Once inside the 6-7-meter-long small intestine, nutrient and mineral absorption begins and continues as materials are moved continuously through the intestines. In the duodenum, which is the largest in diameter of the small intestine sections, food breakdown is assisted by the introduction of pancreatic enzymes like lipase, amylase and trypsinogen, among others. The pancreas also releases bicarbonate, which buffers the very acidic chime just released from the stomach.
Bile also enters the digestive tract in the duodenum, through the bile duct, and is essential to fat absorption. Most duodenal ulcers occur between the bile duct and the pyloric sphincter.3 Bile is a greenish alkaline fluid produced in the liver and stored in the gall-bladder until needed to aid in fat digestion. The duodenum narrows and connects with the jejunum at the ligament of Treitz, which is considered the transition from the upper to lower GI tract. Absorption of nutrients and minerals continues in the jejunum and the ileum, the final and longest section of the small intestine, making up the final three-fifths of the length.3 The ileum has the thinnest walls of the small intestine and is the most vascular.
For all intents and purposes, nutrient and mineral absorption ends at the ileocecal fold, where the ileum passes all remaining solid materials into the cecum, the first section of the 1.5-meter-long large intestine. Water and salts are absorbed by the large intestine, leaving whatever remains as solid waste. As stated, the cecum is the first section of the large intestine that rests in the lower right abdominal quadrant and receives materials from the ileum. Just inferior, or below the cecum, is the appendix. Materials in the cecum actually move superiorly up the ascending colon, across the transverse colon, down the descending colon along the left abdominal flank, across the sigmoid colon and finally to the rectum and anus.
To help increase the surface area for absorption, the intestines are lined with villi, which are closely packed finger-like structures that protrude toward the center of the intestinal lumen. The number and size of villi decrease in the large intestine until the lining becomes smooth near the rectum.
Irritation or injury at any point along this system can result in GI bleeding into the tract. What that blood looks like and where it exits the tract is determined by where the bleeding actually occurs.
Characteristics of GI Bleeding
Depending on the location and rate of bleeding, identifying gastrointestinal bleeding may be fairly simple, or nearly impossible. Regardless of where bleeding is located, the clinical manifestations are dependent on the rate and volume of blood loss. Patients who have experienced slow or minimal blood loss may only present with abdominal discomfort, and bleeding may not be detected until blood tests show the patient has low hemoglobin.
Moderate to severe blood loss from any source causes hypovolemic shock. The human body responds to hypovolemic shock the same way every time through a series of normal compensatory mechanisms. The release of catecholamines, epinephrine and norepinephrine, increases the heart rate and breathing and triggers peripheral vascular resistance, which is what causes pale, cool and clammy skin. Over time, patients may also present with orthostatic blood pressure changes, which signal a blood loss exceeding 1000 mL.1
Blood exiting the GI tract is the best and most definitive indicator of GI bleeding. It comes in three forms: hematemesis, hematochezia and melena. Hematemesis is bloody vomit, which can come as either frank red blood or "coffee-ground" colored. A patient with hematemesis is bleeding above the ligament of Treitz. Gastric acids rapidly convert hemoglobin to a brown-colored hematin, which is why blood takes on a coffee-ground appearance. When frank blood is vomited, it signals that the blood has had little or no time to mix with gastric acids.1
Bleeding anyplace in the GI tract results in blood from the rectum. Melena, black-colored stool caused by the breakdown of blood, signals the blood had to travel a great distance and time; 90% of the time that signifies an upper GI bleed.1 Hematochezia, maroon-colored, clotted or bright red blood passing from the rectum, suggests a lower GI bleed source.