How Does the Stomach Work in Human Digestion?
The stomach is an essential organ of the human digestive system, before understanding the functions of the stomach, it is important to understand the stomach anatomy, stomach definition and its structure. The stomach definition according to biology is the J-shaped thick-walled organ that lies in the digestive cavity of the body. Even though a minimal percentage of digestion occurs in the mouth, the digestive process begins in the buccal cavity itself.
The stomach connects the oesophagus to the first part of the small intestine known as the duodenum. It is an enlargement of the alimentary canal that lies directly inferior to the oesophagus. As we are going to discuss the human stomach and functions of stomach it is important to note that the primary function of the stomach is to store the bolus (bolus is the chewed food mixed with saliva from the buccal cavity), while digestion is considered as the secondary function of the stomach.
The article deals with the comprehensive study of the stomach that includes the discussion around the stomach definition, stomach anatomy and stomach structure. The anatomical study also mentions the stomach location. The article also briefly discusses the common stomach diseases.
The Stomach Structure
Since we have developed an overview of the stomach let us look into the gross stomach anatomy or the stomach structure in the human digestive system. The stomach is divided into four sections namely, fundus, body, antrum and pylorus. The opening of the fundus or simpler terms the very beginning of the stomach is termed the cardia region. Each section of the stomach is associated with a unique function. The cardia is defined as the opening of the oesophagus to the stomach. The fundus follows the cardia, it is the dome-shaped section, it is located inferior to the diaphragm, above and to the left of the cardia.
The fundus is followed by the body of the stomach which is the main part of the stomach. The fundus is the site where the mixing and churning of bolus takes place. The Antrum is also known as pylorus antrum is the lower part of the stomach that is responsible for retaining the food into the stomach, the pylorus is the term used to define the opening of the stomach to the duodenum region of the small intestine. The pyloric sphincter is present here, it controls the movement of the bolus into the duodenum. The stomach structure diagram mentioned below will help in a better understanding of the stomach location and the sections of the stomach.
(Image will be uploaded soon)
Histology of the Human Stomach
The histology of the stomach refers to the study of the tissues of the stomach, in simpler terms it refers to the discussion of the layers of the stomach wall and the different cells of the stomach. As discussed earlier the stomach is a J-shaped thick-walled organ, this thickness is because of multiple layers of the stomach wall. It is also important to note that each section of the stomach consists of a different subset of cells that perform a unique function in the human digestive process.
The stomach wall has four different layers of tissue namely, the mucosa, submucosa, muscularis externa and the serosa, it is interesting to note that this scheme of tissue organisation is followed throughout the digestive system.
The mucosa is the innermost layer of the stomach, it is responsible for the mucous secretion. One of the characteristic features of the mucosa is ridges when the stomach is filled with bolus it flattens the ridges.
The submucosa is the layer after the mucosa, it consists of large blood vessels and lymph vessels, it is the layer that is associated with nerve cells and fibres.
The muscularis layer is the layer above the submucosa, it is further divided into longitudinal and circular smooth muscles. The nerve plexus between the longitudinal and the circular muscles of muscularis is termed the Myenteric or Auerbach nerve plexus.
The serosa is the outermost layer that constitutes the fibrous membrane, it is also referred to as the visceral peritoneum.
The image mentioned below of the stomach anatomy will help in better understanding the histology and stomach structure.
(Image will be uploaded soon)
Cells of the Stomach
Since we have studied the stomach definition and stomach location or stomach position, let us look into the cells of the human stomach which are responsible for the digestive functions of the stomach. The epithelial lining of the gastric mucosa is composed exclusively of superficial mucous cells that secrete a protective coating of alkaline mucus. Numerous gastric pits are scattered on the surface of the epithelium and look like commonly used pincushions, indicating the entrance to each gastric gland that secretes a complex digestive juice called gastric juice. Let us look into the cells of the stomach.
Goblet cells are mucous secreting cells. They are found in the fundus and pyloric region of the stomach, the mucus provides protection against the hydrochloric acid of the stomach.
Parietal cells are also known as oxyntic cells, they are responsible for HCl secretion, they also secrete peptides known as the intrinsic factor. Parietal cells are present in the body and antrum region of the stomach.
Chief cells are found in the body and the antrum region. They are exocrine cells of the stomach, they are responsible for the secretion of pepsinogen and gastric lipase. Pepsinogen is the inactive form of pepsin that gets activated by HCl. The pepsin breaks proteins into peptones and proteases. The gastric lipase breaks the first and third bonds of fatty acids.
Endocrine cells are also found in the body and the antrum region, they are divided into two subsets G cells, concerned with gastrin release and D cells, responsible for somatostatin release. Gastrin is the ligand that induces the release of gastric acid. Somatostatin is the inhibitory hormone that inhibits the release of gastric acid.
The Stomach Diseases
There are various common stomach infections that occur as a result of the dysfunction of the cells of the stomach. Some of the common stomach diseases are mentioned below.
Gastritis is a stomach infection in which the organ gets inflamed, the inflammation results in severe pain and discomfort. This is generally caused by the use of alcohol. A bacteria named H.pyroli can also cause it.
Gastroesophageal reflux disease is a common condition in which acid-containing stomach contents can regurgitate the oesophagus. Symptoms may not appear, or heartburn or cough may occur due to regurgitation.
Zollinger-Ellison Syndrome (ZES) is a condition in which several tumours are formed in the stomach. The tumours secrete hormones to increase acid production. Severe GERD (Gastroesophageal reflux disease) and gastric ulcers result from this rare condition.
Varicose veins are a condition in people with severe liver disease that can cause the veins in the stomach to swell and swell when pressure is increased. These veins, called varicose veins, patients suffering from it have an increased risk of bleeding.
A stomach ulcer, also known as gastric ulcer, is an erosion of the mucosal lining of the stomach that often causes pain and bleeding. Gastric ulcers are most commonly caused by NSAID (non-steroidal anti-inflammatory drugs) or H. pylori infections.
To conclude, we have learnt about the stomach digestive system and the components involved in the functions of the stomach. We have also learnt about common stomach disorders.
FAQs on Stomach: Anatomy, Structure, Location & Functions
1. What is the primary function of the stomach in the human digestive system?
The stomach is a J-shaped muscular organ that plays a crucial role in digestion. Its primary functions include:
- Temporary food storage: It holds food for 2-4 hours, allowing for controlled release into the small intestine.
- Mechanical digestion: The stomach's muscular walls churn and mix the food, a process called maceration, breaking it down into a semi-liquid paste.
- Chemical digestion: It secretes gastric juice containing hydrochloric acid (HCl) and enzymes like pepsin to begin the digestion of proteins.
- Production of chyme: It converts the swallowed food (bolus) into a thick, acidic fluid called chyme.
- Disinfection: The highly acidic environment kills most harmful bacteria and microorganisms present in the food.
2. Where is the stomach located in the human body?
The stomach is located in the upper part of the abdomen, primarily to the left of the midline, just below the diaphragm. It is situated between the oesophagus, which delivers food to it, and the small intestine, where digestion continues.
3. What are the four main anatomical regions of the stomach?
The human stomach is anatomically divided into four main regions, each with a specific location and function:
- Cardiac region: The uppermost part where the oesophagus connects to the stomach. It contains the cardiac sphincter which prevents acid reflux.
- Fundus: A dome-shaped area to the left of the cardiac region, which often contains trapped gas from digestion.
- Body (Corpus): The main, central part of the stomach, where most of the mechanical and chemical digestion occurs.
- Pyloric region: The lower, funnel-shaped part that connects to the duodenum (the first part of the small intestine). It ends in the pyloric sphincter, a valve that controls the exit of chyme.
4. What is chyme and how is it formed in the stomach?
Chyme is the semi-fluid, acidic mixture of partially digested food and gastric secretions that is formed in the stomach. The process begins when the food bolus enters the stomach from the oesophagus. Here, strong muscular contractions (peristalsis) mix the food with gastric juice. This juice contains hydrochloric acid and the enzyme pepsin, which starts protein digestion. This entire mixture is churned into chyme, which is then gradually released into the small intestine for further digestion and absorption.
5. Why doesn't the stomach digest itself, given its highly acidic environment?
The stomach has a sophisticated self-protection mechanism to prevent digestion by its own potent acid and enzymes. The inner lining of the stomach is coated with a thick layer of alkaline mucus, secreted by goblet cells. This mucus forms a protective barrier. Additionally, the bicarbonate ions within the mucus neutralise the acid before it can reach the stomach wall. This dynamic defence system ensures that only the food is digested, not the stomach lining itself.
6. How do the different cells in the stomach lining contribute to digestion?
The gastric glands in the stomach lining contain several types of specialised cells, each with a critical role:
- Parietal Cells (Oxyntic Cells): These cells secrete hydrochloric acid (HCl), which creates the acidic pH needed to kill microbes and activate enzymes. They also produce intrinsic factor, a substance essential for the absorption of vitamin B12.
- Chief Cells (Peptic Cells): These cells produce pepsinogen, which is an inactive form of the protein-digesting enzyme pepsin. HCl converts pepsinogen into its active form, pepsin.
- Mucous Neck Cells: Located in the neck of the gastric pits, these cells secrete a protective layer of mucus that shields the stomach wall from its own acidic environment.
- G-cells: These endocrine cells secrete the hormone gastrin, which stimulates the parietal cells to produce more HCl.
7. What is the difference between mechanical and chemical digestion in the stomach?
Both mechanical and chemical digestion occur simultaneously in the stomach. Mechanical digestion refers to the physical breakdown of food. In the stomach, this is achieved by the powerful churning motions of the muscular walls, which mix and pummel the food into smaller particles, increasing its surface area. Chemical digestion, on the other hand, involves the use of chemicals to break down food molecules. In the stomach, this is primarily carried out by hydrochloric acid and the enzyme pepsin, which specifically targets and breaks down large protein molecules into smaller peptides.
8. What is the importance of hydrochloric acid (HCl) in the stomach beyond just breaking down food?
Hydrochloric acid (HCl) has multiple crucial functions in the stomach. While it helps denature proteins, its most important roles include:
- Activating Enzymes: HCl converts inactive pepsinogen into its active, protein-digesting form, pepsin. Pepsin works optimally in the highly acidic environment (pH 1.5-2.5) created by HCl.
- Killing Pathogens: The strong acidity of HCl acts as a powerful disinfectant, killing most of the harmful bacteria, viruses, and other microorganisms that are ingested with food, thus preventing infections.
- Signalling: The acidic chyme entering the small intestine triggers the release of hormones that regulate further digestion.
9. How does the stomach regulate the passage of food into the small intestine?
The passage of food from the stomach into the small intestine (duodenum) is a tightly controlled process known as gastric emptying. This regulation is managed by the pyloric sphincter, a ring of smooth muscle at the junction of the stomach and duodenum. This sphincter remains tightly closed as the stomach churns the food into chyme. It then opens periodically to allow small amounts of chyme to squirt into the duodenum, ensuring the small intestine is not overwhelmed and has enough time to neutralise the acid and digest the nutrients effectively.
