Significance of the acid-base state
Maintaining the hydrogen ion exponent at a stable level is vital. Even minimal and short-term changes in pH (hydrogen exponent) provoke a whole chain of disorders. Without strict regulation, the following changes occur:
- Activity of cellular enzymes.
- Rate and intensity of oxidation-reduction reactions.
- Sensitivity of cellular receptors to biologically active substances (BAS).
- Permeability of cell membranes.
The physiological meaning of the regulatory system is to prevent a pH shift in the intracellular fluid (where the normal range is 6.8–7.2). This is the only way to ensure optimal tissue metabolism.
Sources of hydrogen ions
Every second, metabolic processes in the body generate acids that threaten to shift the balance. Where does the excess of $H^+$ ions come from?
- Carbon dioxide ($CO_2$). The main product of tissue respiration. By interacting with water, it forms carbonic acid ($H_2CO_3$), which then dissociates into $H^+$ and $HCO_3^-$.
- Intense muscular work. Leads to the accumulation of lactic and pyruvic acids.
- Liver metabolism. The breakdown and restructuring of amino acids (e.g., during gluconeogenesis) or the synthesis of triglycerides are always accompanied by the release of $H^+$ and $OH^-$ ions.
- Digestion. Hydrolysis of nutrients and salts of organic acids.
Two lines of pH defense
The functional system for maintaining pH operates on a feedback principle. As soon as the indicator deviates from normal (e.g., drops below 7.35), a cascade of reactions is triggered.
First line of defense: Local self-regulation mechanisms Blood buffer systems are triggered instantly. The most powerful of these are the bicarbonate and hemoglobin systems. For example, hemoglobin acts as a strong base: it binds excess $H^+$ ions formed during the breakdown of carbonic acid.
Second line of defense: Internal link If the buffer capacity is insufficient, neural centers and effector organs are engaged:
- Lungs — alter ventilation, eliminating volatile acid ($CO_2$).
- Kidneys — excrete excess hydrogen ions in the urine.
- Liver and GI tract — metabolically neutralize decay products.
- Sweat glands — excrete some acids through the skin.
- Cerebral cortex — shapes behavior (e.g., active search for specific food or environment).
Reception mechanisms (how the brain detects the problem)
To activate effector organs, the nervous system must detect an ABB shift. Two types of sensors assist with this:
- Peripheral chemoreceptors. Located in the vascular bed — in the carotid bodies (carotid artery bifurcation) and the aortic arch. They directly detect changes in $H^+$ concentration and the partial pressure of $CO_2$ ($pCO_2$) in the blood. Signals travel from them to the medulla oblongata via the sinus nerves and the aortic branch of the vagus nerve.
- Central chemoreceptors. Located within the medulla oblongata itself. Blood is separated from brain tissue by the blood-brain barrier (BBB), which is nearly impermeable to $H^+$ ions. However, carbon dioxide ($CO_2$) easily diffuses across it. When $CO_2$ rises in the blood, it penetrates the cerebrospinal fluid, alters the hydrogen concentration there, and central receptors respond precisely to this shift, transmitting a humoral signal to the respiratory center neurons.
Briefly: answers to topical questions
Which blood buffer systems participate in regulating the acid-base state?
Four main physicochemical blood buffer systems participate in regulating the acid-base state. They represent the first mobile line of defense, which triggers instantly during pH shifts:
- Bicarbonate — accounts for 7–9% of the total blood buffer capacity.
- Hemoglobin — is the primary blood buffer system that absorbs hydrogen ions.
- Phosphate — responds immediately when the concentration of hydrogen ions changes.
- Protein — also participates in rapid blood buffering.
What are the specific mechanisms of renal participation in maintaining the acid-base state?
The kidneys regulate the acid-base state through processes in the renal tubules. Mechanisms include:
- Reabsorption — the uptake of bicarbonate and other metabolites back from the primary urine.
- Secretion — the release of hydrogen ions into the tubular lumen in exchange for reabsorbed sodium ions.
- Acidogenesis — an energy-dependent process; secretion of hydrogen ions into the tubular lumen in exchange for reabsorbed sodium.
- Ammoniogenesis.
- Phosphate secretion.
- K+, Na+ exchange mechanism.
The intensity of reabsorption and secretion processes for acidic/basic products varies depending on shifts in the acid-base balance.
What classifications of acid-base disorders exist based on their origin?
Based on origin, acid-base disorders are classified into several main types depending on the primary factor of the shift. The following groups of disorders are distinguished:
- Metabolic (non-respiratory) — primary change in the concentration of hydrogen or hydroxyl ions.
- Respiratory — associated with a primary change in the partial pressure of carbon dioxide.
- Mixed — simultaneous combination of signs of gas and non-gas acidosis or alkalosis in a single patient.
Distinguishing the two main types of disorders is simple: metabolic (non-respiratory) shifts are a primary "chemistry" problem (excess acids or bases), while respiratory shifts are a "breathing" problem (change in CO₂ gas levels).
Class and exam questions
What is the difference between acidosis and alkalosis?
Acidosis is the acidification of the environment, a drop in pH below normal due to an increase in hydrogen ion concentration. Alkalosis is alkalinization, when the pH rises above normal and the amount of H⁺ drops.
Why is the intracellular fluid pH lower than that in the blood?
Inside cells (where the pH is 6.8–7.2), metabolic processes occur continuously with the formation of organic acids and carbon dioxide, making the environment more acidic than in arterial blood (7.4).
How does carbon dioxide affect acidity?
Carbon dioxide interacts with water in the internal environment, turning into carbonic acid (H₂CO₃). It dissociates into bicarbonate (HCO₃⁻) and a hydrogen ion (H⁺). The more CO₂, the more H⁺ and the lower the pH.
What else to study on this topic
- Principles of blood buffer systems function (especially bicarbonate)
- The role of hemoglobin as a buffer during gas exchange
- Renal mechanisms of hydrogen ion excretion
- Pathophysiology of metabolic and respiratory acidosis/alkalosis
- Interaction between the respiratory center and the cerebral cortex
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