The concept of acid-base state or equilibrium (CBS) is as follows: this is a fairly constant value of the ratio of acid to the base of the blood plasma of a living organism. Names analogous to it are reaction, equilibrium, equilibrium of acids and alkalis. This indicator is one of the components of homeostasis. The quantitative determination of this equilibrium is calculated by the content of protons, that is, the concentration of hydrogen ions. Otherwise, it is called a pH value.
Acid base condition (CBS) is an important characteristic of blood. It fluctuates throughout life, but not at critical rates. The constancy of this value is necessary for the full value of metabolic processes in the body, to ensure the normal maintenance of enzyme activity, as well as the intensity of metabolism and its orientation.
A bit of physics
Any liquid can be characterized as acidic or alkaline. It depends and is determined by the content of protons in it (the name of free hydrogen ions). The same goes for blood. Today, the acidity of any liquid medium is determined by such a concept as a hydrogen indicator - pH (power hydrogen - “hydrogen power”). The scale and determination of pH (from 0 to 14) in 1908 was introduced by the Danish biochemist and physicist Seren Peter Lauritz Servisen.
The neutral reaction of the liquid — its pH — is 7 units. At lower values, they indicate an increase in acid; higher values turn the liquid into alkaline.
The concept of the acid-base state of the body and its constancy is supported by 2 components - BR (buffer solutions or systems) and physiological compensation due to organs - kidneys, lungs, and liver.
Mechanism
Pathophysiology of the acid-base state - any tissue of a living working organism is always sensitive to pH shifts in any direction. If it is exceeded and the reaction is alkaline, cell destruction begins immediately, the proteins collapse (denature), the enzymes are inactivated, and the body may die.
Blood electrolytes - acids, alkalis and salts, which under the influence of water break up into cations and anions. The constancy or regulation of the acid-base state occurs due to, as has been said, buffer systems. Their main purpose is to counteract sharp fluctuations in the proton content.
These solutions tend to keep the level of hydrogen ions constant even when acids or alkalis are added to them or when they are diluted. The composition of the buffer is a mixture of some weak acid with its base, but with a strong anion, that is, it is an acid-base pair. For example, carbonate acid can be called such a system: H 2 CO 3 and NaHC0 3 .
In the blood, constantly acting and there are several basic buffer systems:
- Bicarbonate (a mixture of H 2 CO 3 and HCO 3 ) - occupies 53% of the buffer capacity of the blood and is the most powerful.
- Hemoglobin system - consists of oxygenated hemoglobin (weak acid) and non-oxygenated (or deoxyhemoglobin). This is a weak base - NN in -KN in O 2 ) - 35%. Oxyhemoglobin gives protons 80 times more to the environment.
- The protein buffer system is, first of all, blood albumin, therefore it is the main one for the internal environment of cells. This buffer occupies only 5-7% of the blood capacity. It works thanks to the amphoteric properties of the protein. In an acidic environment, albumin becomes a cation, in an alkaline medium it acts as an acid. This property is called the ability to ionize.
- The phosphate system (diphosphate monophosphate - NaH 2 PO 4 and NaHPO 4 ) - accounts for 2-5% of blood plasma.
The value of each buffer system
The bicarbonate buffer system (the most manageable among others) is especially important: with an excess of protons, a reaction with bicarbonate ions (HCO 3 -) occurs and H 2 CO 3 - carbonic acid is formed. It is nothing but a solution of carbon dioxide in water. Further, its amount decreases due to the decomposition of this acid with the formation of carbon dioxide, excreted by the lungs. The activity of this buffer is inextricably linked to lung ventilation.
Hemoglobin buffer is dependent on lung function, is associated with oxygenation, that is, oxygen saturation. Oxygen potentiates this buffer, i.e., is determined by the activity of the respiratory system.
The protein system is responsible for the neutralization of metabolic products.
The concentration of phosphate buffer is concentrated mainly in such a place in the kidneys as the tubules and intracellular space. The acid-base reaction of urine - dihydrogen phosphate (H 2 PO 4 ) depends only on it. But NaHCO 3 in the tubules of the kidneys is absorbed back.
Physiological Compensation Processes
The importance of the kidneys in the regulation of CBS is expressed in the fact that they bind and remove hydrogen ions and return sodium and bicarbonate ions to the blood. Therefore, the regulation of the acid-base state by the kidneys depends on the water-salt metabolism. Metabolic renal compensation works slowly - compensation occurs within 9-12 hours.
What happens in the renal tubules: the secretion of hydrogen ions occurs in them. Here they combine with bicarbonate ions (NaHC0 3 and KHCO3). Carbonic acid (H 2 CO 3 ) is formed. It, in turn, easily dissociates into carbon dioxide and water, with an excess of which they are also excreted by the lungs and kidneys. At the same time, the released sodium and potassium cations fill the tubules equivalently again, they again participate in the further formation of bicarbonates.
As a result of all these transformations, the alkalinity of the blood remains. The only disadvantage is the slow action of the kidneys. The constant of the acid-base state is also determined by the active work of the liver. It oxidizes most organic acids, and removes inorganic acids along with bile.
Lactate is converted in the liver to glycogen (animal starch). Pancreatic, intestinal (alkaline reaction) and gastric juices are also involved in metabolic compensation.
The acid-base state of human blood normally manifests itself as a slightly alkaline liquid. The pH of arterial blood is 7.35-7.47, and venous blood is 0.02 lower. Acid becomes a donor of hydrogen ions, their base binds and is called an acceptor.
The lungs in maintaining the constancy of CBS play a major role, because 95% of the acid valencies in the form of carbon dioxide are released through them.
15 thousand mmol of carbon dioxide is secreted by the lungs per day, and the kidneys, for example, can secrete only 40-60 mmol. That is, human breathing - this is the work of the lungs in maintaining homeostasis.
Inadequate ventilation increases the partial pressure of carbon dioxide in the alveolar air and creates alveolar hypercapnia. Accordingly, the amount of CO 2 in arterial blood increases and arterial hypercapnia also arises here. If PaCO 2 is increased too much or hypercapnia is prolonged, the respiratory center is inhibited, decreasing its sensitivity to CO 2 .
With hyperventilation of the lungs, the picture is opposite and it is characterized by hypocapnia - alveolar and arterial. Oscillations of carbon dioxide cause respiratory shifts of acid-base balance.
The pulmonary compensation mechanism is extremely fast (correction of pH changes to the alkaline side with acidosis occurs literally in 1-3 minutes) and is very sensitive. Buffer systems operate much faster - they need only 30 seconds to compensate.
Types of Violations
They develop in many pathological conditions, and regulatory mechanisms in such cases may not work. Depending on the pH shift, acidosis and alkalosis may develop. The causes of displacement are respiratory (respiratory) and metabolic (metabolic) shifts. Accordingly, respiratory or metabolic alkalosis or acidosis develops. The systems of regulation of the acid-base state of the blood tend to eliminate the changes that have arisen, moreover, with respiratory disorders, metabolic compensation mechanisms are involved, and with metabolic disorders, respiratory.
Diagnosis of CBS
For analysis of the acid-base state of the blood, it can be taken from a vein or finger - any. The fact is that blood from a finger can be considered arterialized, since its indicators are close to those from arteries, which are considered the most suitable and pure for research.
Capillary blood is collected in glass flasks with a volume of 50 μl or special tubes with anticoagulants.
More permanent is considered arterial. The volume of blood taken - 0.1-0.2 ml - literally a few drops. The determination of blood pH is carried out electrometrically using glass pH electrodes. Blood acidity can be determined in other ways: by the color of the conjunctiva (V. Karavaev’s system), at home.
The color of the conjunctiva is determined by the pulling of the lower eyelid. Pale pink conjunctiva - acidosis, dark pink - alkalosis, bright - normal.
Urine is not used for determination; it certainly will not show the pH of the body.
There are several methods at home: using a litmus test, an electronic device, the color of the conjunctiva, and the lower pressure and pulse.
These methods are operational and can be used in urgent cases, although not entirely accurate compared to laboratory data.
To study blood gases and determine PaCO 2 in the blood, the Astrup method with the same electrode or the Severinghaus electrode is used. The obtained values are calculated using the nomogram.
The effect of acid-base blood conditions
There are accurate scientific facts and evidence on this score. A study of the acid-base state of the blood confirms that changes in pH greater than 0.4 are incompatible with life. The number of hydrogen ions in the plasma is normally 40 nmol / l, the range is from 36 to 45. This value corresponds to pH 7.4. Full compensation can be said with pH fluctuations in the range of 7.35-7.45.
Further, there is already a violation of the acid-base state and its interpretation can be twofold:
- Acidosis is subcompensated (pH 7.25-7.35), decompensated (pH <7.25).
- With respect to alkalosis, it is subcompensated (pH 7.45-7.55), decompensated (pH> 7.55).
Fluctuations in pH seem insignificant, but such an impression is formed due to the scale of logarithms. In fact, the difference, even just a pH unit, means a 10-fold increase in the concentration of protons.
Metabolic disorders
Buffer Base (Buffer Base, BB) - the sum of all anions in the blood. What anions can be contained in plasma - sodium, phosphorus, chlorine, potassium and iron. They are associated with a decrease or increase in non-volatile acids in the blood. And BE is the difference between EXPLOSIVES and proper maintenance (concentration) of buffer bases. Their number does not depend on the CO 2 voltage.
Normally, the explosive content is expressed as 48.0 ± 2.0 mmol / L. The reference BE content is 2.5 mmol / L. In practice, the main indicator is precisely BE.
In a state of acidosis, bases in deficiency and BE are reduced. Thus, the BE value is the most informative indicator of metabolic disorders of the acid-base state with a + or - sign. Base deficiency is acidosis, excess beyond the normal range is metabolic alkalosis.
So, the types of acid-base disorders can occur in alkalosis, acidosis - respiratory or metabolic.
Metabolic (metabolic) acidosis occurs with the accumulation of under-oxidized decay products, i.e., non-volatile acids. Such a disorder develops with a deficiency of oxygen supply, blood vessels in the blood vessels, carbohydrate metabolism disorders with accumulation of ketone bodies in the blood for diabetes, acute renal and hepatic insufficiency, severe diarrhea, heart failure, any kind of shock, poisoning with wood alcohol, antifreeze, salicylates and other
To compensate, the body connects respiratory alkalosis, which develops with hyperventilation of the lungs against the background of Kussmaul respiration. This pathological respiration is acidic, associated with hyperventilation of the lungs.
Metabolic (metabolic) alkalosis can cause severe electrolyte disturbances. Compared with acidosis, it is less common. Its causes may include the introduction of NaHCO 3 during diffusion of solutions in excess, the use of alkalizing products (vegetable, dairy), indomitable vomiting with loss of chlorides, intake of diuretics that cause loss of potassium and excretion of the same chlorides, excessive production of aldosterone by the adrenal cortex as a result of hypovolemia . Hypercorticism itself also applies to this when transfusing a sufficiently large volume of blood stored with sodium citrate, i.e., containing nitrogen oxides. Respiratory disorders of CBS (acid-base condition) can occur with inadequate ventilation of the lungs and CO 2 fluctuations in the blood.
Respiratory (respiratory) alkalosis occurs with hyperventilation - voluntary and involuntary. In healthy people, this condition can occur with a large climb to the mountains, with marathon running, emotional excitement. In patients with cardiac and pulmonary pathologies, when there is shortness of breath. With severe hypocapnia (PaCO 2 below 20 or 25 mm Hg) and, as a result, respiratory alkalosis, seizures can develop in the absence of measures and death. Respiratory alkalosis is especially unfavorable during hypoxia, i.e., a decrease in oxygen supply - during flight accidents, for example. Hyperventilation occurs with head injuries, brain tumors, intoxication with sepsis, an overdose of salicylates, and liver failure.
Respiratory acidosis
Its essence is the accumulation of CO 2 in the blood as a result of respiratory failure. This is hypercapnia and hypoventilation of the lungs. It can develop as a result of a person being in conditions with a high content of CO 2 .
Hypoventilation is always associated with respiratory failure resulting from inhibition of the respiratory center. The causes of the pathology are: infections, sleeping pills, head injuries, myasthenia gravis, chronic pulmonary pathologies.
The compensatory mechanisms that the body connects, trying to adjust the pH to neutral values, will never act in excess - this is controlled. And it means, for example, that with respiratory disorders, pH compensation will tend to normal, but will never exceed 7.4. It should be noted that full compensation is rarely achievable.
Tips
The CBS shifts that caused the inclusion of compensatory mechanisms are always primary, and compensation is secondary. It should be borne in mind that primary violations of indicators in determining the acid-base state are always expressed to a greater extent than compensatory ones, and it is they that determine the pH shift in one direction or another.
A correct interpretation of the shifts of the primary and compensatory secondary is necessary and mandatory because it determines the further adequate correction of these disorders, i.e., therapy for first aid and treatment in the future.
To eliminate errors in the diagnosis of the acid-base state of the blood, PaO 2 should always be considered along with other components of the disorder and a combination with the clinical manifestations of the pathology.
For clues: any primary disturbance (metabolic or respiratory), regardless of etiology, is parallel to the pH deviation. And the compensatory effect is opposite to it.
The acid-base state of blood plasma in the assessment of urgent states of an organism in intensive care is an extremely important value and indicator. Thanks to him, you can predict the state of the body in an extreme situation.