THE SMALL INTESTINE

The small intestine is the longest part of the alimentary canal.
It starts after the stomach and ends at the large intestine.

Length: about 6 meters
Diameter: about 2–3 cm in an adult man

Functions:

Final digestion of all types of food (carbohydrates, proteins, fats, nucleic acids)
Absorption of digested food into blood and lymph

It is divided into three parts:

Duodenum
Jejunum
Ileum

1. DUODENUM

The first 20–25 cm long portion of the small intestine.
Main site of digestion.
It contains glands that produce alkaline secretion (contains bicarbonate to neutralize acidity of chyme).

Two main secretions enter the duodenum:

a. Pancreatic Juice

Secreted by the pancreas and poured into the duodenum.
It is slightly alkaline (pH 8) due to bicarbonate ions, which neutralize the acidic chyme (partially digested food from stomach).

Major Enzymes and their Functions:

Enzyme	Function
Pancreatic amylase	Converts polysaccharides (complex carbohydrates) into maltose and glucose
Trypsinogen (inactive enzyme)	Activated by enterokinase (enzyme secreted by duodenal wall) into trypsin, which digests proteins → polypeptides
Chymotrypsin	Digests proteins → smaller peptides
Carboxypeptidase	Breaks peptides → amino acids
Pancreatic lipase	Digests fats (lipids) → fatty acids + glycerol
Pancreatic nucleases	Break down DNA and RNA → nucleotides

b. Bile

Produced by the liver and stored in the gallbladder before release.Contains bile salts that emulsify fats (break large fat globules into small droplets → called emulsion).This increases surface area for enzymes (like lipase) to act effectively.Bile also neutralizes acidic chyme.

⚠️ Note: If bile pigments (such as bilirubin) are not excreted properly and accumulate in blood, it causes jaundice (yellowing of skin and eyes).

2. JEJUNUM AND 3. ILEUM

Jejunum: about 2.4 meters long, next to the duodenum.
Ileum: the last and longest part, about 3.5 meters long.
Both perform final digestion and absorption of food.

Intestinal glands in the walls secrete intestinal juice (also called succus entericus) which contains many enzymes.

Major Enzymes and their Actions:

Enzyme	Function
Aminopeptidase	Converts polypeptides → dipeptides
Erypsin	Converts dipeptides → amino acids
Lipase	Converts fats → fatty acids + glycerol
Maltase	Converts maltose → glucose
Sucrase	Converts sucrose → glucose + fructose
Lactase	Converts lactose → glucose + galactose

After the complete action of these enzymes, the chyme (semi-digested food) becomes an alkaline emulsion called chyle.

ABSORPTION OF DIGESTED FOOD AND WATER

Occurs mainly in jejunum and ileum.

Internal Structure for Absorption

The inner wall has large circular folds called plicae circulares.
These folds bear numerous finger-like projections called villi (singular: villus).

Each villus contains:

Blood capillaries (for absorption into blood)
A lymphatic vessel called lacteal (for absorption of fats)
The surface of epithelial cells of villi has microvilli (tiny projections of cytoplasm) that greatly increase the surface area for absorption.
Due to the presence of villi and microvilli, the internal surface appears velvety.

Absorption Pathways

1. Absorption of Simple Sugars and Amino Acids

Simple sugars (like glucose) and amino acids are absorbed into the epithelial cells of villi by:

Diffusion (movement from high to low concentration)
Active transport (using energy against concentration gradient)
From villi → enter blood capillaries → form hepatic portal vein → carry nutrients to the liver.

Liver functions:

Stores extra glucose as glycogen
Stores amino acids as proteins
From liver → nutrients pass via hepatic vein → to heart → distributed to body cells.

2. Absorption of Fatty Acids and Glycerol

Fatty acids and glycerol are absorbed into epithelial cells of villi by passive transport (without energy).
Inside the epithelial cells:
They recombine to form triglycerides.
These are coated with proteins to form chylomicrons (small fat droplets).
Chylomicrons enter lacteals (lymph vessels) → move to thoracic lymphatic duct → enter bloodstream.
In blood plasma, enzymes hydrolyze chylomicrons back into fatty acids and glycerol, which are then used by body cells for energy or storage.

Summary of Small Intestine Functions

Function	Location	Process/Enzyme
Neutralization of acid	Duodenum	Bicarbonate ions (from pancreatic juice & bile)
Digestion of food	Duodenum, Jejunum, Ileum	Enzymes from pancreas and intestinal glands
Absorption of nutrients	Jejunum & Ileum	Villi & microvilli increase surface area
Fat absorption	Ileum (via lacteals)	Formation of chylomicrons

✅ Key Points to Remember

Bile emulsifies fats but does not contain enzymes.
Enterokinase activates trypsinogen → trypsin.
Villi and microvilli maximize absorption surface.
Hepatic portal vein carries absorbed nutrients to the liver.
Chylomicrons carry fat molecules into lymph and then blood.

Multiple Choice Questions on the Small Intestine

Here are 25 MCQs based on the provided text, covering key concepts comprehensively. Each question has four options (A, B, C, D). The correct answer is indicated, followed by a brief explanation referencing the text.

1. What is the approximate length of the small intestine in an adult human? A) 2-3 meters B) 6 meters C) 20-25 cm D) 3.5 meters

Answer: B Explanation: The text states that the small intestine is about 6 m in length in an adult man.

2. Which of the following is NOT a function of the small intestine? A) Final digestion of food B) Absorption of digested food into blood and lymph C) Storage of bile pigments D) Production of intestinal juice

Answer: C Explanation: The small intestine handles digestion and absorption, but bile pigments are stored in the gallbladder and their retention can cause jaundice; storage is not a function of the small intestine.

3. The small intestine consists of how many main parts? A) Two B) Three C) Four D) One

Answer: B Explanation: It consists of three parts: duodenum, jejunum, and ileum.

4. What is the length of the duodenum? A) 2.4 meters B) 3.5 meters C) 20-25 cm D) 6 meters

Answer: C Explanation: The duodenum is the first 20-25 cm long portion of the small intestine.

5. The primary concern of the duodenum is: A) Absorption of water B) Digestion of food C) Storage of chyme D) Production of villi

Answer: B Explanation: The duodenum is concerned with the digestion of food and contains glands producing alkaline secretion.

6. What is the pH of pancreatic juice? A) Acidic (pH 4) B) Neutral (pH 7) C) Slightly alkaline (pH 8) D) Highly alkaline (pH 10)

Answer: C Explanation: Pancreatic juice is slightly alkaline (pH 8) due to bicarbonate, which neutralizes the acidity of chyme.

7. Which enzyme in pancreatic juice digests polysaccharides into maltose and glucose? A) Trypsin B) Pancreatic amylase C) Lipase D) Nucleases

Answer: B Explanation: Pancreatic amylase digests polysaccharides into maltose and even glucose.

8. Trypsinogen is activated into trypsin by which enzyme? A) Enterokinase B) Chymotrypsin C) Carboxypeptidase D) Aminopeptidase

Answer: A Explanation: Enterokinase, secreted by the walls of the duodenum, activates trypsinogen into trypsin, which digests proteins into polypeptides.

9. Which pancreatic enzyme digests lipids into glycerol and fatty acids? A) Pancreatic amylase B) Trypsin C) Pancreatic lipase D) Pancreatic nucleases

Answer: C Explanation: Pancreatic lipase digests lipids to glycerol and fatty acids.

10. What is the role of bile in digestion? A) Neutralizes chyme acidity B) Digests proteins into amino acids C) Emulsifies fats into small droplets D) Digests DNA into nucleotides

Answer: C Explanation: Bile contains salts that emulsify fats, breaking them into small droplets (emulsion) to provide large surface areas for lipase action.

11. Where is bile stored before release into the duodenum? A) Pancreas B) Liver C) Gallbladder D) Jejunum

Answer: C Explanation: Bile is the secretion of the liver but is stored in the gallbladder before release.

12. What condition occurs if bile pigments accumulate in the blood? A) Chyme formation B) Jaundice C) Emulsion breakdown D) Villi enlargement

Answer: B Explanation: If bile pigments are prevented from leaving the digestive tract, they accumulate in blood, causing jaundice.

13. How long is the jejunum? A) 20-25 cm B) 2.4 meters C) 3.5 meters D) 6 meters

Answer: B Explanation: The jejunum is the 2.4 meters long part next to the duodenum.

14. The ileum constitutes approximately what portion and length of the small intestine? A) First fifth, 1.2 meters B) Last three-fifths, 3.5 meters C) Middle half, 2.4 meters D) Entire length, 6 meters

Answer: B Explanation: The ileum is the last three-fifths, about 3.5 meters long part of the small intestine.

15. Which enzyme in intestinal juice digests polypeptides into dipeptides? A) Erypsin B) Maltase C) Lipase D) Aminopeptidase

Answer: D Explanation: Aminopeptidase digests polypeptides into dipeptides.

16. What does sucrase in intestinal juice digest? A) Lactose into glucose and galactose B) Maltose into glucose C) Sucrose into glucose and fructose D) Proteins into amino acids

Answer: C Explanation: Sucrase digests sucrose into glucose and fructose.

17. After the action of intestinal juice enzymes, chyme is converted into: A) Acidic emulsion B) Alkaline emulsion called chyle C) Solid waste D) Bile salts

Answer: B Explanation: The chyme is converted into an alkaline emulsion called chyle.

18. Absorption of digested food primarily occurs in which parts of the small intestine? A) Duodenum only B) Jejunum and ileum C) Stomach and duodenum D) Large intestine

Answer: B Explanation: Absorption of digested food, water, and minerals occurs in jejunum and ileum.

19. What are the finger-like projections on the inner wall of jejunum and ileum called? A) Microvilli B) Lacteals C) Villi D) Folds

Answer: C Explanation: The inner wall contains large circular folds with numerous finger-like projections called villi.

20. Each villus is supplied with blood capillaries and a lymphatic vessel called: A) Hepatic portal vein B) Thoracic duct C) Lacteal D) Chylomicron

Answer: C Explanation: Each villus has blood capillaries and a lacteal (lymphatic vessel).

21. The cytoplasmic projections on epithelial cells of villi that increase surface area are: A) Villi B) Folds C) Microvilli D) Chyle

Answer: C Explanation: Epithelial cells of villi have microvilli, making the total surface area extraordinarily large.

22. Simple sugars and amino acids are absorbed into the epithelial cells of villi by: A) Passive transport only B) Diffusion or active transport C) Emulsification D) Hydrolysis

Answer: B Explanation: Simple sugars (e.g., glucose) and amino acids are absorbed by diffusion or active transport into epithelial cells, then enter blood capillaries.

23. From the blood capillaries of villi, sugars and amino acids are carried to the liver via: A) Thoracic lymphatic duct B) Hepatic portal vein C) Gallbladder D) Pancreas

Answer: B Explanation: They join to form the hepatic portal vein, which carries them to the liver for storage.

24. Fatty acids and glycerol are reformed into triglycerides inside villi and coated with proteins to form: A) Chyme B) Chylomicrons C) Bile salts D) Glycogen

Answer: B Explanation: They combine to form triglycerides, coated with proteins to make chylomicrons, which enter lacteals.

25. Chylomicrons from lacteals eventually enter the bloodstream via: A) Hepatic portal vein B) Thoracic lymphatic duct C) Duodenum walls D) Pancreatic juice

Answer: B Explanation: Chylomicrons move into the thoracic lymphatic duct, from where they enter the bloodstream; blood plasma enzymes then hydrolyze them back into fats and proteins.

Short Questions on the Small Intestine

Below is a comprehensive set of 60 short questions (including true/false, fill-in-the-blanks, one-word answers, and brief short answers) derived directly from the provided text. I've aimed to cover all key details exhaustively while keeping them concise. Each question is followed immediately by its answer and a brief reference to the text for verification.

1. What is the longest part of the alimentary canal? Answer: Small intestine. (It is the longest part of the alimentary canal.)

2. Where does the small intestine start and end? Answer: Starts after the stomach; ends at the large intestine. (It starts after the stomach and ends at the large intestine.)

3. What is the diameter of the small intestine in an adult man? Answer: 2-3 cm. (In adult man it is about 2-3 cm in diameter.)

4. What is the length of the small intestine in an adult man? Answer: 6 m. (It is about 6 m in length.)

5. What are the two main responsibilities of the small intestine? Answer: Final digestion of food; absorption of digested food into blood and lymph. (Responsible for final digestion and absorption.)

6. Name the three parts of the small intestine. Answer: Duodenum, jejunum, ileum. (Consists of three parts: duodenum, jejunum, and ileum.)

7. What is the length of the first portion of the small intestine? Answer: 20-25 cm. (The first 20-25 cm long portion is the duodenum.)

8. What is the primary concern of the duodenum? Answer: Digestion of food. (It is concerned with the digestion of food.)

9. What type of secretion do glands in the duodenum produce? Answer: Alkaline secretion containing bicarbonate. (Contains glands which produce an alkaline secretion containing bicarbonate.)

10. Name the two main secretions poured into the duodenum. Answer: Pancreatic juice; bile. (Two main secretions: pancreatic juice and bile.)

11. What is the source of pancreatic juice? Answer: Pancreas. (It is the secretion of pancreas.)

12. Where is pancreatic juice poured? Answer: Duodenum. (Poured into duodenum.)

13. What is the pH of pancreatic juice? Answer: Slightly alkaline (pH 8). (It is slightly alkaline (pH 8) due to bicarbonate.)

14. What does bicarbonate in pancreatic juice do to chyme? Answer: Neutralizes its acidity. (Neutralizes the acidity of chyme.)

15. Name the enzyme in pancreatic juice that digests polysaccharides. Answer: Pancreatic amylase. (Pancreatic amylase digests polysaccharides into maltose and glucose.)

16. What does pancreatic amylase produce? Answer: Maltose and glucose. (Digests polysaccharides into maltose and even glucose.)

17. What is the inactive form of the enzyme that digests proteins into polypeptides? Answer: Trypsinogen. (Trypsinogen, which is in inactive form.)

18. What activates trypsinogen into trypsin? Answer: Enterokinase. (Enterokinase activates trypsinogen into trypsin.)

19. Where is enterokinase secreted? Answer: Walls of duodenum. (Secreted by the walls of duodenum.)

20. What does trypsin digest? Answer: Proteins into polypeptides. (Trypsin digests proteins into polypeptides.)

21. Name a pancreatic enzyme that digests proteins into smaller peptides. Answer: Chymotrypsin. (Chymotrypsin digests proteins into smaller peptides.)

22. What does carboxypeptidase digest? Answer: Proteins into amino acids. (Carboxypeptidase digests into amino acids.)

23. Which pancreatic enzyme digests lipids? Answer: Pancreatic lipase. (Pancreatic lipase digests lipids to glycerol and fatty acids.)

24. What are the products of lipid digestion by pancreatic lipase? Answer: Glycerol and fatty acids. (Digests lipids to glycerol and fatty acids.)

25. What do pancreatic nucleases digest? Answer: DNA and RNA. (Pancreatic nucleases digest DNA and RNA into nucleotides.)

26. What are the products of pancreatic nucleases? Answer: Nucleotides. (Digest DNA and RNA into nucleotides.)

27. What is bile? Answer: Secretion of liver. (It is the secretion of liver.)

28. Where is bile stored before release? Answer: Gallbladder. (Stored in gallbladder.)

29. What do bile salts act as? Answer: Detergent molecules. (Bile salts act as detergent molecules.)

30. What is the function of bile salts on fats? Answer: Break fats into droplets and keep them separate. (They break fats into droplets and keep them separate.)

31. Why are fats broken into droplets by bile? Answer: Because fats are insoluble in water and cannot be attacked by lipase. (Fats are insoluble in water, so cannot be attacked readily by lipase.)

32. What does the emulsion from bile provide for enzymes? Answer: Large surface areas. (Provide large surface areas for effective action of lipid-digesting enzymes.)

33. True or False: Bile digests fats directly. Answer: False. (Bile emulsifies fats but does not digest them.)

34. What condition is caused by accumulation of bile pigments in blood? Answer: Jaundice. (Accumulate in blood, causing jaundice.)

35. What happens if bile pigments are prevented from leaving the digestive tract? Answer: They accumulate in blood. (If prevented from leaving digestive tract, they may accumulate in blood.)

36. How long is the jejunum? Answer: 2.4 meters. (Jejunum is 2.4 meters long.)

37. What is the position of the jejunum relative to duodenum? Answer: Next to duodenum. (Next to duodenum.)

38. How is the ileum described in terms of portion and length? Answer: Last three-fifths, about 3.5 meters. (Last three fifth i.e., about 3.5 metres long.)

39. What do jejunum and ileum carry out? Answer: Rest of digestion and absorption of food. (Carry out the rest of digestion and absorption.)

40. What do the walls of jejunum and ileum contain? Answer: Glands which secrete intestinal juice. (Contain glands which secrete intestinal juice.)

41. Name an enzyme in intestinal juice that digests polypeptides. Answer: Aminopeptidase. (Aminopeptidase digests polypeptides into dipeptides.)

42. What does erypsin digest? Answer: Dipeptides into amino acids. (Erypsin digests dipeptides into amino acids.)

43. What does lipase in intestinal juice digest? Answer: Fats into fatty acids and glycerol. (Lipase digests fats into fatty acids and glycerol.)

44. What does maltase digest? Answer: Maltose into glucose. (Maltase digests maltose into glucose.)

45. What are the products of sucrase action? Answer: Glucose and fructose. (Sucrase digests sucrose into glucose and fructose.)

46. What does lactase digest? Answer: Lactose into glucose and galactose. (Lactase digests lactose into glucose and galactose.)

47. What is chyme converted into after intestinal juice enzymes? Answer: Alkaline emulsion called chyle. (Converted into an alkaline emulsion, called chyle.)

48. Where does absorption of digested food, water, and minerals occur? Answer: Jejunum and ileum. (Occurs in jejunum and ileum.)

49. What are the large circular folds in the inner wall of jejunum and ileum? Answer: Folds with villi. (Inner wall contains large circular folds with villi.)

50. What are villi? Answer: Finger-like projections. (Numerous finger-like projections called villi.)

51. What supplies each villus? Answer: Blood capillaries and lacteal. (Richly supplied with blood capillaries and lacteal.)

52. What is a lacteal? Answer: Vessel of lymphatic system. (A vessel of lymphatic system.)

53. What covers blood capillaries and lacteal in villi? Answer: Single-cell thick epithelium. (Covered by a single-cell thick epithelium.)

54. What are microvilli? Answer: Cytoplasmic projections on epithelial cells. (Countless cytoplasmic projections called microvilli.)

55. What do villi and microvilli increase? Answer: Total surface area of absorption. (Total surface area becomes extraordinarily large.)

56. How does the internal surface of jejunum and ileum appear? Answer: Velvety. (Appears velvety due to folds and villi.)

57. How are simple sugars and amino acids absorbed? Answer: By diffusion or active transport. (Absorbed by diffusion or active transport.)

58. Where do simple sugars and amino acids enter after epithelial cells? Answer: Blood capillaries of villi. (Enter the blood capillaries of villi.)

59. What vein carries sugars and amino acids to the liver? Answer: Hepatic portal vein. (Join to form hepatic portal vein.)

60. How are fatty acids and glycerol absorbed? Answer: By passive transport. (Absorbed by passive transport into epithelial cells.)

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