Biochemical Foundations of Human Vital Activity - Volkov, N. I., & Nesen, E. N. 2000

Biochemical Foundations of Human Vital Activity
Acid-Base Balance of the Body
Acid-Base Balance of the Body's Internal Environment

The biological fluids of the body contain a specific concentration of hydrogen protons (Н⁺) and hydroxyl ions (ОН-).

Hydrogen protons (Н⁺) are primarily generated during the dissociation (breakdown into ions) of acids:

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Strong acids dissociate into ions almost completely, whereas weak acids dissociate only partially.

Hydroxyl ions (ОН^-) are formed through the dissociation of bases or complex Organic compounds containing ОН^- groups:

NaOH — Na⁺ + OH^-

Water molecules also increase the concentration of Н⁺ and ОН^- in a solution. Water acts as a weak electrolyte and partially dissociates into ions:

Hydrogen protons are readily hydrated by water molecules to form hydronium ions (Н₃О⁺). However, for the sake of simplicity, it is commonly denoted simply as the proton Н⁺.

The concentration of free hydrogen protons in an aqueous environment determines its acidity, while the hydroxyl concentration determines its basicity, or alkalinity. The ratio of the concentration of free hydrogen protons to that of hydroxyls ([Н⁺]/[ОН^-]) determines the active reaction of the medium, i.e., its acid-base state. The constancy of the active reaction within the body's internal environment is referred to as acid-base (alkaline) equilibrium. If the concentration of Н⁺ exceeds that of ОН^-, the aqueous environment is acidic. If there are more hydroxyls than hydrogen protons, the environment is alkaline. When their concentrations are equal, the environment is neutral. To characterize the active reaction of the medium more precisely, the pH value is used.

pH — the hydrogen ion exponent of the acid-base state of the body's aqueous environment

Measurements of water electrical conductivity have established that at room Temperature (22 °C), only one ten-millionth part of 1 mole of water dissociates into ions. This produces 10^-7 g ∙ ion ∙ L^-1 of hydrogen ions (Н⁺) and 10^-7 g ∙ ion ∙ L^-1 of hydroxyl ions (ОН^-). The product of the concentrations of hydrogen and hydroxyl ions represents the ion product of water, which remains constant at a given temperature and equals 1 ∙ 10^-14: К_Н2О = [Н⁺] ∙ [ОН^-]= 10^-7 ∙ 10^-7 = 1 ∙ 10^-14. Consequently, regardless of how the concentrations of Н⁺ and ОН^- change, their product at 22 °C always remains 1 ∙ 10^-14; thus, the concentration of one ion can be determined if the concentration of the other is known.

Fig. 32 pH scale

To characterize the active reaction of a medium, it is customary to use the concentration of Н⁺, designated as the hydrogen exponent, or pH, which represents the negative decimal logarithm of the hydrogen proton concentration:

pH = - lg [Н⁺].

Using pH, the acidity of an environment is expressed in whole numbers. For example, if the concentration of Н⁺ in a solution is 1 ∙ 10^-7 mol, the pH is 7:

pH = - lg [10^-7] = 7.

To determine the acidity or basicity of a medium, The pH scale is used, which illustrates the relationship between the actual concentrations of Н⁺ and ОН^- ranging from 1 ∙ 10⁰ to 1 ∙ 10^-14 mol and the pH value (Fig. 32). As the concentration of Н⁺ decreases, the pH value increases, and the acidity of the aqueous medium decreases. Concurrently, the concentration of ОН^- and the alkalinity of the medium increase. Solutions with a pH of 7 are neutral, those below 7 are acidic, and those above 7 are basic. The pH scale is logarithmic; therefore, A change in pH by one unit results in a 10-fold change in the actual concentration of Н⁺ in the solution.

In the aqueous environments of the body, pH is measured using special indicators or nuclear magnetic Resonance (NMR) spectroscopy.

Determining the Blood and urine pH of an athlete serves as an important diagnostic indicator in sports practice, since physical exertion causes significant shifts in the internal environment's pH, which affects numerous biological processes.

The majority of biochemical processes occur within a strictly defined pH range. Below are the pH values of various biological fluids and certain dietary components.

It has been demonstrated that the pH of the medium affects the following biological processes:

✵ the state of Proteins, particularly Enzymes, and their biological activity: Each enzyme has an optimal pH value at which it exhibits maximum activity; high metabolic activity is typically observed within a specific biological pH range; shifts in pH significantly reduce enzyme activity and The rate of the processes they regulate;

✵ the contractile activity of Actin and Myosin proteins: a drop in pH in skeletal muscle reduces The formation of Actomyosin cross-bridges in myofibrils and decreases Muscle contraction force; a decrease in pH in The Heart muscle can lead to The Development of coronary artery disease;

✵ Ion transport and the excitability of Plasma Membranes: acidification of the environment in nerve and muscle Cells diminishes the conductivity of Na⁺-K⁺ pumps for ions, which affects the excitability of these Tissues; disruption of Na⁺-K⁺ pump function in non-excitable tissues, such as Endocrine glands, is associated with altered hormone secretion into the blood;

✵ the release of Ca²⁺ from the sarcoplasmic reticulum (storage depot) and the speed of skeletal muscle contraction: activation of the muscle contraction process is marked by an increase in intracellular pH, which facilitates the release of Ca²⁺ from the sarcoplasmic reticulum membranes where it is sequestered; during prolonged muscle stimulation, the cellular environment becomes acidic due to the accumulation of lactic acid, which suppresses muscle contraction strength;

✵ the rate of ATP breakdown: as pH rises, the rate of ATP Hydrolysis increases, thereby increasing The amount of Free energy available to perform useful work within cells;

✵ the excitability of pain receptors.

Impairment of Acid-Base Balance

Acid-base balance is an essential prerequisite for the normal functioning of all cells in the Organism. A shift in blood pH from 7.36 down to 6.80 in an untrained organism can be fatal.

Physical exertion accelerates METABOLISM, including those pathways that lead to the accumulation of acidic byproducts. During Glycolysis (the anaerobic oxidation of glucose) in skeletal muscle, lactic acid accumulates. It enters the bloodstream and can alter the body's acid-base equilibrium. With moderate (aerobic) exercise, only minor amounts of lactic acid are produced, meaning no significant changes in blood pH are observed. Intense anaerobic workouts, particularly sprint running and swimming events, lead to a substantial buildup of lactic acid in skeletal Muscles and its release into the blood. Consequently, the pH in skeletal muscle and blood drops to 7.0 or even 6.5. This acidification of the body's internal environment is known as acidosis.

A distinction is made between metabolic acidosis, which occurs due to altered metabolism—such as during muscular activity, fasting, or certain pathological states—and respiratory (gaseous) acidosis, which arises from impaired breathing processes.

The development of exercise-induced metabolic acidosis is more commonly observed in untrained individuals. This acidotic state can be accompanied by muscle pain and a decrease in physical performance. Acidosis is a contributing factor to physical fatigue. Once exercise ceases, muscle and blood pH quickly normalizes (within about 30 minutes). The discomfort and pain associated with shifts in the internal pH environment during physical exertion likewise disappear during this timeframe.

To prevent the onset of acidosis, athletes prior to competition—particularly short-distance runners—sometimes perform pulmonary hyperventilation. Hyperventilation, which involves deep and intensive breathing for several seconds, lowers CO₂ concentrations in the Lungs and blood, driving blood pH up to 7.6, a condition known as alkalosis. Alkalosis of the body's internal environment can also occur due to an excessive Dietary intake of alkaline substances or the loss of acidic components. Furthermore, alkalosis may develop when ascending to altitudes above 3,000 meters or training in high-altitude conditions due to a reduction of acidic products in the blood.

In highly trained athletes, high physical performance is typically maintained despite the development of metabolic acidosis, thanks to the physiological systems' adaptation to Changes in the internal environment, as well as the high efficiency of chemical buffer systems that resist shifts in environmental pH.