Where does the food go once you swallow it? You can have a clue, it is one of the organs which often gets upset with our bad eating habits. Yes, you are right, it’s the stomach. What do you think happens to the food in the stomach? The stomach stores the food for some time to digest it partially. Did you know that the stomach uses muscular movements as well as chemicals to digest the food? Sounds a little extreme, right? Come let’s learn more about this crucial organ of our digestive system and understand its role in the process of digestion.
Stomach is a J-shaped organ which is located in the upper-left part of the abdomen. It is a hollow organ that stores the food for 4-5 hours during which it digests the food. Its inner surface is highly convoluted and can fold up when empty and open out like an expanding balloon as the stomach gets filled with food.
The oesophagus opens into the stomach in the cardiac region. This part of the stomach is present near the heart, and therefore, it is known as the cardiac region.
The opening of the oesophagus into the stomach is guarded by a gastro-oesophageal sphincter which prevents food from entering back into the oesophagus. A sphincter is a specialised muscle that relaxes or tightens to open or close an opening or passage in the body.
This region is filled with air and gas. Fundus is also known as the fundic region.
This is the main part of the stomach. Food is broken down into smaller particles because the enzymes act here.
Pylorus is also known as the pyloric part. It is the posterior part of the stomach that opens into the small intestine, i.e., duodenum.
The opening of the pylorus into the duodenum is guarded by a pyloric sphincter.
There are four types of muscles present in the stomach:
The alimentary canal is composed of four basic layers:
It is the outermost layer of the stomach and is also known as visceral peritoneum. It is composed of thin squamous epithelium also called mesothelium along with some connective tissue.
Muscularis layer is formed by smooth muscles. The cells of the muscularis layer are arranged into inner circular and outer longitudinal layers.
It is formed of loose connective tissues. It contains nerves, blood vessels and lymph vessels.
The food is stored for about 4–5 hours in the stomach. The gastro-oesophageal sphincter regulates the entry of food in the form of bolus into the stomach. The stomach can hold around a litre of liquid. It breaks down the food chemically as well as mechanically.
The muscles of the stomach perform the churning action which helps in the breakdown of food. The churning action of the stomach aids in mixing of the digestive juices with the food. Bolus (food mixed with saliva that comes from the mouth) turns into a thick chyme.
The mucosal layer of the stomach has gastric glands. The gastric gland has three types of cells:
Gastric juice is the mixture of secretion of mucus, chief, and parietal cells.
Digestion of carbohydrates and nucleic acids do not occur in the stomach. Proteins, lipids, and milk proteins are digested in the stomach. Pepsinogen is an inactive form of enzyme and known as zymogen. It is converted into its active form pepsin, in the presence of the acidic pH (6.8) provided by the HCl in the stomach.
Proteins are digested by the action of the enzyme known as pepsin. Pepsin converts proteins into proteoses and peptones (peptides). However, all the protein molecules are not converted into polypeptides.
Small amounts of lipases are secreted by the gastric glands that act on chyme and bring about little digestion of lipids. Lipases break down lipids into fatty acids and glycerol. Lipases are secreted only in small amounts, therefore, all the lipid molecules are not broken down in the stomach.
Soluble milk proteins present in the stomach are digested by an enzyme known as rennin. Rennin is also called chymosin. Rennin converts the soluble milk protein (casein) into insoluble milk protein (calcium paracaseinate).
Further, the insoluble milk proteins are digested by pepsin. Rennin is present in the gastric juice of infants. As the individual grows up, the quantity of rennin is reduced and pepsin functions as the major enzyme that digests milk proteins.
You must have seen infants spitting up curdled milk. This curdled milk is produced due to the activity of rennin. Rennin coagulates the milk proteins in the stomach. This produces curdled milk, which the baby might spit up.
Question 1. The opening of oesophagus in the cardiac region of the stomach is guarded by which sphincter?
A. Pyloric sphincter
B. Gastro-oesophageal sphincter
C. Anal sphincter
D. Urethral sphincter
Solution: The oesophagus opens into the stomach in the cardiac region. The opening is guarded by a gastro-oesophageal sphincter. This sphincter prevents the food from entering back to the oesophagus. Hence, the correct option is b.
Question 2. Study the diagram below and choose the correct option.
A. A is the fundus
B. C is the cardiac region
C. D is the pylorus
D. B is the body
Solution: The different parts of the stomach are:
Cardiac: It is the upper and tubular part of the stomach where oesophagus opens. It is represented by A.
Fundus: The fundic region of the stomach is filled with air and gas. It is represented by B.
Body: Body is the main part of the stomach where large food molecules are broken down into smaller ones by the action of enzymes. It is represented by C.
Pylorus: It is the posterior part of the stomach where it is connected to the duodenum of the small intestine. It is represented by D.
Thus, the correct option is c.
Question 3. Determine the correct sequence of stomach layers from outside to inside.
A. Serosa, muscularis, submucosa, mucosa
B. Muscularis, submucosa, mucosa, serosa
C. Mucosa, submucosa, muscularis, serosa
D. Submucosa, mucosa, muscularis, serosa
Solution: The correct sequence of layers of stomach from outside to inside is serosa, muscularis, submucosa and mucosa. Serosa is the outermost layer of the stomach also known as visceral peritoneum. Mucosa is the innermost layer that secrete mucus. Hence, the correct option is a.
Question 4. Rohit saw an infant spit up curdled milk after being fed bottled milk. He remembered that his teacher had told them about the enzyme which causes milk to curdle in an infant’s stomach. Which enzyme do you think it is?
D. Salivary amylase
Solution: The curdled milk in infants is produced due to the activity of rennin. Rennin coagulates the milk proteins in the stomach by converting soluble milk protein casein to insoluble calcium paracaseinate. This produces curdled milk, which the baby might spit up.
Question 1. What is the primary function of the stomach?
Solution: The stomach plays an important role in digestion. It stores food for about 4-5 hours. During this interval, some molecules of proteins, lipids and milk proteins are converted into peptides, glycerol and fatty acids, insoluble milk proteins, respectively.
Question 2. How many types of muscles are present in the stomach?
Solution: There are three types of muscles present in the stomach which are listed below:
Question 3. What are the sphincters that guard the openings of the stomach?
Solution: The opening of the oesophagus into the stomach is guarded by a gastro-oesophageal sphincter which prevents food from entering back into the oesophagus. A sphincter is a specialised muscle that relaxes or tightens to open or close an opening or passage in the body.
The opening of the pylorus ( posterior part of the stomach that opens into the small intestine) into the duodenum is guarded by a pyloric sphincter.
Question 4. What is the role of the stomach in the process of food digestion?
Solution: The digestion in the stomach occurs in two ways: mechanical and chemical digestion. In mechanical digestion the food bolus is converted into chyme (a mixture of food particles and gastric juices) as a result of vigorous churning of food caused due to contraction of stomach muscles.. In chemical digestion, macromolecules, such as proteins, lipids and milk proteins (in infants) are broken down into peptides, glycerol and fatty acids, insoluble milk protein, respectively.
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