Relationship : These changes in arterial pCO 2 cause changes in pH as defined in the Henderson-Hasselbalch equation :. The key point is that these 2 equations can be used to calculate the effect on pH of a given change in ventilation provided of course the other variables in the equations eg body's CO 2 production are known.
The next question to consider is how all this is put together and controlled, that is, how does it work? The respiratory system is the body's second defense against acid-base imbalance. Carbon dioxide combines with water molecules to form carbonic acid.
The body alters the rate and depth of respiration to make needed adjustments in the acid-base regulation. The respiratory system responds within minutes of imbalance and reaches maximum effectiveness in hours. The body's ability to use respiratory compensation is dependent on normal function of all components of the respiratory system. Chemoreceptors and respiratory neurons in the brainstem must be functioning optimally.
It is important to recognize that this mechanism responds more slowly with aging. Respiratory regulation also depends on the motor nerve innovation of the respiratory muscles, including the diaphragm. In addition, the chest wall, airways, lungs, and pulmonary circulation must be functioning normally if respiratory regulation is to work effectively. The rate and depth of respiration are influenced strongly by chemoreceptors that sense alteration in the partial pressure of arterial carbon dioxide and hydrogen in the blood.
The rate and depth of respiration increases and excess carbonic acid is exhaled in an attempt to correct the pH imbalance. If too little carbonic acid is present in the blood, the rate and depth of the respiration decrease, and hypoventilation occurs. This will result in retention of carbon dioxide until it is once more present in normal amounts in the body.
Cells continually produce metabolic acids during normal metabolism. The function of the kidneys is to excrete all acids from the body, with the exception of carbonic acid, which can only be excreted by the lungs through exhalation. If metabolic acids begin to accumulate in the blood, the kidneys compensate by increasing acid excretion.
If alkalemia occurs, the kidneys slow their excretion mechanism to allow acid to accumulate in the blood to normal levels. Given the complex nature of this process, renal regulation takes 2 to 3 days to respond maximally to an acid-base imbalance; however, it can respond indefinitely for chronic imbalances. The kidneys have several mechanisms to achieve acid excretion. Fluid filtered from the blood enters the glomerular capsule at the beginning of the nephron.
The epithelial cell lining of the renal tubule acts as a filter. This allows the cells to secrete certain substances into the renal tubular fluid for excretion and move other substances into the interstitial fluid for retention in the body.
At the proximal tubules in the kidney, renal tubular epithelial cells excrete metabolic acid by secreting hydrogen ions into the tubule lumen. For each hydrogen ion that is secreted into the renal tubular fluid, 1 bicarbonate ion is moved into the interstitial fluid.
The fluid filtered from the blood at the glomerulus contains many bicarbonate ions. Most or all of the bicarbonate ions are reabsorbed into the blood during secretion of hydrogen ions. Renal tubular cells are able to secrete additional hydrogen ions into the tubular fluid to aid in removal of large amounts of hydrogen ions from metabolic acid.
Once the hydrogen ions are in the renal tubular fluid, they combine with other substances: bicarbonate ions, phosphate urine buffers, and ammonia. When the kidneys need to excrete excess hydrogen ions, renal tubular cells increase their production of ammonia.
Hydrogen ions combine with ammonia produced by renal tubular cells. Ammonia is a gas that moves easily into the renal tubular fluid when it combines with hydrogen ions to become ammonium ions. Ammonium ions are non-lipid soluble and do not easily cross from the renal tubular fluid back into the blood, resulting in excretion.
Although the kidneys are unable to excrete carbonic acid, they can compensate for carbonic acid imbalances by adjusting the excretion of metabolic acids. This assists in keeping the pH of the blood from becoming too abnormal. If a deficit of carbonic acid in the blood is prolonged, the kidneys will decrease the excretion of metabolic acids. As the metabolic acids accumulate in the blood, they will compensate for the lack of carbonic acid and return the pH of the blood toward normal.
If the body is unable to achieve or maintain homeostasis for the amount of time and degree needed by the body, clinical decompensation occurs. The rate of decompensation depends on the degree of reserve and one's baseline organ function.
For the very young, the very old, and those with chronic disease, this decompensation can occur more quickly. On the other hand, normal, healthy adults can use their compensatory mechanisms for a period of time before failure ensues. Acid-base homeostasis, as well as the degree and type of compensation, are measured in the blood with an arterial blood gas test.
The lab values of particular interest to acid-base imbalance are the pH, arterial partial pressure of carbon dioxide PaCO 2 , and bicarbonate HCO 3 -. Figure 2 lists the normal range of arterial blood gas values. The pH is a measure of the hydrogen ion concentration.
Metabolic acidosis results from a relative excess of acid in the body. Clinical conditions that result in an increase in acid in the body include diabetic ketoacidosis, starvation, alcoholism, severe hyperthyroidism, severe infection, burns, circulatory shock with tissue anoxia, and oliguric renal failure.
When the pH of the interstitial and intracellular fluids declines, the protein structure and enzyme activity in the cells become altered, resulting in the signs and symptoms of metabolic acidosis. A decrease in pH of the cerebrospinal and interstitial fluid of the brain result in headache, confusion, drowsiness, or alteration in level of consciousness. Both the pH and bicarbonate level are decreased with metabolic acidosis.
The body attempts to compensate for the metabolic derangement with respiratory regulation. Although hyperventilation does not remove metabolic acid from the body, it does change the ratio of bicarbonate ions to carbonic acid in a favorable direction. Respiratory acidosis is a condition that tends to cause a relative excess of carbonic acid, resulting from impaired gas exchange or impaired respiratory function.
Inadequate neuromuscular function can be the cause of impaired respiratory function and result in respiratory acidosis. Guillain-Barre syndrome, acute chest injury, hypokalemic respiratory failure, severe kyphoscoliosis, or respiratory muscle fatigue can all result in excess carbon dioxide retention and respiratory acidosis.
Neurologic abnormalities, including blurred vision, tremor, vertigo, disorientation, restlessness followed by lethargy, and somnolence, are more likely to occur with respiratory acidosis than with other types of imbalances, as carbonic acid passes more easily through the blood-brain barrier. Severe respiratory acidosis causes peripheral dilation and hypotension. Reflexive tachycardia can result, particularly if a cardiac dysrhythmia is also present. Carbon dioxide is a slightly acidic compound.
The amount of carbon dioxide you exhale is a function of how deeply you inhale or exhale. Your brain constantly monitors this in order to maintain the proper pH balance in your body. The kidneys help the lungs maintain acid-base balance by excreting acids or bases into the blood. A blood pH imbalance can lead to two conditions: acidosis and alkalosis.
Respiratory acidosis is caused by your lungs not being able to remove enough carbon dioxide when you exhale. This can occur when your lungs are affected by a disease or other disorder. Respiratory acidosis can also be caused by taking narcotics or sleep medications.
Brain and nervous system disorders that cause breathing problems may also lead to respiratory acidosis. Metabolic acidosis is a buildup of acid in the body that originates in the kidneys. Specific causes include:. Causes of respiratory alkalosis include hyperventilation due to anxiety , aspirin overdose, high fever , and possibly even pain. Symptoms of respiratory alkalosis are muscle cramping and twitching.
Level of acidic compounds in the body rises through increased intake or production, or decreased elimination. Level of basic alkaline compounds in the body falls through decreased intake or production, or increased elimination. Blood alkalinity increases when the level of acid in the body decreases or when the level of base increases. The blood's acid-base balance is precisely controlled because even a minor deviation from the normal range can severely affect many organs. The body uses different mechanisms to control the blood's acid-base balance.
These mechanisms involve the. One mechanism the body uses to control blood pH involves the release of carbon dioxide from the lungs. Carbon dioxide, which is mildly acidic, is a waste product of the processing metabolism of oxygen and nutrients which all cells need and, as such, is constantly produced by cells. It then passes from the cells into the blood. The blood carries carbon dioxide to the lungs, where it is exhaled. As carbon dioxide accumulates in the blood, the pH of the blood decreases acidity increases.
The brain regulates the amount of carbon dioxide that is exhaled by controlling the speed and depth of breathing ventilation. The amount of carbon dioxide exhaled, and consequently the pH of the blood, increases as breathing becomes faster and deeper. By adjusting the speed and depth of breathing, the brain and lungs are able to regulate the blood pH minute by minute. The kidneys are able to affect blood pH by excreting excess acids or bases.
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