How does aging affect the digestive system? The first observation is that the digestive process becomes less efficient, over the years recordings of functionality changes in different parts such as:
The process of digestion starts in the mouth, where food is chewed and broken into smaller pieces. Saliva is mixed with the food during chewing, which makes it easier to move around in the mouth. An enzyme called salivary amylase starts to digest carbohydrates like sugar and starches.
Once the food has been chewed, it is pushed by the tongue to the back of the mouth. From here, muscles move it further down the esophagus and continue past a one-way valve, which is called the oesophageal sphincter. The muscles of the esophagus are strong and gravity-defying – one can swallow even if standing upside down or lying down.
The stomach, under normal conditions, performs three main functions.
The cephalic phase is the stage in which the stomach responds to the senses observation such as smell, taste, or even the thought of food. About 30% of total acid secretion occurs before food enters the stomach. These sensory and mental inputs converge on the hypothalamus, which relays signals to the medulla oblongata. Vagus nerve fibers from the medulla stimulate the parasympathetic nervous system of the stomach which, in turn, stimulates gastric secretion. Sensory stimuli from food activate dorsal motor nucleus of the vagus nerve in the medulla (activating the parasympathetic nervous system). Insulin-induced hypoglycemia also stimulates the vagus nerve. This results in four distinct physiological events.
1.) In the body of the stomach, the vagal postganglionic muscarinic nerves release acetylcholine(ACh) which stimulates H+ secretion.
2.) In the lamina propria of the body of the stomach, the ACh released from the vagal endings triggers histamine secretion from ECL cells. Histamine also stimulates H+ secretion from parietal cells.
3.) In the antrum, peptidergic postganglionic parasympathetic vagal neurons and other enteric nervous system neurons release GRP which stimulates antral G cells to produce and release gastrin. Gastrin stimulates acid secretion by directly stimulating parietal cells as well as by promoting histamine secretion by ECL cells.
4.)In both the antrum and corpus, the vagus nerve inhibits D cells, thus reducing their release of somatostatin and reducing background inhibition of gastrin release.
The glands at different locations of the stomach wall may secrete different substances. These glands are therefore termed according to its location – cardia glands (in the cardia of the stomach), fundic glands (in the fundus of the stomach) and pyloric glands (in the pylorus of the stomach).
The secretions from these glands are as follows:
The two enzyme-producing cells in the stomach are the chief cells and parietal cells. Chief cells secrete pepsin while parietal cells secrete gastric acid and intrinsic factor.
Although gastric acid is secreted by the stomach wall, it is not considered to be an enzyme. Acid indiscriminately breaks down substances, which are not resistant to it. Mucus is also secreted by the stomach wall and one of the main functions of this mucus is to protect the lining of the stomach wall. Gastrin, a hormone, is secreted by cells of the pyloric glands. These other substances are important for digestion despite not being enzymes.
Enzymes speed up and/or facilitate chemical reactions and are usually specific for certain substances. The two main types of stomach enzymes include:
Pepsin is the active form of pepsinogen which is secreted by the chief cells in the stomach wall. Pepsinogen is converted to pepsin by the action of stomach acid. Pepsin digest protein. It breaks down large protein chains (polypeptides) into smaller proteins (dipeptides and peptides).
Pepsin is most active when it is in an acidic (pH 1.5 to 3.5) environment. Coupled with the fact that it is secreted in an inactive form means that it does not damage or digest the tissue of wall. The mucus barrier that separates the stomach contents from the stomach wall also prevents auto-digestion.
Although the stomach acid is effective in destroying bacteria that may be consumed with food or beverages, pepsin may assist in this regard to some extent.
Intrinsic factor is a glycoprotein that is secreted by the parietal cells in the stomach wall. It is an enzyme-like substance which is responsible for vitamin B12 absorption. However, an intrinsic factor only acts within the small intestine despite being secreted by the stomach wall.
Vitamin B12 that is released from food in the stomach is bound by specific binding proteins, and not an intrinsic factor. When this bound vitamin B12 enters the small intestine, enzymes from the pancreas releases the vitamin B12 from the binding proteins.
Activation of Gastric Chief Cells
Gastric chief cells are primarily activated by Ach (Acetylcholine). However, the decrease in pH caused by activation of parietal cells further activates gastric chief cells. Alternatively, the acid in the duodenum can stimulate S cells to secrete secretin which acts on an endocrine path to activate gastric chief cells.
Gastric Phase, 50-60% of total gastric acid secretion occurs during this phase. The gastric phase is a period in which swallowed food and semi-digested protein (peptides and amino acids) activate gastric activity. Ingested food stimulates gastric activity in two ways: by stretching the stomach and by gastric contents stimulating receptors in the stomach. Stretch activates two reflexes: a short reflex mediated through the myenteric nerve plexus, and a long reflex mediated through the vagus nerves and brainstem.
Distention (Stretching) Path
1.) Vagovagal Reflex Distention activates an afferent pathway which in turn stimulates an efferent response from the dorsal nucleus of the vagus nerve. Stimulation of acid secretion occurs as it does in the cephalic phase.
2.) Local ENS Pathway Activated ENS releases ACh stimulating parietal cells to secrete acid.
From the stomach food and chime move through the porter into the other parts of the digestive system. Slow waves of smooth muscle contraction known as peristalsis flow down the length of the gastrointestinal tract to push chyme through the duodenum. Each wave begins at the stomach and pushes chyme a short distance toward the jejunum. It takes many peristaltic contractions over the course of an hour for chyme to travel through the entire length of the duodenum. Small regional contractions of the intestinal wall, known as segmentation, help to mix chyme with the digestive secretions in the duodenum and increase the rate of digestion. Segmentation also increases the contact of chyme with the mucosal cells to increase the absorption of nutrients through the intestinal wall.
The small intestine consists of the duodenum, the jejunum, and the ileum, which share the same wall structure formed by, from inside to outside,
The mucosa of the small intestine, comprising simple columnar epithelium and a lamina propria, forms wave-like projections, villi, which protrude into the lumen. The most important cell in the epithelium is the absorptive enterocyte with microvilli on its apical membrane.
The jejunum and ileum are histologically identical, except for their villi and the presence of Paneth cells. The villi of the jejunum are tall and cylindrical, while they are short and cylindrical in the ileum.
The duodenum continues the process of digestion of food that begins in the stomach. Its main function is to receive the chyme which is a combination of partially digested food and stomach acids. The chyme is released into the duodenum through pylorus (porter), which is a small valve located between the stomach and the duodenum. The duodenum accepts the chyme from the stomach and continues the digestion. This is done with the help of digestive enzymes and intestinal juices secreted by the crypts in the intestinal wall. Also, the duodenum receives bile drained from the liver and gallbladder and pancreatic juice secreted by the pancreas. These secretions aid in the digestion of food.
Apart from digesting foods, the duodenum regulates the rate of gastric emptying. Gastric emptying represents stomach emptying which is the process of food going from the stomach to the duodenum. The duodenum also triggers the hunger signals. Both of these functions are performed with the help of hormones that are produced and released by the duodenal epithelium. The epithelium includes cells that secrete two hormones known as secretin and cholecystokinin. When excess acid is present in the small intestine or duodenum, the hormone secretin is released. On the other hand, cholecystokinin is released in the presence of fatty acids and amino acids. Both secretin and cholecystokinin encourage secretion of bile and pancreatic juice.