The Small Intestine — The Final Stage of Digestion and Absorption
The small intestine is a very important part of your digestive system — it’s the longest tube inside your body where most of the digestion and absorption of food happens. Imagine it as a long, winding tube that starts right after your stomach and ends where the large intestine begins.
- Length: About 6 meters long (that’s around 20 feet!).
- Diameter: About 2 to 3 centimeters wide (a bit less than the width of your little finger).
Despite its name “small,” it’s really quite long, but it’s called small because its diameter (width) is smaller than that of the large intestine.
The Three Parts of the Small Intestine
The small intestine has three sections, each with a special role:
- Duodenum — the first part, right after the stomach.
- Jejunum — the middle part.
- Ileum — the last and longest part.
1. Duodenum — Where Digestion Happens
The duodenum is about 20-25 cm (around 8-10 inches) long. It’s the first part of the small intestine, and its main job is to continue the digestion of food that started in the stomach.
Here’s what happens in the duodenum:
- The food that leaves your stomach is called chyme — a thick, acidic, semi-liquid mixture.
- Since chyme is acidic, it needs to be neutralized before it can go further into the intestines. The duodenum produces a special alkaline secretion that contains bicarbonate — a chemical that neutralizes acid, kind of like how baking soda neutralizes vinegar in the kitchen.
Important Secretions in the Duodenum
There are two main important fluids that enter the duodenum to help digestion:
a. Pancreatic Juice
This juice comes from the pancreas, an organ near your stomach.
- It’s slightly alkaline (pH about 8), thanks to bicarbonate, so it neutralizes the acidic chyme.
- It contains many enzymes — special proteins that speed up chemical reactions — to break down carbohydrates, proteins, fats, and nucleic acids in your food:
| Enzyme | Function |
| Pancreatic amylase | Breaks down starch (complex carbs) into simpler sugars like maltose and glucose. |
| Trypsinogen | Inactive protein-digesting enzyme, activated by another enzyme called enterokinase to become trypsin, which breaks proteins into smaller pieces called polypeptides. |
| Chymotrypsin & Carboxypeptidase | Further break down polypeptides into even smaller pieces, amino acids. |
| Pancreatic lipase | Breaks down fats into glycerol and fatty acids. |
| Pancreatic nucleases | Break down DNA and RNA (genetic material) into nucleotides. |
Think of pancreatic juice like a cleaning crew with special tools for every kind of mess (carbs, proteins, fats, DNA).
b. Bile
- Produced by the liver and stored in the gallbladder until needed.
- Bile is not an enzyme but contains bile salts which break large fat droplets into smaller droplets, a process called emulsification.
- This increases the surface area of fats so enzymes like pancreatic lipase can work more efficiently to digest fats.
Imagine trying to wash greasy dishes — if you break up the grease into smaller droplets, soap works better. That’s what bile salts do in your intestine.
2. Jejunum and Ileum — Digestion and Absorption
These two parts come after the duodenum and are mainly responsible for:
- Completing digestion
- Absorbing nutrients (the digested pieces of food) into your bloodstream.
- The jejunum is about 2.4 meters (8 feet) long.
- The ileum is about 3.5 meters (11.5 feet) long and is the final section before food passes into the large intestine.
Intestinal Juice and Its Enzymes
The walls of the jejunum and ileum have glands that secrete intestinal juice — a watery fluid containing enzymes that help finish digestion:
| Enzyme | Function |
| Aminopeptidase | Breaks polypeptides (small protein pieces) into dipeptides (even smaller chains). |
| Erypsin | Breaks dipeptides into amino acids (the building blocks of proteins). |
| Lipase | Breaks fats into fatty acids and glycerol. |
| Maltase | Converts maltose (a sugar) into glucose (simple sugar). |
| Sucrase | Converts sucrose (table sugar) into glucose and fructose. |
| Lactase | Converts lactose (milk sugar) into glucose and galactose. |
Formation of Chyle
- After these enzymes work on the chyme, the mixture is transformed into a milky, alkaline fluid called chyle.
- This chyle contains digested nutrients ready to be absorbed by the intestine walls.
Absorption — Taking Nutrients into the Body
The jejunum and ileum have special structures to absorb nutrients efficiently.
Structure of the Absorbing Surface: Villi and Microvilli
- The inner walls of the jejunum and ileum are folded into big circular folds.
- On these folds are tiny finger-like projections called villi (singular: villus).
- Each villus is covered by epithelial cells (thin skin cells).
- The surface of these cells has even tinier projections called microvilli.
Why are these important?
They massively increase the surface area of the intestine — imagine a flat table versus a table covered in small spikes. More surface means more absorption, just like more solar panels catch more sunlight.
What’s inside a Villus?
- Each villus contains a network of blood capillaries (tiny blood vessels).
- It also contains a small lymphatic vessel called a lacteal.
- These vessels help carry absorbed nutrients away from the intestine to the rest of the body.
How Absorption Works in Villi
Absorption of Simple Sugars and Amino Acids
- Sugars like glucose and amino acids from digested proteins are absorbed by the villi.
- They enter the epithelial cells by active transport or diffusion (methods of moving molecules across cell membranes).
- From these cells, they enter the blood capillaries.
- The blood capillaries join to form the hepatic portal vein, which carries these nutrients directly to the liver.
Role of the liver:
- The liver stores excess glucose as glycogen (a storage form of sugar).
- It also stores excess amino acids as proteins.
- When needed, the liver sends nutrients into the bloodstream via the hepatic vein, which eventually reaches the heart and then the rest of the body.
Absorption of Fatty Acids and Glycerol
- Fat digestion products like fatty acids and glycerol enter the villi by passive transport.
- Inside villi cells, they recombine to form triglycerides (fat molecules).
- These triglycerides are coated with proteins, forming tiny droplets called chylomicrons.
- Chylomicrons enter the lacteals (lymph vessels).
- From lacteals, chylomicrons travel through the lymphatic system, eventually reaching the bloodstream near the heart via the thoracic duct.
Once in the bloodstream:
- Enzymes break down chylomicrons back into fats and proteins.
- The fats are then used or stored by body cells.
Summary with Simple Analogy
Think of your small intestine like a super-efficient factory assembly line and distribution center:
- The duodenum is where the heavy machinery (enzymes and bile) break down complex food into simple parts.
- The jejunum and ileum are where the factory workers (intestinal enzymes) finish the job and pack the nutrients.
- The villi and microvilli are the conveyor belts and loading docks, absorbing and sending nutrients into the delivery trucks (blood and lymph vessels).
- The blood and lymph vessels are highways delivering nutrients all over the body for energy, growth, and repair.