Textbook - BIOLOGICAL CHEMISTRY - Hubsky Y.I. - 2000

Chapter VI. BIOCHEMISTRY OF PHYSIOLOGICAL FUNCTIONS AND SPECIALIZED TISSUES

CHAPTER 28. BIOCHEMISTRY AND PATHOCHEMISTRY OF BLOOD

28.3. ACID-BASE STATUS. BLOOD BUFFER SYSTEMS

Blood plays a major role in maintaining acid-base Homeostasis, which is the specific ratio between hydrogen and hydroxyl ion concentrations in biological fluids. In the blood of a healthy human, a constant hydrogen ion concentration is maintained, corresponding to 10^-7.36 in arterial plasma, or pH = 7.36, meaning that the reaction of normal blood is slightly alkaline.

The Regulation of the acid-base status (ABS) or acid-base balance in the body is carried out through the participation of Blood Buffer Systems and the functioning of the Lungs and Kidneys. Buffer systems are mixtures of weak acids with salts of these acids and strong bases that neutralize bases and acids, respectively, which may accumulate in the body during METABOLISM, thereby counteracting pH deviations from the physiological level.

The pH of any buffer system can be calculated using the equation:

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Blood buffer systems

The body's buffer systems include: the bicarbonate system, the phosphate system, the Blood Plasma protein system, and the Hemoglobin-oxyhemoglobin system. The bicarbonate buffer system and the hemoglobin system are of the greatest importance in maintaining the acid-base balance of the blood and the entire body.

The bicarbonate buffer system consists of free carbonic acid (in the form of carbon dioxide) dissolved in plasma and erythrocytes, and dissolved bicarbonates—sodium (in plasma) and potassium (in erythrocytes).

Acidity in the bicarbonate system can be calculated using the Henderson-Hasselbalch equation:

There is a constant ratio between the concentrations of carbon dioxide and sodium bicarbonate in blood plasma:

that is, the content of bicarbonates exceeds the content of free carbonic acid by 18-20 times, which determines the mild alkalinity of the blood. The blood bicarbonate reserve (i.e., chemically bound carbonic acid) is termed the alkaline reserve of blood, which is an important biochemical component that counteracts shifts in blood pH toward the acidic side when acidic products accumulate in the body, such as lactic acid under conditions of heavy physical exertion.

The decrease in the alkaline reserve of the blood, which occurs due to the neutralization of acids by bicarbonates, is accompanied by The formation of an equivalent amount of H₂CO₃; however, this does not accumulate in the blood, but rapidly decomposes into Water and carbon dioxide, which is eliminated by the lungs. The ratio between the concentrations of carbonic acid and bicarbonates thus remains constant. Therefore, the engagement of an additional physiological mechanism (pulmonary ventilation, which regulates gas exchange between the blood and alveolar air) helps maintain blood pH at a physiological level even under conditions of acid metabolite accumulation and a reduced alkaline reserve.

Hemoglobin buffer system

The ability of hemoglobin and oxyhemoglobin to act as components of respective buffer systems is related to their properties as acids capable of forming unstable potassium salts within erythrocytes.

Potassium hemoglobinate (KHb) in tissue capillaries can interact with carbonic acid (as well as other acidic metabolites) to form free hemoglobinic acid (HHb) and the salt of the corresponding acid. Oxyhemoglobin present in arterial blood is a stronger acid than hemoglobin, and its potassium salt can decompose bicarbonates to yield carbonic acid, which is eliminated by the lungs in the form of CO₂ (see above).

The lungs and kidneys participate in maintaining the ABS by regulating the excretion or retention of excess acidic or basic metabolic products in the body. Specifically, the elimination of carbon dioxide by the lungs during Respiration counteracts the accumulation of acid equivalents in the blood. The kidneys are capable of both excreting acidic products (such as acidic phosphates) or basic products (Na₂HPO₄, ammonium ions) and retaining basic equivalents through bicarbonate reabsorption.

Acid-base balance disorders are pathological states characterized by the excessive accumulation of acidic (acidosis) or basic (alkalosis) compounds in the body.

Depending on the biochemical and pathophysiological mechanisms of development, the following forms of acid-base balance disorders are distinguished:

- metabolic acidosis — characterized by a decrease in HCO₃^- concentration due to the accumulation of acids or loss of bases in the body;

- metabolic alkalosis — characterized by an increase in HCO₃^- concentration due to the loss of acids or, less commonly, the accumulation of bases by the body;

- respiratory acidosis — results from an increased carbonic acid concentration caused by impaired pulmonary CO₂ excretion;

- respiratory alkalosis — is characterized by a decreased blood carbonic acid content due to pulmonary hyperventilation and increased CO₂ elimination.