The right atrium takes oxygen-depleted blood from the body via the superior and inferior vena cava and pumps it to the right ventricle. The tricuspid valve permits oxygen-depleted blood from the right atrium to pass forward to the right ventricle. The pulmonary valve allows only oxygen-rich blood to leave the right ventricle and enter the pulmonary artery. The pulmonary vein then carries this blood to the lungs for replacement with new blood cells. In severe cases of hypoxia, such as those resulting from high altitude exposure or lung disease, more than one round of breathing is required before enough oxygen reaches the blood to cause tissue damage. This is called "superimposed" or "recapillary" hypoxia. The human body has a number of adaptations that allow it to function under low oxygen conditions. The most important is the use of oxygen stores in form of hemoglobin in red blood cells. When these are used up, people will eventually become unconscious even when exposed to air with less than 10% oxygen. However something worse can happen if you stay out in these conditions for too long... People who work or travel above about 5,000 feet (1,524 m) experience milder effects of high altitude because more oxygen is available at higher elevations. At greater heights, however, severe effects occur because there is not enough oxygen at lower levels of activity.