(4) Small Intestine
The small intestine is the main organ for food digestion. In the small intestine, food undergoes chemical digestion by pancreatic juice, bile, and intestinal juice. Most nutrients are absorbed in the small intestine, while undigested food residues pass from the small intestine into the large intestine. The small intestine is located below the stomach and is 5 to 7 meters long. From top to bottom, it is divided into the duodenum, jejunum, and ileum. The duodenum is about 25 cm long, with a slightly wider section in the middle lower part called the duodenal papilla. This is where the common bile duct opens, allowing pancreatic juice and bile to enter the small intestine through this opening. Around the opening is a ring of smooth muscle that acts as a sphincter, known as the sphincter of Oddi, preventing the return flow of intestinal contents into the bile ducts.
1. Movement of the Small Intestine
(1) Tonic Contraction: The tonic contraction of the smooth muscles of the small intestine serves as the basis for other forms of movement. When the tension of the small intestine decreases, the intestinal cavity expands, and the mixing and transport of intestinal contents slow down; conversely, when the tension increases, the mixing and transport of chyme in the small intestine accelerate.
(2) Rhythmic Segmentation: Completed by the circular muscles, at the segment of the intestine containing chyme, the circular muscles contract simultaneously at many points, dividing the chyme into several segments. Subsequently, the originally contracted areas relax, while the originally relaxed areas contract, splitting the original segments into halves, which then merge adjacent halves into new segments. This process repeats continuously, separating and mixing the chyme. The forward propulsion effect of segmentation is minimal. Its role lies in: ① thoroughly mixing chyme with digestive juices to facilitate chemical digestion; ② bringing chyme into close contact with the intestinal wall to create conditions for absorption; ③ squeezing the intestinal wall to aid in blood and lymphatic return.
(3) Peristalsis: Peristalsis is a movement that propels chyme towards the large intestine, completed by the circular muscles. Since peristalsis in the small intestine is weak, it usually only covers a short distance before disappearing, so the speed of chyme propulsion in the small intestine is very slow, at 1-2 cm/min.
2. Digestive Juices Entering the Small Intestine
(1) Pancreatic Juice: Pancreatic juice is secreted by the exocrine portion of the pancreas and enters the pancreatic duct, flowing through the common bile duct into the duodenum. Pancreatic juice is a colorless, odorless, weakly alkaline liquid with a pH of 7.8-8.4, similar in water content to saliva. The inorganic components mainly consist of bicarbonate, whose function is to neutralize gastric acid entering the duodenum, protecting the intestinal cell membrane from strong acid erosion, while also providing the optimal pH for various digestive enzymes in the small intestine. Organic components are proteins composed of multiple enzymes. ① Amylase: an enzyme for starch digestion; ② Lipase class: Enzymes digesting lipids in pancreatic juice include lipase, phospholipase A2, cholesterol esterase, and colipase; ③ Protease class: Proteases in pancreatic juice are basically divided into two categories, endopeptidases and exopeptidases. Trypsin, chymotrypsin, and elastase belong to endopeptidases, while carboxypeptidase A and carboxypeptidase B are exopeptidases. Initially, all proteases secreted by pancreatic cells exist in inactive proenzyme forms and are activated by enteropeptidase upon entering the duodenum.
Besides the three major classes of enzymes mentioned above, pancreatic juice also contains ribonuclease and deoxyribonuclease. The optimal pH for all enzymes in pancreatic juice is around 7.0.
(2) Bile: Bile is synthesized by liver cells and stored in the gallbladder. After being concentrated, it is discharged into the duodenum. Bile is a golden yellow or orange-brown bitter-tasting viscous liquid. It contains water and inorganic components such as sodium, potassium, calcium, bicarbonate, as well as organic components like bile salts, bilirubin, fatty acids, phospholipids, cholesterol, and cellular protein. Bile salts are formed by the combination of bile acids synthesized by the liver using cholesterol with glycine or taurine, forming sodium or potassium salts, which are the primary components of bile involved in digestion and absorption. Generally, it is believed that bile does not contain digestive enzymes. The functions of bile are: ① Bile salts can activate lipase, accelerating its catalysis of fat decomposition; ② Bile salts, cholesterol, and lecithin in bile can act as emulsifiers, emulsifying fats into fine particles, increasing the action area of lipase, greatly accelerating its decomposition of fats; ③ Bile salts combine with fat decomposition products such as free fatty acids and monoglycerides to form water-soluble complexes, promoting fat absorption; ④ By promoting fat absorption, it indirectly aids in the absorption of fat-soluble vitamins. Additionally, bile is the primary pathway for the excretion of cholesterol and bilirubin metabolic products from the body.
(3) Intestinal Juice: Intestinal juice is a weakly alkaline liquid secreted by duodenal gland cells and intestinal gland cells, with a pH of approximately 7.6. Digestive enzymes in the intestinal juice include aminopeptidase, dextrinase, maltase, lactase, sucrase, and phosphatase. The main inorganic component is bicarbonate. Intestinal juice also contains enteropeptidase, which can activate trypsinogen.
(5) Large Intestine
There is no significant digestive activity in the human large intestine. The main function of the large intestine is to absorb water, and it also provides temporary storage for undigested food residues. Generally, the large intestine does not perform digestion, and the breakdown of substances within it is mostly due to bacterial action. Bacteria can use relatively simpler substances in the intestine to synthesize B vitamins and vitamin K, but more often they decompose undigested carbohydrates, proteins, and fats in food residues, producing metabolites that are mostly harmful to the human body.
1. Movement of the Large Intestine The movements of the large intestine are few and slow, responding to stimuli more slowly, which facilitates the temporary storage of feces.
(1) Haustral Reciprocating Movements: Caused by irregular contractions of the circular muscles, these movements cause short-distance shifts in the contents of the haustra in both directions without advancing forward.
(2) Segmental or Multihaustral Propulsive Movements: Completed by the contraction of one haustrum or a segment of the colon, pushing the intestinal contents to the next segment of the colon.
(3) Peristalsis: Composed of stable forward contraction waves, the muscles in front of the contraction wave relax, while those behind contract, closing and emptying this segment of the intestine.
2. Bacterial Activity in the Large Intestine The bacteria in the large intestine originate from air and food, surviving on food residues, while...