Egestion

Not every component of the food that is eaten can be digested and absorbed. A typical diet, particularly one rich in vegetables, fruit and whole grains, contains a substantial amount of indigestible plant material — chiefly cellulose and other components of dietary fibre — which no enzyme secreted anywhere along the human alimentary canal is able to break down, since the human body produces no cellulase. This indigestible residue, together with water, some unabsorbed minerals, bile pigments (which give it its characteristic colour), sloughed-off epithelial cells from the lining of the gut, and very large numbers of bacteria (many of which normally inhabit the large intestine and are not harmful), passes on from the ileum, through the ileo-caecal valve, into the large intestine.

The large intestine has essentially no digestive role of its own — no enzyme of any consequence is secreted along its length, and its mucosa carries out no absorption of digested nutrients, since by the time material reaches the caecum, digestion and nutrient absorption are already complete. Its function is instead almost entirely concerned with processing this residual material before it is eliminated. As the material passes slowly along the caecum and the ascending, transverse and descending colon, the colonic mucosa actively reabsorbs a large proportion of the water that remains in it (along with some sodium and other electrolytes), progressively converting the initially watery, semi-liquid residue into a firmer, semi-solid mass. This reabsorption is important for the body's overall water economy: several litres of fluid pass through the gut each day (from ingested liquid and from digestive secretions themselves), and only a small fraction of this is normally lost in the faeces, most being recovered here. The large intestine also harbours a substantial resident population of bacteria, which, among other activities, synthesise small amounts of certain vitamins, notably vitamin K and some B-complex vitamins, a portion of which may be absorbed and used by the body. …