Medically speaking, a watershed area is a region of the body with a blood supply derived from the ends of two or more arteries. Multiple blood supplies can confer certain advantages, but these areas are also at increased risk of ischemia because they are located at the very end of the circulation. Several regions, including parts of the brain and transverse colon, are watershed areas. This quirk of circulation can be important in medical diagnosis and treatment.
The advantage to a watershed area is that in the event of an interruption of blood supply to one vessel, the other vessel can still meet the need. If the patient has a block or similar problem, the tissue remains fully perfused because it still gets enough blood. Oxygen and nutrient needs can be met, and the patient may not experience symptoms like pain or a decline in function. This confers obvious benefits, particularly for older patients at risk of blockages and other problems.
Blood supply can, however, be interrupted if a patient has low blood pressure, low blood volume, or other issues that limit circulation as a whole. In this case, the dual blood supply cannot make up for the lack of blood. The location at the distal end of the patient's circulation means the vessel has already supplied blood to a number of locations by the time it reaches the watershed area. Consequently, the patient's organs may not get enough blood. This can lead to tissue death and other complications.
One potential issue is a so-called “watershed stroke.” Patients who do not get enough blood in the watershed area of the brain can develop tissue ischemia in the brain cells that rely on a healthy and abundant supply of blood. This can cause declines in brain function that may lead to symptoms like slurred speech and confusion. If the patient does not receive prompt treatment, there is a risk of serious brain damage or death as the cells go into an ischemic cascade. In this phenomenon, dying cells pump out cellular toxins that bathe neighboring cells, killing them off and causing the problem to spread.
Problems with a watershed area can also arise in locations like the colon. The patient's transverse colon may not get enough blood, which causes tissue to die. Patients can experience symptoms like cramps, diarrhea, abdominal pain, and fever. If the condition is not addressed, the severity of the symptoms can increase and the patient may be at risk for complications like peritonitis, an inflammation of the abdominal cavity that can be fatal if not treated as aggressively as possible.