A bowel movement is the last stop in the movement of food through your digestive tract. Your stool passes out of your body through the rectum and anus. Another name for stool is feces. It is made of what is left after your digestive system (stomach, small intestine, and colon) absorbs nutrients and fluids from what you eat and drink.

Sometimes a bowel movement isn't normal. Diarrhea happens when stool passes through the large intestine too quickly. Constipation occurs when stool passes through the large intestine too slowly. Bowel incontinence is a problem controlling your bowel movements. Other abnormalities with bowel movements may be a sign of a digestive problem.

The processes of digestion include six activities: ingestion, propulsion, mechanical or physical digestion, chemical digestion, absorption, and defecation.

The first of these processes, ingestion, refers to the entry of food into the alimentary canal through the mouth. There, the food is chewed and mixed with saliva, which contains enzymes that begin breaking down the carbohydrates in the food plus some lipid digestion via lingual lipase. Chewing increases the surface area of the food and allows an appropriately sized bolus to be produced.

Food leaves the mouth when the tongue and pharyngeal muscles propel it into the esophagus. This act of swallowing, the last voluntary act until defecation, is an example of propulsion, which refers to the movement of food through the digestive tract. It includes both the voluntary process of swallowing and the involuntary process of peristalsis. Peristalsis consists of sequential, alternating waves of contraction and relaxation of alimentary wall smooth muscles, which act to propel food along (see image below). These waves also play a role in mixing food with digestive juices. Peristalsis is so powerful that foods and liquids you swallow enter your stomach even if you are standing on your head.

Peristalsis moves food through the digestive tract with alternating waves of muscle contraction and relaxation.

Digestion includes both mechanical and chemical processes. Mechanical digestion is a purely physical process that does not change the chemical nature of the food. Instead, it makes the food smaller to increase both surface area and mobility. It includes mastication, or chewing, as well as tongue movements that help break food into smaller bits and mix food with saliva. Although there may be a tendency to think that mechanical digestion is limited to the first steps of the digestive process, it occurs after the food leaves the mouth, as well. The mechanical churning of food in the stomach serves to further break it apart and expose more of its surface area to digestive juices, creating an acidic “soup” called chyme. Segmentation, which occurs mainly in the small intestine, consists of localized contractions of circular muscle of the muscularis layer of the alimentary canal. These contractions isolate small sections of the intestine, moving their contents back and forth while continuously subdividing, breaking up, and mixing the contents. By moving food back and forth in the intestinal lumen, segmentation mixes food with digestive juices and facilitates absorption.

In chemical digestion, starting in the mouth, digestive secretions break down complex food molecules into their chemical building blocks (for example, proteins into separate amino acids). These secretions vary in composition, but typically contain water, various enzymes, acids, and salts. The process is completed in the small intestine.

Food that has been broken down is of no value to the body unless it enters the bloodstream and its nutrients are put to work. This occurs through the process of absorption, which takes place primarily within the small intestine. There, most nutrients are absorbed from the lumen of the alimentary canal into the bloodstream through the epithelial cells that make up the mucosa. Lipids are absorbed into lacteals and are transported via the lymphatic vessels to the bloodstream (the subclavian veins near the heart). The details of these processes will be discussed later.

In defecation, the final step in digestion, undigested materials are removed from the body as feces.

In some cases, a single organ is in charge of a digestive process. For example, ingestion occurs only in the mouth and defecation only in the anus. However, most digestive processes involve the interaction of several organs and occur gradually as food moves through the alimentary canal (image below).

Some chemical digestion occurs in the mouth. Some absorption can occur in the mouth and stomach, for example, alcohol and aspirin.

The final step in digestion is the elimination of undigested food content and waste products. The undigested food material enters the colon, where most of the water is reabsorbed. Recall that the colon is also home to the microflora called “intestinal flora” that aid in the digestion process. The semi-solid waste is moved through the colon by peristaltic movements of the muscle and is stored in the rectum. As the rectum expands in response to storage of fecal matter, it triggers the neural signals required to set up the urge to eliminate. The solid waste is eliminated through the anus using peristaltic movements of the rectum.

Common Problems with Elimination

Diarrhea and constipation are some of the most common health concerns that affect digestion. Constipation is a condition where the feces are hardened because of excess water removal in the colon. In contrast, if enough water is not removed from the feces, it results in diarrhea. Many bacteria, including the ones that cause cholera, affect the proteins involved in water reabsorption in the colon and result in excessive diarrhea.

Emesis

Emesis, or vomiting, is elimination of food by forceful expulsion through the mouth. It is often in response to an irritant that affects the digestive tract, including but not limited to viruses, bacteria, emotions, sights, and food poisoning. This forceful expulsion of the food is due to the strong contractions produced by the stomach muscles. The process of emesis is regulated by the medulla.

Radiographic examination of the process of defecation after the instillation of a CONTRAST MEDIA into the rectum.

Infants' bowel movements go through many changes in color and consistency, even within the first few days after birth. It's important to keep track of your infant's bowel movements. Some things to look for include:

• Color. A newborn's first bowel movements usually consist of a thick, black or dark green substance called meconium (pronounced mi-KOH-nee-uhm). After the meconium is passed, the stools ("poop") will turn yellow-green. The stools of breastfed infants look mustard-yellow with seed-like particles.
• Consistency. Until the infant starts to eat solid foods, the consistency of the stool can range from very soft to loose and runny. Formula-fed infants usually have stools that are tan or yellow in color and firmer than those of a breastfed infant. Whether your baby is breastfed or bottle-fed, hard or very dry stools may be a sign of dehydration.
• Frequency. Infants who are eating solid foods can become constipated if they eat too many constipating foods, such as cereal or cow's milk, before their system can handle them. The U.S. Food and Drug Administration (FDA) and the American Academy of Pediatrics (AAP) do not recommend cow's milk for babies under 12 months.

Also, because an infant's stools are normally soft and a little runny, it's not always easy to tell when a young infant has mild diarrhea. The main signs are a sudden increase in the number of bowel movements (more than one per feeding) and watery stools.

Diarrhea can be a sign of intestinal infection, or it may be caused by a change in diet. If the infant is breastfeeding, diarrhea can result from a change in the mother's diet. The main concern with diarrhea is the possibility that dehydration can develop. If fever is also present and your infant is less than 2 months old, you should call your health care provider. If the infant is over 2 months old and the fever lasts more than a day, check the infant's urine output and rectal temperature and consult a health care provider. Make sure the infant continues to feed often.

Starting around the age of 3 to 6 weeks, some breastfed babies have only one bowel movement a week. This is normal because breast milk leaves very little solid waste to pass through the digestive system. Formula-fed infants should have at least one bowel movement a day. If a formula-fed infant has fewer bowel movements than this and appears to be straining because of hard stools, constipation may be the cause. Check with your health care provider if there are any changes in or problems with your infant's bowel movements.

The seven types of stool are:

• Type 1: Separate hard lumps, like nuts (hard to pass)
• Type 2: Sausage-shaped, but lumpy
• Type 3: Like a sausage but with cracks on its surface
• Type 4: Like a sausage or snake, smooth and soft
• Type 5: Soft blobs with clear cut edges (passed easily)
• Type 6: Fluffy pieces with ragged edges, a mushy stool
• Type 7: Watery, no solid pieces. Entirely liquid

Types 1–2 indicate constipation, with 3 and 4 being the ideal stools (especially the latter), as they are easy to defecate while not containing any excess liquid, and 5, 6 and 7 tending towards diarrhoea.