Acids, Bases and Buffers

Water

• Water is the solvent of life

• Consistently makes up approx. 60% of human body weight

• The partial negative on oxygen and partial positive on hydrogen create a polar molecule that can act as a solvent for other polar molecules

• Can form weak hydrogen bonds that can break and reform. 

• This reforming ability helps solutes move through water

3 Functions of Water

• Dissolves and transports molecules

• Component of many chemical reactions

• It helps dissipate heat and control body temp (sweat)

Compartmentalization of Water

• Water is stored in various “Compartments”. Some are stored intracellularly (inside cells) and others are stored extracellularly (in blood or interstitial fluid). 

• This compartmentalization helps control the relative concentration of molecules in different areas

• Water can move between compartments and impact the concentrations of different molecules in different areas.

• This contributes to homeostasis and aids in important chemical reactions. 

Electrolytes and Osmolality

• Water dissolves many anions and cations (collectively named electrolytes)

• Osmolality is the concentration of all dissolved solutes in blood (electrolytes, proteins, etc:)

• Water's small size, dissolving ability, and ability to move between compartments helps maintain osmolality.

Kd of Water

• Water is in equilibrium because it constantly breaks apart and reforms. 

• The degree to which a substance dissociates can be calculated using the following formula

• Kd = [A]{B]/[AB] where A and B are the concentrations of each dissociated part and AB is the remaining undissociated part. 

• So if 10 water molecules are added in solution and 5 dissociate, the Kd for water would be ((5)(5))/5=5

• The higher the Kd the easier the molecule is to break apart

pH

• pH stands for power of hydrogen and it measures acidity or alkalinity of a solution based on how many hydrogen ions are in that solution

• pH=-log[H⁺]

• Blood pH is normally 7.35-7.45

Acids and Bases

• Acids are molecules that release H⁺ into a solution.

• Bases are molecules that can accept H⁺ from a solution

• Strong acids are very electronegative, and when they form a bond with Hydrogen, the pull the electron completely away from hydrogen leading the hydrogen to completely separate in solution

• Weak acids don’t completely dissociate, and therefore exist in an equilibrium, with only some acid molecules releasing their hydrogens entirely.

Dissociation Constant for Weak Acids

• Therefore, weak acids also have their own dissociation constant. This is referred to as K_a. 

• The formula for K_a is as follows

• Ka = [H +][A -]/[HA]

• Strong Acids Fully Dissociate

• Higher the Ka, the stronger the acid

Henderson-Hasselbalch Equation

• pH=pKa + log([A-]/[HA])

• Therefore, if the pH=pKa, 50% of the acid has dissociated

• This means that pKa is the pH at which 50% of an acid has dissociated

• Buffers are most useful when the pH of a solution is = or +-1 of the pKa of the buffer

Buffers

• Buffers are a combination of weak acids and their conjugate base in equilibrium.

• They are most useful when pH=pKa because that means there is an equal amount of weak acid to donate protons, and conjugate base to accept protons

Biologically Important Buffers

• Dihydrogen Phosphate-Hydrogen phosphate system (Active inside cells)

  • H₂PO₄^- ⇌ HPO₄^2- + H⁺ [pKa = 6.8] 

• Carbonic Acid-Bicarbonate Buffering System

  • H₂CO₃ ⇌ HCO₃^- + H⁺ [pKa = 7.4]

  • CO2, a byproduct of metabolism, is combined with water through an enzyme called carbonic anhydrase (generated by the kidneys and RBC) to form carbonic acid required for this buffer system. 

  • Exhaling CO₂ or eliminating HCO₃^- and H⁺ through the kidneys can help keep this buffering system in equilibrium

Metabolic Acidosis

• Excess ketones or lactic acids or diarrhea (which results in a loss of bicarbonate). This all results in excess hydrogen. The body tries to compensate by hyperventilating which does 2 things. Firstly, it breathes out CO₂ (so H and bicarb can combine into carbonic acid which can become CO₂ which can be breathed out). Secondly, it can prevent the reaction from shifting to the right because there’s no CO₂ to make H₂CO₃ so there can’t be more H+ being made.

Metabolic Alkalosis

Caused by ingesting basic compounds, bicarb retention, or excess vomiting (protons lost in vomiting). This all results in decreased hydrogen. To compensate, the body hypoventilates. This retains CO₂ to make more H₂CO₃ which makes more H+.

Respiratory Acidosis

• Caused by conditions in which the body cannot exhale enough CO₂. This means the reaction shifts too much to the right, causing too much bicarb and hydrogen. To compensate, the kidneys pee it out. 

Respiratory Alkalosis

• : Caused by hyperventilation, which causes not enough CO₂ to make hydrogen. The body compensates by peeing out bicarb and retaining hydrogen.