Acid-base homeostasis
Acid -base balance is the general name for a variety of physiological control mechanisms according to the principle of homeostasis. Keep the sequence of the required metabolic processes upright at a pH value of 7.4 (± 0.05) in the blood. For the regulation of acid-base balance, the buffering properties of the blood and tissue and the gas exchange in the lungs and excretion mechanisms of the kidney contribute. Disturbances in acid -base balance of the body lead to acidosis ( acidosis ) or alkalosis ( Untersäuerung ) and may affect life-threatening.
Metabolic processes
CO2 as the end product of cellular respiration falls particularly on physical work in large quantities. In the blood, it reacts with water to form carbonic acid, which dissociates immediately to bicarbonate and oxonium ions in the organism:
Buffer systems
The task of part - buffer systems in the blood is the maintenance of a constant pH. They are summarized under the name of blood buffer. Buffering capacity of a system describes the amount of acid or base may be added without changing the pH to a greater extent. The greater the buffer capacity, the more stable the system is sensitive to changes in pH. In general, a system has its greatest buffering capacity in the area of its pK value. For the blood, this means that the pK value of the buffer system should be as close as possible at the desired pH = 7.4. Also important is the concentration of the buffer system.
- The most significant part of the buffer system in the body protein Anion buffer (protein buffer), which due to the high concentration of proteins has its importance. It accounts for about 24 % of the buffering effect of the blood.
Above all histidine residues of proteins play a role, since histidine is the only amino acid whose isoelectric point is at neutral pH. Thus histidine proteins can act as proton donors and proton acceptors, to stabilize the pH. However, the physiologically important bicarbonate buffer system comprising an open buffer system about 75 % of the total buffer capacity of the blood. The fact that CO2 can be exhaled, the body's pH, the depth of breathing, as well as the reabsorption of bicarbonate in the kidney is capable of running on the respiratory rate, influence, whatever its meaning, despite the unfavorable pK value of 6, 1 explained - because the pH is determined only by the ratio of bicarbonate / carbonic acid. (Henderson -Hasselbalch Equation )
The other part of buffer systems are preferred for their lower significance are often grouped together as a non- bicarbonate buffer, NBP. There are closed systems, the total concentration of the buffer substances can not change quickly:
- Dihydrogen phosphate / phosphate buffer
- Ammonia / ammonium buffer
Both buffer systems play especially in the area of the kidney a role to maintain the pH of the urine is constant. There, above all, to excrete acids in metabolic derailments or, depending on food situation to be able to without the urine is too acidic, for example, a acidosis. The pH in the blood could be otherwise removed from the physiological pH ( 7.35-7.45 ).
Disorders
In the above formula, it can be seen that an increase in the concentration of CO2 on the left side to the growth in concentrations of bicarbonate ( HCO3-) and H leads (acidosis ).
Reinforced " exhaling " of CO2 ( Panting after jogging ) decreases in the sequence, the concentrations of bicarbonate ( HCO3-) and H ( alkalosis).
Acidosis without clinical significance occurs about heavy physical work, as the muscles for a direct H from glycolysis releases, on the other hand, because the CO2 production increases greatly. Also without clinical significance is the respiratory Höhenalkalose. Ascends to a mountain, the air pressure decreases: the air is "thinner" (see barometer formula ). In order to still breathe enough oxygen, respiratory rate and depth of breathing must be improved. Here, more CO2 is exhaled and automatically increase according to the above formula, the blood pH.
Depending on whether the cause of acidosis or alkalosis in breathing ( = respiration ) is to be found, it is called
- Respiratory and
- Non-respiratory ( synonym: metabolic ) disorders.
Parameter for the assessment
The following parameters are used in the clinic to classify acidosis or alkalosis on their origin back and figure out how the body compensates for this ( partial).
Bicarbonate
Clinical Significance The HCO3 concentration significantly in determining the " non-respiratory components " in the event of disturbance in the acid - base balance. Changes to this concentration aid the clinician in identifying the origin of acidosis or alkalosis. In the clinic everyday two versions are used.
News bicarbonate About the Henderson -Hasselbalch equation are the pH, partial pressure of CO2 and the bicarbonate concentration in the blood current in context. If pH and pCO2 measured, the actual bicarbonate can be calculated from it.
- Thus, the actual bicarbonate shows the HCO3- concentration, as it actually is present with known pH and pCO2 values.
- Changes in metabolic and respiratory disorders
Standard bicarbonate to determine the HCO3std, originally had the blood samples at 37 ° C, 100% oxygen saturation and a CO2 partial pressure are investigated by 40 mm Hg. However, all modern analyzers are now able to calculate this parameter from the current sample blood. ( Van Slyke and Cullen )
- The HCO3std represents the bicarbonate content of the plasma is, would be present at a pCO2 of 40 mm Hg
- Changes in non-respiratory disorders
- Remains unchanged in respiratory disorders
Base excess and total buffer bases
Base excess ( base excess ):
- Indicates the deviation from the reference value of the total buffer bases. " 1" Thus means a value of the total buffer bases in the amount of 49 mmol / l
- Positive values : metabolic alkalosis ( or metabolically compensated respiratory acidosis )
- Negative values : metabolic acidosis ( or metabolically compensated respiratory alkalosis )
Total buffer bases:
- Sum of standard bicarbonate and all other basic buffers in the blood. Reference value for 100% oxygen saturated blood: 48 mmol / l
- Does not change in respiratory, but remains a non-respiratory disorders.
Anion gap
The anion gap is a calculated parameter that can be used for differential diagnosis of metabolic acidosis. See main article anion gap.