Phase 1 · Understand the Body — Why the disease behaves the way it does.
The One-Line Truth
The whole disease in one chain
Everything on this page exists to explain this line.
Bleeding from anywhere in the gut below the ligament of Treitz (the first bend of the small intestine, right past the stomach).
A Leaking Water Main Under the Street
Picture a water main (the colon's blood vessels) running under a street (the intestinal wall). Over decades, pressure and wear weaken one section—maybe a side pocket bursts (diverticulum), or the pipe develops a thin, corroded spot (angiodysplasia). Water starts gushing into the street (blood leaks into the intestinal space). The city's water pressure drops, so the pump station (your heart) speeds up to keep pressure steady in the remaining pipes. Valves close off water flow to gardens and parks (skin and gut get less blood) to keep the main streets (brain and heart) pressurized. But if the leak is huge or keeps going, the whole system fails: pressure collapses, the pump can't catch up, and the city goes dark. You see that water in the street (red or maroon stool), and it tells you exactly which pipe broke and how fast.
ACG Clinical Guideline: Management of Patients with Acute Lower Gastrointestinal Bleeding (2016) · American Society for Gastrointestinal Endoscopy (ASGE) guideline on lower GI bleeding · American College of Gastroenterology (ACG) · UpToDate / NEJM clinical reviews · reviewed 2026-07-07