Blood Buffers

Objectives Definition of acid, base and buffer Maintenance of H+ Buffer Systems for regulation of H+ The Henderson –Hassel Balch Equation Acid- Base disorders : Acidosis and Alkalosis

Back to Basics: An acid is a substance that increases H+ concentration, thus reducing pH. A base is a proton acceptor ., bases decrease H+ concentrations and raise the pH.

Acids are produced continuously during normal metabolism, although the blood concentration of free hydrogen ion ( H+) vary between narrow limits. The acids handled by the body daily are about 20,000 mmol of volatile and 40- 80 mmol of non-volatile acids.

Relatively constant H+ concentrations are important physiologically, as small changes in pH affect enzyme activity and thus metabolism. The immediate defense against changing H+ concentrations is provided buffers, while excretion is regulated by adaptive responses in the lungs and kidney.

The changes in ECF (H+) conc. or (pH) are regulated by : Buffers: v. rapid temporarily traps acids or bases Respiratory response: rapid gets rid of or retains CO2 Renal response: slow excretes of fixed acids & retains or excretes HCO3-

Acid Production in the Body

Daily Production (mmol) Acid Source Normal Fate Daily Production (mmol) Carbon Dioxide (carbonic acid) Cellular respiration Excretion by lungs 20 000 Organic acids Lactic acid Ketone bodies Anaerobic glycolysis Fatty acid oxidation Gluconeogenesis Tissue oxidation 1000 Mineral acids Sulphuric acid Phosphoric acid Sulphur-containing amino acids Organic phosphorus- containing compounds Renal excretion 70

Volatile Acids: CO2: The product of oxidation of substrate for utilization, mainly CHO and fat, is carbon dioxide. Although CO2 is not an acid, it dissolves in H2O to form H2CO3 ,so accumulation of CO2 may lower body pH . As CO2 is excreted through lungs it can be viewed as being volatile acid

CO2 H+ + HCO3- ↔ H2CO3 ↔ CO2 + H2O Removed by lungs

These are of 2 types: organic and inorganic Non Volatile Acids: These are of 2 types: organic and inorganic

Organic acids : mainly lactic acid and ketone bodies. Lactate is produced continuously from the anaerobic metabolism of glucose, particularly in erythrocytes( no mitochondria) and skeletal muscle (strenuous exercising) These are converted to glucose in the liver Ketone bodies production increases with prolonged fasting and also in diabetic ketoacidosis

Ketone bodies are formed of fatty acids metabolism in the liver As organic acids are almost fully metabolized , under normal circumstances they contribute little to net acid excretion

Inorganic Acids: They are two main sources, sulphur-containing amino acids and phosphorus –containing organic compounds.

Oxidation of sulphyhydryl groups in cysteine and methionine results in the synthesis of sulphuric acid while hydrolysis of phosphesters produces phosphoric acid. Inorganic acidic anion must be excreted from the body by kidney

Buffers Buffers are solutions of weak acids or bases which contain both dissociated and undissociated forms.

Buffers Buffers limit the change of pH that would be caused by addition of strong acid or base Buffers act effectively at pH = pK ( also its concentration determine its efficiency

The conc. of H+ in blood is usually of ECF expressed as the negative logarithm10 (pH) = - log H The pH of the ECF is 7.35-7.45 It is molar concentration is 35-45 nmol/L

Buffers in Blood and Intracellular Fluid The main extracelluler buffers: Carbonic – bicarbonate system Hb (hemoglobin) The main intracellular buffers Proteins (Ptn) Phosphate buffer system

Buffer → H Buffer (base) (acid) H+ + HCO3- → H2CO3 → CO2 + H2O H+ + HPO24 → H2 PO-4 H+ + Hb- → H.Hb H+ + Prot- → H.Prot

Hemoglobin: Plays an important role more than other proteins as a buffer due to : Relatively high concentration Relatively rich histidine (pk=7.0) Role in transport of blood gases

Tissues CO2 O2 Erythrocyte ECF CO2 O2 + HHb CO2 + H2O + HbO2 HCO3- +H+ Carbonic anhydrase HCO3- HCO3- +H+ Cl - Cl - O2 + HHb

Carbonic Acid-Bicarbonate system: High concentration Ratio rapidly corrected by respiration Components easily measured.

All the blood buffers are in equilibrium and changes in (H+) that affects one system produce corresponding changes in the others. H2CO3/ HCO3- proved the most appropriate to be used to investigate the acid –base status

Back to Basics: K: The relative strength of weak acids are expressed quantitatively as dissociation constants, that express the tendency to ionize. pK= is the pH at which equal quantities of acid and its conjugate base exit.

Back to Basics: HA ↔ H+ + A- K =[ H+][A- ] [HA]

[ H+]= K [HA] [A-]

Take the log of both sides: Log [ H+]= log K [HA] [A-] Multiply through by -1 -Log [ H+]= - log K -log [HA]

pH = pK + log [A-] [HA]

The Henderson- Hasselbalch eq. pH = pK + log [A-] [HA] Note: we can replace H2CO3 for 0.03 xPCO2 Where 0.03 is the solubility coefficient of CO2 and PCO2 is the partial pressure of CO2( since H2CO3 is in equilibrium with the dissolved co2

IF A- = HA pH = pK + log 1 1

pH = pK + 0 so pK is the pH at which 50% of the acid is dissociated or it is a pH at which equal amounts of the acid and its conjugate base exist.

For Carbonic Acid-Bicarbonate system: 7.4 = 6.1 + log [HCO3-] [H2CO3]

For Carbonic Acid-Bicarbonate system: 7.4 -6.1 = log [HCO3-] [H2CO3] 1.3 = log [HCO3-]

For Carbonic Acid-Bicarbonate system: [HCO3-] = 20/1 [H2CO3]

Collection and transport of specimens: Arterial blood specimens are the most appropriate. Arterialized capillary blood could also be used. It is essential for the capillary blood to flow freely with no vasoconstriction or sluggish blood Patients must be relaxed Blood is collected into containers that contain sufficient heparin as an anticoagulant Specimens transferred to lab immediately better chilled on ice to avoid glycolysis

Disturbance of the Acid-Base Status Acidosis : pH < 7.4 HCO3- < 20/1 H2CO3- ↓ HCO3- = metabolic ↑ H2CO3- = respiratory Alkalosis: pH > 7.4 HCO3- > 20/1 H2CO3- ↑ HCO3- = metabolic ↓ H2CO3- = respiratory