Feces (Faeces) – Formation, Composition, Color and Odor

Feces (UK ~ faeces) is the waste material passed out from the bowels through the anus. It is usually solid to semi-solid in consistency but can be hard in constipation or watery with diarrhea. Fecal matter is also referred to as stool and the process of passing feces is known as defecation or bowel movement.

Formation of Feces

Fluid to Solid Feces

Fecal matter is the remaining material after food is digested along with water, bacteria and other substances secreted into the gastrointestinal tract. About 1.5 liters of fluid chyme passes from the small intestine into the large intestine each day. Most of the nutrients from the food has been absorbed at this stage.

As the chyme moves through the first half of the colon, large amounts of water and electrolytes are absorbed. Despite this, water makes up about 70% of the fecal weight. Water absorption transforms the fluid chyme into a mush-like consistency by the time it passes through the transverse colon. It solidifies further along its passage down the descending colon.

Colonic Bacteria

The bacteria in the colon play integral roles in nutrient absorption and the formation of feces. Colonic bacteria digest cellulose thereby releasing residual nutrients which are absorbed by the colon. In addition, the action of colonic bacteria contribute to the formation and absorption of vitamin B12, thiamin, riboflavin and vitamin K. The bacteria also produce the gases, carbon dioxide, hydrogen and methane, which make up flatus.The action of the bacteria plays a major part in determining the color and odor of fecal matter.

Water and Electrolytes in the Colon

Electrolytes like bicarbonate are secreted by the wall of the large intestine into the lumen. This helps to neutralize any acidic byproducts of bacterial metabolism. At the same time sodium and chloride are  absorbed by the intestinal wall which creates a concentration gradient to facilitate water absorption.

However, the large intestine can only absorb about 8 liters of water in a day. Any extra fluid will remain in the colon, giving a liquid consistency to the feces (loose stool). It also creates additional bulk and increases defecation frequency by triggering the local defecation reflex. This is seen as watery diarrhea. Excess or a deficiency of water will also change the consistency of the stool as outlined in the Bristol Stool Chart which features the different types of feces.

Composition of Feces

About 75% of fecal weight is made up of water. The other 25% is composed of solid matter which contains :

  • Undigested fiber and solidified components of digestive juices (30%)
  • Bacteria (30%)
  • Fat (10% to 20%)
  • Inorganic matter (10% to 20%)
  • Protein (2% to 3%)

Color of Feces

Feces usually has a brown color, ranging from a tan hue to a darker-brown color.  Bilirubin is passed out in the bile and the action of bacteria and air in the gut breaks it down into stercobilin and urobilin, which gives stool its typical color. Additionally, certain foods with a strong colorants or other staining agents may also influence the color of feces.

Odor of Feces

The odor of feces may differ among people and is influenced significantly by the foods that are consumed. The gas byproducts of bacterial action in the colon largely accounts for the odor of the feces. Hydrogen sulphide is one of the most prominent odoriferous compounds responsible for the characteristic smell of the stool. Other compounds like indole, skatole and mercaptans are also responsible for the odor.

Related Articles

  1. Loose Stool and Hard Stool
  2. Bleeding in the Stool

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