Module 17: Fluid, Electrolyte, and Acid-Base Balance

Lesson 5: Disorders of Acid-Base Balance

Rối Loạn Cân Bằng Toan-Kiềm

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Dưới đây là danh sách những thuật ngữ Y khoa của module Fluid, Electrolyte, and Acid-Base Balance.
Khái quát được số lượng thuật ngữ sẽ xuất hiện trong bài đọc và nghe sẽ giúp bạn thoải mái tiêu thụ nội dung hơn. Sau khi hoàn thành nội dung đọc và nghe, bạn hãy quay lại đây và luyện tập (practice) để quen dần các thuật ngữ này. Đừng ép bản thân phải nhớ các thuật ngữ này vội vì bạn sẽ gặp và ôn lại danh sách này trong những bài học (lesson) khác của cùng một module.

Medical Terminology: Fluid, Electrolyte, and Acid-Base Balance

antidiuretic hormone (ADH)
also known as vasopressin, a hormone that increases the volume of water reabsorbed from the collecting tubules of the kidney
dehydration
state of containing insufficient water in blood and other tissues
dihydroxyvitamin D
active form of vitamin D required by the intestinal epithelial cells for the absorption of calcium
diuresis
excess production of urine
extracellular fluid (ECF)
fluid exterior to cells; includes the interstitial fluid, blood plasma, and fluids found in other reservoirs in the body
fluid compartment
fluid inside all cells of the body constitutes a compartment system that is largely segregated from other systems
hydrostatic pressure
pressure exerted by a fluid against a wall, caused by its own weight or pumping force
hypercalcemia
abnormally increased blood levels of calcium
hypercapnia
abnormally elevated blood levels of CO2
hyperchloremia
higher-than-normal blood chloride levels
hyperkalemia
higher-than-normal blood potassium levels
hypernatremia
abnormal increase in blood sodium levels
hyperphosphatemia
abnormally increased blood phosphate levels
hypocalcemia
abnormally low blood levels of calcium
hypocapnia
abnormally low blood levels of CO2
hypochloremia
lower-than-normal blood chloride levels
hypokalemia
abnormally decreased blood levels of potassium
hyponatremia
lower-than-normal levels of sodium in the blood
hypophosphatemia
abnormally low blood phosphate levels
interstitial fluid (IF)
fluid in the small spaces between cells not contained within blood vessels
intracellular fluid (ICF)
fluid in the cytosol of cells
metabolic acidosis
condition wherein a deficiency of bicarbonate causes the blood to be overly acidic
metabolic alkalosis
condition wherein an excess of bicarbonate causes the blood to be overly alkaline
plasma osmolality
ratio of solutes to a volume of solvent in the plasma; plasma osmolality reflects a person’s state of hydration
respiratory acidosis
condition wherein an excess of carbonic acid or CO2 causes the blood to be overly acidic
respiratory alkalosis
condition wherein a deficiency of carbonic acid/CO2 levels causes the blood to be overly alkaline
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Normal arterial blood pH is restricted to a very narrow range of 7.35 to 7.45. A person who has a blood pH below 7.35 is considered to be in acidosis (actually, “physiological acidosis,” because blood is not truly acidic until its pH drops below 7), and a continuous blood pH below 7.0 can be fatal. Acidosis has several symptoms, including headache and confusion, and the individual can become lethargic and easily fatigued (Figure 1). A person who has a blood pH above 7.45 is considered to be in alkalosis, and a pH above 7.8 is fatal. Some symptoms of alkalosis include cognitive impairment (which can progress to unconsciousness), tingling or numbness in the extremities, muscle twitching and spasm, and nausea and vomiting. Both acidosis and alkalosis can be caused by either metabolic or respiratory disorders.

As discussed earlier in this chapter, the concentration of carbonic acid in the blood is dependent on the level of CO2 in the body and the amount of CO2 gas exhaled through the lungs. Thus, the respiratory contribution to acid-base balance is usually discussed in terms of CO2 (rather than of carbonic acid). Remember that a molecule of carbonic acid is lost for every molecule of CO2 exhaled, and a molecule of carbonic acid is formed for every molecule of CO2 retained.
Metabolic acidosis occurs when the blood is too acidic (pH below 7.35) due to too little bicarbonate, a condition called primary bicarbonate deficiency. At the normal pH of 7.40, the ratio of bicarbonate to carbonic acid buffer is 20:1. If a person’s blood pH drops below 7.35, then the person is in metabolic acidosis. The most common cause of metabolic acidosis is the presence of organic acids or excessive ketone bodies in the blood. Table 1 lists some other causes of metabolic acidosis.

The first three of the eight causes of metabolic acidosis listed are medical (or unusual physiological) conditions. Strenuous exercise can cause temporary metabolic acidosis due to the production of lactic acid. The last five causes result from the ingestion of specific substances. The active form of aspirin is its metabolite, sulfasalicylic acid. An overdose of aspirin causes acidosis due to the acidity of this metabolite. Metabolic acidosis can also result from uremia, which is the retention of urea and uric acid. Metabolic acidosis can also arise from diabetic ketoacidosis, wherein an excess of ketone bodies are present in the blood. Other causes of metabolic acidosis are a decrease in the excretion of hydrogen ions, which inhibits the conservation of bicarbonate ions, and excessive loss of bicarbonate ions through the gastrointestinal tract due to diarrhea.
Metabolic alkalosis is the opposite of metabolic acidosis. It occurs when the blood is too alkaline (pH above 7.45) due to too much bicarbonate (called primary bicarbonate excess).

A transient excess of bicarbonate in the blood can follow ingestion of excessive amounts of bicarbonate, citrate, or antacids for conditions such as stomach acid reflux—known as heartburn. Cushing’s disease, which is the chronic hypersecretion of adrenocorticotropic hormone (ACTH) by the anterior pituitary gland, can cause chronic metabolic alkalosis. The oversecretion of ACTH results in elevated aldosterone levels and an increased loss of potassium by urinary excretion. Other causes of metabolic alkalosis include the loss of hydrochloric acid from the stomach through vomiting, potassium depletion due to the use of diuretics for hypertension, and the excessive use of laxatives.
Respiratory acidosis occurs when the blood is overly acidic due to an excess of carbonic acid, resulting from too much CO2 in the blood. Respiratory acidosis can result from anything that interferes with respiration, such as pneumonia, emphysema, or congestive heart failure.
Respiratory alkalosis occurs when the blood is overly alkaline due to a deficiency in carbonic acid and CO2 levels in the blood. This condition usually occurs when too much CO2 is exhaled from the lungs, as occurs in hyperventilation, which is breathing that is deeper or more frequent than normal. An elevated respiratory rate leading to hyperventilation can be due to extreme emotional upset or fear, fever, infections, hypoxia, or abnormally high levels of catecholamines, such as epinephrine and norepinephrine. Surprisingly, aspirin overdose—salicylate toxicity—can result in respiratory alkalosis as the body tries to compensate for initial acidosis.
Various compensatory mechanisms exist to maintain blood pH within a narrow range, including buffers, respiration, and renal mechanisms. Although compensatory mechanisms usually work very well, when one of these mechanisms is not working properly (like kidney failure or respiratory disease), they have their limits. If the pH and bicarbonate to carbonic acid ratio are changed too drastically, the body may not be able to compensate. Moreover, extreme changes in pH can denature proteins. Extensive damage to proteins in this way can result in disruption of normal metabolic processes, serious tissue damage, and ultimately death.

A. Respiratory Compensation

Respiratory compensation for metabolic acidosis increases the respiratory rate to drive off CO2 and readjust the bicarbonate to carbonic acid ratio to the 20:1 level. This adjustment can occur within minutes. Respiratory compensation for metabolic alkalosis is not as adept as its compensation for acidosis. The normal response of the respiratory system to elevated pH is to increase the amount of CO2 in the blood by decreasing the respiratory rate to conserve CO2. There is a limit to the decrease in respiration, however, that the body can tolerate. Hence, the respiratory route is less efficient at compensating for metabolic alkalosis than for acidosis.

B. Metabolic Compensation

Metabolic and renal compensation for respiratory diseases that can create acidosis revolves around the conservation of bicarbonate ions. In cases of respiratory acidosis, the kidney increases the conservation of bicarbonate and secretion of H+ through the exchange mechanism discussed earlier. These processes increase the concentration of bicarbonate in the blood, reestablishing the proper relative concentrations of bicarbonate and carbonic acid. In cases of respiratory alkalosis, the kidneys decrease the production of bicarbonate and reabsorb H+ from the tubular fluid. These processes can be limited by the exchange of potassium by the renal cells, which use a K+-H+ exchange mechanism (antiporter).

C. Diagnosing Acidosis and Alkalosis

Lab tests for pH, CO2 partial pressure (PCO2), and HCO3– can identify acidosis and alkalosis, indicating whether the imbalance is respiratory or metabolic, and the extent to which compensatory mechanisms are working. The blood pH value, as shown in Table 2, indicates whether the blood is in acidosis, the normal range, or alkalosis. The PCO2 and total HCO3– values aid in determining whether the condition is metabolic or respiratory, and whether the patient has been able to compensate for the problem. Table 3 lists the conditions and laboratory results that can be used to classify these conditions. Metabolic acid-base imbalances typically result from kidney disease, and the respiratory system usually responds to compensate.

Metabolic acidosis is problematic, as lower-than-normal amounts of bicarbonate are present in the blood. The PCO2 would be normal at first, but if compensation has occurred, it would decrease as the body reestablishes the proper ratio of bicarbonate and carbonic acid/CO2.

Respiratory acidosis is problematic, as excess CO2 is present in the blood. Bicarbonate levels would be normal at first, but if compensation has occurred, they would increase in an attempt to reestablish the proper ratio of bicarbonate and carbonic acid/CO2.

Alkalosis is characterized by a higher-than-normal pH. Metabolic alkalosis is problematic, as elevated pH and excess bicarbonate are present. The PCO2 would again be normal at first, but if compensation has occurred, it would increase as the body attempts to reestablish the proper ratios of bicarbonate and carbonic acid/CO2.

Respiratory alkalosis is problematic, as CO2 deficiency is present in the bloodstream. The bicarbonate concentration would be normal at first. When renal compensation occurs, however, the bicarbonate concentration in blood decreases as the kidneys attempt to reestablish the proper ratios of bicarbonate and carbonic acid/CO2 by eliminating more bicarbonate to bring the pH into the physiological range.

OpenStax. (2022). Anatomy and Physiology 2e. Rice University. Retrieved June 15, 2023. ISBN-13: 978-1-711494-06-7 (Hardcover) ISBN-13: 978-1-711494-05-0 (Paperback) ISBN-13: 978-1-951693-42-8 (Digital). License: Attribution 4.0 International (CC BY 4.0). Access for free at openstax.org.

Symptoms of acidosis affect several organ systems. Both acidosis and alkalosis can be diagnosed using a blood test.

CauseMetabolite
DiarrheaBicarbonate
UremiaPhosphoric, sulfuric, and lactic acids
Diabetic ketoacidosisIncreased ketone bodies
Strenuous exerciseLactic acid
MethanolFormic acid*
Paraldehydeβ-Hydroxybutyric acid*
IsopropanolPropionic acid*
Ethylene glycolGlycolic acid, and some oxalic and formic acids*
Salicylate/aspirinSulfasalicylic acid (SSA)*

*Acid metabolites from ingested chemical.

pHPCO2Total HCO3–
Metabolic acidosis↓N, then ↓↓
Respiratory acidosis↓↑N, then ↑
Metabolic alkalosis↑N, then↑↑
Respiratory alkalosis↑↓N, then ↓

Reference values (arterial): pH: 7.35–7.45; pCO2: male: 35–48 mm Hg, female: 32–45 mm Hg; total venous bicarbonate: 22–29 mM.
N denotes normal; ↑ denotes a rising or increased value; and ↓ denotes a falling or decreased value.

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Dưới đây là video và các luyện tập (practice) của bài này. Nghe là một kĩ năng khó, đặc biệt là khi chúng ta chưa quen nội dung và chưa có nhạy cảm ngôn ngữ. Nhưng cứ đi thật chậm và đừng bỏ cuộc.
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Script:
  1. The maintenance of normal arterial blood pH within the narrow range of 7.35 to 7.45 is crucial for human health, with deviations towards acidosis or alkalosis posing serious risks.
  2. Acidosis, characterized by a blood pH below 7.35, can lead to symptoms such as headache, confusion, lethargy, and fatigue.
  3. On the other hand, alkalosis, with a pH above 7.45, can cause cognitive impairment, muscle twitching, and nausea.
  4. Both conditions can stem from metabolic or respiratory disorders, with various underlying causes including organic acids, ketone bodies, or respiratory issues like pneumonia or hyperventilation.
  5. In metabolic acidosis, a primary deficiency of bicarbonate leads to blood acidity, often triggered by factors like excess ketone bodies in diabetes or certain medications.
  6. Conversely, metabolic alkalosis results from excessive bicarbonate levels, commonly due to conditions like prolonged vomiting or Cushing’s disease.
  7. Respiratory acidosis arises from carbonic acid excess caused by impaired respiration, while respiratory alkalosis stems from carbonic acid deficiency, often due to hyperventilation.
  8. Compensatory mechanisms involving buffers, respiration, and renal adjustments attempt to maintain pH balance, but disruptions can lead to severe consequences.
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