Oxygen saturation of the hemoglobin of arterial blood
Oxygen saturation as measured by pulse oximetry
Oxygen content of arterial blood
Symbol relating the hydrogen ion concentration or activity of a solution to that of a standard solution; approximately equal to the negative logarithm of the hydrogen ion concentration. pH is an indicator of the relative acidity or alkalinity of a solution
An excess of acid is called acidosis or acidaemia and an excess in bases is called alkalosis or alkalemia. The process that causes the imbalance is classified based on the etiology of the disturbance (respiratory or metabolic) and the direction of change in pH (acidosis or alkalosis). This yields the following four basic processes:
The presence of only one of the above derangements is called a simple acid–base disorder. In a mixed disorder more than one is occurring at the same time. Mixed disorders may feature an acidosis and alkosis at the same time that partially counteract each other, or there can be two different conditions affecting the pH in the same direction. The phrase "mixed acidosis", for example, refers to metabolic acidosis in conjunction with respiratory acidosis. Any combination is possible, except concurrent respiratory acidosis and respiratory alkalosis, since a person cannot breathe too fast and too slow at the same time.
Acid–base imbalances that overcome the buffer system can be compensated in the short term by changing the rate of ventilation. This alters the concentration of carbon dioxide in the blood, shifting the above reaction according to Le Chatelier's principle, which in turn alters the pH. For instance, if the blood pH drops too low (acidemia), the body will compensate by increasing breathing, expelling CO2, and shifting the following reaction to the right such that less hydrogen ions are free – thus the pH will rise back to normal. For alkalemia, the opposite occurs.
The kidneys are slower to compensate, but renal physiology has several powerful mechanisms to control pH by the excretion of excess acid or base. In responses to acidosis, tubular cells reabsorb more bicarbonate from the tubular fluid, collecting duct cells secrete more hydrogen and generate more bicarbonate, and ammoniagenesis leads to increased formation of the NH3 buffer. In responses to alkalosis, the kidney may excrete more bicarbonate by decreasing hydrogen ion secretion from the tubular epithelial cells, and lowering rates of glutamine metabolism and ammonia excretion.