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Digestion is the process of mechanically and enzymatically breaking down food into substances for absorption into the bloodstream. The food contains three macronutrients that require digestion before they can be absorbed: fats, carbohydrates, and proteins. Through the process of digestion, these macronutrients are broken down into molecules that can traverse the intestinal epithelium and enter the bloodstream for use in the body. Digestion is a form of catabolism or breaking down of substances that involves two separate processes: mechanical digestion and chemical digestion. Mechanical digestion involves physically breaking down food substances into smaller particles to more efficiently undergo chemical digestion. The role of chemical digestion is to further degrade the molecular structure of the ingested compounds by digestive enzymes into a form that is absorbable into the bloodstream. Effective digestion involves both of these processes, and defects in either mechanical digestion or chemical digestion can lead to nutritional deficiencies and gastrointestinal pathologies.
Through the gastrointestinal system, nutritional substances, minerals, vitamins, and fluids, enter the body. Lipids, proteins, and complex carbohydrates are broken down into small and absorbable units (digested), principally in the small intestine. The products of digestion, including vitamins, minerals, and water, which cross the mucosa and enter the lymph or the blood (Absorption).
Digestion of the major food macronutrients is an orderly process involving the action of a large number of digestive enzymes. Enzymes from the salivary and the lingual glands digest carbohydrates and fats, enzymes from the stomach digest proteins, and enzymes from the exocrine glands of the pancreas digest carbohydrates, proteins, lipids, RNA, and DNA. Other enzymes that help in the digestive process are found in the luminal membranes and the cytoplasm of the cells that lines the small intestine. The action of the enzymes is promoted by the hydrochloric acid (HCl), which is secreted by the stomach, and bile from the liver.
Cephalic Phase
The cephalic phase of gastric secretion occurs before food enters the stomach due to neurological signals.
Key Points
The cephalic phase of gastric secretion is initiated by the sight, smell, thought or taste of food.
Neurological signals originate from the cerebral cortex and in the appetite centers of the amygdala and hypothalamus.
This enhanced secretory activity is a conditioned reflex.
This phase of secretion normally accounts for about 20 percent of the gastric secretion associated with eating a meal.
Key Terms
- conditioned reflex: A response, to a stimulus, that has been acquired by operant conditioning.
- cephalic phase: This occurs before food enters the stomach, especially while it is being eaten.
The cephalic phase of gastric secretion occurs before food enters the stomach, especially while it is being eaten. It results from the sight, smell, thought, or taste of food; and the greater the appetite, the more intense is the stimulation.
Neurogenic signals that initiate the cephalic phase of gastric secretion originate from the cerebral cortex, and in the appetite centers of the amygdala and hypothalamus. They are transmitted through the dorsal motor nuclei of the vagi, and then through the vagus nerve to the stomach.
This phase of secretion normally accounts for about 20% of the gastric secretions that are associated with eating a meal. Since this enhanced secretory activity is brought on by the thought or sight of food it is a conditioned reflex—it only occurs when we like or want food. When one’s appetite is depressed this part of the cephalic reflex is inhibited.
The cephalic phase causes ECL cells to secrete histamine and increase HCl acid in the stomach. There will also be an influence on G cells to increase gastrin circulation.
Chain of Events for the Nervous System and Hormone System
- Thinking of food (i.e., smell, sight) stimulates the cerebral cortex.
- The cerebral cortex sends messages to the hypothalamus, the medulla, and the parasympathetic nervous system via the vagus nerve, and to the stomach via the gastric glands in the walls of the fundus and the body of the stomach.
- The gastric glands secrete gastric juice.
- When food enters the stomach, the stomach stretches and activates stretch receptors.
- The stretch receptors send a message to the medulla and then back to the stomach via the vagus nerve.
- The gastric glands secrete more gastric juice.
- Chemical stimuli (i.e., partially digested proteins, caffeine) directly activate G cells (enteroendocrine cells) that are located in the pyloric region of the stomach to secrete gastrin; this, in turn, stimulates the gastric glands to secrete gastric juice.
Gastric Phase
The gastric phase is a period in which swallowed food activates gastric activity in the stomach.
Key Points
The gastric phase accounts for about two-thirds of gastric secretions.
Ingested food stimulates gastric activity by stretching the stomach and raising the pH of its contents; this causes a cascade of events that leads to the release of hydrochloric acid by the parietal cells that lower the pH and break apart the food.
Gastric secretion is stimulated chiefly by three chemicals: acetylcholine (ACh), histamine, and gastrin.
Below pH of 2, stomach acid inhibits the parietal cells and G cells; this is a negative feedback loop that winds down the gastric phase as the need for pepsin and HCl declines.
Key Terms
- gastric phase: The second phase of digestion follows mastication (chewing) and takes place in the stomach.
The gastric phase is a period in which swallowed food and semi-digested protein ( peptides and amino acids ) activate gastric activity. About two-thirds of gastric secretion occurs during this phase.
Ingested food stimulates gastric activity in two ways: by stretching the stomach and by raising the pH of its contents.
Stretching activates two reflexes: a short reflex is mediated through the myenteric nerve plexus; and a long reflex is mediated through the vagus nerves and brainstem.
Gastric Secretion
Gastric secretion is stimulated chiefly by three chemicals:
- Acetylcholine (ACh). This is secreted by the parasympathetic nerve fibers of both the short and long reflex pathways.
- Histamine. This is a paracrine secretion from the enteroendocrine cells in the gastric glands.
- Gastrin. This is a hormone produced by enteroendocrine G cells in the pyloric glands.
All three of these stimulate parietal cells to secrete hydrochloric acid and intrinsic factor. The chief cells secrete pepsinogen in response to gastrin and especially ACh, and ACh also stimulates mucus secretion.
As dietary protein is digested, it breaks down into smaller peptides and amino acids that directly stimulate the G cells to secrete even more gastrin: this is a positive feedback loop that accelerates protein digestion.
Small peptides also buffer the stomach acid so the pH does not fall excessively low. As digestion continues and these peptides empty from the stomach, the pH drops lower and lower. Below pH of 2, stomach acid inhibits the parietal cells and G cells: this is a negative feedback loop that winds down the gastric phase as the need for pepsin and HCl declines.
The gastric phase of digestion: During the gastric phase, gastrin is secreted. The stomach stretches and churns while enzymes break down proteins.
Intestinal Phase
The intestinal phase occurs in the duodenum as a response to the arriving chyme, and it moderates gastric activity via hormones and nervous reflexes.
Key Points
Stretching of the duodenum (the first segment of the small intestine ) enhances gastric function via the vagal nerve, as the chyme causes the secretion of gastrin, which stimulates the stomach.
The acid and semi-digested fats in the duodenum trigger the intragastric reflex: the duodenum sends inhibitory signals to the stomach by way of the enteric nervous system.
The newly arrived chyme also stimulates enteroendocrine cells of the intestine to release compounds that stimulate the pancreas and gall bladder, while also suppressing gastric secretion and motility to allow the duodenum to process the chyme before receiving more from the stomach.
Key Terms
- chyme: The thick, semifluid mass of partly digested food that is passed from the stomach to the duodenum.
- intragastric reflex: One of three extrinsic reflexes of the gastrointestinal tract that is stimulated by the presence of acid levels in the duodenum or in the stomach that cause the release of gastrin from the G cells in the antrum of the stomach.
- enteroendocrine cells: Specialized endocrine cells of the gastrointestinal tract that produce hormones such as serotonin, somatostatin, motilin, cholecystokinin, gastric inhibitory peptide, neurotensin, vasoactive intestinal peptide, and enteroglucagon.
EXAMPLES
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The intestinal phase occurs in the duodenum as a response to the arriving chyme, and it moderates gastric activity via hormones and nervous reflexes. The duodenum initially enhances gastric secretion, but soon inhibits it. The stretching of the duodenum accentuates vagal reflexes that stimulate the stomach, and peptides and amino acids in the chyme stimulate the G cells of the duodenum to secrete more gastrin, which further stimulates the stomach.
Soon, however, the acid and semi-digested fats in the duodenum trigger the enterogastric reflex. That is, the duodenum sends inhibitory signals to the stomach by way of the enteric nervous system, while also sending signals to the medulla that inhibit the vagal nuclei. This reduces vagal stimulation of the stomach and stimulates sympathetic neurons that send inhibitory signals to the stomach.
Duodenum: The intestinal phase of digestion occurs in the duodenum, the first segment of the small intestine.
Chyme
Chyme also stimulates duodenal enteroendocrine cells to release secretin and cholecystokinin. These hormones primarily stimulate the pancreas and gallbladder, but they also suppress gastric secretion and motility. The effect of this is that gastrin secretion declines and the pyloric sphincter contracts tightly to limit the admission of more chyme into the duodenum. This gives the duodenum time to work on the chyme it has received before being loaded with more.
The enteroendocrine cells also secrete glucose-dependent insulinotropic peptides. Originally called a gastric-inhibitory peptide, it is no longer thought to have a significant effect on the stomach. Rather, it probably stimulates insulin secretion in preparation for processing the nutrients that are about to be absorbed by the small intestine.
Hormones of the Digestive System
There are five main hormones that aid and regulate the digestive system in mammals.
Key Points
The five major hormones are gastrin ( stomach ), secretin ( small intestine ), cholecystokinin (small intestine), gastric inhibitory peptide (small intestine), and motilin (small intestine).
Gastrin is in the stomach and stimulates the gastric glands to secrete pepsinogen (an inactive form of the enzyme pepsin) and hydrochloric acid. The secretion of gastrin is stimulated by food arriving in the stomach.
Secretin is in the duodenum and signals the secretion of sodium bicarbonate in the pancreas and it stimulates the secretion of bile in the liver.
Cholecystokinin (CCK) is in the duodenum and stimulates the release of digestive enzymes in the pancreas and the emptying of bile from the gall bladder.
Gastric inhibitory peptide (GIP) is in the duodenum and decreases the stomach-churning in order to slow the emptying of the stomach.
Motilin is in the duodenum and increases the migrating myoelectric complex component of gastrointestinal motility and stimulates the production of pepsin.
Key Terms
- motilin: A polypeptide that has a role in fat metabolism.
- gastrin: A hormone that stimulates the production of gastric acid in the stomach.
- secretin: A peptide hormone secreted by the duodenum that serves to regulate its acidity.
There are five main hormones that aid in the regulation of the digestive system in mammals. There are variations across the vertebrates, such as birds, so arrangements are complex and additional details are regularly discovered. For instance, more connections to metabolic control (largely the glucose-insulin system) have been uncovered in recent years.
- Gastrin is in the stomach and stimulates the gastric glands to secrete pepsinogen (an inactive form of the enzyme pepsin) and hydrochloric acid. The secretion of gastrin is stimulated by food arriving in the stomach. The secretion is inhibited by low pH.
- Secretin is in the duodenum and signals the secretion of sodium bicarbonate in the pancreas and it stimulates the secretion of bile in the liver. This hormone responds to the acidity of the chyme.
- Cholecystokinin (CCK) is in the duodenum and stimulates the release of digestive enzymes in the pancreas and stimulates the emptying of bile in the gallbladder. This hormone is secreted in response to the fat in chyme.
- Gastric inhibitory peptide (GIP) is in the duodenum and decreases stomach-churning in order to slow the emptying of the stomach. Another function is to induce insulin secretion.
- Motilin is in the duodenum and increases the migrating myoelectric complex component of gastrointestinal motility and stimulates the production of pepsin.
Appetite-Regulating Hormones
There are hormones secreted by tissues and organs in the body that are transported through the bloodstream to the satiety center, a region in the brain that triggers impulses that give us feelings of hunger or aid in suppressing our appetite. Ghrelin is a hormone that is released by the stomach and targets the pituitary gland, signaling to the body that it needs to eat.
PYY is a hormone that is released by the small intestine to counter ghrelin. It is released by the hypothalamus and signals that you have just eaten and helps to suppress our appetite.
The pancreas releases the hormone insulin that targets the hypothalamus and also aids in suppressing our appetite after we have just eaten and there is a rise in blood glucose levels.
The last hormone is leptin, which also helps to suppress appetite. Leptin is produced by adipose fat tissue and targets the hypothalamus.
Digestive hormones: The action of the major digestive hormones.
Function
Digestion is a process that converts nutrients in ingested food into forms that can be absorbed by the gastrointestinal tract. Proper digestion requires both mechanical and chemical digestion and occurs in the oral cavity, stomach, and small intestine. Additionally, digestion requires secretions from accessory digestive organs such as the pancreas, liver, and gallbladder. The oral cavity, stomach, and small intestine function as three separate digestive compartments with differing chemical environments. The oral cavity provides significant mechanical digestive functions and minor chemical digestion at a pH between 6.7 and 7.0. The oral cavity requires separation from the acidic environment of the stomach with a pH of 0.8 to 3.5. As such, enzymes such as alpha-amylase secreted by salivary glands in the oral cavity and also by the pancreas cannot function in the stomach, and thus digestion of carbohydrates does not occur in the stomach. However, in the stomach, significant digestion of proteins into polypeptides and oligopeptides occurs by the action of pepsin, which functions optimally at a pH of 2.0 to 3.0.
Minor digestion of lipids into fatty acids and monoacylglycerols also occurs by the action of gastric lipase secreted by chief cells in oxyntic glands of the body of the stomach. Importantly, this acidic environment of the stomach is also separated from the more basic environment of the small intestine by the tonically constricted pylorus. This functions to create an environment where the digestive enzymes produced by the pancreas and duodenum can function optimally at a pH of 6 to 7, a more basic environment than the stomach created by bicarbonate secreted by the pancreas. These separate yet coordinated digestive functions are essential to the body’s ability to absorb and utilize necessary nutrients. A defect in any aspect of this process can result in malabsorption and malnutrition amongst other gastrointestinal pathologies.
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