Acid-Base BalanceAcid-Base Balance
Acid - Base Balance
Blood - normal pH of 7.2 – 7.45 < 7.2 = acidosis > 7.45 =
alkalosis 3 buffer systems to maintain
normal blood pH1. Buffers2. Removal of CO2 by lungs3. Removal of H+ ions by kidneys
The Negative logarithmic relationship between [H+] and pH
Acids are H+ donors. Bases are H+ acceptors, or give up OH- in
solution. Acids and bases can be:
Strong – dissociate completely in solution
HCl, NaOH Weak – dissociate only partially in
solutionLactic acid, carbonic acid
The Body and pH
Homeostasis of pH is tightly controlled
Extracellular fluid = 7.4 Blood = 7.35 – 7.45 < 6.8 or > 8.0 death occurs Acidosis (acidemia) below 7.35 Alkalosis (alkalemia) above 7.45
Small changes in pH can produce major disturbances
Most enzymes function only with narrow pH ranges
Acid-base balance can also affect electrolytes (Na+, K+, Cl-)
Can also affect hormones
The body produces more acids than bases
Acids take in with foods Acids produced by metabolism of
lipids and proteins Cellular metabolism produces CO2.
CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3
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Control of Acids
1. Buffer systemsTake up H+ or release H+ as
conditions changeBuffer pairs – weak acid and a baseExchange a strong acid or base for
a weak oneResults in a much smaller pH
change
Bicarbonate buffer
Sodium Bicarbonate (NaHCO3) and carbonic acid (H2CO3)
Maintain a 20:1 ratio : HCO3- : H2CO3
HCl + NaHCO3 ↔ H2CO3 + NaCl
NaOH + H2CO3 ↔ NaHCO3 + H2O
Phosphate buffer
Major intracellular buffer H+ + HPO4
2- ↔ H2PO4-
OH- + H2PO4- ↔ H2O + H2PO4
2-
Protein Buffers Includes hemoglobin, work in blood and ISF Carboxyl group gives up H+ Amino Group accepts H+
Side chains that can buffer H+ are present on 27 amino acids.
2. Respiratory mechanisms
Exhalation of carbon dioxide Powerful, but only works with
volatile acids Doesn’t affect fixed acids like lactic
acid CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3
-
Body pH can be adjusted by changing rate and depth of breathing
3. Kidney excretion
Can eliminate large amounts of acid Can also excrete base Can conserve and produce bicarb
ions Most effective regulator of pH If kidneys fail, pH balance fails
Hydrogen Ion Excretion in Kidney
Buffering of hydrogen ions in urine
Increase in H ion secretion and HCO3 ion reabsorption
Decrease H ion secretion and HCO3 ion reabsorption
PCO2 PCO2
H , HCO3 H , HCO3
ECF volume ECF volume
Angiotensin II Angiotensin II
Aldosterone Aldosterone
Hypokalemia Hyperkalemia
Factors that increase or decrease H secretion and HCO3 Reabsorption by renal tubules :
Rates of correction
Buffers function almost instantaneously
Respiratory mechanisms take several minutes to hours
Renal mechanisms may take several hours to days
Buffers
Protein Buffer Systems Amino Acid buffers Hemoglobin buffers Plasma Protein buffers Phosphate Buffer Systems Carbonic Acid – Bicarbonate Buffer
System
Buffer systems are used to keep the body in pH balance (homeostasis) It consists of a weak acid (H+)and its dissociation products (an anion) 3 major buffer systems in human
Protein buffer system (includes hemoglobin buffer system) Regulates ICF & ECF (both plasma & interstitial fluid) Most important in ICF & hemoglobin
Hemoglobin buffer system = carbonic anhydrase in RBC* it absorbs CO2 from ECF & get
immediate effect Amino acids have carboxyl group (gives up H+) and Amino acids have amino group(can accept H+)
Carbonic acid-bicarbonate buffer system Important in ECF Lots of carbon dioxide from metabolic acids It mixes with water & get carbonic acid which dissociates into
H+ & HCO3-
Metabolic acids have H+ ; Our body has “bicarbonate reserve” Bicarbonate reserve = ample supply of bicarb in ECF These combine to form CO2 + H2O CO2 excreted via lungs
Think of CO2 as an acid since it readily combines with water to become carbonic acid
Maintenance of Acid-Base Balance
Respiratory System: removal of CO2 by lungs – stabilizes the ECF, has direct effect on Carbonic Acid – Bicarbonate Buffer System
Urinary System: removal of H+ ions by kidneys
Regulation of blood pH by the respiratory system
Kidney excretion of H+
Metabolic reactions produce nonvolatile acids One way to eliminate this huge load is to excrete H+ in
urine In the proximal convoluted tubule, Na+/H+ antiporters
secrete H+ as they reabsorb Na+
Intercalated cells of collecting duct include proton pumps that secrete H+ into tubule fluid; reabsorb K+ and HCO3
-
Urine can be up to 1000 times more acidic than blood 2 other buffers can combine with H+ in collecting duct
HPO42- and NH3
Secretion of H+ by intercalated cells in the collecting duct