 In this video, I will describe the regulation of blood pH by the respiratory and urinary systems. The respiratory system provides a short-term mechanism to maintain pH homeostasis by regulating the excretion of carbon dioxide. We can either increase or decrease the pulmonary ventilation rate in order to regulate the rate of external respiration to control the rate at which carbon dioxide is removed from the body. The chemoreceptors detect changes in the pH of the brain or arterial blood and regulate the respiratory drive as a short-term mechanism to help maintain pH homeostasis in response to acidosis detected by the chemoreceptors. There will be an increase in the respiratory drive stimulating an increase in pulmonary ventilation which helps to increase the rate of excretion of carbon dioxide from the body helping to bring back pH homeostasis helping to prevent the acidosis by raising the blood pH. In contrast, if the chemoreceptors detect alkalosis, this will stimulate a decrease in the respiratory drive leading to decreased pulmonary ventilation rate and a decreased rate of carbon dioxide excretion from the body which helps to have an effect of lowering the blood pH restoring pH homeostasis. So the respiratory system provides a rapid short-term regulation over the pH balance of the body. The urinary system provides a long-term mechanism to maintain pH homeostasis by regulating the excretion of hydrogen ions coupled to the conservation of bicarbonate ions in order to respond to chronic long-term acidosis or conversely, in response to alkalosis, the urinary system can stimulate the excretion of bicarbonate ions. So acidosis will stimulate an increased excretion of hydrogen ions and conservation of bicarbonate ions leading to an increase in blood pH and alkalosis leads to excretion of bicarbonate ions which will have the effect of decreasing the blood pH. The illustration here is showing us the mechanism occurring at the proximal convoluted tubule that secretes hydrogen ions and conserves bicarbonate ions. Hydrogen ions are actively transported into the nephron, into the filtrate inside of the nephron. Meanwhile, bicarbonate ions are actively transported into the blood stream. This illustration shows us the mechanism occurring in the distal convoluted tubule and collecting duct for the secretion of hydrogen ions and conservation of bicarbonate in response to acidosis. So you can see this mechanism is very similar to the mechanism of the proximal convoluted tubule. The major difference is that potassium ions are exchanged for hydrogen ions at the apical surface of the type A intercalated cell instead of sodium ions. So in response to acidosis, the long-term mechanism in the kidneys will be to conserve bicarbonate and secret hydrogen ions, whereas the short-term mechanism in the lungs will be to increase the pulmonary ventilation rate to stimulate an increase in the rate of carbon dioxide excretion. So together, the urinary and respiratory systems can work to increase the blood pH, restoring the pH homeostasis preventing acidosis. In contrast, in response to alkalosis, the long-term response in the kidney will be to secrete bicarbonate ions, whereas the short-term response of the respiratory system will be decreasing the pulmonary ventilation rate, leading to a decrease in the rate of carbon dioxide excretion. Together, the urinary and respiratory systems will have the effect of decreasing the blood pH in response to alkalosis in order to restore pH homeostasis. This illustration shows us the mechanism of secretion of bicarbonate ions that occurs in the distal convoluted tubule and collecting duct in the kidneys, where bicarbonate ions are secreted by bicarbonate chloride antiporter protein. Meanwhile, hydrogen ions are reabsorbed in this mechanism. This works to decrease the blood pH as a long-term response to help restore homeostasis in response to alkalosis.