## Regulation of Bile Release and Bile Salt Functions ### Bile Release Bile, crucial for fat digestion, doesn't continuously flow into the duodenum. Its release is carefully regulated: * **Cholecystokinin (CCK):** This hormone triggers gallbladder contraction, releasing bile into the duodenum. Think of CCK as the "on" switch for bile release. * **Stimulus:** The presence of proteins and fats in the small intestine stimulates CCK release. A fatty meal will trigger a significant release of CCK and subsequent bile release. * **Hepatopancreatic Sphincter:** This sphincter controls bile flow. It remains closed until a peristaltic wave approaches, causing it to relax and allow bile entry into the duodenum. The hepatopancreatic sphincter acts like a valve, preventing premature bile release. The coordinated action of CCK and the hepatopancreatic sphincter ensures bile is released only when needed for fat digestion. ### Bile Salt Functions Bile salts are essential for efficient fat digestion and absorption: * **Emulsification:** Bile salts break down large fat globules into smaller droplets. This increases the surface area of fats, making them more accessible to digestive enzymes. Don't confuse emulsification with digestion. Emulsification increases the surface area for enzymes to act upon, while digestion is the actual breakdown of molecules. * **Increased Solubility:** Emulsification increases the solubility of fats in water, allowing them to mix better with the aqueous environment of the small intestine. * **Enhanced Lipase Activity:** The increased surface area facilitates the action of lipases, enzymes that break down fats. * **Absorption:** Bile salts aid in the absorption of fatty acids, cholesterol, and fat-soluble vitamins. Remember that fat-soluble vitamins (A, D, E, and K) require bile salts for absorption. ## The Small Intestine: Structure and Function The small intestine is a long, tubular organ responsible for the majority of nutrient digestion and absorption. ### Small Intestine Functions: * **Chyme Reception:** Receives partially digested food (chyme) from the stomach. * **Secretion Reception:** Receives pancreatic juice from the pancreas and bile from the liver and gallbladder. * **Digestion Completion:** Completes the digestion of carbohydrates, proteins, and fats. * **Absorption:** Absorbs the end products of digestion (monosaccharides, amino acids, fatty acids, etc.). * **Residue Transport:** Transports undigested material to the large intestine. ### Parts of the Small Intestine: The small intestine is divided into three sections: 1. **Duodenum:** The shortest and most fixed part; receives chyme directly from the stomach. The duodenum is where most of the chemical digestion occurs. 2. **Jejunum:** Makes up the proximal two-fifths; has a larger diameter than the ileum and is more active in digestion. 3. **Ileum:** The most distal portion; primarily involved in absorption. The jejunum and ileum are suspended by the **mesentery**, a double-layered fold of peritoneum containing blood vessels, nerves, and lymphatic vessels. The greater omentum, another fold of peritoneum, drapes over the intestines. ## Digestive Enzymes: A Summary The following table summarizes the major digestive enzymes and their functions: | Enzyme | Source | Digestive Action | |----------------------|-----------------------------|---------------------------------------------------------------------------------| | Salivary Amylase | Salivary glands | Begins carbohydrate digestion (starch to disaccharides) | | Pepsin | Gastric chief cells | Begins protein digestion | | Pancreatic Amylase | Pancreas | Breaks down starch into disaccharides | | Pancreatic Lipase | Pancreas | Breaks down fats into fatty acids and glycerol | | Trypsin, Chymotrypsin, Carboxypeptidase | Pancreas | Break down proteins or partially digested proteins into peptides | | Nucleases | Pancreas | Break down nucleic acids into nucleotides | | Peptidase | Intestinal mucosal cells | Breaks down peptides into amino acids | | Sucrase, Maltase, Lactase | Intestinal mucosal cells | Break down disaccharides into monosaccharides | | Intestinal Lipase | Intestinal mucosal cells | Breaks down fats into fatty acids and glycerol | | Enterokinase | Intestinal mucosal cells | Converts trypsinogen into trypsin | ## Absorption in the Small Intestine The small intestine is the primary site of nutrient absorption. This process is highly efficient, thanks to specialized structures: * **Villi:** Finger-like projections on the intestinal lining that increase surface area. * **Microvilli:** Tiny projections on the surface of each villus cell, further increasing surface area. Absorption mechanisms vary depending on the nutrient: * **Monosaccharides:** Absorbed via active transport or facilitated diffusion into blood capillaries. * **Amino Acids:** Absorbed via active transport into blood capillaries. * **Fatty Acids & Glycerol:** Diffuse into villi cells. Triglycerides are resynthesized, packaged into chylomicrons, and absorbed into lacteals (lymphatic vessels) before entering the bloodstream. Almost all absorbable nutrients are absorbed before reaching the end of the small intestine.