Logan Steele
Defecation is the act of expelling faeces from the body, and it is a necessary biological process that occurs in all organisms. It involves the coordination of multiple systems, including the gastrointestinal, nervous, and musculoskeletal systems. The process is initiated by the defecation reflex, which is triggered when the rectum is filled with stool, activating stretch receptors that signal the brain. This results in the contraction of rectal muscles and relaxation of the internal anal sphincter, followed by the voluntary contraction of the external anal sphincter and abdominal muscles, which work together to expel faeces from the body. While defecation is typically under voluntary control in adults, certain medical conditions and disorders can impair the defecation reflex, leading to constipation, diarrhoea, or faecal incontinence.
| Characteristics | Values |
|---|---|
| Definition | The act of expelling feces from the digestive tract via the anus |
| Other Names | Bowel movement, pooping, crapping, doing number two, dropping a deuce, taking a dump, going poo-poo, making doo-doo, defaecation, defecation |
| Frequency | Varies from a few times daily to a few times weekly |
| Process | Begins with mass movement from the colon to the rectum, initiating the defecation reflex, which involves rectal contraction and internal and external anal sphincter relaxation |
| Control | Involuntary from the colon to the internal anal sphincter; under conscious or subconscious control at the level of the external anal sphincter |
| Reflex Initiation | Deep breath and contraction of abdominal muscles to increase abdominal pressure |
| Prevention | Voluntary tightening of the external anal sphincter |
| Related Conditions | Constipation, fecal incontinence, diarrhea |
| Contributing Factors | Diet, lifestyle, and medical conditions |
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The defecation reflex
Defecation is the act of expelling faeces from the digestive tract via the anus. This complex function requires coordination between the gastrointestinal, nervous, and musculoskeletal systems. The process begins with mass movement from the colon to the rectum, initiating the defecation reflex, which involves rectal contraction and internal and external anal sphincter relaxation. The defecation reflex is a coordinated function of the digestive tract, nervous system, and musculoskeletal system.
There are two main types of defecation reflexes: the myenteric defecation reflex and the parasympathetic defecation reflex. The myenteric defecation reflex is responsible for increasing peristalsis and propelling stool toward the rectum. This eventually signals the internal anal sphincter to relax and reduce sphincter constriction. The parasympathetic defecation reflex, on the other hand, can be voluntarily controlled, but the myenteric reflex cannot. It is possible to experience the myenteric defecation reflex without the parasympathetic reflex, in which case the urge to defecate may be weaker.
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Muscles involved
Defecation is a complex function that requires the coordination of the gastrointestinal, nervous, and musculoskeletal systems. It is the process of expelling faeces from the digestive tract via the anus. This process involves the movement of faecal matter from the colon to the rectum, which induces the urge to defecate. The rectum is responsible for the temporary storage of faeces before defecation. As it fills up, the rectal walls expand and stretch receptors stimulate the desire to defecate.
The defecation reflex involves rectal contraction and internal and external anal sphincter relaxation. The internal anal sphincter is controlled by parasympathetic fibres, which relax involuntarily. The external anal sphincter, on the other hand, is a skeletal muscle controlled by somatic nerve supply, allowing conscious control of defecation. This means that individuals can prevent defecation by voluntarily tightening the external anal sphincter.
The levator ani and puborectalis muscles also play a crucial role in defecation. They work together to allow complete faecal expulsion. Additionally, the Valsalva manoeuvre and abdominal muscle contractions increase intra-abdominal pressure to expel faeces faster. The Valsalva manoeuvre involves attempting forced expiration of breath against a closed airway, contracting the expiratory chest muscles, diaphragm, abdominal wall muscles, and pelvic diaphragm to exert pressure on the digestive tract.
Disorders of the nerves and muscles involved in defecation can lead to constipation, which is reduced defecation frequency, generally three or fewer times per week. This can be caused by a low-fiber diet or prolonged suppression of the defecation urge. Pelvic floor disorders can also impact defecation, as the pelvic floor muscles are responsible for pooping, and their dysfunction can lead to conditions like rectal prolapse.
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Impairments
The complex process of defecation involves the coordination of the gastrointestinal, nervous, and musculoskeletal systems. Impairments in the muscles involved in this process can lead to various complications, including constipation and faecal incontinence.
One such impairment is anismus, or dyssynergic defecation, which is a type of pelvic floor dysfunction that makes it difficult to defecate. It occurs when the muscles and nerves in the pelvic floor fail to coordinate correctly, resulting in chronic constipation. Biofeedback therapy is the most effective treatment for this condition, teaching individuals to activate and relax their sphincter muscles through guided exercises.
Faecal incontinence, or the loss of control over defecation, can also be caused by impairments in the muscles involved. This condition can be the result of physical injury, nerve damage, prior surgeries, constipation, diarrhoea, reduced rectal capacity, or intense fright. It is more common in older individuals but can also occur in younger patients as a complication of labour and anorectal surgery.
Constipation, characterised by reduced defecation frequency and hardened stool, can be caused by disorders of the nerves and muscles involved in defecation. Obstipation refers to severe constipation, where faeces become so dry and impacted that defecation becomes extremely challenging, often requiring medical or surgical intervention.
Additionally, the Valsalva manoeuvre, a technique used to induce defecation, can lead to cardiovascular complications in rare cases. This involves the contraction of expiratory chest muscles, the diaphragm, abdominal wall muscles, and the pelvic diaphragm, increasing pressure on the digestive tract. However, it can cause a significant rise in blood pressure, potentially leading to cardiac arrest, aneurysm rupture, or the dislodging of blood clots.
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Constipation
Dyssynergic defecation, or anismus, is a common cause of chronic constipation, affecting the coordination of muscles and nerves in the pelvic floor. Patients with dyssynergic defecation often exhibit impaired abdominal or rectal muscle contraction, along with anal muscle tightening instead of relaxation during defecation. This lack of coordination, or dyssynergia, is responsible for this condition. Additionally, many patients with dyssynergic defecation show a decreased sensation in the rectum, impairing their ability to perceive the arrival of a bowel movement. Rectal hyposensitivity can be addressed through biofeedback therapy, which helps train sensory awareness.
To diagnose dyssynergic defecation, healthcare providers conduct rectal exams, such as a digital rectal exam (DRE) and sigmoidoscopy, to rule out structural and metabolic causes of constipation. If these initial tests do not reveal any obvious causes, more specific tests are performed, including anorectal manometry, which measures anal and rectal muscle contractions and relaxations. Treatment for constipation may include lifestyle modifications such as increasing water intake, consuming more fibre, exercising, and adopting regular bathroom habits. Laxatives and pharmaceutical interventions may also be recommended if other methods are unsuccessful.
The defecation process involves the coordination of various muscles. When the rectum is full, the pressure increase forces the anal canal walls apart, allowing faecal matter to enter. The rectum shortens as the internal and external anal sphincters, along with the puborectalis muscle, facilitate faecal expulsion by pulling the anus up and over the exiting faeces. The external anal sphincter is under voluntary control, while the internal sphincter is involuntary. In infants, defecation occurs as a reflex action without voluntary control of the external anal sphincter, which is gradually learned through toilet training.
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Fecal incontinence
Defecation is the act of expelling faeces from the body. It is a complex function that requires coordination between the gastrointestinal, nervous, and musculoskeletal systems. The process begins with mass movement from the colon to the rectum, initiating the defecation reflex, which involves rectal contraction and internal and external anal sphincter relaxation. While the internal anal sphincter is involuntary, the external anal sphincter is under voluntary control.
The diagnosis of fecal incontinence involves taking a detailed health history, conducting a physical examination, and assessing emotional well-being and quality of life. Treatment options include medication, muscle training, biofeedback, anal plugs, nerve stimulation, and surgery. Muscle training and biofeedback aim to strengthen the pelvic floor muscles and improve control over bowel movements.
It is important to seek medical advice and treatment for fecal incontinence. While it is a challenging condition, proper diagnosis and a combination of treatments can help manage the symptoms and improve overall well-being.
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Frequently asked questions
Defecation is the act of expelling faeces from the body through the anus.
The internal and external anal sphincters are the primary muscles involved in defecation. The internal anal sphincter is a smooth muscle that contracts involuntarily, while the external anal sphincter is a skeletal muscle that can be controlled voluntarily to delay defecation. The pelvic floor muscles also play a role in supporting the process.
The rectum stores faeces temporarily, and when it fills up, the rectal muscles contract, creating the urge to defecate. The internal anal sphincter relaxes, and the external anal sphincter initially contracts. When defecation occurs, the pelvic floor relaxes, allowing the muscles in the rectum to push out the stool.
