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

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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

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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