Chapter 42: Normal Acid–Base Regulation

INTRODUCTION

Blood pH is a result of the interplay between the continuous production of acid though metabolism, the buffering of the acid load, and the ultimate elimination of that load from the body. When this balance is disrupted, clinically significant acidosis or alkalosis can develop.

Acidosis has been known to cause myocardial depression, decreased responsiveness to catecholamines, increased risk of cardiac arrhythmias, decreases in systemic vascular resistance but increased pulmonary vascular resistance, as well as insulin resistance and impaired immune function. Significant alkalosis also may cause myocardial depression and arrhythmias, as well as inhibiting respiratory drive and decreasing oxygen delivery through a leftward shift in the oxygen–hemoglobin disassociation curve.

ACID–BASE REGULATION PHYSIOLOGY

The quickest and most efficient method the body naturally regulates pH is through respiration. Carbon dioxide (CO₂) is produced as the byproduct of aerobic metabolism. Carbon dioxide is additionally formed when bicarbonate buffers an acid forming carbonic acid (H₂CO₃) which breaks down into water and CO₂:

Carbon dioxide is then carried in the blood dissolved directly in solution, buffered with water as carbonic acid or attached to hemoglobin within erythrocytes as carbaminohemoglobin (75% of total CO₂). Carbon dioxide is ultimately eliminated from the body through exhalation at a rate equal to the minute ventilation, defined as the product of the tidal volume and respiratory rate. Concomitantly, the bicarbonate buffer levels are maintained by its increased absorption within kidneys.

The kidneys also maintain acid–base regulation at the level of the collecting ducts where Type A intercalated cells excrete protons in exchange for potassium. In times of alkalosis, these cells are effectively reversed so that bicarbonate and potassium is lost while protons are reabsorbed into the blood stream.

The gastrointestinal tract is a net excreter of bicarbonate through the actions of carbonic anhydrase within the intestinal epithelium. When electrolyte transport in these cells is pathologically increased, such as during diarrhea, significant amounts of bicarbonate can be lost.

CAUSES OF ACID–BASE ABNORMALITIES

Respiratory acidosis or alkalosis is the result of minute ventilation which is pathologically too low or high, respectively, for the amount of carbon dioxide produced. A very high minute ventilation may be appropriate if the total amount of carbon dioxide within the blood is very high such as during times intense metabolic activity. Pathology related to acid–base disturbance occurs when there is a mismatch between the elimination of carbon dioxide and its formation. Metabolic alkalosis results from the pathologic gain of bicarbonate or loss of protons. Milk-alkali syndrome is a metabolic alkalosis from the excessive consumption of stomach antacids. Iatrogenic metabolic alkalosis can also result from infusions of acetate or citrate (such as occurs with massive blood transfusions) which are metabolized by the liver into bicarbonate. Another common cause of ...