There are five layers in the gastrointestinal wall (Figure 35.1):

Figure 35.1 Cross-section of small intestine.

The blood flow from the small intestine is significant because it passes directly to the liver via the hepatic portal vein. Most of the water-soluble nutrients absorbed from the gut are transported to the liver in this way. Fat-soluble substances are absorbed by the intestinal lymphatic system and distributed to the blood by the thoracic duct, thus bypassing the liver.

The stomach mucosa contains many mucus-secreting cells; the mucus creates a protective layer over the lining of the stomach wall. The lining of the stomach is highly glandular and consists of four types of gland:

Different areas of the stomach vary in their secretions.

The gastrointestinal tract is a complex and self-contained ecosystem of over 400 bacterial species. Anaerobic bacteria (bacteria that live in an environment where there is little or no oxygen) outnumber aerobic bacteria (those that require the presence of oxygen to function). Fungi are also present but, due to the population of the normal commensals, tend not exceed limits, except for a problem such as that caused by an overgrowth of, for example, Candida albicans.

Due to the existence of the commensals, a situation exists where host, bacteria and fungi live together without any adverse effects on one another. In many cases in the gastrointestinal tract, bacteria provide a very useful service, producing vitamins that are vital for health or taking part in chemical reactions necessary to activate compounds (see Chapter 13 ‘Vitamins and minerals’, p. 108). These bacteria are usually non-pathogenic. Most of the gut flora is found in the colon and lives in the lumen of the bowel; it does not normally penetrate the gut wall.

Ingestion of certain substances, e.g. antibiotics, can disturb the normal balance of these commensals, which is why carefully consideration of gut health after a course of antibiotics is important.