1. A very important organ in the digestive system is the stomach.

2. The stomach is located just below the esophagus in the digestive system. Partially digested food from the esophagus passes into the stomach, where more digestion takes place. The Stomach

3. Here we’ll look at a cross-sectional view of a stomach.

4. The stomach wall consists of 3 layers of muscle fibres. 3 Layers of Muscle

5. Inside the muscle layers is a layer called the mucosa layer. Mucosa Layer 3 Layers of Muscle

6. This mucosa layer contains cells that produce mucous, Mucosa Layer 3 Layers of Muscle  Cells that produce mucous  Cells that produce hydrochloric acid  Cells that produce pepsinogen

7. Cells that produce hydrochloric acid, Mucosa Layer 3 Layers of Muscle  Cells that produce mucous  Cells that produce hydrochloric acid  Cells that produce pepsinogen

8. And cells that produce a substance called pepsinogen Mucosa Layer 3 Layers of Muscle  Cells that produce mucous  Cells that produce hydrochloric acid  Cells that produce pepsinogen

9. Food enters the stomach through a tube called the esophagus. Mucosa Layer 3 Layers of Muscle  Cells that produce mucous  Cells that produce hydrochloric acid  Cells that produce pepsinogen Esophagus

10. A ring of muscle at the bottom of the esophagus is called the lower esophageal sphincter. The sphincter muscles can open or close the passage from the esophagus into the stomach. The lower esophageal sphincter is sometimes called the cardiac sphincter. Mucosa Layer 3 Layers of Muscle  Cells that produce mucous  Cells that produce hydrochloric acid  Cells that produce pepsinogen Esophagus Lower Esophageal Sphincter

11. When there is no food entering the stomach, this passage remains closed. This prevents the highly acidic stomach contents from entering the esophagus and damaging it. Mucosa Layer 3 Layers of Muscle  Cells that produce mucous  Cells that produce hydrochloric acid  Cells that produce pepsinogen Esophagus Lower Esophageal Sphincter Closed

12. There is another sphincter at the bottom of the stomach, called the pyloric sphincter. This opens and closes to control the passage of partially digested food from the stomach to small intestine. We see that it is closed here. Mucosa Layer 3 Layers of Muscle  Cells that produce mucous  Cells that produce hydrochloric acid  Cells that produce pepsinogen Esophagus Lower Esophageal Sphincter Pyloric Sphincter

13. When a food particle, or bolus comes down the esophagus, Bolus

14. The lower esophageal sphincter opens and allows it to pass into the stomach.

15. The presence of food triggers cells in the mucosa layer to secrete a substance called pepsinogen. pepsinogen

16. And other cells secrete hydrochloric acid, or HCl pepsinogen + HCl

17. HCl reacts with pepsinogen to form an enzyme called pepsin pepsinogen + HCl pepsin

18. The enzyme pepsin breaks down large protein molecules into smaller protein molecules. pepsin l arge protein molecules small protein molecules

19. This is a type of chemical digestion that takes place in the stomach. The enzyme pepsin speeds up the chemical breakdown of large protein molecules. pepsin l arge protein molecules small protein molecules chemical digestion

20. While this is happening, the muscles of stomach wall contract in a peristaltic fashion, moving the bolus and enzymes around. This, along with the chemical digestion, changes the bolus into a liquid called chyme. bolus chyme

21. This churning action helps mix the enzymes with the food and speeds up digestion. bolus chyme

22. The physical breaking up of food in the stomach, caused by the muscular contractions is called mechanical digestion. mechanical digestion

23. Now, we’ll review digestion in the stomach. Digestion in the Stomach.  Cells in the mucosa layer secrete mucous, pepsinogen and hydrochloric acid. The mixture formed by these is called gastric juice.  Pepsinogen and hydrochloric acid combine to form the enzyme pepsin.  The enzyme pepsin breaks down large protein molecules in food into smaller ones. This is called chemical digestion.  While this is going one, muscles in the stomach wall contract in a peristaltic fashion to churn food around and break it up. It mixes with gastric juices, forming a liquid called chyme. The physical breaking down of food in the stomach is called mechanical digestion.

24. When food enters the stomach, cells in the mucosa layer secrete, or give off mucous, pepsinogen and hydrochloric acid. Mucous helps to lubricate everything. Digestion in the Stomach.  Cells in the mucosa layer secrete mucous, pepsinogen and hydrochloric acid. The mixture formed by these is called gastric juice.  Pepsinogen and hydrochloric acid combine to form the enzyme pepsin.  The enzyme pepsin breaks down large protein molecules in food into smaller ones. This is called chemical digestion.  While this is going one, muscles in the stomach wall contract in a peristaltic fashion to churn food around and break it up. It mixes with gastric juices, forming a liquid called chyme. The physical breaking down of food in the stomach is called mechanical digestion.

25. The mixture formed by all these things is called gastric juice. Because it contains hydrochloric acid, gastric juice is very acidic. Mucous helps protect the stomach wall from the corrosive effects of the acid. Digestion in the Stomach.  Cells in the mucosa layer secrete mucous, pepsinogen and hydrochloric acid. The mixture formed by these is called gastric juice.  Pepsinogen and hydrochloric acid combine to form the enzyme pepsin.  The enzyme pepsin breaks down large protein molecules in food into smaller ones. This is called chemical digestion.  While this is going one, muscles in the stomach wall contract in a peristaltic fashion to churn food around and break it up. It mixes with gastric juices, forming a liquid called chyme. The physical breaking down of food in the stomach is called mechanical digestion.

26. The Pepsinogen and hydrochloric acid combine to form an enzyme called pepsin. Digestion in the Stomach.  Cells in the mucosa layer secrete mucous, pepsinogen and hydrochloric acid. The mixture formed by these is called gastric juice.  Pepsinogen and hydrochloric acid combine to form the enzyme pepsin.  The enzyme pepsin breaks down large protein molecules in food into smaller ones. This is called chemical digestion.  While this is going one, muscles in the stomach wall contract in a peristaltic fashion to churn food around and break it up. It mixes with gastric juices, forming a liquid called chyme. The physical breaking down of food in the stomach is called mechanical digestion.

27. Pepsin helps break down large protein molecules in food into smaller ones. These small protein molecules are broken down further as the food passes through the intestines. Digestion in the Stomach.  Cells in the mucosa layer secrete mucous, pepsinogen and hydrochloric acid. The mixture formed by these is called gastric juice.  Pepsinogen and hydrochloric acid combine to form the enzyme pepsin.  The enzyme pepsin breaks down large protein molecules in food into smaller ones. This is called chemical digestion.  While this is going one, muscles in the stomach wall contract in a peristaltic fashion to churn food around and break it up. It mixes with gastric juices, forming a liquid called chyme. The physical breaking down of food in the stomach is called mechanical digestion.

28. Breaking down protein molecules is a chemical reaction, so this process is called chemical digestion. Enzymes are needed for all types of chemical digestion in our body. Different enzymes break down different types of food. Digestion in the Stomach.  Cells in the mucosa layer secrete mucous, pepsinogen and hydrochloric acid. The mixture formed by these is called gastric juice.  Pepsinogen and hydrochloric acid combine to form the enzyme pepsin.  The enzyme pepsin breaks down large protein molecules in food into smaller ones. This is called chemical digestion.  While this is going one, muscles in the stomach wall contract in a peristaltic fashion to churn food around and break it up. It mixes with gastric juices, forming a liquid called chyme. The physical breaking down of food in the stomach is called mechanical digestion.

29. While this is going one, muscles in the stomach wall contract in a peristaltic fashion to churn food around and break it up. Digestion in the Stomach.  Cells in the mucosa layer secrete mucous, pepsinogen and hydrochloric acid. The mixture formed by these is called gastric juice.  Pepsinogen and hydrochloric acid combine to form the enzyme pepsin.  The enzyme pepsin breaks down large protein molecules in food into smaller ones. This is called chemical digestion.  While this is going one, muscles in the stomach wall contract in a peristaltic fashion to churn food around and break it up. It mixes with gastric juices, forming a liquid called chyme. The physical breaking down of food in the stomach is called mechanical digestion.

30. The food mixes with gastric juices, and the combination of churning and chemical breakdown of proteins forms a liquid called chyme. Digestion in the Stomach.  Cells in the mucosa layer secrete mucous, pepsinogen and hydrochloric acid. The mixture formed by these is called gastric juice.  Pepsinogen and hydrochloric acid combine to form the enzyme pepsin.  The enzyme pepsin breaks down large protein molecules in food into smaller ones. This is called chemical digestion.  While this is going one, muscles in the stomach wall contract in a peristaltic fashion to churn food around and break it up. It mixes with gastric juices, forming a liquid called chyme. The physical breaking down of food in the stomach is called mechanical digestion.

31. The physical mixing and churning of food in the stomach is called mechanical digestion. Digestion in the Stomach.  Cells in the mucosa layer secrete mucous, pepsinogen and hydrochloric acid. The mixture formed by these is called gastric juice.  Pepsinogen and hydrochloric acid combine to form the enzyme pepsin.  The enzyme pepsin breaks down large protein molecules in food into smaller ones. This is called chemical digestion.  While this is going one, muscles in the stomach wall contract in a peristaltic fashion to churn food around and break it up. It mixes with gastric juices, forming a liquid called chyme. The physical mixing and churning of food in the stomach is called mechanical digestion.

32. Acknowledgements for Images Used "Digestive system diagram en" by Mariana Ruiz Villarreal(LadyofHats) - Own work. Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Digestiv e_system_diagram_en.svg#mediaviewer/File:Dig estive_system_diagram_en.svg