If the chemical buffers are unable to achieve stabilization of the pH

Physiologic Buffers

If the chemical buffers are unable to achieve stabilization of the pH, the physiologic buffers—respiratory & renal systems—respond. The respiratory system changes its rate and depth of breathing to retain or excrete carbon dioxide (CO₂), respiration is the only method of excreting carbon dioxide. The renal system conserves or excretes acids or bases in the urine, along with replenishing bicarbonate levels. Carbon dioxide and other acid waste products are continually formed as a result of cellular metabolism.

Assume the pH has fallen (CO₂has risen):

	Respirations increase to eliminate more CO₂ from the body
	As venous blood passes through the lung capillaries the CO₂ diffuses out of the venous blood into the lungs and is exhaled
	This leaves less CO₂ to enter the arterial blood vessels—making it less acidic and raising the pH.

Conversely, assume the pH has risen (CO₂ has fallen):

	Respirations decrease to retain more CO₂ in the body
	As venous blood passes through the lung capillaries less CO_s diffuses out of the venous blood into to the lungs
	This leaves more CO₂ to enter the arterial blood vessels—making it more acidic and lowering the pH.

However, for respirations to initiate this process, something must stimulate respirations. Here’s how the process works:

	Neurons located in the respiratory center of the medulla oblongata are sensitive to changes in arterial blood CO₂ and the pH
	Arterial CO₂ increases—or—arterial blood pH decreases
	Respiratory center is stimulated
	Respirations increase in rate and depth
	More CO₂ is eliminated and the pH rises.

If the situation is reversed:

	Arterial CO₂ decreases—or—arterial blood pH increases
	Respiratory center is stimulated
	Respirations decrease in rate and depth
	CO₂ is retained and pH is lowered.

The respiratory mechanism can maintain acid-base balance twice as effectively as the chemical buffers. It is capable of responding to changes in pH quickly—within a matter of minutes. Also, it can handle twice the amount of acids and bases. However, the lungs can restore normal pH only temporarily, it is a quick short-term fix. Another of the physiologic buffers—the kidneys—accomplishes the long-term restoration of pH.

Text Box: Memory Jogger:
Bicarbonate and pH move in the same direction. When the HCO3 rises or falls, so does the pH. The more powerful but slower renal mechanism is stimulated when the respiratory mechanism is unable to stop the shift in pH. The kidneys respond by conserving varying amounts of acids or bases or by excreting varying amounts of acids or bases into the urine. They also produce bicarbonate to replenish lost supplies. By excreting more or less H⁺ in exchange for reabsorbing more or less sodium ions, the kidneys control urine pH; in doing so, they play an important role in blood pH control. Here’s an example:

	Blood pH decreases
	Renal tubules secrete more H⁺ from blood to urine
	Tubules reabsorbs sodium ions from urine to blood (1 sodium for each H⁺)
	Urine pH decreases—more importantly—blood pH increases towards normal.

This mechanism depends on the kidneys excreting varying amounts of H⁺ from the body to match the amounts entering the blood and proves to be imminently more effective in balancing input against output than the respiratory mechanism. However, adjustments to the pH made by the kidneys can take from hours to days to be accomplished.