Briefly describe the physical and chemical digestion that occurs in the stomach.

Physical and chemical digestion in the stomach begins with the act of eating. Here's a step-by-step breakdown:

1. Physical Digestion: This starts with the mechanical breakdown of food in the mouth through chewing. The food then travels down the esophagus into the stomach. In the stomach, the food is physically broken down further by the churning action of the stomach muscles. This churning mixes the food with the gastric juices produced by the stomach, turning it into a semi-fluid mass called chyme.

2. Chemical Digestion: This involves the breakdown of food on a molecular level. In the stomach, this is primarily facilitated by gastric juice, which contains hydrochloric acid and the enzyme pepsin. Hydrochloric acid helps to break down the food and also provides the acidic environment needed for pepsin to work. Pepsin begins the process of protein digestion by breaking down the protein in food into smaller units called peptides.

3. The chyme is then gradually released into the small intestine, where further digestion and absorption of nutrients occur.

It's important to note that not all nutrients are digested in the stomach. For example, fats are primarily digested in the small intestine, and carbohydrates begin their digestion in the mouth. The stomach's main role is in the digestion of proteins.