Acid-Base Balance for Allied Health Majors

Using the Henderson-Hasselbach Equation

H2O + CO2 H2CO3 H+ + HCO3 -

pH = pK + logHCO3 -

pCO2 (α)α =0.03

Acid• An acid is a substance that will disassociate a H+ and

become more negatively charged (electron acceptor).• When hydrogen ions accumulate in a solution, it

becomes more acidic ([H+] increases = more acidity).

72

pH

HCl

H + Cl-

H + Cl-

H + Cl-

H + Cl-

H + Cl-

Hydronium ions in solutionConcentration of hydrogen ions increases, pH drops

Base• A base is chemical that will remove hydrogen ions from

the solution• the base has a negative charge (or extra electrons) to

donate to hydrogen ions and thus create a bond with hydrogen

72

pH

H + Cl-

H + Cl-

H + Cl-

H + Cl-

H + Cl-NaOH Na+ OH-

Na+ OH-

Na+ OH-

Na+ OH-

Acids and basis neutralize eachother

72

pHNa+ Cl-

Na+ Cl-

Na+ Cl-Na+ Cl-

H+ OH- H2O

pH log[H] log1

H

A change of 1 pH unit corresponds to a 10-fold change in hydrogen ion concentration

Acids are being created constantly through metabolism

• Inorganic phosphates (mostly from ATP, etc.)

• Anaerobic respiration of glucose produces lactic acid

• Fat metabolism yields organic acids and ketone bodies

• Carbon dioxide!!!!!

Acids must be buffered, transported away from cells, and eliminated from the body

Phosphate: important renal tubular bufferHPO4

- + H+ H2PO 4

Ammonia: important renal tubular bufferNH3 + H+ NH4

+

Proteins: important intracellular and plasma buffersH+ + Hb HHb Histidine in proteins

is particularly good at neutralizing hydrogen ions

Bicarbonate: most important ECF buffer

H2O + CO2 H2CO3 H+ + HCO3 -CA

From metabolism Hydrogen is buffered by hemoglobin

Buffering is good, but it is a temporary solution. Excess acids and bases must be

eliminated from the body

H2O + CO2 H2CO3 H+ + HCO3 -CA

Lungs eliminate carbon dioxide

Kidneys can remove excess non-volatile acids and bases

gas aqueous

Excessive Acids and Bases can cause pH changes---denature proteins

• Normal pH of body fluids is 7.40• Alkalosis or alkalemia – arterial blood pH rises above

7.45• Acidosis or acidemia – arterial pH drops below 7.35

(physiological acidosis)• For our class, we will stick to 7.40 as normal!

• Acidosis: – too much acid– Too little base

• Alkalosis– Too much base– Too little acid

Compensation for deviation• Lungs (only if not a respiratory problem)

– If too much acid (low pH)—respiratory system will ventilate more (remove CO2) and this will raise pH back toward set point

– If too little acid (high pH)—respiratory will ventilate less (trap CO2 in body) and this will lower pH back toward set point

• Kidneys– If too much acid (low pH)—intercalated cells

will secrete more acid into tubular lumen and make NEW bicarbonate (more base) and raise pH back to set point.

– If too little acid/excessive base (high pH)- proximal convoluted cells will NOT reabsorb filtered bicarbonate (base) and will eliminate it from the body to lower pH back toward normal.

How would your ventilation change if you

had excessive acid?

[H+]AlveolarVentilation

pCO2

H2O + CO2 H2CO3 H+ + HCO3 -

This is too high and this means the buffer system swings this way!

CO2 vented out

How would your ventilation change if you

had too little acid?

[H+]AlveolarVentilation

pCO2

H2O + CO2 H2CO3 H+ + HCO3 -

This is too low and this means the buffer system swings this way!

CO2 trapped

How can the kidneys control acids and bases?

• Bicarbonate is filtered and enters nephron at Bowman’s capsule

• Proximal convoluted tubule– Can reabsorb all

bicarbonate (say, when you need it to neutralize excessive acids in body)

OR

– Can reabsorb some or NONE of the bicarbonate (maybe you have too much base in body and it needs to be eliminated)

How can the kidneys control acids and bases?

• Acidosis• Intercalated cells

– Secrete excessive hydrogen

– Secreted hydrogen binds to tubular buffers (ammonia and phosphate bases)

– Secretion of hydrogen leads to gain of bicarbonate (NEW!)

HPO4-

NH3

What would happen if the respiratory system had a problem with ventilation?

Respiratory Acidosis and Alkalosis

PCO2 levels-Normal PCO2 fluctuates between 35 and 45 mmHg

• Respiratory Acidosis (elevated CO2 greater than 45mmHg)

• Depression of respiratory centers via narcotic, drugs, anesthetics

• CNS disease and depression, trauma (brain damage)

• Interference with respiratory muscles by disease, drugs, toxins

• Restrictive, obstructive lung disease (pneumonia, emphysema)

• Respiratory Alkalosis (less than 35mmHg- lowered CO2)

• Hyperventilation syndrome/ psychological (fear, pain)

• Overventilation on mechanical respirator

• Ascent to high altitudes• Fever

What if your metabolism changed? • Metabolic acidosis • bicarbonate ion

levels below normal (22 mEq/L)

• Metabolic alkalosis• bicarbonate ion levels higher

(greater than 26mEq/L)

• Ingestion, infusion or production of more acids (alcohol)

• Carbonic anhydrase inhibitors (decreased H+ secretion)

• Salicylate overdose (aspirin)

• Diarrhea (loss of intestinal HCO3

-)• Accumulation of lactic

acid in severe Diabetic ketoacidosis

• starvation

• Excessive loss of fixed acids due to ingestion, infusion, or renal reabsorption of bases

• Loss of gastric juice during vomiting

• Intake of stomach antacids (Leisure world syndrome)

• Diuretic abuse (loss of H+ ions)

• Severe potassium depletion (increased aldosterone)

• Steroid therapy (mineralcorticoid excess)

H2O + CO2 H2CO3 H+ + HCO3 -

pH = pK + logHCO3 -

pCO2

(α)Acidosis: pH < 7.4

- metabolic: HCO3 -

- respiratory: pCO2

Alkalosis: pH > 7.4- metabolic: HCO3 -

- respiratory: pCO2

Mechanisms of Acidosis and Alkalosis

α =0.03

Compensation

Acidosis (pH<7.40)

Alkalosis

(pH >7.40)

Problem Compensation

Too much acid (High CO2)

Too little acid (low CO2)

Respiratory Kidneys

Too little base

(low HCO3-)

Too much base

(high HCO3-)

Metabolic Respiratory (quick!)

Kidneys

(long-term)

Analysis of Simple Acid-Base Disorders

Figure 30-10; Guyton and Hall

Step One:

Step 3

Step Two

<22 >26

>26 <22

<35

<35 >45

>45

Resp and renal compensation

Resp and renal compensation

Let’s practice

What is the problem?: pH pCO2 HCO3-

PCO2 complete HCO3- reabs.

+excess tubular H+

H+ secretion

pH Buffers (NH4+, NaHPO4

-)

H+ +new HCO3

-

Buffers -

H2O + CO2 H2CO3 H+ + HCO3 -

What is the correction and which organ system does this?

What is the problem?: pH pCO2 HCO3-

PCO2 H+ secretion

pH

HCO3- reabs.

+excess tubular HCO3

-

HCO3

-

+ H+ excretion

excretion

Let’s practice

What is the correction and which organ system does this?

H+

What is the problem?: pH pCO2 HCO3-

HCO3- complete HCO3

- reabs. +excess tubular H+

pH

HCO3-

filtration

+new HCO3

-

Buffers -

Buffers (NH4+, NaHPO4-)

What is the respiratory system doing at the same time?

Let’s practice

What is the correction and which organ system does this?

pH pCO2 HCO3-

HCO3-

pH

HCO3-

filtration

HCO3- reabs.

excess tubular HCO3-

excretion

H+ excretion

HCO3-

+

Let’s practice

What is the problem?:

What is the correction and which organ system does this?

What is the respiratory system doing at the same time?

Anion Gap-- protein , phosphate, citrate, sulfate

Na+Cl-

Aniongap

HCO3-

Meta. acidosis: too little baseMore Cl- and anion gap sameDiarrhea, renal acidosis

Meta. acidosis: too little baseMore Cl- and anion gap biggerKetoacidosis, salicylate, lactateChronic renal failure

Na+ Cl-

Aniongap

HCO3-

Na+Cl-

Aniongap

HCO3-

electroneutrality

Overuse of Diuretics

Metabolic Alkalosis

K+ depletionangiotensin II

aldosterone

extracell. volume

tubular H+ secretion

HCO3 reabsorption+ new HCO3 Production

Clinical Correlation: Abuse of Diuretics or Conn’s disease

Conn’s disease