Constipation is the infrequent or difficult passage of feces. Feces may be excessively hard, firm, or dry. Obstipation is intractable constipation and results in severe fecal impaction throughout the rectum and colon. Obstipated animals cannot eliminate the impacted fecal mass. Megacolon is a clinical disorder characterized by marked dilatation and hypomotility of the rectum and colon. Megacolon may cause obstipation and fecal impaction. In dogs and cats, the disorder is usually acquired; congenital spinal cord defects may cause chronic constipation in bulldogs and Manx cats.

The clinical signs of constipation include tenesmus with little or no passage of feces. Defecation may be painful. The fecal mass can be palpated through the abdomen or by digital examination of the rectum.

Flatulence is distention of the stomach or the intestines with air or gases. Flatus is the gas expelled from a body opening. Ordinarily, the terms flatus and flatulence pertain to the expulsion of intestinal gas through the anus. The problem is easily recognized in house pets from the quantity or quality of gas expelled or by the constant rumbling noises created by gas movement in the intestines.

In dogs and cats, the colon absorbs water and electrolytes from ingesta passing from the small intestine. The colon stores fecal matter until it can be eliminated through the process of defecation. The proximal portion of the colon serves primarily an absorptive function, whereas the distal colon is more involved with storage. Rhythmic segmental contractions mix ingesta within the colon and promote the absorption of water and electrolytes. Segmental contractions thus tend to decrease the rate of movement of luminal contents through the colon. Occasionally, these contractions may weakly propel the fecal mass toward the rectum. The primary propulsive wave within the colon is a “mass movement.” These movements occur only a few times each day and propel feces into the rectum, stimulating defecation.

When feces enter the rectum, a weak defecation reflex is stimulated. Distention of the rectal wall stimulates afferent signals that spread through the myenteric plexus to initiate mass movements in the descending colon and rectum. Neurologically coordinated relaxation of the anal sphincter allows the fecal column to pass through the anus. This intrinsic defecation reflex is enforced by a second reflex mediated via parasympathetic nerves from the sacral spinal cord segments. This sacral reflex greatly enforces the strength of the colonic movements and may allow complete evacuation of the colon. In housetrained pets, relaxation of the anal sphincter is voluntarily controlled by the cerebral cortex. Therefore, house-trained pets can inhibit the defecation reflex until they find an appropriate place.