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6.3D: Chemical Composition of Bone

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    Acid-base imbalances, including metabolic acidosis and alkalosis, can produce severe, even life-threatening medical conditions.

     

    LEARNING OBJECTIVES

     

    Differentiate among the acid-base disorders

     

    KEY TAKEAWAYS

    Key Points

     

    • Metabolic acidosis can produce, among other symptoms, chest pains, altered mental states, nausea, abdominal pain, and muscle weakness.
    • Rapid, deep breathing during metabolic acidosis is an attempt to lower carbon dioxide levels and return pH to normal.
    • Extreme acidemia can lead to coma, seizures, heart arrhythmias, and low blood pressure.
    • Slowed breathing, which results in retaining more CO2, is the primary method of reducing metabolic alkalosis.
    • Chronic respiratory acidosis is a result of COPD, obesity hypoventilation syndrome, ALS, and thoracic deformities.
    • Respiratory alkalosis can be caused by excessive mechanical ventilation, psychiatric problems, stroke, drug use, traveling to high altitude regions, lung disease, fever, and pregnancy, among other factors.

     

    Key Terms

     

    • metabolic alkalosis: A metabolic condition in which the pH of tissue is elevated beyond the normal range ( 7.35 to 7.45 ). This is the result of decreased hydrogen ion concentration, leading to increased bicarbonate concentration, or a direct result of increased bicarbonate concentration.
    • respiratory acidosis: A medical condition in which decreased ventilation (hypoventilation) causes increased blood carbon dioxide concentration and decreased pH (a condition generally called acidosis).
    • metabolic acidosis: A condition that occurs when the body produces too much acid or when the kidneys are not removing enough acid from the body.

     

    EXAMPLES

     

    Traveling to a high altitude can cause an acid-base imbalance due to reduced levels of oxygen in the atmosphere, and, therefore, in the blood. To compensate for this, the traveler begins to hyperventilate, trying to expel excess carbon dioxide and bring pH back to normal. However, if the traveler stays at high altitude, it may take several days for their pH to fully return to normal.

     

    Acid-Base Disorders

     

    Acid-base imbalance is an abnormality of the human body’s normal balance of acids and bases that causes the plasma pH to deviate out of normal range (7.35 to 7.45). In the fetus, the normal range differs based on which umbilical vessel is sampled (umbilical vein pH is normally 7.25 to 7.45; umbilical artery pH is normally 7.18 to 7.38). Acid-base imbalances can exist in varying levels of severity, some life-threatening.

    An excess of acid is called acidosis and an excess in bases is called alkalosis. The process that causes the imbalance is classified based on the etiology of the disturbance (respiratory or metabolic) and the direction of change in pH (acidosis or alkalosis).

    Mixed disorders may feature an acidosis and alkalosis excess at the same time that partially counteract each other, or there can be two different conditions affecting the pH in the same direction. The phrase mixed acidosis, for example, refers to metabolic acidosis in conjunction with respiratory acidosis.

     

    Metabolic Acidosis

     

    In medicine, metabolic acidosis is a condition that occurs when the body produces too much acid or when the kidneys are not removing enough acid from the body. If unchecked, metabolic acidosis leads to acidemia, that is, blood pH is less than 7.35 due to increased production of hydrogen by the body, or because of the body’s inability to form bicarbonate (HCO3-) in the kidneys.

    Acidosis refers to a low pH in tissue. Acidemia refers to a low pH in the blood. Symptoms may include chest pain, palpitations, headache, altered mental status such as severe anxiety due to hypoxia, decreased visual acuity, nausea, vomiting, abdominal pain, altered appetite (either loss of or increased) and weight loss (longer term), muscle weakness, and bone pains.

    Rapid deep breaths increase the amount of carbon dioxide exhaled, thus lowering the serum carbon dioxide levels, resulting in some degree of compensation. Overcompensation via respiratory alkalosis to form an alkalemia does not occur.

    Neurological complications include lethargy, stupor, coma, seizures. Cardiac complications include arrhythmias (ventricular tachycardia) and decreased response to epinephrine; both lead to hypotension (low blood pressure).

     

    Metabolic Alkalosis

     

    Metabolic alkalosis is a metabolic condition in which the pH of tissue is elevated beyond the normal range (7.35 to 7.45). This is the result of decreased hydrogen ion concentration, leading to increased bicarbonate concentration, or as a direct result of increased bicarbonate concentrations. Alkalosis refers to a high pH in tissue.

    Alkalemia refers to a high pH in the blood. The causes of metabolic alkalosis can be divided into two categories, depending upon urine chloride levels. Chloride-responsive causes result from the loss of hydrogen ions via vomiting or the kidneys. Vomiting results in the loss of hydrochloric acid (hydrogen and chloride ions) with the stomach contents.

    The kidneys compensate for these losses by retaining sodium in the collecting ducts at the expense of hydrogen ions (sparing sodium/potassium pumps to prevent further loss of potassium), and leads to metabolic alkalosis. The excess sodium increases extracellular volume and the loss of hydrogen ions creates a metabolic alkalosis.

    Later, the kidneys respond through the aldosterone escape to excrete sodium and chloride in urine. Compensation for metabolic alkalosis occurs mainly in the lungs, which retain carbon dioxide (CO2) through slower breathing, or hypoventilation (respiratory compensation).

    CO2 is then consumed towards the formation of the carbonic acid intermediate, thus decreasing pH. Renal compensation for metabolic alkalosis, less effective than respiratory compensation, consists of increased excretion of HCO3– (bicarbonate), as the filtered load of HCO3– exceeds the ability of the renal tubule to reabsorb it.

     

    Respiratory Acidosis

     

    Respiratory acidosis is a medical condition in which decreased ventilation (hypoventilation) causes an increase in blood carbon dioxide concentration and decreased pH (a condition generally called acidosis). Carbon dioxide is produced constantly as the body’s cells respire, and this CO2 will accumulate rapidly if the lungs do not adequately expel it through alveolar ventilation.

    Acute respiratory acidosis occurs when an abrupt failure of ventilation occurs. This failure in ventilation may be caused by depression of the central respiratory center by cerebral disease or drugs, an inability to ventilate adequately due to neuromuscular disease (e.g., myasthenia gravis, amyotrophic lateral sclerosis, Guillain-Barré syndrome, muscular dystrophy), or airway obstructions related to asthma or chronic obstructive pulmonary disease (COPD) exacerbation.

     

    Respiratory Alkalosis

     

    Respiratory alkalosis is a medical condition in which increased respiration (hyperventilation) elevates the blood pH (a condition generally called alkalosis). There are two types of respiratory alkalosis: chronic and acute.

    Acute respiratory alkalosis occurs rapidly. During acute respiratory alkalosis, the person may lose consciousness whereupon the rate of ventilation will resume to normal.

    Chronic respiratory alkalosis is a more long-standing condition. Respiratory alkalosis may be produced accidentally (iatrogenically) during excessive mechanical ventilation. Other causes include: psychiatric causes, drug use, fever, and pregnancy.

     

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