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Acid-base balance

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Acid base balance the mechanisms that the body uses to keep its fluids close to neutral so that the body can function properly. Having the right amount of acid and base in the blood and other body fluids is important for the body to work the way it should. Acid base balance is also called acid-base equilibrium.

Acid base terms

pH refers to hydrogen ion (H+,) levels, hence the ‘H’ in pH. H+ levels are important because a lack of (deficit) or too much (excess) will tell you if the patient is acidotic or alkolotic. One confusing point about pH is that it is an INVERSE ratio, which means that the more H+ present, the lower the pH and vice versa.

Normal pH

The normal pH is between 7.35 - 7.45


Can give away a H+ or can separate (dissociate) hydrogen from its ion, so the hydrogen is not positive and therefore no longer an acid. Acids are end products of metabolism and must be buffered or excreted to achieve a normal pH 20 parts base / 1 part acid.


Unlike Acids, bases can accept a H+ and bond with hydrogen. They are all negative and like to ‘buffer’ body acids.

Base excess / Base deficit

Represents an increase or decrease in the amount of base compared with the amount of acids present -2 to +2mmol/L.


Concentration of hydrogen carbonate in blood. Used to determine along with pH and CO2 source of acid base imbalance. Normal HCO3 is 22-26mmol/L


Carbon dioxide partial pressure (tension). Reflects alveolar ventilation as it diffuses across the alveolar capillary membrane and “blown off”. Normal pCO2 is 35-45 mmhg.


Arterial oxygen tension. In other words how well the lungs are able to pick up oxygen, i.e. supply, but not demand (this is shown in a mixed venous gas. Normal range is 75-100mmhg.


(Lactic Acid) When cells no longer have enough O2 for ‘normal’ aerobic metabolism (cell hypoxia) Anaerobic metabolism takes over resulting in lactate production, leading to lactic aci-dosis. Normal is 0.5 - 2.0mmol/L


(Haemoglobin) Amount of haemoglobin in blood possibly capable of carrying oxygen.


This shows us how much red blood cells there are in a sample of blood - i.e. how watered down (or not)blood is. Normal hematocrit is 36-44%.

Anion gap

This value is used in metablic acidosis to find the cause. It reflects unmeasured anions (negatively chargedions) such as proteins, sulfates, phosphates etc. The equation to find the anion gap is (Na + K+) - (cl + [HCO3-])a high value can be caused by keto or lactic acidosis, or renal failure. Low values may indicate hyponatremia or decreasedplasma protiens. Normal anion gap is 10-20mmol/L.


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