1 Chapter 24 Water Electrolyte and Acid Base balance Acids Bases and Buffers One of the most important aspects of homeostasis Metabolism depends on enzymes and enzymes are sensitive to pH Slight deviation from the normal pH can shut down entire metabolic pathways Slight deviation from normal pH can alter the structure and function of macromolecules 7 35 7 45 is the normal pH range of blood and tissue fluid Challenges to acid base balance Metabolism constantly produces acid Lactic acids from anaerobic fermentation Phosphoric acid from nucleic acid catabolism Fatty acids and ketones from fat catabolism Carbonic acid from carbon dioxide pH of a solution is determined solely by its hydrogen ions H Acids any chemical that releases H in solution Strong acids such as hydrochloric acid HCl ionize freely Gives up most of its H Markedly lowers pH of a solution Weak acids such as carbonic acid H2CO3 ionize only slightly Keeps most H chemically bound Small effects on pH Bases any chemical that accepts H Strong bases such as the hydroxide ion OH have a strong tendency to bind H markedly raising pH Weak bases such as the bicarbonate ion HCO3 bind less available H and have less effect on pH Buffer any mechanism that resists changes in pH convert strong acids or bases to weak ones Physiological buffer system that controls output of acids bases or CO2 Urinary system buffers greatest quantity of acid or base takes several hours days to exert its effect Respiratory system buffers within minutes cannot alter pH as much as the urinary system Chemical buffer a substance that binds H and removes it from solution as its concentration begins to rise or releases H into solution as its concentration falls Restore normal pH in fractions of a second Functions as mixtures called buffer systems composed of weak acids and weak bases Three major chemical buffers bicarbonate phosphate and protein systems Amount of acid or base neutralized depends on the concentration of the buffers and the pH of the working environment 2 The bicarbonate buffer system a solution of carbonic acid and bicarbonate ions Carbonic acid and bicarbonate ions Reversible reaction important in ECF Functions best in the lungs and kidneys to constantly remove CO2 To lower pH kidneys excrete HCO3 To raise pH kidneys excrete H and lungs excrete CO2 Respiratory control of pH Basis for the strong buffering capacity of the respiratory system The addition of CO2 to the body fluids raises the H concentration and lowers pH The removal of CO2 has the opposite effects Respiratory system neutralizes 2 3 times as much acid as chemical buffers CO2 is constantly produced by aerobic metabolism Normally eliminated by the lungs at an equivalent rate Increased CO2 and decreased pH stimulate pulmonary ventilation while an increased pH inhibits pulmonary ventilation The kidneys can neutralize more acid or base than either the respiratory system of chemical buffers Renal tubules secrete H into the tubular fluid Most binds to bicarbonate ammonia and phosphate buffers Bound and free H are excreted in the urine actually expelling H from the body Other buffer systems only reduce its concentration by binding it to other chemicals