Respiratory Unit: Acid-Base Balance

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Acid-base

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Acid-base refers to the maintenance of the normal hydrogen ion concentration in the body fluids.

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ph:

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Is an expression of the hydrogen ion concentration of a solution.

The broad range of pH is 0-14. The normal blood pH is 7.35-7.45. Hydrogen (H+) and pH have an inverse relationship, ie. as one increases the other decreases.

- Incr. in hydrogen ion concentration makes a solution more acidic resulting in an decr. in pH. In acidosis the pH of the blood is below 7.35.

- Decr. in H+ ion concentration makes a solution more alkaline = incr. in pH. In alkalosis the pH of the blood will be above 7.45.

WE ARE ACID MAKING MACHINES:

Through the process of cellular metabolism the body produces acids. To keep the body pH in balance these metabolic acids must be either excreted or neutralized at the same rate in the same amount as the acids are produced.

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2 Types of acids produced during metabolic processes:

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- Volatile Acids

-Nonvolatile Acids

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Volatile Acids:

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- *Can only be excreted by lungs

- *CO2 is an acid

- Volatile acids that can be eliminated as carbon dioxide gas (eg.CO2)

- Carbonic acid (eg. H2CO3) is a weak acid that in the presence of the enzyme Carbonic anhydrase easily dissociates into CO2 where it can be eliminated via the lungs.

CO2 + H2O <---> H2CO3 <---> HCO3 + H+

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Nonvolatile Acids:

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These are metabolic acids that must be excreted via the kidneys. Examples of these metabolic acids are:

- Lactic acid produced through anaerobic metabolism (as in cardiac arrest)

- Ketoacid created when carbohydrates cannot be used for cellular metabolism as in insulin dependent diabetes mellitus (eg. ketoacidosis) or starvation.

- Phosphoric and sulfuric acids that are produced from the metabolism of proteins.

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Control Mechanisms for Acid-Base Balance (4)

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1) Blood Buffer System

2) Respiratory System

3) Intracellular Buffering

4) Renal System

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Blood Buffer System:

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- for fixing minor acid/base changes -- early so don't have big change in pH.

- Immediately available to combine with excess acid or base.

- Acts within seconds

- Prevents big swings in pH.

- Known as the carbonic acid/bicarbonate system.

CO2 + H2O <---> H2CO3 <---> H+ +HCO3

- To maintain acid-base balance, the ratio of carbonic acid to base bicarbonate is 1:20.

1 (H2CO3) to 20 (HCO3) = 7.40

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Respiratory System:

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- Fairly efficient, but not 100% (50-75%)

- Acts within 1-3 mins.

- Controls pH through regulation of CO2 (*only works on metabolic problems)

- CO2 formed continuously as a byproduct of metabolism.

Eg.

1) Incr. rate of metabolism = incr.CO2 production.

(CO2 + H2O <---> H2CO3 production)

2) Increase in acid = decr. in pH

3) Decr. in pH = stimulates medulla = Incr. ventilation (breath faster and deeper)

4) Incr. ventilation = CO2 being blown off (lose acid)

OR VICE VERSA

1) Decr. rate of metabolism = decr. CO2 (decr. rate of H2CO3 production)

2) Decr. acid (eg. more base) = incr. pH

3) Incr. pH = decr. medulla stimulation = decr. ventilation

4) Decr. ventilation = less CO2 is blown off (retain acid)

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Intracellular Buffering

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* K+ shift is more pronounced in acidosis. ie. works better in acidosis

- acts within 2-4 hours

- cells exchange H+ across the cell membrane for K+ and vice versa.

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Intracellular Buffering in Acidosis:

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1) K+ leaves the cell in exchange for H+ ions in the blood which now enter the cell.

2) The loss of H+ ions from the blood helps to correct the acidosis.

3) The extra K+ ions in the blood result in hyperkalemia

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Intracellular Buffering in Alkalosis:

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1) K+ enters the cell in exchange for the H+ ions which leave the cells for the blood.

2) The extra H+ ions in the blood helps to correct the alkalosis.

3) The loss of K+ ions in the blood results in hypokalemia.

** The K+ imbalances are temporary and will reverse when the acid-base imbalance is rectified.

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Renal System:

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* Can only fix respiratory issues not metabolic.

- Slowest control mechanism to respond - takes hours to days.

- 100% efficient in returning pH to normal.

- alters the excretion of H+ or HCO3 in the urine

Eg:

- In alkalosis (incr. pH, too much base) kidneys retain H+ ions and excrete bicarbonate (HCO3).

- In acidosis (decr. pH, too much acid) kidneys will retain bicarbonate and excrete H+.

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Why acidosis is a problem:

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- CNS depression --> lethargic, confused, disoriented, coma.

- ** Warm and flushed --> dilation of bld. vessels - less responsive to SNS.

- Incr. in serum K+

- pH falls below 7.0 --> bradycardia, decr. contractility, cardiac arrest.

- less important: weakness, abd. cramping, fatigue, headaches

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Why alkalosis is a problem:

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- CNS irritability --> tetany (muscle locks), seizures

- cool and pale --> bld. vessels constrict

- Decr. in serum K+

- myocardium is hyper-responsive -->tachycardia, dysrhythmias

- less important: anorexia, nausea/vomiting

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Disorders of Acid-Base Balance: Respiratory Acidosis (CO2 retention - decr. pH, incr. pCO2)

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Causes:

1) Depressed CNS: head injury, drug overdose (eg. narcotics).

2) Disease Process: neuromuscular, chest trauma.

3) Disease airways/impaired gas exchange: COPD, asthma, pneumonia, pulmonary edema, atelectasis

Compensation:

eg. COPD

1) CO2 + H2O <---> H2CO3 <---> H+ = HCO3 (bld. buffering)

2) intracellular buffering

3) Renal system (save HCO3, get rid of H+ and NH+4 ammonia.

Treatment: treat the cause and the acid-base will correct themselves.

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Disorders of Acid-Base Balance: Respiratory Alkalosis (CO2 blown off - incr. pH, decr. pCO2)

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Cause: CNS stimulation

eg.

1) Hysteria/anxiety (hyperventilating)

2) Pain = panting, short rapid breathing

3) Encephalitis, meningitis, head injury

Compensation:

- kidneys retain H+ and excrete HCO3

Treatment: Treat the cause

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Disorders of Acid-Base Balance: Metabolic Acidosis

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Causes:

A) Increase in metabolic acids:

1) Lactic acidosis - eg. in cardiac arrest (#1cause)

2) Ketoacidosis - complications of type 1 diabetes

3) ASA poisoning *** - adults initially resp. alkalosis then met. acidosis; kids: metabolic acidosis

4) Renal Failure (acute or chronic)

B) Decrease in bicarbonate:

- loss of intestinal secretions

1) diarrhea

2) intestinal suction

3) Intestinal fistula

Compensation: resp. system has to fix it.

- kussmal's resps. - incr. resps to blow off CO2

Treatment: Treat the cause!

eg. ketoacidosis - give insulin to decr. sugars

eg. renal failure - dialisis

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Disorders of Acid-Base Balance: Metabolic Alkalosis (incr. pH, TCO2, or HCO3)

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Causes:

A) increase in bicarbonate:

- ingestion of HCO3 (eg. antacids)

B) Loss of acids:

1) vomiting (upper tract is acidic)

2) gastric suction

Compansation:

Decr. resps - retain CO2

Treatment: Treat the cause!

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Blood Gases:

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** Practice ABG's

pH:

- norms: 7.35-7.45

- above 7.45 = alkaline (B)

- blow 7.35 = acididic (A)

pCO2:

- norms: 35-45

- above 45 = CO2 is retained (hypoventilating) (A)

- below 35 = CO2 blown off (hyperventilating) (B)

TCO2 aka HCO3:

- norm: 22-26

- above 26 = alkaline (B)

- below 22 = acidic (A)

Assess pO2:

- below 90 = not happy