<P> Too little carbon dioxide, and, to a lesser extent, too much oxygen in the blood can temporarily halt breathing, a condition known as apnea, which freedivers use to prolong the time they can stay underwater . </P> <P> The partial pressure of carbon dioxide is more of a deciding factor in the monitoring of pH . However, at high altitude (above 2500 m) the monitoring of the partial pressure of oxygen takes priority, and hyperventilation keeps the oxygen level constant . With the lower level of carbon dioxide, to keep the pH at 7.4 the kidneys secrete hydrogen ions into the blood, and excrete bicarbonate into the urine . This is important in the acclimatization to high altitude . </P> <P> The kidneys measure the oxygen content rather than the partial pressure of oxygen in the arterial blood . When the oxygen content of the blood is chronically low, oxygen - sensitive cells secrete erythropoietin (EPO) into the blood . The effector tissue is the red bone marrow which produces red blood cells (RBCs) (erythrocytes). The increase in RBCs leads to an increased hematocrit in the blood, and subsequent increase in hemoglobin that increases the oxygen carrying capacity . This is the mechanism whereby high altitude dwellers have higher hematocrits than sea - level residents, and also why persons with pulmonary insufficiency or right - to - left shunts in the heart (through which venous blood by - passes the lungs and goes directly into the systemic circulation) have similarly high hematocrits . </P> <P> Regardless of the partial pressure of oxygen in the blood, the amount of oxygen that can be carried, depends on the hemoglobin content . The partial pressure of oxygen may be sufficient for example in anemia, but the hemoglobin content will be insufficient and subsequently as will be the oxygen content . Given enough supply of iron, vitamin B12 and folic acid, EPO can stimulate RBC production, and hemoglobin and oxygen content restored to normal . </P>

How does ph plays a role in homeostasis and disease prevention