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Questions and Answers
Acids yield hydroxyl ions (OH-) when dissolved in water.
False
The normal pH range for extracellular body fluid is from 7.34 to 7.44.
True
Acidosis occurs when the pH is greater than 7.45.
False
Bicarbonate acts as a weak base in the bicarbonate-carbonic acid buffering system.
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Alkalemia occurs when there is an excess of acid in the blood.
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Plasma protein has a net positive charge and is incapable of binding H+.
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Respiratory acidosis is caused by increased alveolar ventilation.
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Compensated acid-base disorders imply that the pH has returned to the normal range.
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Metabolic acidosis results from an increase in bicarbonate levels.
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The kidneys respond to acid-base imbalances more quickly than the lungs.
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Hypoventilation can lead to metabolic alkalosis by increasing CO2 retention.
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Mixed respiratory and nonrespiratory disorders can lead to serious medical conditions due to failed compensation.
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An increase in HCO3 in the blood will generally lower the pH.
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The Henderson-Hasselbalch equation only accounts for lung function in regulating pH.
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Bicarbonate reclamation primarily occurs in the proximal tubules of the kidneys.
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A normal blood pH is maintained at a 10:1 ratio of HCO3 to H2CO3.
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Increased ventilation can be a response to nonrespiratory disturbances affecting blood pH.
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The kidneys produce a net excess of 50–100 mmol/L of acid each day that doesn't need to be excreted.
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Dihydrogen phosphate (H2PO4-) and ammonium (NH4-) are forms in which the kidney excretes hydrogen ions.
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Acidemia indicates a deficiency of H concentration in the blood.
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The normal anion gap range is from 10 to 20 mmol per L.
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Study Notes
Definitions
- An acid is a substance that can yield a hydrogen ion (H+) or hydronium ion when dissolved in water.
- A base is a substance that can yield hydroxyl ions (OH-).
- A buffer is a system that resists changes in pH, typically a combination of a weak acid or base and its salt. An example in plasma is the bicarbonate-carbonic acid system.
Maintenance of H Ions
- The normal concentration of H+ in extracellular body fluid ranges from 36–44 nmol/L (pH, 7.34–7.44).
- The buffering system, lungs, and kidneys control and excrete H+ to maintain pH homeostasis.
- pH is the negative log of the concentration of H+. An increase in H+ concentration decreases the pH, while a decrease in H+ concentration increases the pH.
- Acidosis is a pH below 7.35.
- Alkalosis is a pH above 7.45.
Buffer Systems
- The body's first line of defense against extreme changes in H+ concentration.
- Present in all body fluids. All buffers consist of a weak acid (like carbonic acid H2CO3) and its salt or conjugate base (like bicarbonate HCO3).
- When an acid is added to the bicarbonate-carbonic acid system, HCO3- combines with the H+ from the acid to form H2CO3.
- When a base is added, H2CO3 combines with the OH- group to form H2O and HCO3.
The Interrelationship of Hemoglobin and H+
- Hemoglobin in red blood cells interacts with the H+ from the bicarbonate buffering system.
- The phosphate buffer system plays a role in plasma and red blood cells, and is involved in the exchange of sodium ions in urine H+ filtrate.
- Plasma proteins, especially the imidazole groups of histidine, also form an important buffer system in plasma. Most circulating proteins have a net negative charge and are capable of binding H+.
Regulation of Acid-Base Balance: Lungs and Kidneys
- The interrelationship of the lungs and kidneys in maintaining pH is depicted by the Henderson-Hasselbalch equation.
- The numerator (HCO3-) denotes kidney function, while the denominator (pCO2, which represents H2CO3) denotes lung function.
- The Henderson-Hasselbalch equation expresses acid-base relationships mathematically.
- pK' is the pH at which there is an equal concentration of protonated and unprotonated species.
- In plasma and at body temperature (37°C), the pK of the bicarbonate buffering system is 6.1.
The Lungs
- The lungs (respiratory component) participate rapidly in the regulation of blood pH through hypoventilation or hyperventilation.
- Changes in the H+ concentration of blood resulting from nonrespiratory disturbances cause the respiratory center to respond by altering the rate of ventilation in an effort to restore blood pH to normal.
The Kidneys
- The kidneys (non-respiratory or metabolic component) control the bicarbonate concentration.
- Without reclamation, the loss of HCO3 in the urine would result in an excessive acid gain in the blood.
- The main site for HCO3 reclamation is the proximal tubules.
- In health, with properly functioning kidneys and lungs, a 20:1 ratio of HCO3 to H2CO3 is maintained (resulting in a pH of 7.40).
- Under normal conditions, the body produces a net excess (50–100 mmol/L) of acid per day that must be excreted by the kidney.
- Since the minimum urine pH is approximately 4.5, the kidney excretes little non-buffered H+.
- The remaining urinary H+ combines with dibasic phosphate (HPO4-) and ammonia (NH3) and is excreted as dihydrogen phosphate (H2PO4-) and ammonium (NH4-).
How the Kidneys Reabsorb HCO3
- In the proximal tubule.
Anion Gap
- Anion Gap = (Na+K) - (Cl+HCO3)
- Normal Range: 10 – 20 mmol per L
Acid-Base Disorders
- Acidemia reflects excess acid or H+ concentration.
- Alkalemia reflects excess base or pH greater than the reference range.
- Respiratory acidosis or alkalosis is a disorder caused by ventilatory dysfunction (a change in the pCO2, the respiratory component).
- Nonrespiratory (metabolic) disorders result from a change in the bicarbonate level (a renal or metabolic function).
Compensation
- The body tries to restore acid-base homeostasis by altering the factor not primarily affected.
- Mixed respiratory and non-respiratory disorders occasionally arise from more than one pathologic process and represent the most serious medical conditions since compensation for the primary disorder is failing.
Compensation in Acid-Base Disorders
- The lungs can compensate immediately, but the response is short-term and often incomplete.
- The kidneys are slower to respond (2-4 days), but the response is long-term and potentially complete.
- Fully compensated implies that the pH has returned to the normal range (the 20:1 ratio has been restored).
- Partially compensated implies that the pH is approaching normal.
Metabolic Acidosis
- Decrease in bicarbonate (less than 24 mmol/L), resulting in a decreased pH.
- Compensated by hyperventilation.
Respiratory Acidosis
- Decrease in alveolar ventilation (hypoventilation) causes a decreased elimination of CO2 by the lungs, leading to hypercarbia (elevated pCO2).
- Compensated by the kidneys: increase the excretion of H+ and increase the reclamation of HCO3-.
- Note: Although renal compensation begins immediately, it takes days to weeks for maximal compensation to occur. When HCO3 in the blood increases as a result of the kidneys, the base-to-acid ratio will be altered, and the pH will return toward normal.
Metabolic Alkalosis
- Increase in bicarbonate, resulting in an increased pH.
- Compensated by hypoventilation (increase in CO2 retention).
Respiratory Alkalosis
- Increased rate of alveolar ventilation causes excessive elimination of CO2 from the lungs.
- Compensated by the kidneys: excrete HCO3 in the urine and reclaim H+ to the blood.
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Description
Explore the fundamental concepts of acids, bases, and buffer systems in relation to pH regulation in the human body. This quiz covers definitions, maintenance of H+ ions, and physiological conditions such as acidosis and alkalosis.
