Constipation : refers to infrequent or difficult defecation caused by decreased motility of the intestines. Feces remains in the colon for increased time and as a result absorb more water making the feces dry and hard.
Phases of Digestion
3 overlapping phases:
1) Cephalic Phase
Initial taste of food activates neural centers in the cerebral cortex, hypothalamus, and brain stem. The brain stem then activates the facial (VII), glossopharyngeal (IX), and vagus (X) nerve. The result is stimulation of salivary glands, and gastric juice secretion. The purpose of this phase is to prepare the mouth and stomach for the food to be eaten.
2) Gastric Phase
This phase begins once food reaches the stomach. Neural and hormonal mechanisms regulate this phase.
Neural Phase
As food distends the stomach, this activates stretch receptors in its walls. Chemoreceptors monitor pH of the stomach chyme. Both the above mechanisms activate a negative feedback loop.
Hormonal Regulation
Regulated by the hormone Gastrin released by the G cells of the gastric glands in response to: distention of the stomach by chyme, partially digested proteins in chyme, high pH of chyme due to presence of food in the stomach, acetycholine released from parasympathetic neurons. Once released, Gastrin enters the bloodstream making a round-trip through the body and finally reaches target organs of the digestive system. Gastrin stimulates gastric glands to secrete gastric juice and it also strengthens the contraction of the lower esophageal sphincter to prevent acid reflux. It also relaxes the pyloric sphincter promoting gastric emptying. Gastrin secretion if inhibited when pH drops below 2.0 and is again stimulated when pH rises.
3) Intestinal Phase
This phase begins when food enters the small intestine. In contrast, reflexes in this phase have inhibitory effects that slow the release of chyme from the stomach . This prevents the duodenum from being overloaded with more chyme than it can handled effectively.
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