Anatomy
The small intestine is a long, coiled tube, approximately 20 feet in length, divided into three main sections: the duodenum, jejunum, and ileum.
1. Duodenum:
Roughly 10 inches long, it is the first segment that receives partially digested food (chyme) from the stomach, along with bile from the gallbladder and digestive enzymes from the pancreas.
The duodenum's primary role is to continue the chemical digestion of chyme.
2. Jejunum:
Extending about 8 feet, this middle section is primarily involved in the absorption of nutrients and water from the processed chyme.
The jejunum's lining is richly folded with villi and microvilli, increasing its surface area to maximize absorption.
3. Ileum:
The final and longest segment, about 12 feet in length, continues the process of nutrient absorption and then passes the remaining undigested material into the large intestine.
The ileum contains Peyer's patches, important for immune surveillance.
Functions
1. Digestion:
The small intestine completes the digestion of carbohydrates, proteins, and lipids, breaking them down into their simplest forms (sugars, amino acids, and fatty acids, respectively) through the action of enzymes.
2. Absorption:
Nutrients, along with water and electrolytes, are absorbed through the walls of the small intestine into the bloodstream.
This is facilitated by the extensive surface area provided by villi and microvilli.
3. Transport:
Peristaltic movements help to mix the chyme with digestive juices and move it along the intestine, while segmentation movements aid in its mechanical breakdown.