29.3: Introduction to the Small and Large Intestines
By the end of this section, you should be able to:
- 29.3.1 Describe the structure and function of the small and large intestines.
- 29.3.2 Discuss common conditions that affect the small and large intestines.
Small and Large Intestines
The largest GI organ is the small intestine (Figure 29.6). The duodenum, jejunum, ileum, and ileocecal valve comprise the almost 20 feet of the small intestine. The primary functions of the small intestine are absorption and digestion; the small intestine is responsible for 90%–95% of nutrient absorption. The muscles of the small intestine further mix the chyme with digestive fluids for continued breakdown. The small intestine receives digestive juices from the liver, gallbladder, and pancreas. Bile acids from the liver and gallbladder enter the small intestine and break down fat for absorption in the small intestine. The pancreatic enzymes lipase , protease , and amylase travel through the pancreatic duct to the small intestine to facilitate digestion. Lipase, together with bile salts, breaks down fats and aids absorption of fat-soluble vitamins (A, D, E, K). Poor fat absorption may result in steatorrhea (fatty bowel movements). Protease breaks down protein for absorption. Amylase is released from the pancreas into the proximal small intestine to break down and absorb carbohydrates.
| Active Site | Enzyme | Effect on Nutrients |
|---|---|---|
| Mouth | Salivary amylase | Breaks down starches into disaccharides |
| Stomach | Pepsin | Breaks down proteins into large peptides |
| Pancreas | Amylase | Continues the breakdown of starches |
| Pancreas | Chymotrypsin, trypsin | Continue the breakdown of proteins |
| Pancreas | Lipase | Breaks down fats into fatty acids and glycerol |
| Small intestine | Maltase, sucrase, lactase | Break down disaccharides into monosaccharides |
| Small intestine | Peptidase | Breaks down dipeptides into amino acids |
Clinical Tip
Assess for Gastrointestinal Health Practices
When taking a client’s health history, nurses should ask about:
- Dietary practices: food preparation, food shopping and accessibility, healthy and unhealthy food choices, cultural or religious food preferences, meal timing, food allergies or intolerances
- Nutritional supplements: vitamins, minerals, herbal or natural supplements, need for nutritional counseling
- Oral health: dental history, tooth or gum disease, oral mucosal integrity
- Appetite: recent changes
- Preventive health: hydration habits, exercise habits, colonoscopy history
- Bowel movements: changes in patterns
- Weight: recent unintended weight gain or loss