Feces
Solid waste discharged during defecation, used in agriculture and medicine.
Feces (also called faeces, fæces, or poop) are the solid or semi-solid leftovers of food that the small intestine couldn't digest, later broken down by bacteria in the large intestine. They contain small amounts of metabolic waste, including bacterially altered bilirubin and dead cells shed from the gut lining. These materials exit the body through the anus or cloaca during defecation. In agriculture, feces can serve as fertilizer or soil conditioner; they can also be burned for fuel, dried for construction, or used medicinally—for instance, in fecal transplants. Together, urine and feces are known as excreta.
The characteristic smell of feces comes from skatole (produced from tryptophan via indoleacetic acid and decarboxylation), thiols (sulfur-containing compounds), amines, and carboxylic acids. This unpleasant odor is thought to deter humans from touching or consuming feces, which could cause sickness or infection.
Defecation involves pressures that can reach 100 mmHg (13.3 kPa) in humans and 450 mmHg (60 kPa) in penguins. Muscular contractions and gas buildup in the gut generate the force needed to expel feces, prompting the sphincter to release the pressure.
After digestion, an animal’s waste retains a significant amount of energy—often about 50% of the original food’s energy. This means a large portion of consumed energy remains available for decomposers in ecosystems. Many organisms feed on feces, including bacteria, fungi, and insects like dung beetles, which can detect odors from far away. Some species specialize in feces, while others use it as a dietary supplement. This behavior, called coprophagia, occurs in young elephants (to gain gut flora from their mothers), as well as in dogs, rabbits, and monkeys.
Feces and urine reflect ultraviolet light, which helps raptors like kestrels—able to see near-ultraviolet—locate prey by their middens and territorial markers. Seeds are also commonly found in feces. Frugivores (fruit-eating animals) disperse seeds when they eat fruit and later excrete them. This method of seed dispersal is highly successful: seeds far from the parent plant face less predation and often come with their own fertilizer, provided they survive the digestive tract.
Organisms that consume dead organic matter or detritus are called detritivores. They play a key role in recycling organic matter into simpler forms that plants and other autotrophs can absorb, a process known as the biogeochemical cycle. For soil nutrients to be maintained, feces should ideally return to the area where the food originated—something that often doesn’t happen in human societies, where food is transported from rural areas to cities and feces are disposed of into rivers or seas.
The "fecal resistome" is the total gene pool in human and animal feces that confers antimicrobial resistance. Antibiotics taken by humans and animals mostly leave the body through feces and urine, which contain antibiotic resistance genes (ARGs). These genes act as environmental pollutants, spreading through agricultural practices like manure application and through wastewater systems. When contaminated feces enter new ecosystems, their microbes promote horizontal gene transfer, spreading ARGs to native microbes. ARGs persist longer in soil when manure is used, and they can remain in the environment due to co-resistance with metals, which continue to exert selective pressure. Even without antibiotics, fecal matter alters soil composition and encourages the growth of pre-existing resistant bacteria.
Human defecation frequency varies: some people go several times a day, others once every two or three days. Extended hardening of feces that disrupts this routine for days or more is called constipation. The appearance of human feces depends on diet and health. Normally, it is semisolid with a mucus coating. Bile and bilirubin from dead red blood cells give it a typical brown color. After the first stool (meconium), a newborn’s feces contain only bile, appearing dark yellow-green. Breastfed babies produce soft, pale yellowish, and relatively mild-smelling matter; once the baby starts eating solid food and bilirubin from dead red blood cells appears, the stool turns brown. Throughout life, stool color and texture vary: rapid passage through the intestines makes it greenish, while a lack of bilirubin gives it a clay-like appearance.
Feces are a key indicator of the gut microbiome, which is an important marker of health and disease.
- composition
- Undigested food remains, bacteria, bilirubin, dead epithelial cells
- odor_cause
- Skatole, thiols, amines, carboxylic acids
- energy_retention
- Often 50% of original food energy
- color_cause
- Bile and bilirubin from dead red blood cells
- first_stool
- Meconium
Lore & Background
Feces are produced after digestion, when food not absorbed in the small intestine is broken down by bacteria in the large intestine. The distinctive odor comes from skatole, thiols, amines, and carboxylic acids. Feces retain about 50% of the energy of original food, supporting decomposers in ecosystems. Many organisms feed on feces, including bacteria, fungi, dung beetles, and animals practicing coprophagia, such as young elephants, dogs, rabbits, and monkeys. Seeds in feces aid plant dispersal, and feces reflect ultraviolet light, helping raptors like kestrels locate prey.
Reader's Guide
Feces play a critical role in ecology and human health. Ecologically, they recycle nutrients through detritivores and biogeochemical cycles, and serve as food for many organisms. In agriculture, animal feces like guano and manure are used as fertilizer, and dried dung is burned as fuel. Human feces are studied for the gut microbiome via 16S ribosomal RNA or shotgun sequencing, and fecal biomarkers can indicate diseases such as Inflammatory Bowel Disease. The fecal resistome—the gene pool for antimicrobial resistance in feces—spreads antibiotic resistance genes through manure and wastewater, persisting in soil. Coprolites (fossilized feces) and paleofeces provide direct evidence of ancient diets and health. Feces thus link digestion, ecology, medicine, and environmental science.
Did You Know?
- Feces contain about 50% of the energy of the original food eaten.
- Skatole, produced from tryptophan, is a key compound in the odor of feces.
- Fecal transplants are a medicinal use of human feces.
The Three-Part Physiological Breakdown
Jaundice is not a disease in itself but a visible warning that the body's heme-metabolism pipeline has been disrupted at some point. Clinicians divide the underlying pathology into three categories based on where the problem sits. Prehepatic jaundice stems from an abnormally rapid destruction of red blood cells, which floods the blood with unconjugated bilirubin. Sickle-cell anemia, spherocytosis, thalassemia, and glucose-6-phosphate dehydrogenase deficiency are classic examples, and infections such as malaria and bartonellosis can produce the same accelerated turnover. Hepatic jaundice implicates the liver directly: hepatocytes are injured by acute or chronic hepatitis, cirrhosis, drug-induced toxicity, alcoholic liver disease, or inherited conditions like Gilbert's syndrome (affecting roughly five percent of the population) and the rarer Crigler-Najjar syndromes. Posthepatic, or obstructive, jaundice occurs when bile ducts are physically blocked. Gallstones lodged in the common bile duct are the most common cause, but pancreatic-head cancer, acute or chronic pancreatitis, biliary strictures, and parasitic liver flukes can all create the same mechanical obstruction.
What the Body Shows: Signs, Symptoms, and the Itch
The hallmark of jaundice is a yellowish tint—occasionally greenish—appearing in the skin and the whites of the eyes. Scleral icterus, the yellowing of the eye's white area, typically becomes visible once serum bilirubin reaches at least three milligrams per deciliter. In reality, the term is a misnomer: bilirubin deposits in the conjunctival membranes that overlay the avascular sclera, making conjunctival icterus the technically accurate label. Because the conjunctiva is rich in elastin, it has a particularly high affinity for bilirubin, allowing even slight elevations to be spotted early. Accompanying signs include dark urine from bilirubin excretion and pale, fatty stools that lack their normal color. A hallmark complaint is intense, persistent itchiness, caused by bilirubin acting as a direct skin irritant. In people with darker skin tones, the yellowing on the skin can be subtle, so clinicians are advised to examine the sclera, palms, soles, and oral mucosa for a more reliable read. In a rare pediatric presentation, elevated bilirubin during tooth calcification can leave a yellow or green stain on developing teeth.
A Spectrum From Benign to Life-Threatening
The causes of jaundice span a wide range of severity. On the milder end, Gilbert's syndrome produces only transient, mild elevations in unconjugated bilirubin and is found in about five percent of people. Prolonged fasting, large bruises, and thyroid problems can also nudge unconjugated levels upward without indicating serious disease. At the other extreme, jaundice can signal cirrhosis, acute hepatitis, pancreatic cancer, or sepsis from leptospirosis—conditions that can be fatal if untreated. The bilirubin itself splits into two biochemical forms: unconjugated, which accumulates when red-cell breakdown outpaces liver processing, and conjugated, which builds up when the liver or bile ducts cannot excrete it properly. Normal blood levels sit below one milligram per deciliter; values above two to three generally cross the threshold into visible jaundice. It is also worth noting that not every yellowish skin discoloration is true jaundice. Carotenemia, from consuming large quantities of carotene-rich foods, and certain medications such as rifampin can produce a similar hue without any bilirubin elevation at all.
Treating the Root and the Newborn Challenge
Because jaundice is a sign rather than a disease, treatment is always directed at the underlying cause. When a bile duct is physically obstructed—by gallstones, a tumor, or a stricture—surgical intervention is usually necessary to restore flow. In all other cases, management is medical: treating an infectious source, discontinuing an offending drug, or addressing the metabolic disorder. For the persistent itchiness that accompanies many cases, clinicians may drain the gallbladder, prescribe ursodeoxycholic acid, or use opioid antagonists such as naltrexone. Newborns present a unique and time-sensitive challenge. An estimated eighty percent of babies develop some degree of jaundice during their first week of life. When bilirubin climbs into the four-to-twenty-one milligram-per-deciliter range, treatment escalates to phototherapy or, in more severe or premature cases, an exchange transfusion. The word jaundice itself carries a linguistic heritage, descending from the French jaunisse, literally yellow disease, a name that has tracked the condition's most recognizable feature for centuries.
Frequently Asked Questions
What is Feces in the context of Physiology & Metabolism 1-17?
Feces are the solid or semi-solid remnants of food that the small intestine could not absorb, which are then further processed by the bacterial community in the large intestine. They leave the body through the anus or cloaca during the act of defecation.
What gives feces its characteristic smell?
The odor is produced by a combination of compounds—skatole, thiols, amines, and carboxylic acids—that arise when gut bacteria break down organic material in the colon.
Why does feces typically appear brown?
The coloration comes from bile pigments and bilirubin, which are byproducts of the body's recycling of dead red blood cells. These pigments pass through the digestive tract and tint the final waste product.
What is actually inside feces?
The material consists of undigested food particles, bacteria, bacterially altered bilirubin, and shed epithelial cells from the intestinal lining. Surprisingly, it can still hold roughly half of the caloric energy originally present in the food.
What are the practical applications of feces?
Agriculturally it works as fertilizer or a soil conditioner, and it can also be burned for fuel or dried for construction purposes. In medicine, fecal transplants transfer donor stool to reseed a patient's gut with a healthy microbiome.
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