Chapter 3. Digestion, Absorption, and Elimination
Chapter Review
IMPORTANT: Key Takeaways
- Hunger is a non-specific physiological sensation that drives us to eat when we need food, whereas appetite is a psychological desire to consume a specific food.
- The sections of the GI tract that play a role in digestion, absorption, and elimination include the mouth, esophagus, small intestine, and large intestine.
- The cephalic phase of digestion occurs when you begin thinking about food, which sends signals to your brain, causing you to salivate in preparation for your meal.
- In the mouth, chewing (mastication) stimulates the mechanical breakdown of food. The chemical breakdown of food involves enzymes such as salivary amylase and lingual lipase. Amylases break down carbohydrates, lipases break down lipids, and proteases break down proteins.
- Peristalsis occurs in the esophagus which pushes the food bolus down to the stomach.
- Cells lining the stomach secrete the hormone gastrin, which stimulates the gastric glands (which contain two cell types: parietal cells and chief cells) to secrete digestive juices. Parietal cells produce hydrochloric acid and intrinsic factor while chief cells produce pepsinogen and gastric lipase (Table 4.1).
- Chyme enters the small intestine and two types of muscular contractions, called peristalsis and segmentation, control the movement and mixing of chyme and enhance the absorption of nutrients in the small intestine.
- Indigestible fibres and incompletely broken down food move from the small intestine into the large intestine. The large intestine is primarily responsible for absorbing much of the remaining water, as no further chemical or mechanical breakdown of food occurs (unless it is accomplished by bacteria residing in the large intestine).
- Accessory organs that play a key role in the chemical digestion and absorption of nutrients include the liver, the pancreas, and the gallbladder.
- As chyme enters the duodenum, it signals the release of secretin and CCK into the duodenum, which causes the pancreas and gallbladder to release digestive juices.
- The liver produces bile.
- The gallbladder stores and secretes bile into the duodenum, which emulsifies fats and breaks them down into smaller droplets.
- Following CCK secretion, the pancreas releases pancreatic juices that contain enzymes (ie. proteases, pancreatic amylase, and pancreatic lipase) to further chemically digest proteins, carbohydrates, and lipids. The pancreatic juices also contain bicarbonate ions that neutralize the acidity of chyme. Additionally, the pancreas releases the hormones insulin and glucagon, which play a role in the regulation of blood glucose levels.
- GERD is a gastrointestinal disorder that occurs when the gastroesophageal sphincter remains partially opened, allowing gastric juice (which contains hydrochloric acid) to move into the esophagus, causing heartburn.
- Ulcers occur when areas of the GI tract become eroded by hydrochloric acid and pepsin, which may result in burning pain, vomiting of blood, and bloody stool.
- In affected individuals, celiac disease produces an immune reaction which damages the villi of the small intestine and consequently results in the malabsorption of nutrients.
Practice Questions
- What is the difference between appetite and hunger? How might the distinction between the two allow us to develop healthier eating habits?
- Explain the role of the hypothalamus and nerve cells in producing feelings of hunger and satiety.
- How does digestion begin before you even put food into your mouth? What is this phase of digestion called?
- What is the difference between the mechanical and chemical breakdown of food?
- How do peristalsis and segmentation differ in propelling food down the GI tract?
- How is the stomach lining protected against the highly acidic environment?
- What is the primary role of the large intestine in the digestive process?
- How do gastrointestinal disorders like ulcers and celiac disease affect one’s health and dietary habits?
- What structural characteristic(s) allows the small intestine to be the primary site of nutrient absorption?
- What is the role of accessory organs such as the liver, pancreas, and gallbladder in the digestive process?
- What are the digestive health benefits of probiotic foods? Provide some examples of these foods.
