Introduction
The gastrointestinal system of the physical body comprises a gaggle of organs working together to convert food into energy for the body. Anatomically, the gastrointestinal system is formed from the alimentary canal , alongside accessory organs like the liver, pancreas and gallbladder. The hollow organs that structure the alimentary canal (GI tract) include the mouth, stomach, oesophagus, intestine and enormous intestine that contains the rectum and anus.
The digestion process involves the alimentary tract alongside various accessory organs and organ systems. Due to the monogastric nature of the human body, the process becomes very easy. This means that we've a one-chambered stomach, unlike other animals like cows, which have four chambers. A combination of nerves, bacteria, hormones, blood and other organs of the gastrointestinal system completes the task of digestion.
Gastrointestinal Tract Anatomy
The alimentary tract in humans begins at the mouth, continuing through the esophagus, stomach, and thus the tiny and massive intestines. The alimentary canal is about 9 meters long. There are supporting organs like the liver which plays an important role by secreting enzymes that are necessary for the digestion of food.
The GI tract of the human being can be bifurcated into 2 halves -
Upper GI tract
Lower GI tract
Upper Gastrointestinal Tract
The upper GI consists of the following organs:
Mouth
It includes the teeth, tongue, and buccal mucous membranes containing the ends of the salivary glands that continue with the taste bud , floor of the mouth, and underside of the tongue. Mouth functions by chewing the food, constantly by the muscular action of the tongue, cheeks, teeth through the lower and upper jawbone .
Pharynx
The pharynx is enclosed within the neck and throat which functions as a part of both the gastrointestinal system and therefore the systema respiratorium . The food is protected from entering the lungs and trachea.
Esophagus
It is a muscular tube-like structure which carries food to the stomach as its function. After the food reaches the esophagus from the mouth, the swallowing becomes involuntary and the esophagus takes the charge.
Stomach
Stomach may be a place where most of the digestion occurs. The stomach is a J-shaped bag-like organ that helps to store the food for the timing till it breaks it down, mixes and churns it with enzymes and other digestive fluids and eventually passes it along to the intestine.
Lower Gastrointestinal Tract
The lower GI consists of the following organs:
Small Intestine
The small intestine could also be a coiled thin tube, about 6 meters long , where most of the absorption of nutrients takes place. Food is mixed with enzymes from the liver and therefore the pancreas within the intestine . The surfaces of the tiny intestine functions by absorbing the nutrients from the food into the bloodstream, which carries them to the remainder of the body
Large Intestine
The large intestine, also mentioned because the Colon, could also be a thick tubular organ wrapped around the intestine . Its primary function is to process the waste products and absorb any remaining nutrient and water heater into the system. Stool is made within the body by the remaining waste that's sent to the rectum to urge faraway from the body.
Digestion Process
The alimentary tract or the alimentary canal may be a series of hollow organs and tubes that begins from the mouth cavity and continues into the pharynx, through the stomach, small intestines, large intestines, and eventually ending at the anus. While travelling through the various compartments of the alimentary canal, the food gradually gets into the process of digestion. The digestion process takes place within the following steps.
Ingestion
The very first step involves mastication (chewing). The salivary glands, alongside the tongue, helps to moisten and lubricate food, before being pushed down into the food pipe.
Mixing and Movement
It involves the process of lubricating and manipulating food and pushing it down the food through the food pipe (using peristalsis), and into the stomach.
Secretion
The stomach, small intestine, liver, and pancreas secrete enzymes and acids to assist the method of digestion. It functions by breaking down food particles into simple components and simply absorbable components.
Digestion
When the complex food particles get converted into simpler substances in the presence of enzymes and acids secreted by different digestive organs, it is termed as the process of digestion.
Absorption
This process begins within the intestine where most of the nutrients and minerals are absorbed. The excess water in the indigestible matter is absorbed by the large intestines.
Excretion
The process of removing indigestible substances and waste by-products from the body through the method of defecation.
Disorders of the Human Digestive System
Vomiting: it's the ejection of stomach contents through the mouth.
Diarrhoea: It is the abnormal watery bowel movement. Prolonged diarrhoea eventually leads to dehydration.
Constipation: A condition during which the faeces are clutched within the rectum thanks to an irregular movement .
Indigestion: When there is a pain or discomfort in the stomach which is caused when food is not digested properly, resulting in the feeling of fullness that is known as the process of Indigestion. This is especially caused due to the inadequate enzyme secretion, gastrointestinal disorder, anxiety, overeating and eating spicy foods.
Gastric Secretion
The gastric mucosa secretes 1.2 to 1.5 litres of digestive juice per day. Gastric juice makes the food particles soluble, helps in digestion (particularly of proteins), and turns the gastric contents to a semiliquid mass called chyme, thus preparing it for further digestion in the small intestine. Gastric juice may be a variable mixture of water, acid , electrolytes (sodium, potassium, calcium, phosphate, sulfate, and bicarbonate), and organic substances (mucus, pepsins, and protein). This juice is very acidic due to its acid content, and it's rich in enzymes. As noted above, the stomach walls are shielded from digestive juices by the membrane on the surface of the epithelial cells bordering the lumen of the stomach; this membrane is rich in lipoproteins, which are immune to attack by acid.
FAQs on Phases of Gastric Secretion
1. What are the three phases of gastric secretion?
The process of gastric secretion is regulated in three distinct phases based on where the stimulus originates:
- Cephalic Phase: This phase is triggered by the sight, smell, taste, or even the thought of food. It is a neural response mediated by the vagus nerve, preparing the stomach for the arrival of food by stimulating the secretion of gastric juice.
- Gastric Phase: This phase begins once food enters the stomach. It is stimulated by the distension (stretching) of the stomach wall and the presence of proteins. The hormone gastrin is released, which is the primary driver of HCl and pepsinogen secretion in this phase.
- Intestinal Phase: This phase starts when partially digested food (chyme) enters the small intestine. It has both an initial excitatory effect and a subsequent, more dominant inhibitory effect to slow down gastric activity, preventing the intestine from being overloaded.
2. What are the main components of gastric juice secreted during these phases?
The gastric juice secreted by the stomach glands is a complex mixture containing several key components essential for digestion:
- Hydrochloric Acid (HCl): Secreted by parietal cells, HCl creates a highly acidic environment (pH 1.5-3.5) that kills most bacteria and activates pepsin.
- Pepsinogen: An inactive enzyme secreted by chief cells. HCl converts it into its active form, pepsin, which begins the digestion of proteins into smaller peptides.
- Mucus: Secreted by mucous neck cells, it forms a thick protective layer over the stomach lining, preventing it from being damaged by HCl and pepsin.
- Intrinsic Factor: A glycoprotein secreted by parietal cells, which is crucial for the absorption of vitamin B12 in the small intestine.
3. What triggers the cephalic phase of gastric secretion before food even enters the stomach?
The cephalic phase is a fascinating example of the brain's role in digestion. It is triggered entirely by sensory and mental inputs. The sight, smell, taste, or even thinking about food sends signals from the cerebral cortex and hypothalamus to the medulla oblongata. From there, the vagus nerve (a cranial nerve) carries parasympathetic signals directly to the stomach's submucosal plexus, stimulating gastric glands to secrete acid, pepsinogen, and mucus in anticipation of the meal.
4. How is the stomach protected from being digested by its own acidic secretions?
The stomach has a sophisticated defence mechanism to prevent self-digestion, often called the gastric mucosal barrier. This barrier has two key components:
- A thick, alkaline mucus layer physically separates the stomach lining from the acidic gastric juice. The bicarbonate trapped in this mucus neutralises any acid that penetrates it.
- The epithelial cells of the stomach lining have tight junctions between them, which prevent gastric juice from leaking into the underlying tissue. These cells also have a high turnover rate, being replaced every 3-6 days.
5. What is the role of the hormone gastrin in the gastric phase?
Gastrin is the principal hormone of the gastric phase and plays the most significant role in stimulating secretion. When food, particularly proteins and peptides, enters the stomach and causes distension, specialised G-cells in the stomach antrum release gastrin into the bloodstream. Gastrin then travels to the parietal cells, strongly stimulating them to secrete hydrochloric acid (HCl), and also stimulates chief cells to secrete pepsinogen.
6. Why is the intestinal phase of gastric secretion mostly inhibitory?
While the intestinal phase has a brief initial excitatory effect, its primary function is to inhibit gastric secretion and motility. This is a crucial feedback mechanism called the enterogastric reflex. As acidic, fatty chyme enters the duodenum, it triggers the release of hormones like secretin and cholecystokinin (CCK). These hormones signal the stomach to slow down, ensuring the small intestine is not overwhelmed and has enough time to properly neutralise the acid and digest the fats.
7. How do the phases of gastric secretion differ from the process of gastric emptying?
It's a common point of confusion, but these are two distinct, though related, processes. Gastric secretion refers to the chemical process of producing and releasing gastric juice (acid, enzymes, mucus). The three phases (cephalic, gastric, intestinal) describe the regulation of this chemical secretion. In contrast, gastric emptying is the mechanical process of the stomach physically pushing its contents (chyme) into the small intestine. The rate of emptying is controlled by the same hormonal and neural signals that regulate the intestinal phase of secretion.
8. What is 'chyme' and how is it formed in the stomach?
Chyme is the semi-liquid, pulpy, acidic mass of partially digested food and gastric secretions that is formed in the stomach. Its formation involves both mechanical and chemical processes. The stomach's muscular walls perform powerful churning movements (mixing waves) that macerate the food and thoroughly mix it with gastric juice (HCl, pepsin, and mucus). This process converts the solid food bolus into the soupy consistency of chyme, preparing it for its onward journey into the small intestine.
