Biology 12-3.1B From the Mouth to the Stomach

hydroxide

a compound of a metal with the hydroxide ion OH− (as in many alkalis) or the group —OH.

chemical digestion

1. the process in the alimentary canal by which food is broken up physically, as by the action of the teeth, and chemically, as by the action of enzymes, and converted into a substance suitable for absorption and assimilation into the body. 2. the function or power of digesting food.

The Mouth

The mechanical digestion and chemical digestion of food begins in the mouth. Mechanical digestion involves the cutting and crushing of food by the teeth. The tongue aids mechanical digestion by moving and positioning food for chewing and swallowing, and mixing food with saliva. Saliva is released from salivary glands inside the mouth. Salivary glands produce about one litre of saliva each day. Saliva contains mainly water used to moisten food and aid hydrolysis, as well as salivary amylase, an enzyme protein responsible for the chemical digestion of starch, which is converted to maltose. The mechanical and chemical processes that take place in the mouth produce a food ball or bolus that can then be swallowed.

The Stomach

The stomach is a muscular organ that mechanically and chemically digests food. The muscular walls of the stomach contract to mix the food with acid and enzymes, resulting in the production of acid chyme. Cells in the stomach lining produce hydrochloric acid, the pepsinogen enzyme, and mucus. The low pH caused by the production of hydrochloric acid kills bacteria and activates pepsinogen, which, as pepsin, digests proteins into peptides. Pepsin has optimal activity in the low pH of the stomach. The mucus protects the stomach from its acidic contents. The following chart summarizes the components and function of gastric juice. The pyloric sphincter controls the amount of acid chyme that enters the duodenum by releasing small amounts at regular intervals. The duodenum is important for the digestion and absorption of nutrients, and it makes enzymes like maltase and peptidases. From the pancreas it receives enzymes and bicarbonate that neutralize acid. From the liver, via the gall bladder, it receives bile that emulsifies fats. The function of the small intestine, accessory organs, and the large intestine are the topics of later lessons in this section.

Know these things

describe the role of the mouth in the digestion of food describe the role of the throat in the swallowing of a bolus understand the role of the esophagus in digestion and the process of peristalsis list the composition of gastric juice and the role of the stomach in digestion

pepsin

the chief digestive enzyme in the stomach, which breaks down proteins into polypeptides.

swallowing

to take into the stomach by drawing through the throat and esophagus with a voluntary muscular action, as food, drink, or other substances.

mucus

a slimy substance, typically not miscible with water, secreted by mucous membranes and glands for lubrication, protection, etc.

hydrochloric acid

a strongly acidic solution of the gas hydrogen chloride in water.

pepsinogen

a substance that is secreted by the stomach wall and converted into the enzyme pepsin by gastric acid.

gastric juice

a thin, clear, virtually colourless acidic fluid secreted by the stomach glands and active in promoting digestion.

salivary amylase

an enzyme in the saliva that converts starch into dextrin and maltose.

physical digestion

Physical digestion is the physical process of breaking down food into smaller pieces that can easily be accessed by digestive enzymes, without making any chemical changes, whereas chemical digestion refers to the process through which the mammalian body further breaks down food substances into small, soluble chemicals that can be absorbed into the blood. The chewing of food and movement in the stomach contribute to physical digestion while natural catalysts aid chemical digestion.

The Esophagus

The esophagus, a food tube about 25 centimetres long, moves the food bolus to the stomach by peristalsis. Once the food ball enters the esophagus, it is pushed towards the cardiac sphincter by smooth muscle contractions called peristalsis. Food travels from the mouth to the stomach in about 4 to 8 seconds. Peristalsis occurs throughout the length of the digestive tract and is responsible for keeping things moving and for the occasional strange sounds that arise. The digestive tract is surrounded by both circular and longitudinal smooth muscle that allows for rhythmic contractions or peristalsis. This movement is like squeezing the last little bit from the bottom of a tube of toothpaste. The cardiac or gastroesophageal sphincter is a ring of muscle located at the entrance to the stomach that keeps the food and acid chyme in the stomach, preventing reflux during mechanical digestion. If reflux occurs and chyme moves back into the esophagus, a burning sensation in the throat is experienced. This irritation of the esophagus is commonly known as heartburn.

Describe in your own words the mechanical and chemical processes in the mouth that result in the formation of a bolus.

The mechanical digestion and chemical digestion of food begins in the mouth. Mechanical digestion involves the cutting and crushing of food by the teeth. The tongue aids mechanical digestion by moving and positioning food for chewing and swallowing, and mixing food with saliva. Saliva is released from salivary glands inside the mouth. Salivary glands produce about one litre of saliva each day. Saliva contains mainly water used to moisten food and aid hydrolysis, as well as salivary amylase, an enzyme protein responsible for the chemical digestion of starch, which is converted to maltose. The mechanical and chemical processes that take place in the mouth produce a food ball or bolus that can then be swallowed

The Pharynx

The pharynx or throat is a common passage for food to the esophagus and air to the trachea. The epiglottis meets with the glottis during swallowing to cover the opening to the trachea and send food into the esophagus. Mechanical digestion of food (mouth, teeth, and tongue) and mixing it with saliva produces a food ball or bolus that can be swallowed. As the food moves from the mouth to the pharynx past the uvula (dangly thing at the back of the mouth and goaltender for our digestive system) our control of the process ends. Swallowing is a reflex action that happens automatically. This reflex pushes the bolus into the esophagus. The trachea or windpipe lies in front of the esophagus and shares the pharynx as a passage for air. When swallowing occurs the soft palate of the mouth pushes down and the trachea pushes up (visible by movement of the Adam's apple), bringing the glottis and epiglottis together to close the trachea. The epiglottis covers the opening to the trachea to ensure food moves into the esophagus.

digestive tract

The series of organs in the digestive system through which food passes, nutrients are absorbed, and waste is eliminated. In higher vertebrates, it consists of the esophagus, stomach, small and large intestines, rectum, and anus.

peristalsis

The wavelike muscular contractions in tubular structures, especially organs of the digestive system such as the esophagus and the intestines. Peristalsis is characterized by alternate contraction and relaxation, which pushes ingested food through the digestive tract towards its release at the anus.