Both arteries and veins are located in a special area in the brain called the subarachnoid space. Arteries carry blood from the heart to the brain, and branch into smaller capillaries where oxygen and nutrients are delivered to the brain. Veins bring the leftover blood back to the heart. Arteries have a thick wall that can handle the constant pumping of blood from the heart. Veins are more flexible and allow a slower return of blood to the heart. When blood is found in the subarachnoid space, it is typically from the rupture of an artery, vein, or abnormal connection between the two.
Patients suffering a subarachnoid hemorrhage who make it to the hospital alive are generally able to stop the bleed on their own. But much like a fresh unprotected scar, there is a risk for re-rupture. The risk of a re-rupture depends on the blood pressure, the patient's ability to form a clot, and the inherent vulnerability of the underlying source of the bleed. Vulnerable sources include a weak spot in the artery (i.e. aneurysm), or an abnormal tangle of arteries or vessels (i.e. arteriovenous malformation). Ruptures from veins typically seal on their own, and are not at an increased risk of re-rupture.
Depending upon the cause and severity of a subarachnoid hemorrhage, treatment may involve observation, alleviating the pressure in the head, investigating the underlying source through imaging, securing any vulnerable spots, and preventing further damage to the brain.