When food passes through the large intestine too slowly, large amounts of water are absorbed as the faeces remains in the colon for too long. Usually faeces is about 99% water, when too much water is absorbed the faeces becomes a hard mass of undigested residues.

Constipation can occur when people are on drugs or suffering from some form of brain disease. The brain does not receive or process the signal to tell it that there are faeces in the colon which needs to be expelled. The faeces remain in the colon and water continues to be absorbed, making the mass harder. In other people a problem in the pelvic muscle means they can't build up the pressure needed to defecate, this can be a result of childbirth and some forms of surgery.

In most people constipation is caused by lack of fibre in the diet and can be easily corrected.

Bibliography

Encyclopaedia Britannica, 15th Edition. (Chicago: University of Chicago, 1986)

Encyclopaedia Britannica, Volume 7. (Chicago: Encyclopaedia Britannica, 1970)

The Book of Popular Science Encyclopaedia. (New York: Grolier, 1961)

The Software Toolworks Multimedia Encyclopaedia, Release 6. (New York: Grolier, 1996)

Encarta 96 Encyclopaedia. (Redmond: Microsoft, 1996)