name the stages in gastric function and describe there physiology.
The stages of gastric function can be divided into three main phases: the cephalic phase, the gastric phase, and the intestinal phase. Each phase involves specific physiological processes that contribute to the overall function of the stomach.
1. Cephalic phase:
The cephalic phase is the initial stage of gastric function that occurs in anticipation of food ingestion. It is triggered by sensory stimuli like the sight, smell, taste, or even the thought of food. During this phase, the brain signals the stomach to prepare for food digestion. The key physiological events in the cephalic phase include:
- Increased salivation: The brain stimulates the salivary glands to produce saliva, which contains enzymes that initiate the breakdown of carbohydrates.
- Gastric acid secretion: The brain sends signals to the stomach lining to release gastrin, a hormone that stimulates the production of gastric acid to lower the pH in the stomach for optimal digestion.
- Gastric motility: The muscles of the stomach contract and relax, mixing and churning the gastric contents.
2. Gastric phase:
The gastric phase begins when food enters the stomach. It is characterized by the release of gastric juices and the mechanical breakdown of food. The main physiological processes during this phase include:
- Gastric secretion: The stomach lining releases gastric juices containing hydrochloric acid, pepsinogen (which converts into the active enzyme pepsin), and mucus. Hydrochloric acid helps to further lower the stomach pH, while pepsin breaks down proteins.
- Gastric motility: Contractions known as peristalsis occur, moving the food in a grinding motion within the stomach. This helps mix the food particles with gastric secretions and promotes digestion.
- Gastric emptying: The partially digested food, known as chyme, is gradually emptied into the small intestine for further digestion and absorption.
3. Intestinal phase:
The intestinal phase takes place in the small intestine, primarily triggered by the presence of chyme. It focuses on the digestion and absorption of nutrients. The key physiological processes include:
- Pancreatic secretion: The small intestine releases hormones that stimulate the pancreas to secrete digestive enzymes, including amylase (for carbohydrate digestion), lipase (for fat digestion), and proteases (for protein digestion).
- Bile secretion: The liver releases bile into the small intestine, aiding in the emulsification and digestion of fats.
- Intestinal motility: Peristalsis propels the chyme through the small intestine, allowing for efficient digestion and absorption of nutrients.
It is important to note that these stages overlap and blend together during the digestion process. The cephalic and gastric phases mainly focus on the early stages of digestion in the stomach, while the intestinal phase emphasizes the absorption of nutrients in the small intestine.