chapter 24 5 – deglutition & the stomach. deglutition a bolus is a soft, rounded mass of food...

Chapter 24

5 – Deglutition & The Stomach

Deglutition

• A bolus is a soft, rounded mass of food that is formed in the mouth when food is mixed with saliva.

Deglutition

– Once the bolus is swallowed, it passes from the mouth into the pharynx.

Deglutition

• The pharynx is a funnel-shaped tube that starts at the back of the mouth and runs down the neck where it opens into the esophagus posteriorly and the larynx anteriorly.

Deglutition

• The esophagus is a hollow muscular tube that connects the pharynx to the stomach.

Deglutition

• Deglutition (swallowing) is the movement of food from the mouth into the stomach.

Deglutition

• Deglutition occurs in three stages:

1. The voluntary stage

2. The pharyngeal stage

3. The esophageal stage

Deglutition

1. The voluntary stage starts when the bolus is forced to the back of the oral cavity and into the pharynx by the movement of the tongue.

Deglutition

2. The pharyngeal stage begins when the bolus moves into the pharynx and the brain stem sends impulses to the epiglottis to close the opening of the larynx which prevents the bolus from entering the trachea.

Deglutition

3. The esophageal stage begins once the bolus enters the esophagus; during this phase peristalsis pushes the bolus into the stomach.

Deglutition

• Peristalsis is a progression of successive muscular contractions that occurs in the muscularis of the GI tract.

The Stomach

• The stomach is a J-shaped enlargement of the GI tract that connects the esophagus to the duodenum.

The Stomach

• The stomach has four main regions:

1. The cardia

2. The fundus

3. The body

4. The pylorus

The Stomach

1. The carida surrounds the superior opening of the stomach.

The Stomach

2. The fundus is the rounded portion superior to and to the left of the cardia.

The Stomach

3. The body is the large central portion of the stomach inferior to the fundus.

The Stomach

4. The pylorus is the region of the stomach that connects the duodenum; it communicates with the duodenum via a sphincter called the pyloric sphincter.

The Stomach

• **The stomach wall is composed of the same four basic layers of the GI tract, with certain modifications.

The Stomach

• **Recall – the four layers of the GI tract are: the mucosa, the submucosa, the muscularis, and the serosa.

The Stomach

• **The mucosa of the stomach contains secretory cells called ‘gastric glands’ that empty their secretions into narrow channels called gastric pits.

The Stomach

• Gastric glands are glands in the mucosa of the stomach; the four types of cells are chief cells, parietal cells, mucous neck cells and G cells.

The Stomach

1. Chief cells secrete pepsinogen & gastric lipase

2. Pareital cells secrete hydrochloric acid (HCl)

3. Mucous neck cells secrete mucus

4. G cells secrete gastrin

The Stomach

• **The digestion that occurs in the stomach is both mechanical and chemical.

The Stomach

1. Mechanical digestion in the stomach consists of peristaltic movements called ‘mixing waves’ that intensify as food reaches the pylorus.

The Stomach

• Mixing waves macerate food, mix it with secretions of the gastric glands, and reduce it to chyme.

The Stomach

• Chyme is the semifluid mixture of partly digested food and digestive secretions found in the stomach and small intestine during the digestion of a meal.

The Stomach

• **As food reaches the pylorus, each mixing waves periodically forces about 3 mL of chyme into the duodenum through the pyloric sphincter.

The Stomach

– Most of the chyme is forced back into the stomach where mixing continues; the next wave pushes the chyme forward again and forces a little more into the duodenum.

The Stomach

2. Chemical digestion in the stomach consists of the conversion of proteins into peptides by pepsin, and triglycerides into fatty acids and glycerol by the enzymes lingual lipase & gastric lipase.

The Stomach

• **Pepsin, lingual lipase & gastric lipase are all activated in extremely acidic environments.

The Stomach

• **Recall – the acid (HCl) is secreted by the stomach’s parietal cells; it creates a very acidic environment (pH 2) in the stomach.

The Stomach

• **Recall – ‘pepsinogen’ is secreted by the stomach’s chief cells.

The Stomach

• **Pepsinogen is converted into active ‘pepsin’ when it comes in contact with HCl.

The Stomach

• The stomach wall is impermeable to most substances; however, some water, electrolytes, certain drugs (especially aspirin), and alcohol can be absorbed through the stomach lining.

The Stomach

• From the stomach, chyme passes into the small intestine; chemical digestion in the small intestine is not autonomous, it depends on activities of accessory digestive organs.

Homework

• Finish handout:

– “Deglutition & The Stomach”

• Study for mini-quiz