3.2: Mouth to the Stomach

Digestion begins in the mouth, both mechanically and chemically. Mechanical digestion is called mastication, which is the chewing and grinding of food into smaller pieces. The salivary glands release saliva, mucus, and the enzymes, salivary amylase, lingual lipase and lysozyme.

Figure \(\PageIndex{1}\): The mouth (public Domain; Alan Hoofring (NIH) via Wikipedia)

Salivary amylase cleaves the alpha 1-4 glycosidic bonds in the carbohydrate (typically starch) molecules, amylose and amylopectin. However, salivary amylase cannot cleave the branch points in amylopectin where there are alpha 1-6 glycosidic bonds, as shown in the figure below. Overall this enzyme accounts for a minor amount of carbohydrate digestion.

Figure \(\PageIndex{2}\): Enzymatic action of salivary amylase. Purple arrows point to alpha 1-4 glycosidic bonds that can be cleaved. The yellow arrows point to the alpha 1-6 glycosidic bonds that cannot be cleaved

Another enzyme, lingual lipase, is also released in the mouth. Although it is released in the mouth, it is most active in the stomach where it preferentially cleaves short-chain fatty acids in the sn-3 position of triglycerides. Lingual lipase has a small role in digestion in adults, but may be important for infants to help break down triglycerides in breast milk2. Lysozyme is a non-digestive enzyme that breaks down bacteria cell walls to prevent a possible infection.

Query \(\PageIndex{1}\)

Swallowing

Now that the food has been thoroughly chewed and formed into a bolus (a ball of masticated food and saliva), it can proceed down the pharynx where it reaches a "fork in the road" with the larynx as one road and the esophagus as the other. The esophagus road leads to the stomach; this is the direction that food should go. The other road, through the larynx, leads to the trachea and ultimately the lungs. This is definitely not where you want your food or drink going, as this is the pathway for the air you breathe.

Figure \(\PageIndex{3}\): Cross section of face. The epiglottis covers the larynx to prevent food and drink from entering the lungs3

Fortunately, our body was designed in such a way that a small tissue, called the epiglottis, covers the opening to the trachea. It directs the food down the correct road as shown below.

Figure \(\PageIndex{4}\): Epiglottis is like a traffic cop guiding food down the correct digestion road.

Query \(\PageIndex{2}\)

Esophagus

Before being correctly guided into the esophagus, the bolus of food will travel through the upper esophageal sphincter. Sphincters are circular muscles that are found throughout the gastrointestinal tract that essentially serve as gates between the different sections. Once in the esophagus, wavelike muscular movements, known as peristalsis, occur, as shown in the following video and the animation in the link below.

Video \(\PageIndex{1}\): This endoscopy videoclip shows the peristalsis in the lower part of the stomach, the antrum. Peristalis is a wavelike, constant, rhytmic movement caused by muscular contraction carrying food from the stomach through the pylorus into the duodenum, which is the first part of the small bowel. https://www.youtube.com/watch?v=o18UycWRsaA

Web Link

Peristalsis Animation

Query \(\PageIndex{3}\)

At the end of the esophagus the bolus will encounter the lower esophageal sphincter. This sphincter keeps the harmful acids of the stomach out of the esophagus. However, in many people this sphincter is leaky, which allows stomach acid to reflux, or creep up, the esophagus. Stomach acid is very acidic (has a low pH). The ruler below will give you an idea of just how acidic the stomach is. Notice that the pH of gastric (term used to describe the stomach) fluid is lower (more acidic) than any of the listed items besides battery acid.

Figure \(\PageIndex{5}\): pH of some common items4

The leaking of the very acidic gastric contents results in a burning sensation, commonly referred to as "heartburn." If this occurs more than twice per week and is severe, the person may have gastroesophageal reflux disease (GERD). The following videos explain more about these conditions.

Video \(\PageIndex{2}\): Gastro Esophageal Reflux Disease (GERD) or Acid Reflux is an abnormal reflux (Backflow) of gastric contents into the esophagus, creating a mucosal damage. https://www.youtube.com/watch?v=SW-QfyDSY5I

Video \(\PageIndex{3}\): What on earth is GERD and what does it have to do with you? https://www.youtube.com/watch?v=FqdOvZkrSYk&feature=rec-lis-watch-cur_emp-farside_rn

Table \(\PageIndex{1}\): Review of Chemical Digestion in the Mouth

Macronutrient	Action
Carbohydrates	Salivary amylase cleaves 1,4-glycosidic bonds
Lipids	Release of lingual lipase
Protein	None

Query \(\PageIndex{4}\)

References

1. Alan Hoofring, http://visualsonline.cancer.gov/deta...m?imageid=4371
2. Shils ME, Shike M, Ross AC, Caballero B, Cousins RJ, editors. (2006) Modern nutrition in health and disease. Baltimore, MD: Lippincott Williams & Wilkins.
3. http://en.Wikipedia.org/wiki/File:Il..._head_neck.jpg
4. upload.wikimedia.org/wikipedi...6/PH_scale.png