The Stomach (Easy Notes with Chemical Composition)
The stomach is a J-shaped muscular organ located on the left side of the abdominal cavity just below the diaphragm. It connects the oesophagus to the small intestine (duodenum) and performs both mechanical and chemical digestion of food. It acts as a temporary storage chamber and converts swallowed food into a semi-liquid mixture called chyme.
The stomach consists of three main parts. The first part is the cardiac region which is present next to the oesophagus and receives food. The second part is the fundus, a dome-shaped swollen region that stores undigested food and trapped gases. The last part is the pyloric region, which opens into the duodenum and controls the passage of chyme through the pyloric sphincter. At the upper end, the entry of food is controlled by the cardiac sphincter which prevents the backflow of acidic food into the oesophagus and prevents heartburn.
The wall of the stomach is thick and composed of five layers. The outermost serosa protects the stomach. Below it, there are three muscular layers (chemical composition: contractile proteins actin + myosin) – an outer longitudinal, a middle circular and an inner oblique layer. These muscles help in mechanical digestion by powerful churning movements. The next layer is submucosa (chemical composition: collagen + elastin fibers) that contains blood vessels, lymph vessels and nerves. The innermost layer is mucosa, which contains millions of gastric glands that secrete digestive substances.
Mechanical digestion in the stomach occurs by rhythmic contractions called peristalsis which mix food and break it into small particles. The heat produced during these contractions also melts solid lipids.
Chemical digestion occurs by gastric juice (chemical composition: HCl + pepsinogen + mucus + water + salts) secreted by gastric glands. These glands contain three types of secretory cells:
Mucous cells secrete mucus (chemical composition: glycoproteins + water) that forms a protective layer over the inner lining of the stomach and prevents it from being digested by enzymes.
Parietal or oxyntic cells secrete hydrochloric acid HCl (chemical composition: hydrogen ions H⁺ + chloride ions Cl⁻). HCl maintains acidic pH (2–3), softens food, kills pathogens and converts inactive pepsinogen into active pepsin.
Chief cells secrete pepsinogen (chemical composition: inactive proteolytic enzyme – protein in nature). In acidic medium, it changes into pepsin which breaks proteins into peptides.
In infants, an enzyme rennin (chemical composition: protein enzyme) is present which coagulates milk protein casein to assist digestion.
A protein called intrinsic factor (chemical composition: glycoprotein) produced in the stomach helps absorb vitamin B₁₂ in the small intestine.
The stomach is protected from its own acid by a thick mucus layer and bicarbonate ions (HCO₃⁻) which neutralize excess acid. Failure of this protection causes ulcers. Backflow of acid into oesophagus causes pyrosis (heartburn).
Secretion of gastric juice is controlled by both nerves and hormones. The smell, taste or sight of food stimulates nerves to release small amounts of gastric juice. Presence of proteins in food stimulates G-cells in the stomach to release gastrin hormone (chemical composition: peptide hormone) which increases gastric secretion. When food leaves the stomach, a hormone somatostatin stops further release of acid.
In about 3–4 hours, the food is converted into chyme and slowly released into the duodenum through the pyloric sphincter.
Stomach Online Quiz | FSc Biology
Stomach Online Quiz | FSc Biology
Detailed Quiz on Digestion in the Stomach. 30 MCQs with 30-second timer per question. Features: Progress bar, randomization option, detailed results with grades, all questions reviewed at end with your answers, correct answers, and explanations. Good luck!
1. What is the primary function of the stomach in digestion?
a) Carbohydrate breakdown
b) Protein digestion
c) Fat emulsification
d) Vitamin absorption
Time: 30s
Explanation: The stomach primarily begins the digestion of proteins through the action of pepsin and hydrochloric acid, converting them into peptides. Correct: b
2. Which gland in the stomach secretes hydrochloric acid (HCl)?
a) Chief cells
b) Parietal cells
c) Mucous cells
d) G cells
Time: 30s
Explanation: Parietal cells, also known as oxyntic cells, secrete HCl, which creates an acidic environment (pH 1.5-3.5) in the stomach. Correct: b
3. What is the role of HCl in the stomach?
a) Activates pepsinogen to pepsin
b) Digests carbohydrates
c) Neutralizes food
d) Produces mucus
Time: 30s
Explanation: HCl lowers the pH, denatures proteins, kills bacteria, and converts pepsinogen (inactive) to pepsin (active enzyme). Correct: a
4. Pepsin is an enzyme that digests:
a) Proteins
b) Lipids
c) Nucleic acids
d) Carbohydrates
Time: 30s
Explanation: Pepsin is a protease that hydrolyzes proteins into smaller polypeptides and peptides in the acidic stomach environment. Correct: a
5. The inactive form of pepsin is called:
a) Pepsinogen
b) Trypsinogen
c) Chymotrypsinogen
d) Procarboxypeptidase
Time: 30s
Explanation: Pepsinogen is secreted by chief cells and activated to pepsin by HCl in the stomach lumen. Correct: a
6. What protects the stomach lining from being digested by HCl and pepsin?
a) Mucus
b) Bile
c) Pancreatic juice
d) Saliva
Time: 30s
Explanation: Mucous cells secrete a thick mucus layer that forms a barrier, preventing self-digestion and ulceration. Correct: a
7. The stomach's shape is described as:
a) J-shaped
b) U-shaped
c) S-shaped
d) Straight tube
Time: 30s
Explanation: The human stomach is J-shaped, located in the upper left abdomen, between the esophagus and small intestine. Correct: a
8. Which part of the stomach is responsible for the most secretion of gastric juice?
a) Fundus and body
b) Cardia
c) Pylorus
d) Antrum
Time: 30s
Explanation: The fundus and body contain the majority of gastric glands that secrete digestive enzymes and acid. Correct: a
9. The hormone that stimulates gastric secretion is:
a) Gastrin
b) Secretin
c) Cholecystokinin
d) Insulin
Time: 30s
Explanation: Gastrin, secreted by G cells in the pyloric antrum, stimulates parietal cells to release HCl and chief cells to release pepsinogen. Correct: a
10. What is the mixture of partially digested food and gastric juice called?
a) Chyme
b) Bolus
c) Feces
d) Micelles
Time: 30s
Explanation: Chyme is the semi-liquid mass formed in the stomach that passes into the duodenum for further digestion. Correct: a
11. The pyloric sphincter controls the release of:
a) Chyme into the duodenum
b) Food into the esophagus
c) Bile into the stomach
d) Pancreatic juice
Time: 30s
Explanation: The pyloric sphincter regulates the flow of chyme from the stomach to the small intestine, preventing backflow. Correct: a
12. Which cells secrete pepsinogen?
a) Chief cells
b) Parietal cells
c) Mucous neck cells
d) Enteroendocrine cells
Time: 30s
Explanation: Chief (zymogenic) cells in the gastric glands produce and secrete pepsinogen. Correct: a
13. The pH in the stomach is maintained at:
a) 1.5 to 3.5
b) 6 to 7
c) 8 to 9
d) 4 to 5
Time: 30s
Explanation: The acidic pH is due to HCl secretion, optimal for pepsin activity and protein denaturation. Correct: a
14. Intrinsic factor, secreted by parietal cells, is essential for the absorption of:
a) Vitamin B12
b) Iron
c) Calcium
d) Glucose
Time: 30s
Explanation: Intrinsic factor binds to vitamin B12, enabling its absorption in the ileum. Correct: a
15. The cardiac sphincter prevents:
a) Backflow of stomach contents into the esophagus
b) Entry of food into the stomach
c) Release of chyme
d) Acid secretion
Time: 30s
Explanation: Also known as the lower esophageal sphincter, it prevents gastroesophageal reflux. Correct: a
16. Which phase of gastric secretion is triggered by the sight and smell of food?
a) Cephalic phase
b) Gastric phase
c) Intestinal phase
d) Colonic phase
Time: 30s
Explanation: The cephalic phase is neural, mediated by the vagus nerve, preparing the stomach for food arrival. Correct: a
17. During the gastric phase, distension of the stomach wall stimulates:
a) Local reflexes and gastrin release
b) Secretin release
c) Bile secretion
d) Insulin production
Time: 30s
Explanation: Stretch receptors trigger increased gastric secretion via neural and hormonal pathways. Correct: a
18. The intestinal phase of gastric secretion is mostly:
a) Inhibitory
b) Stimulatory
c) Neutral
d) Absent
Time: 30s
Explanation: Hormones like secretin and CCK from the duodenum inhibit gastric secretion to prevent overload. Correct: a
19. Pepsin works optimally at pH:
a) 1.5-2.5
b) 6-7
c) 4-5
d) 8-9
Time: 30s
Explanation: Pepsin is active in the highly acidic environment of the stomach, denaturing proteins for easier hydrolysis. Correct: a
20. Gastric lipase begins the digestion of:
a) Short-chain triglycerides
b) Proteins
c) Carbohydrates
d) Nucleic acids
Time: 30s
Explanation: Though minor, gastric lipase initiates fat digestion, especially in infants, producing fatty acids and diglycerides. Correct: a
21. The stomach absorbs small amounts of:
a) Water, alcohol, and aspirin
b) Glucose and amino acids
c) Vitamins A, D, E, K
d) Minerals like calcium
Time: 30s
Explanation: The stomach's simple epithelium allows passive absorption of these substances, but nutrient absorption mainly occurs in the small intestine. Correct: a
22. Excessive acid production in the stomach can lead to:
a) Peptic ulcers
b) Constipation
c) Diabetes
d) Appendicitis
Time: 30s
Explanation: Imbalance between acid/pepsin and protective mucus can erode the mucosa, causing ulcers. Correct: a
23. Which neurotransmitter stimulates gastric secretion via the vagus nerve?
a) Acetylcholine
b) Norepinephrine
c) Serotonin
d) Dopamine
Time: 30s
Explanation: Parasympathetic stimulation via acetylcholine from the vagus nerve enhances gastric motility and secretion. Correct: a
24. The fundus of the stomach primarily functions as a:
a) Storage site for food
b) Mixing chamber
c) Absorption area
d) Secretion site only
Time: 30s
Explanation: The expandable fundus stores undigested food, allowing for larger meals. Correct: a
25. Rennin, present in infants' stomachs, coagulates:
a) Milk proteins
b) Starch
c) Fats
d) Vitamins
Time: 30s
Explanation: Rennin (chymosin) curdles milk casein, slowing gastric emptying and aiding digestion in infants. Correct: a
26. Gastric glands are located in:
a) Gastric pits
b) Villi
c) Crypts of Lieberkuhn
d) Peyer's patches
Time: 30s
Explanation: Gastric pits are invaginations in the gastric mucosa where glands open and secrete juices. Correct: a
27. The vagus nerve influences the stomach during the:
a) Cephalic phase
b) Intestinal phase
c) Colonic phase
d) Pancreatic phase
Time: 30s
Explanation: Vagal stimulation anticipates food intake, increasing secretion even before food enters the stomach. Correct: a
28. Somatostatin inhibits:
a) Gastric acid secretion
b) Bile release
c) Insulin secretion
d) Glucagon release
Time: 30s
Explanation: Secreted by D cells, somatostatin suppresses gastrin and HCl release, regulating acidity. Correct: a
29. The stomach's muscular layers enable:
a) Churning and mixing of food
b) Absorption of nutrients
c) Peristalsis only
d) Segmentation
Time: 30s
Explanation: The three layers (oblique, circular, longitudinal) contract to mix food with gastric juice into chyme. Correct: a
30. Helicobacter pylori infection is associated with:
a) Gastritis and peptic ulcers
b) Gallstones
c) Pancreatitis
d) Hepatitis
Time: 30s
Explanation: H. pylori bacteria weaken the mucus layer, allowing acid to damage the stomach lining, leading to inflammation and ulcers. Correct: a
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