Describe how the pH of blood and tissue fluids in mammals is maintained.

pH of blood and tissue fluids is maintained by mechanisms operating in the kidneys, lungs and the blood plasma, to regulate the H⁺ ion concentration.

Role of the kidneys

• When the blood pH starts to lower, hydrogen ions are actively secreted into the lumen of the tubule and collecting ducts by an ATP-dependent Na⁺/H⁺ The hydrogen ions an arise from dissolution of carbon dioxide in plasma or from metabolic wastes such as lactic acid.
• In the tubules, the H⁺ ions are buffered by sodium hydrogen phosphate which takes up H⁺ ions to form sodium dihydrogen phosphate. This reverse occurs when pH rises.
• When the acidity of renal fluid is exceptionally high, the cells of the distal convoluted tubules produce ammonia from glutamic amino acids.
• The ammonia then combines with the excess hydrogen ions to form ammonium ions which are excreted.

Role of the lungs/respiratory system

• An increase in blood carbon dioxide concentration (or low pH); stimulates the chemoreceptor cells in the carotid sinus and aortic bodies which send an impulses to the respiratory centre. An instruction is sent to the respiratory muscles to contract more rhythmically leading to an increase in rate and depth of breathing.

This rids the blood and body fluids of carbon dioxide and so restores the pH to normal. The reverse occurs when there is an increase in pH or reduction in carbon dioxide concentration.

Role of plasma proteins

• Plasma proteins usually act as buffers of blood pH. In acidic conditions, they bind the H⁺ ions from the plasma while alkaline conditions, they release H⁺ ions into the plasma to neutralize the excess hydroxyl ions. This maintains blood plasma pH.