Dark Stool
Dark, tarry stools, often known as melena, point to a gastrointestinal cause. Major gastrointestinal bleeding often presents with melena as the primary symptom. Sites above the ligament of Treitz near the duodenum account for over 90 percent of all cases of statistically significant gastrointestinal bleeding. Melena typically indicates a hemorrhage at this spot. More than 50 mL of blood in the stomach will cause the feces to become black. When one or two liters of blood are ingested orally, it may take anywhere from 4 to 24 for the first bloody or tarry stool to emerge, lasting up to five days. Thus, a melenic stool indicates recent hemorrhage without indicating the presence or rate of bleeding at the time of transit.
It is common for the body to withstand a 15 percent drop in blood volume by contracting big veins and drawing fluid from extravascular locations to make the difference. Postural hypotension and tachycardia follow as volume loss accelerates, with blood flow being diverted away from crucial organs and toward less important tissues like skin and bone. The patient may feel nauseous and dehydrated. Decreased blood flow to key organs, hypoxemia, lactic acidosis, and finally, death occurs once blood volume has been reduced by 40 to 50 percent.
