The Stomach - Human Digestive System PDF

The Stomach The Stomach Cardia This surrounds the lower oesophageal sphincter. Fundus The rounded portion above and to the left of the cardia Body The large central portion of the stomach Pylorus The narrow inferior region of the stomach that communicates with the duodenum of the small inte...

The Stomach The Stomach Cardia This surrounds the lower oesophageal sphincter. Fundus The rounded portion above and to the left of the cardia Body The large central portion of the stomach Pylorus The narrow inferior region of the stomach that communicates with the duodenum of the small intestine via a sphincter called the **pyloric sphincter**  The Stomach Linings The stomach wall is composed of the same 4 basic layers as the rest of the GIT, with certain modifications The Mucosa The mucosa lies in large folds called **rugae** when the stomach is empty. It is a layer of **simple columnar epithelium** containing many narrow openings that extend down into the lamina propria. These pits are the **gastric glands.** **Gastric Glands** The gastric glands are lined with several kinds of secreting cells: **ZYMOGENIC (PEPTIC) CELLS** These secrete the enzyme precursor, pepsinogen **. PARIETAL (OXYNTIC) CELLS** These secrete **hydrochloric acid** and **intrinsic factor.** **MUCOUS CELLS** These secrete **mucus.** **ENTEROENDOCRINE CELLS** These secrete stomach **gastrin**  Gastric Glands The Submucosa Composed of **loose areolar connective tissue,** which connects the mucosa to the muscularis. **The Muscularis** Unlike other areas in the GI tract, the **muscularis** has 3 layers of smooth muscle: an outer longitudinal layer a middle circular layer an inner oblique layer This arrangement allows the stomach to contract in a variety of ways to churn food, break it into small particles, mix it with gastric juice and pass it into the duodenum.  **Gastric Juice** Gastric juice is a collection of secretions of the zymogenic (peptic) cells the parietal cells the mucous cells 2500ml of gastric juice are produced daily **Composition of Gastric Juice** **Hydrochloric Acid** This is found within gastric juice from the combination of H+ and Cl-ions. Hydrochloric acid converts pepsinogen to the active enzyme pepsin. The glands in the body of the stomach secrete it. It has the following functions: - kills many ingested bacteria - aids protein digestion - stimulates the flow of bile and pancreatic juice **Regulation of Gastric Secretion** The secretion of gastric juice is regulated by both nervous and hormonal mechanisms. Parasympathetic impulses from nuclei in the medulla are transmitted via the vagus (X) nerves and stimulate the gastric glands to secrete Pepsinogen, Hydrochloric acid, Mucus Stomach gastrin (This is also secreted by gastric glands in response to certain foods that enter the stomach.) **Emotions** such as anger, fear and anxiety may **slow down digestion in the stomach** because they stimulate the **sympathetic nervous system.** **Stimulation of Gastric Secretion** Regulation is divided into 3 phases CEPHALIC (REFLEX) PHASE GASTRIC PHASE INTESTINAL PHASE-This phase produces relatively small amounts of gastric juice  **Cephalic Phase** **Gastric Phase**  Protein food and alcohol stimulate the pyloric mucosa to secrete the hormone stomach gastrin that: - stimulates the gastric glands to secrete large amounts of gastric juice - contracts the lower oesophageal sphincter - increases the motility of the GIT - relaxes the pyloric sphincter and ileocaecal sphincter. **Inhibition of Gastric Secretion** - The presence of food in the small intestine during the intestinal phase initiates an enterogastric reflex. - Nerve impulses carried to the medulla from the duodenum return to the stomach and inhibit gastric secretion. - These impulses ultimately inhibit parasympathetic stimulation and stimulate sympathetic activity. **Stimuli that indicate this reflex are:** Distension of the duodenum The presence of acid or partially digested proteins in food in the duodenum Irritation of the duodenal mucosa **Hormonal Inhibition of Gastric Secretion** The intestinal mucosa secretes hormones that inhibit gastric secretion and decrease motility of the GIT: Secretin Cholecystokinin (CCK) Gastric inhibiting peptide (GIP) Secretion of these hormones is stimulated by: Presence of acid or partially digested proteins Fats Hypertonic or hypotonic fluids Irritating substances in chime **Digestion in the stomach** Digestion in the stomach occurs in 2 ways: -**MECHANICALLY** **-CHEMICALLY** **Mechanical Digestion** Several minutes after food enters the stomach, gentle, ripping peristaltic movements called mixing waves pass over the stomach every **15 to 25 seconds**. These waves - macerate food, - mix it with the secretions of the gastric glands - reduce it to a thin liquid called chime - Foods remain in the fundus of the stomach for an hour or more without being mixed with gastric juice. During this time, salivary digestion continues. - As digestion proceeds in the stomach, more vigorous mixing waves begin at the body of the stomach and intensify as they reach the pylorus - At the pylorus, each mixing wave forces a small amount of the gastric contents into the duodenum through the slightly open pyloric sphincter. - Most of the food is forced back into the body of the stomach, where it is subjected to further mixing. **Chemical Digestion** **-**The principal chemical activity of the stomach is to start the digestion of proteins -In the adult, digestion of proteins is primarily achieved through the enzyme **pepsin** **How does pepsin work?** Pepsin is most effective in the very acidic environment of the stomach (pH 2). Pepsin breaks certain peptide bonds between the amino acids making up proteins. The zymogenic cells produce pepsinogen. The parietal cells secrete hydrochloric acid. When pepsinogen comes into contact with the hydrochloric acid, it is converted into active pepsin. Mucus protects the stomach mucosa from being digested by pepsin. **Further Chemical Digestion** **-Gastric lipase** is also present in the stomach. -This operates best at Ph 5 to 6 -It has a limited role of splitting the butterfat molecules found in milk. -The infant stomach also secretes **rennin,** which is important in the digestion of milk. - Rennin and calcium act on the casein of milk to produce a curd. - The coagulation prevents too rapid passage of milk from the stomach. **Regulation of Gastric Emptying** Gastric emptying is stimulated by 2 principal factors: Nerve impulses in response to distension Stomach gastrin releases in the presence of certain types of food The stomach empties all its contents into duodenum within 2 to 6 hours after ingestion. - Foods rich in carbohydrates spend the least time in the stomach. - Protein foods are somewhat slower. - Emptying is slowest after a meal containing large amounts of fat.  **Inhibition of Gastric Emptying** **-**Stomach emptying is inhibited by: - the enterogastric reflex - the hormones - secretin, CCK and GIP The rate of the stomach emptying is limited to the amount of chime that the small intestine can process. The diagram below summaries the **factors that inhibit gastric emptying.** **Pancreas, Liver and Gallbladder**  **Pancreas** Soft tubuloacinar gland about 12.5 cm long and 2.5 cm thick Posterior to the greater curvature of the stomach Connected by a duct or two to the duodenum. -Pancreatic secretions pass from the secreting cells in the pancreas to small ducts that convey the secretions into the small intestine. -The pancreas is made up of small clusters of glandular epithelial cells. About 1% of the cells, the pancreatic islets (islets of Langerhans), form the endocrine portion of the pancreas The remaining 99% of the cells called acini are the exocrine portions of the organ. Secreting cells of the acini release a mixture of digestive enzymes called pancreatic juice. **Pancreatic Juice** **-**Each day the pancreas produces 1,200 to 1,500 ml **pancreatic juice,** which colourless liquid Consists mostly of: Water Some salts Sodium bicarbonate-gives pancreatic juice a slightly alkaline Ph(7.1 TO 8.2) Enzymes: **PANCREATIC AMYLASE** -- a carbohydrate-digesting enzyme **TRYPSIN** -- a protein-digesting enzyme **CHYMOTRYPSIN** - a protein-digesting enzyme **CARBOXYPOLYPEPTIDASE -** a protein-digesting enzyme **PANCREATIC LIPASE** -- the principal fat-digesting enzyme in the body **RIBONUCLEASE** - a nucleic acid-digesting enzyme **DEOXYRIBONUCLEASE -** a nucleic acid-digesting enzyme **Regulation of Pancreatic Secretions** Pancreatic secretion is regulated by: nervous mechanisms through vagus nerve that releases pancreatic juice hormonal mechanisms triggered by presence of chime in the small intestine -Secretin pancreas secretes pancreatic juice rich in sodium bicarbonate ions. -Cholecystokinin (CCK) stimulates a pancreatic secretion rich in digestive enzymes **The Liver**  **Functions of The Liver** 1. The liver manufactures bile salts for the emulsification and absorption of fats, cholesterol, phospholipids and lipoproteins. 2. The liver manufactures the anticoagulant heparin and most of the other plasma proteins -- prothrombin, fibrinogen and albumin. 3. The Kupffer cells of the liver phagocytize worn-out red and white blood cells and some bacteria 4. Liver cells contain enzymes that either break down poisons or transform them into less harmful compounds. 5. Excess monosaccharides are converted into glycogen or fat in the liver. The liver can also convert glycogen, protein and fat into glucose. 6. The liver stores glycogen, copper, iron and vitamins A, B12, D, E and K. 7. The liver and kidneys participate in the activation of vitamin D. **Bile** **-** Each day the hepatic cells secrete 800 to 1,000 ml of bile. This is a yellow, brownish or olive-green liquid with a pH of 7.6 to 8.6. -Bile consists mostly of: Water Bile salts Cholesterol Lecithin Bile pigments especially bilirubin Several ions Bile is partially an **excretory product** and partially a **digestive secretion** Bile salts assume a role in emulsification and absorption of fats following their digestion. Bilirubin is the principal bile pigment that is released when red blood cells are broken down. Bilirubin is broken down in the intestine and one of its breakdown products gives faeces their colour. **Regulation of Bile Secretion** The rate at which bile is secreted is determined by several factors: 1. **Vagal Stimulation** increases the production of bile to more than twice the normal rate 2. **Secretin** stimulates the secretion of bile. 3. The **presence of large amounts of bile salts** in the blood also increases the rate of bile production. **Gallbladder** **-**The gallbladder stores and concentrates bile until it is needed the small intestine Water and many ions are absorbed by the gallbladder mucosa Bile from the liver enters the small intestine through the common bile duct When the small intestine is empty, the sphincter of Oddi, around the ampulla of Vater, closes and the backed-up bile overflows into the cystic duct to the gallbladder for storage \- In order for the gallbladder to eject bile into the small intestine to participate in the digestive process, the muscularis must contract to force bile into the common bile duct and the sphincter of the ampulla of Vater must relax. This is brought about by CCK that is secreted by the intestinal mucosa in response to the presence of chyme in the duodenum. **THE SMALL INTESTINE** Modifications For Absorption and Digestion The mucosa contains many pits lined with glandular epithelium. These pits are the intestinal glands or crypts of Lieberkühn. They secrete the intestinal juice. The submucosa of the duodenum contains duodenal or Brünner glands. These secrete alkaline mucus to protect the wall of the small intestine from the action of the enzymes to aid neutralise the acid in the chime Some of the cells in the mucosa and submucosa have been transformed into goblet cells, which secrete additional mucus. The mucosa of the small intestine also contains solitary lymph nodules and especially in the ileum, aggregated lymphatic nodules (Peyer's patches).  **Intestinal Juice** A clear yellow fluid secreted in amounts of about 2 to 3 litres a day. Has a pH of 7.6 Contains water and mucus Passage of digested nutrients from the alimentary canal into the blood and lymph is called absorption. About 90% of all absorption of nutrients takes place throughout the length of the small intestine. The other 10% occurs in the stomach and large intestine. Any undigested or unabsorbed material is passed on to the large intestine. \- Absorption of materials in the small intestine occurs specifically through the villi and depends on diffusion facilitated diffusion osmosis active transport **The Large Intestine**  **Functions of The Large Intestine** 1\. Completion of absorption 2\. The manufacture of certain vitamins 3\. The formation of faeces 4\. The expulsion of faeces from the body **Action In the Large Intestine** The last stage of digestion occurs through bacterial, not enzymatic, action. The glands of the large intestine secrete mucus, but no enzymes are secreted. Chyme is prepared for elimination by the action of bacteria. These bacteria ferment any remaining carbohydrates and release hydrogen, carbon dioxide and methane gas. These gases contribute to flatus in the colon. They also convert the remaining proteins to amino acids and break down the amino acids into simpler substances -- indole, skatole, hydrogen sulphide and fatty acids. Some of the indole and skatole is carried off in the faeces and contributes to their odour. The rest are absorbed and transported to the liver, where they are converted to less toxic compounds and excreted in the urine. Several vitamins needed for normal metabolism, including some B vitamins and vitamin K, are synthesised by bacterial action and absorbed. By the time the chyme has remained in the large intestine 3 to 10 hours, it has become solid or semisolid as a result of absorption and is now known as faeces Chemically, faeces consists of: Water Inorganic salts Sloughed off epithelial cells from the mucosa of the GIT Bacteria Products of bacterial decomposition Undigested parts of food -The large intestine absorbs enough water to make it an important organ in maintaining the body's water balance -Absorption is greatest in the **caecum and ascending colon.** -The large intestine also reabsorbs electrolytes, including sodium and chloride