In many respects the small intestine is where most of the important functions of the digestive system are performed. In this compartment the chemical digestion (initiated in the mouth and stomach) is rapidly accelerated and it is the only part of the human digestive tract where significant levels of absorption of nutrients occurs. Before we go on and look at these processes let's have a quick look at some of the basic features of the small intestine.
A. MICROSCOPIC STRUCTURE
The epithelium lining the small intestine is a reflection of its role in digestion and absorption. Its wall exhibits small (around 1 mm high) finger-like projections known as villi that help increase the surface area available for absorption.
The lumen is lined with simple columnar epithelial cells that are coupled by tight junctions that ensure that almost everything that moves through the epithelium does so by passing through the epithelial cells themselves.
The luminal surface of these epithelial cells also have tiny folds on their luminal surface known as microvilli. These increase the surface area of the small intestine for absorption and because they look (under a microscope) a bit like the bristles of a brush are sometimes referred to as the brush border. The microvilli also have enzymes attached to their their luminal surface that contribute to the process of digestion.
In between these columnar epithelial cells are mucus-secreting goblet cells that secrete a thick, sticky mucus that helps project the small intestine from damage.
Specialised paneth cells at the base of these glands secrete lysozyme, an antibacterial enzyme that helps protect the small intestine from any ingested bacteria that have survived their journey through the stomach.
The proximal portions of the small intestine are covered in microscopic pits known as intestinal glands. These glands secrete a watery alkaline mucus that helps protect the small intestine from the acidic chyme arriving from the stomach.
Like the stomach these intestinal glands also contain stem cells that divide to reconstitute the epithelium.
The duodenal mucosa also contains two very important enteroendocrine cells:
(i) S cells - secrete the hormone secretin in response to a low (acidic) pH and the products of protein and fat digestion.
(ii) I cells - secrete cholecystokinin in response to fats and the products of protein digestion.
B. MOTILITY
Chyme arriving from the stomach through the pyloric sphincter is transported through the small intestine by peristalsis. The basic rhythm of these contractions are intrinsic to the small intestine and originate from pacemaker smooth muscle cells. However this intrinsic rhythm is modified by the autonomic nervous system and neurones of the enteric nervous system:
- In general, the parasympathetic division of the autonomic nervous system tends to increase the rate and force of contraction while the sympathetic division depresses it.
- Circuits of enteric nervous system neurones produce localised responses to both mechanical and chemical stimuli.
In addition to the propulsive peristaltic motility of the small intestine there is also a lot of mixing produced by segmentation. One of the functions of this type of movement is to produce effective mixing of the food with the exocrine secretions of the liver and pancreas.
Before we go on and talk about the major functions of the small intestine let’s have a look at these accessory gland secretions and their control systems.