion.

3. After food enters the body through ingestion, the next stage of food processing begins.
Digestion is the process of breaking food down into molecules small enough for the body to absorb.
For example, polysaccharides such as the starch in the pizza crust are broken down into monosaccharides (simple sugars).

4. Digestion occurs in two steps.
Mechanical digestion (such as chewing)
chops and grinds food into smaller pieces, increasing its surface area.
Chemical digestion breaks the chemical bonds within the large molecules that make up food, producing smaller building-block molecules.
An example of chemical digestion is the action of acids and enzymes in your stomach.

5. The last two stages of food processing occur after food is digested
The last two stages of food processing occur after food is digested. In the third stage, absorption, certain cells take up (absorb) the small molecules.
The circulatory system then transports the nutrients throughout the body.
Finally, undigested material passes out of the body in the stage called elimination.

6. Digestion Occurs in a Tube
Digestion occurs in a tube called the alimentary canal.
Food moves in one direction through the alimentary canal, which is organized into specialized regions that carry out digestion and absorption in a step-by-step process.

7. Epithelial tissue lines the alimentary canal.
One function of the epithelial cells is to secrete mucus that lubricates the canal and helps prevent the body from digesting itself.
This is important because the digestive juices in your stomach, for example, have a pH of 2—acidic enough to dissolve iron nails.

8. Despite the mucous layer, the epithelial cells in the stomach are constantly eroded.
Enough new cells are generated through mitosis to completely replace your stomach lining every three days.

9. Organs of the Digestive System
Six main organs make up the alimentary canal: the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
Accessory glands and organs include the salivary glands, pancreas, liver, and gallbladder, which secrete digestive juices into the alimentary canal.

10. Mouth Functions both in ingestion and in the beginning of digestion.
Your teeth and tongue are responsible for mechanical digestion.
The various shapes of different types of teeth cut, smash, and grind food into smaller pieces. This makes the food easier to swallow and exposes more surface area to digestive enzymes.

11. Chemical digestion also begins in your mouth.
Salivary glands in your mouth region secrete more than one liter of liquid.
This liquid is called saliva,
contains digestive enzymes, mucus, and other chemicals.

12. The salivary enzyme called amylase begins the chemical digestion of the polysaccharide starch in foods like pizza crust, pasta, and bread.
Other chemicals in saliva kill bacteria and neutralize certain acids in foods, protecting your teeth from decay.

13. Pharynx
The tongue pushes each chewed clump of food, called a bolus, down the throat.
The upper portion of the throat, is the pharynx,
the function of the alimentary canal and the passageway by which air enters the lungs. When you swallow, a cartilage flap called the epiglottis temporarily seals off the airway and prevents food from entering.

14. Esophagus
The bolus enters a long, muscle-encased tube called the esophagus.
This connects the pharynx to the stomach. Although the esophagus is oriented vertically in your body, gravity is not the reason that food moves toward your stomach.

15. Food is pushed through the esophagus by a series of muscle contractions called peristalsis.
The muscles at the very top of the esophagus are striated (voluntary), which means that swallowing begins voluntarily.
The muscle layers around the rest of the esophagus are smooth (involuntary).

16. Once a bolus of food reaches the pharynx these smooth muscles trigger the swallowing reflex.
The smooth muscles contract in a wave-like motion that forces the bolus of food toward the stomach.
Food continues to move along the alimentary canal by peristalsis.

17. Stomach
The stomach is an elastic, muscular sac capable of stretching to hold up to 2 liters of food.
A liquid called gastric juice, secreted by glands in the stomach lining, bathes the bolus after it enters the stomach.
Gastric juice is a mixture of mucus, hydrochloric acid, and enzymes.

18. Hydrochloric acid breaks apart the cells in food
Hydrochloric acid breaks apart the cells in food. It also kills many of the bacteria swallowed with food.
One of the gastric enzymes, pepsin, hydrolyzes large protein molecules into smaller polypeptides.

19. Meanwhile, mechanical digestion turns the bolus into an acidic liquid called chyme.
Stomach muscles contract, creating a churning motion that stirs the chyme and eventually forces it into the small intestine.

20. Most of the time, the stomach is pinched closed at both ends
The passageway between the esophagus and the stomach opens when peristalsis delivers a bolus.
But in some people, the passageway may open at inappropriate times, allowing acidic chyme to flow backward into the esophagus. This creates a burning sensation called heartburn.

21. At the opposite end of the stomach, a muscular valve called the pyloric sphincter
This regulates the flow of chyme into the small intestine.
It typically takes 2 to 6 hours after a meal for the stomach to empty.

22. Small Intestine, Liver, and Pancreas
From the stomach, liquid chyme passes into the small intestine.
The small intestine is a long (6 m), narrow (2.5 cm) tube where digestion is completed and absorption of most nutrients takes place.
Peristalsis moves chyme along the small intestine.
Digestion mostly occurs in the first portion of the small intestine, while absorption occurs along the rest of its length.

23. The first section of the small intestine is called the duodenum.
As chyme enters the duodenum, it mixes with several digestive juices.
One of these digestive juices is a liquid called bile.

24. Bile
is produced outside of the small intestine by the body's largest internal organ, the liver.
Bile is stored in a sac-like structure called the gallbladder until it is secreted into the duodenum.

25. Fats tend to clump together into globs, making it difficult for enzymes to reach the molecules.
Bile separates small fat droplets, preventing them from clumping into globs.
This enables digestive enzymes to break the fats down more efficiently

26. The pancreas produces and secretes pancreatic juice into the duodenum.
Pancreatic juice neutralizes the acid chyme and also contains enzymes that hydrolyze carbohydrates, proteins, and lipids.

27. Large Intestine
By the time food reaches the end of the small intestine, the nutrients have all been broken down and absorbed.
Undigested material passes through another sphincter from the small intestine into the large intestine.

28. Also called the colon, the large intestine is a wide (5 cm), short (1
Also called the colon, the large intestine is a wide (5 cm), short (1.5 m) tube from which water is absorbed into the body.
The large intestine also contains certain bacteria that produce vitamin K and several B vitamins.

29. a major function of the large intestine is to reabsorb water.
Saliva, gastric juice, and other digestive juices all contain large amounts of water.

30. Much of that water is reabsorbed along with nutrients in the small intestine.
The large intestine finishes the job by absorbing most of the remaining water.
Together the small intestine and large intestine reclaim 90 percent of the water that enters the alimentary canal.

31. Undigested food material and other waste products are called feces.
Reabsorption of water causes the feces to become more solid as it moves through the large intestine.
Once again, peristalsis is the mechanism that moves this material through the large intestine. It generally takes 12 to 24 hours for waste material to travel through the colon.