A person with HHT has a tendency to form blood vessels that lack the capillaries between an artery and vein. This means that arterial blood under high pressure flows directly into a vein without first having to squeeze through the very small capillaries. This place where an artery is connected directly to a vein, tends to be a fragile site that can rupture and result in bleeding. We usually call a blood vessel that is abnormal in this way a telangiectasis (tel-AN-jee-eck-TAZE-ee-ya), if it involves small blood vessels. We tend to call it an arterioveneous malformation (AVM) if involves larger blood vessels. So, an AVM might be thought of as a big telangiectasis. The basic abnormality in the blood vessel is the same.
thats part of it...
But it will affect people differently..not every has the problem in the same area. The person who i know who has it had a stroke becuase of it and also has heart/liver/GI problems and bleeding. There is a lot more info on the site too.