Why is human excreta in most cases brown in colour?
The human feces are normally about three fourths water and one-fourth solid matter. The solid matter is composed of about 30 per cent dead bacteria, 10 to 20 per cent inorganic matter, 2 to 3 per cent protein and 30 per cent undigested roughage of the food and dried constituents of digestive juices, such as bile pigments and sloughed epithelial cells.
In most cases the normal colour of human feces is brown. The brown colour is caused by stercobilin and urobilin which are derivatives of bilirubin.
The bilirubin is the breakdown product of blood pigment haemoglobin found in the age old fragile red blood cells which get destroyed in the reticuloendothelial system.
The stercobilin and urobilin thus formed are brought to the intestinal system where they impart brown colouration to the final fecal matter stored in the distal storage colon. The entire process occurs as follows:
The red blood cells which have lived out their life span (120 days) become too fragile. When they reach the reticuloedndothelial tissue their membranes rupture and the pigment haemoglobin released is phagocytised by tissue macrophages.
The haemoglobin is first split into globin and haeme. The haeme ring is opened to give (1) free iron which is transported to blood for further use and (2) a straight chain of four pyrrole nuclei. This chain is the substrate from which bilirubin will be formed.
The free bilirubin thus formed is released from the macrophages into the blood plasma. In the plasma the free bilirubin immediately combines with the plasma albumin and is transported in this combination throughout the blood and interstitial fluids.
Within hours the bilirubin is absorbed by the hepatic cells (liver cells) leaving the plasma protein, globulin.
On reaching the hepatic cells the bilirubin combines with glucuronic acid and becomes conjugated bilirubin. In this form the bilirubin is excreted by an active transport from the liver into bile canaliculi and then into the intestine through the bile duct along with bile.
Once in the intestine, the conjugated bilirubin is converted into urobilirogen by the bacteria which live in the colon. In the feces it gets oxidized in to urobilin or becomes altered and oxidized into stercobilin. These two products make the feces appear brown.
Editor, Research Journal of Biological Sciences
J.J. College of Arts and Science
Pudukkottai, Tamil Nadu
Keywords: human excreta