|Human Physiology 6/e Fox|
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Acidosis and Alkolosis
The normal pH value for the body fluids is between pH 7.35 and 7.45. When the pH value of body fluids is below 7.35, the condition is called acidosis, and when the pH is above 7.45, it is called alkalosis.
Metabolism produces acidic products that lower the pH of the body fluids. For example, carbon dioxide is a by-product of metabolism, and carbon dioxide combines with water to form carbonic acid. Also, lactic acid is a product of anaerobic metabolism, protein metabolism produces phosphoric and sulfuric acids, and lipid metabolism produces fatty acids. These acidic substances must continuously be eliminated from the body to maintain pH homeostasis. Rapid elimination of acidic products of metabolism results in alkalosis, and the failure to eliminate acidic products of metabolism results in acidosis.
The major effect of acidosis is depression of the central nervous system. When the pH of the blood falls below 7.35, the central nervous system malfunctions, and the individual becomes disoriented and possibly comatose as the condition worsens.
A major effect of alkalosis is hyperexcitability of the nervous system. Peripheral nerves are affected first, resulting in spontaneous nervous stimulation of muscles. Spasms and tetanic contractions and possibly extreme nervousness or convulsions result. Severe alkalosis can cause death as a result of tetany of the respiratory muscles.
Although buffers in the body fluids help resist changes in the pH of body fluids, the respiratory system and the kidneys regulate the pH of the body fluids. Malfunctions of either the respiratory system or the kidneys can result in acidosis or alkalosis.
Acidosis and alkalosis are categorized by the cause of the condition. Respiratory acidosis or respiratory alkalosis results from abnormalities of the respiratory system. Metabolic acidosis or metabolic alkalosis results from all causes other than abnormal respiratory functions.
Inadequate ventilation of the lungs causes respiratory acidosis. The rate at which carbon dioxide is eliminated from the body fluids through the lungs falls. This increases the concentration of carbon dioxide in the body fluids. As carbon dioxide levels increase excess carbon dioxide reacts with water to form carbonic acid. The carbonic acid dissociates to form hydrogen ions and bicarbonate ions. The increase in hydrogen ion concentration causes the pH of the body fluids to decrease. If the pH of the body fluids falls below 7.35, symptoms of respiratory acidosis become apparent.
Buffers help resist a decrease in pH, and the kidneys help compensate for failure of the lungs to prevent respiratory acidosis by increasing the rate at which they secrete hydrogen ions into the filtrate and reabsorb bicarbonate ions. However, the capacity of buffers to resist changes in pH can be exceeded, and a time period of 1 or 2 days is required for the kidney to become maximally functional. Thus the kidneys are not effective if respiratory acidosis develops quickly, but they are very effective if respiratory acidosis develops slowly or if it lasts long enough for the kidneys to respond. For example, kidney cannot compensate for respiratory acidosis occurring in response to a severe asthma attack that begins quickly and subsides within hours. However, if respiratory acidosis results from emphysema, which develops over a long period of time, the kidneys play a significant role in helping to compensate.
Respiratory alkalosis results from hyperventilation of the lungs. This increases the rate at which carbon dioxide is eliminated from the body fluids and results in a decrease in the concentration of carbon dioxide in the body fluids. As carbon dioxide levels decrease, hydrogen ions react with bicarbonate ions to form carbonic acid. The carbonic acid dissociates to form water and carbon dioxide. The resulting decrease in the concentration of hydrogen ions cause the pH of the body fluids to increase. If the pH of body fluids increases above 7.35, symptoms of respiratory alkalosis become apparent.
The kidneys help to compensate for respiratory alkalosis by decreasing the rate of hydrogen ions secretion into the urine and the rate of bicarbonate ion reabsorption. If an increase in pH occurs, a time period of 1 or 2 days is required for the kidneys to be maximally effective. Thus the kidneys are not effective if respiratory alkalosis develops quickly. However, they are very effective if respiratory alkalosis develops slowly. For example, the kidneys are not effective in compensating for respiratory alkalosis that occurs in response to hyperventilation triggered by emotions, which usually begins quickly and subsides within minutes or hours. However if alkalosis results from staying at a high altitude over a 2 or 3 day period, the kidneys play a significant role in helping to compensate.
Metabolic acidosis results from all conditions that decrease the pH of the body fluids below 7.35, with the exception of conditions resulting from altered function of the respiratory system. As hydrogen ions accumulate in the body fluids, buffers first resist a decline in pH. If the buffers cannot compensate for the increase in hydrogen ions, the respiratory center helps regulate the body fluid pH. The reduced pH stimulates the respiratory center, which causes hyperventilation. During hyperventilation, carbon dioxide is eliminated at a greater rate. The elimination of carbon dioxide also eliminates excess hydrogen ions and helps maintain the pH of the body fluids within a normal range.
If metabolic acidosis persists for many hours and if the kidneys are functional, the kidneys can also help compensate for metabolic acidosis. They begin to secrete hydrogen ions at a greater rate and increase the rate of bicarbonate ion reabsorption. Symptoms of metabolic acidosis appear if the respiratory and renal systems are not able to maintain the pH of the body fluids within its normal range.
Metabolic alkalosis results from all conditions that increase the pH of the body fluids above 7.45, with the exception of conditions resulting from altered function of the respiratory system. As hydrogen ions decrease in the body fluids, buffers first resist an increase in pH. If the buffers cannot compensate for the decrease in hydrogen ions, the respiratory center helps regulate the body fluid pH. The increased pH inhibits respiration. Reduced respiration allows carbon dioxide to accumulate in the body fluids. Carbon dioxide reacts with water to produce carbonic acid. If metabolic alkalosis persists for several hours, and if the kidneys are functional, the kidneys reduce the rate of hydrogen ion secretion to help reverse alkalosis.
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