Acid Base Balance

Carbon dioxide is continually produced by cellular aerobic metabolism of glucose and fatty acids. Carbon dioxide diffuses down its concentration gradient from the cell to the blood, which carries it to the lungs. It can interact with water to form carbonic acid (H2CO3), a process catalyzed by carbonic anhydrase, an enzyme present in erythrocytes. Carbonic acid can then dissociate to liberate bicarbonate ion (HCO2) and hydrogen ion (H+) as follows:

This process reverses in the lung, where hydrogen ion and bicarbonate ion combine to form carbonic acid, which then breaks down to form water and carbon dioxide, the latter diffusing into the alveolar space down its concentration gradient for removal by ventilation.

A close relationship exists in the blood between carbon dioxide levels and hydrogen ion concentrations such that increases in carbon dioxide cause increases in blood hydrogen ion levels and, as a result, decreases in blood pH. Ventilation has a direct influence on blood carbon dioxide concentrations and thereby affects blood pH as shown:

! Î blood pH ! respiratory alkalosis

! # blood pH ! respiratory acidosis

Alterations in ventilation, therefore, influence blood pH. Impaired ventilation, as may occur during central nervous system depression or airway obstruction, can result in respiratory acidosis. Conversely respiratory alkalosis can be caused by hyperventilation, as might occur during ascent to high altitude or by fever. In general, renal mechanisms function to compensate for inordinate respiratory alternations in blood pH. In addition, feedback control mechanisms exist in the body that alter respiration in the face of changes in blood pH. For example, changes in blood pH as a consequence of nonrespiratory mechanisms (such as may occur in severe diarrhea, altered renal function, and ingestion of acids or bases) or respiratory mechanisms may be returned toward normal (pH = 7.4) by altering the rate and depth of ventilation. Increases in blood hydrogen ion (and carbon dioxide) concentration stimulate carotid chemoreceptors (located in the bifurcation of the common carotid arteries) to elicit a central nervous system reflex in ventilation. A decrease in the blood concentration of hydrogen ions depresses ventilation through the same central nervous system reflex. In addition, ventilation is also regulated by chemoreceptors in the medulla of the brainstem sensitive to changes in hydrogen ion and carbon dioxide concentrations in the cerebrospinal fluid.

Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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