PHYSIOLOGY: Digestion , Absorption & Defecation. Reference. Guyton A., and Hall, J. Textbook of Medical Physiology. 12 th ed. Seeley, R., Stephens, T., and Tate, P., Anatomy & Physiology. 8 th ed. McGraw Hill Company Inc., (2008). Functions of the Digestive System. Motility Functions
Ingestion is taking food into the mouth.
Secretion is the act of expelling a liquid. The cells lining the GI tract secrete about 9 liters (9.5 quarts) of water, acid, buffers, and enzymes each day to lubricate the canal and aid in the process of digestion.
Mechanical digestionis chewing up the food and your stomach and smooth intestine churning the food
Chemical digestion is the work the enzymes do when breaking large carbohydrate, lipid, protein and nucleic acid molecules down into their subcomponents -these and others are the nutrients.
Defecationthe act or process by which solid or semisolid waste material (feces) from the digestive tract are eliminated via the anus.
DEGLUTITION (SWALLOWING) accomplishes the propulsion of ingested food the mouth to the stomach.
tongue moves upward and backward to push the food toward the pharynx.
coordinated involuntary actions which direct food to the esophagus closing the airway passages
peristaltic waves propel food from the esophagus to the stomach
Stomach-digestion takes place
-secretes pepsin + HCl
ileum (functional reserve)
Large intestine-stores and concentrates undigested material
- colon: 3 limbs -ascending
As the rectal walls expand, stretch receptors from the nervous system located in the rectal walls stimulate the desire to defecate.
Vagus nerve- cranial nerve X in PNS
Superior mesenteric plexus – SNS cord segment T9-L2
The rectum and anal canal innervation:
Pelvic splanchnic nerves- parasympathetic S2-4
Hypogastric nerve- sympathetic T11-L2
Internal anal sphincter- pelvic splanchnic nerves and hypogastric nerves
External anal sphincter – pudendal nerve
If defecation is delayed for a prolonged period the fecal matter may harden, resulting in constipation.
Blood pressure rises.
Not good for high- risk cardiac patients.
Sphincter aniexternus muscle for anal and urethral are both closely linked by the same nerves
When one feels the urge to have a bowel movement, they may not be able to hold it until they can get to a toilet, or stool may leak from the rectum unexpectedly.
may be caused by physical injury (such as damage to the anal sphincter that may result from an episiotomy)
intense fright, inflammatory bowel disease, impaired water absorption (diarrhea), and psychological or neurological factors.
Fecal material passes to rectum by the longitudinal muscular contraction.
When there is distension in the sigmoid colon and rectum, the internal sphincter relaxes at the same time as the external sphincter contracts.
This allows some of the fecal contents to enter the anal canal and be sampled by its sensitive epithelium to determine if its solid, liquid or gas.
If the individual decides to continue with defecation, the intrarectal pressure increases on straining. Added pressure from abdominal straining orValsalva maneuver aids defecation. Straining requires intact innervation of the lower thoracic cord T6-T12.
The pressure should be sufficient to overcome the external sphincter which relaxes with the pelvic floor that causing the rectum to straighten up reducing the rectal angle.
This increase in intraabdominal pressure forces stool into the rectum with relaxation of the extenal anal sphincter allowing defecation.
Fecal matter stimulate the anal canal further relaxing the external anal sphincter.
If the individual decides not to continue with defecation, the rectum relaxes further to allow accommodation of the contents which further stimulates the external sphincter to contract.
Upper motor neuron lesion- damage above the defecation reflex center in the sacral cord
Lower motor neuron lesion – damage within the reflex defecation center
The bulbocavernosus reflex is a palpable or visible contraction of the anal sphincter when pressure is applied to the glans penis or clitoris.
When contraction is present, (+) result. This indicates that the reflex activity of the sacral cord is intact and therefore the SCI is an UMN lesion.
Should be tested soon after SCI, before the spinal shock passes.
The anal reflex is a visible contraction of the anal sphincter in response to a pinprick. A positive response indicates an UMN lesion.