Faeces, also known as stool, is the solid component of human waste. Almost half of its dry mass is bacterial biomass, with the remainder comprised of undigested dietary matter, exfoliated cells of the gut, intestinal secretions, small metabolites and mucus.
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Composition
Faecal matter is semisolid in consistency and comprises as much as 75% water in well subjects. Organic material comprises approximately 90% of the solid components. Excreted gut bacteria forms about half of this, and the remainder is made up of protein/nitrogenous compounds, carbohydrates - including non-digested dietary fibre (mainly cellulose-based) - and undigested lipids. The proportions of these components changes markedly with diet 1,2.
Over 300 volatile organic compounds (VOCs) have been identified in human faeces, these compounds are important contributors to the offensive nature of the stool 1,3.
About 10% of the solid component is inorganic, and this fraction comprises a mix of calcium and iron phosphates, GI secretions, epithelial cells from the gut and mucus 2. This inorganic fraction also includes thousands of small molecules, for example simple sugars, amino acids and acids which together constitute the faecal metabolome 1.
Typically human stool is brown in colour, the predominant contributor to this being stercobilin, a byproduct of the catabolism of haemoglobin.
Radiographic features
Faeces can be seen on all imaging modalities, usually with a heterogeneous appearance due to the mix of water, fat, soft tissue, and gas components. The faeces are often outlined by the large amounts of normal large bowel gas.
Plain radiograph
Faeces throughout the large bowel are well seen on abdominal radiographs, with a mixed radiographic density, predominantly soft tissue with gas and fatty components.
Ultrasound
Faeces are often difficult to see on ultrasound due to large amounts of surrounding gas in the large bowel. Nevertheless even if not directly-seen their presence can be inferred component reflecting the ultrasound waves;
CT
Faeces have a mixed attenuation on CT and often the fatty component serves as a useful corroboration that a visualised colonic filling defect is faecal and not a polyp.
History and etymology
The term faeces is the pluralisation of the Latin word "faex" which means dregs or sediments 4,5.
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