Blood pH and Buffering

• pH measures hydrogen ion concentration, indicating acidity or alkalinity.
• Acidity depends on free hydrogen ions.

Blood pH

• Blood pH regulation is critical as blood contacts nearly every body cell.
• Normal blood pH range is narrow (7.35-7.45).
• Outside this range, it can be fatal; the living range is pH 7.0-7.8.
• Acidosis, normal, and alkalosis describe pH levels.

Acids in the Body

• Some acids enter the body through foods.
• Most acids are generated by protein breakdown, incomplete fat/glucose oxidation, and CO2 loading/transport in blood.
• Urine pH range is larger than blood's.

Buffering

• Buffers resist pH swings in body fluids.
• Buffers release H+ (act as acids) when pH rises and bind H+ (act as bases) when pH drops.
• pH dropping indicates H+ increase; pH rising indicates OH- increase.

Acids and Bases

• Strong acids completely dissociate in water, releasing all H+ ions.
• Strong bases completely dissociate in water, releasing all OH- ions.
• Weak acids partially dissociate in water.
• Weak bases partially release OH- or accept H+.

Water

• Water is a large fraction of body weight.
• Infants are 73% or more water, while water is about 45% of body mass in old age.
• Healthy young men are about 60% water; healthy young women, about 50%.

Ionization of Water

• Pure water is a 55.6M solution.
• Water dissociates to a small extent: H2O → H+ + OH-.
• [H+] x [OH-] = 10^-14 M² (ionic product of water).
• At neutrality, [H+] = [OH-] = 10^-7 M.
• Blood pH of 7.4 implies [H+] = 3.98 x 10^-8 M. pH = -log[H+]

Dissociation of Acids

• Acids are proton (H+) donors, and bases are proton acceptors.
• Strong acids dissociate completely (e.g., HCl → H+ + Cl-).
• Weak acids dissociate incompletely (e.g., H2CO3 → H+ + HCO3-).
• Strong bases are more effective proton acceptors than weak ones.

pKa

• pKa = -log Ka, where Ka is the dissociation constant.
• pKa is the pH at which an acid is half dissociated, meaning there are equal amounts of undissociated acid and its conjugate base.
• Lower pKa indicates stronger acid.

Henderson-Hasselbalch Equation

• pH = pKa + log ([conjugate base]/[acid]).
• Buffers are mixtures of weak acids and conjugate bases, with the ability to resist changes in pH upon adding acid or alkali.
• At the pKa, there are equal amounts of dissociated and non-dissociated forms of the acid, where buffering is most effective.
• Conjugate bases pick up added H+ and acids donate protons to form H2O upon adding OH-, maintaining a stable pH.
• Knowing the pKa of an acid indicates pH at which it buffers best.
• A physiologically important buffer in blood and saliva is H2CO3 ↔ HCO3- with pKa = 6.1.

Additional Buffers

• Other buffers include H2PO4- ↔ HPO4^2- (pKa=6.8) and protein ↔ protein-.
• H2CO3 is proportional to pCO2, which helps distinguish between metabolic or respiratory causes of acidosis.

Metabolic Acidosis

• Bicarbonate is low

Respiratory Acidosis

• Carbon dioxide is high

Amino Acids

• Glycine is not a good buffer, buffering best at pH 2.3 and 9.6.
• Alpha carboxyl and alpha amino groups are not good physiological buffers.
• R groups of amino acids are important.

Hemoglobin

• Hemoglobin is a buffer in the blood.
• Most amino acid side chains do not buffer in the physiological range because their pKa is outside this range.
• Histidine residues make hemoglobin a good blood buffer.

pKa Changes

• The pKa of histidine in Hb differs from that of free histidine (pKa 6).
• Neighboring groups affect the pKa.
• Oxyhemoglobin: pKa = 6.8.
• Deoxyhemoglobin: pKa = 7.8.
• Deoxyhemoglobin is a better buffer for H+ produced in metabolism, picking up and carrying ions at blood pH.

Deoxyhemoglobin vs Oxyhemoglobin

• Has a higher Pka than blood pH.
• At pH 6.8, oxyhemoglobin is 50% with H+ and 50% without, but at blood pH it is without.
• At pH 7.8, deoxyhemoglobin is 50% with H+ and 50% without.
• At 7.4, more deoxyhemoglobin binds with H+.
• Hemoglobin carries oxygen to cells and picks up H+ ions produced in metabolism.
• Different pKa values for oxy- and deoxyhemoglobin mean they are protonated to differing degrees at blood pH.

OxyHb

• 7.4 - 6.8 = 0.6 + Log of base/acid, which is 0.6.
• The ratio of base to acid is 3.98:1

DeoxyHb

• 7.4 – 7.8 = -0.4
• Log of base/acid is -0.4.
• The ratio of base to acid is 1:2.51. Lipid-soluble molecules diffuse freely across biological membranes, while substances that interact strongly with water do not.