Small Intestine - Anatomy & Physiology

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Small Intestine

Introduction

The small intestine extends from the pylorus of the stomach to the caecum. The small intestine recieves chyme from stomach. Most chemical degradation of the chyme and absorption takes place in the small intestine. Fats are exclusively broken down in this part of the alimentary tract. Carbohydrates and proteins that are not degraded in the small intestine are available for microbial fermentation in the large intestine. The small intestine produces enzymes for digestion of protein, carbohydrate and fat and absorbs the products of their digestion. Enzymes are produced by glands in the intestinal wall and the pancreas. The gall bladder produces bile which emulsifies fats for digestion. Absorption is facilitated by ridges in the small intestine and by the presence of villi and microvilli.

The small intestine consists of three parts:

• Duodenum - Anatomy & Physiology
• Jejunum - Anatomy & Physiology
• Ileum - Anatomy & Physiology

Structure

• Attached along it's whole length to the dorsal abdominal wall by mesentry.
• The mesentry is relatively long for its most part, giving the small intestine a great deal of mobility.

Function

• To mix content and transport chyme.
• To digest chyme and absorb the products of digestion.

• Contraction of the two muscle layers facilitates mixing and transportation.
• There are two types of muscle contraction:
  • Segmental
    • Rhythmic contraction of circular muscle.
    • Creates ring like contractions
    • Divides content into many segments
    • Moves segments backwards and forth causing mixing with digestive juices
    • This type of contraction predominates during digestion
  • Peristaltic
    • Antagonistic contraction of longitudinal and circular muscle
    • Transports chyme through the small intestine but contractions can be weak to allow time for absorption
• When a peristaltic ‘’wave’’ of contraction reaches the end of the ileum, another starts in the duodenum. This is known as a migrating myoelectric complex.