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Acid- base balance

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Introduction

• The body normally maintains a steady balance between acids produced during metabolism and bases that neutralise and promote the excretion of the acids

• Brown & Edwards 2012 (p.368)

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Acid-base Imbalance – who is at risk?

• People who live with diabetes mellitus, chronic obstructive pulmonary disease (COPD) and kidney disease frequently develop acid – base imbalances

• Brown & Edwards 2012 (p.368)

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Acid-base Imbalance – who is at risk?

• People experiencing vomiting and diarrhoea can lose acids and bases in addition to loss of fluids and electrolytes

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Acid-base Imbalance – who is at risk?

• Any patient with a serious illness is at risk of acid-base imbalance

• Brown & Edwards 2012 (p.368)

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Acid-base Imbalance – who is at risk?

• The older adult is at risk - their kidneys and lungs do not compensate so well anymore for acid-base imbalance

• Brown & Edwards 2012 (p.368)

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pH and Hydrogen ion Concentration

• The acidity or alkalinity of a solution depends upon its hydrogen (H+) ion concentration

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pH and Hydrogen ion Concentration

• An increase in H+ concentration leads

to acidity

• A decrease in H+ concentration leads to alkalinity

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pH and Hydrogen ion Concentration

• Despite the fact that acids are produced in the body everyday , the hydrogen ion concentration of body fluids is small (0.0004mmol/L

• This tiny amount is maintained within a narrow range to ensure optimum cellular function

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pH and Hydrogen ion Concentration

• Hydrogen ion concentration is usually expressed as pH

• The lower the pH the higher the H+ concentration

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pH and Hydrogen ion Concentration

• The pH of a chemical solution may range from 1-14

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pH and Hydrogen ion Concentration

• An acid solution has a pH of less than (<) 7

• An alkaline solution has a pH of greater

than (>) 7

• A neutral solution has a pH of 7

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pH and Hydrogen ion Concentration

• Blood is slightly alkaline (pH 7.35 – 7.45)

• If the pH of blood drops below 7.35 the person is said to have acidosis

• If the pH of blood is greater than 7.45 the person has alkalosis

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pH and Hydrogen ion Concentration

• Relative to a pH of 7, a pH of 8 represents a 10-fold decrease in H+ concentration

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Acid-base Regulation

• The body’s metabolic processes are constantly producing acids

• These acids must be neutralised and excreted to maintain acid-base balance

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Acid-base Regulation

• Normally the body has three (3) mechanisms by which it regulates acid-base balance to maintain the arterial pH between 7.35 – 7.45

• These mechanisms are: • The buffer systems• The respiratory system• The renal system

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The Buffer Systems

• The buffer systems are the fastest acting systems and are the primary regulators of acid-base balance.

• Buffers act chemically to change strong acids into weaker acids or to bind acids to neutralise their effects

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What is a Buffer?

• A buffer consists of a weakly ionised acid or base and its salt that functions to minimise the effects of acids on blood pH until they can be excreted from the body

• Buffers alone cannot maintain blood pH and they need an adequately functioning respiratory and renal system

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The Respiratory System

• Lungs help maintain a normal pH by excreting CO2 and water which are by-products of cellular metabolism

• The rate of C02 excretion is controlled by the respiratory centre in the medulla in the brainstem

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The Respiratory System

• If increased C02 or H+ are detected the respiratory centre stimulates an increased rate and depth of breathing

• Respiratory rate and depth are inhibited if the centre detects low H+ or C02 levels

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The Respiratory System

• If a respiratory condition is the cause of an acid base imbalance (respiratory failure, for example) the respiratory system loses its ability to correct a pH alteration

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The Renal System

• Under normal circumstances the kidneys reabsorb and conserve all of the bicarbonate they filter.

• The kidneys can also generate more bicarbonate and eliminate excess H+ to compensate for acidosis

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The Renal System

• The kidneys usually excrete acidic urine (average pH6)

• If the renal system is the cause of an acid-base imbalance (renal failure, for example) it loses its ability to correct an acid-base imbalance

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Reference

• Brown, D. & Edwards, H. (2012) Lewis’s Medical Surgical Nursing: Assessment and Management of Clinical Problems, Mosby Elsevier, Sydney, Australia