Normal Acid Base Balance
In healthy state, the pH of the blood or , blood hydrogen ion concentration lies within the range pH 7.3 6—7.44.
If the pH is less than the lower value of this range, it is termed as acidic and if more than the higher value of the range, it is called alkaline.
Blood acidity increases when the
- Level of acidic compounds in the body rises. This could be due to increased intake or production, or decreased elimination
- Level of basic compounds in the body falls (through decreased intake or production, or increased elimination)
Blood alkalinity increases when the level of acid in the body decreases or when the level of base increases.
The terms acidosis and alkalosis in clinical practice indicate a change or a tendency to a change in the pH of the blood in a particular direction.
In acidosis, there is an accumulation of acid or a loss of a base causing a fall or a tendency to a fall in the pH. The opposite occurs in alkalosis.
Regulation of Blood PH
The pH of the blood is regulated and controlled by lungs, kidneys and various buffering systems essentially consisting of weak acids and bases, of which the most important is the bicarbonate:carbonic acid ratio HCO3:H2C03.
Carbon dioxide is mildly acidic. It end product of the metabolism of oxygen. It is constantly produced by the cells and gets excreted into the blood. The blood carries carbon dioxide to the lungs, where it is exhaled.
When concentration of carbon dioxide increases in the blood, the pH of the blood decreases (acidity increases).
The brain regulates the amount of carbon dioxide that is exhaled by controlling the speed and depth of breathing. By adjusting the speed and depth of breathing, the brain and lungs are able to regulate the blood pH minute by minute.
Role of the kidneys
The kidneys are able to affect blood pH by excreting excess acids or bases. The kidneys are slower to respond and this compensation generally takes several days.
The pH buffer systems are combinations of naturally occurring weak acids and weak bases which exist in pairs that are in balance under normal pH conditions. The pH buffer systems work chemically to minimize changes in the pH of a solution by adjusting the proportion of acid and base.
The most important pH buffer system in the blood involves carbonic acid and bicarbonate ions.
The ratio of bicarbonate to carbonic acid is normally 20:1. Alteration in this ratio alters the pH regardless of the absolute values of the bicarbonate and carbonic acid. A decrease in the ratio leads to increased acidity and vice versa.
The bicarbonate level can be altered by metabolic factors, while the carbonic acid level is subject to alteration by respiratory factors.
The change in one is followed by a compensatory change in the other, so that the ratio (HCO3:H2C03) and therefore the pH of the blood remains constant.
Acid base disorders are diagnosed by measurement of different values.
PCO2 is a measurement of the tension or partial pressure of carbon dioxide in the blood. The normal arterial PCO2 is 4.1—5.6 kPa (31—42 mmHg).
P02 is a measurement of the tension or partial pressure of oxygen in the blood. The normal arterial P02 is 10.5—14.5kPa (80—110 mmHg).
Standard bicarbonate is the concentration of the serum bicarbonate after fully oxygenated blood has been equilibrated with carbon dioxide at 40 mmHg (5.3 kPa) at 380C. This eliminates respiratory causes and respiratory compensation for altered bicarbonate levels. Normal levels are 22—25 mmol/litre.
Base excess or deficit expresses, in mmol, the total buffer anions present in the blood in excess of deficit of normal. (Normal base excess or deficit +2.5.) Base excess or deficit multiplied by 0.3 times the body weight in kg gives the total extracellular excess or deficit of base in mmol.
Metabolic causes of acid—base disturbances are indicated by changes in the standard bicarbonate level and base excess or deficit. Respiratory causes of acid—base disturbances are indicated by changes in the PCO2 and P02.
Disorders of Acid Base Balance
The disturbances of pH leads to Alkalosis or acidosis. Depending on the causation, these are further classified as respiratory or metabolic.
So we have following acid base balance disorders
- Metabolic alkalosis
- Respiratory alkalosis
- Metabolic acidosis
- Respiratory acidosis
Metabolic alkalosis is , a condition of base excess or a deficit of any acid other than H2C03. It can be caused by excessive ingestion of absorbable alkali, repeated vomiting or aspiration, cortisone excess
The body tries to compensate by
- retention of carbon dioxide by the lungs
- Excretion of bicarbonate base by the kidneys. This results in alkaline urine.
In severe alkalosis, the respiration pattern alters. The respiration becomes is Cheyne—Stokes respiration. The alkalosis accompanied by low potaasium [due to loss in vomiting for example] is called hypokalemic alkalosis.
Metabolic alkalosis without hypokalaemia seldom requires direct treatment. The cause of the alkalosis should be removed where possible and a high urinary output encouraged.
Hypokalaemic alkalosis required replacement of poatassium.
Respiratory alkalosis is a condition where the arterial PCO2 is below the normal range of 31—42 mmHg (4.1—5.6 kPa). It is caused most commonly by excessive pulmonary ventilation as seen in high altitudes, hyperpyrexia, a lesion of the hypothalamus and hysteria.
In an attempt to decrease blood pH, there is increased renal excretion of bicarbonate, usually is inadequate. Severe respiratory alkalosis may lead to supression of respiration.
Respiratory suppression due to alkalosis is rectified by insufflation of carbon dioxide.
Metabolic acidosis, a condition where there is a deficit of base or an excess of any acid other than H2C03, occurs as a result of increase in fixed acids [ketone bodies as in diabetes or starvation, renal insufficiency, anaerobic tissue metabolism] or loss of bases [diarrhoea, ulcerative colitis, gastrocolic fistula, a high intestinal fistula or prolonged intestinal aspiration.]
Hyperpnoea occurs due to over stimulation of the respiratory centre by the reduction in pH of the blood [ an attempt to eliminate as much as possible of the acid substance H2C03]. Kidney excretes acids and the urine is strongly acidic. The standard bicarbonate level is lowered and there is a base deficit.
Correction of tissue hypoxia by restoration of adequate tissue perfusion is the general treatment. Bicarbonate solutions will correct the measured metabolic acidosis but not treat the problem which should be addressed separately.
Respiratory acidosis, a condition where the PCO2 is above the normal range, is caused by impaired alveolar ventilation.
This occurs when there is inadequate ventilation of the anaesthetised patient, or when the effects of muscle relaxants have not worn off or been fully reversed at the end of the anaesthetic. There is also a risk of respiratory acidosis when the patient undergoing surgery already has pre-existing pulmonary disease.