Significance of Acid-Base Status
Maintaining pH at a stable level is vital. Even minimal and short-term pH shifts trigger a cascade of dysfunctions. Without strict regulation, the following are altered:
- Activity of cellular enzymes.
- Rate and intensity of oxidation-reduction reactions.
- Sensitivity of cell membrane receptors to biologically active substances (BAS).
- Cell membrane permeability.
The physiological goal of regulatory systems is to prevent pH shifts 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
Metabolic processes continuously generate acids that threaten to disrupt equilibrium. Sources of excess $H^+$ include:
- Carbon dioxide ($CO_2$). The primary product of tissue respiration. Reacting with water, it forms carbonic acid ($H_2CO_3$), which then dissociates into $H^+$ and $HCO_3^-$.
- Intense muscular activity. Leads to the accumulation of lactic and pyruvic acids.
- Hepatic metabolism. Amino acid breakdown and rearrangement (e.g., during gluconeogenesis) or triglyceride synthesis are always accompanied by the release of $H^+$ and $OH^-$ ions.
- Digestion. Hydrolysis of nutrients and organic acid salts.
Two Lines of Defense for pH
The functional pH maintenance system operates via negative feedback. As soon as the parameter 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 respond instantly. The most powerful among them are the bicarbonate and hemoglobin buffers. For example, hemoglobin acts as a strong base, binding excess $H^+$ ions produced during the breakdown of carbonic acid.
Second line of defense: Systemic mechanisms If buffer capacity is exceeded, neural centers and effector organs are recruited:
- Lungs — alter ventilation to eliminate volatile acid ($CO_2$).
- Kidneys — excrete excess hydrogen ions in the urine.
- Liver and GI tract — metabolically neutralize waste products.
- Sweat glands — eliminate a fraction of acids through the skin.
- Cerebral cortex — drives behavioral adaptations (e.g., actively seeking a specific environment or diet).
Mechanisms of Reception (How the Brain Detects the Problem)
To activate effector organs, the nervous system must detect acid-base shifts using two types of sensors:
- Peripheral chemoreceptors. Located within the vascular bed — in the carotid bodies (at the carotid artery bifurcation) and the aortic arch. They directly detect changes in $H^+$ concentration and the partial pressure of carbon dioxide ($pCO_2$) in the blood. Signals travel via the sinus nerves and the aortic branch of the vagus nerve to the medulla oblongata.
- Central chemoreceptors. Located within the medulla oblongata itself. Blood is separated from brain tissue by the blood-brain barrier (BBB), which is virtually impermeable to $H^+$ ions. However, carbon dioxide ($CO_2$) diffuses across it freely. When blood $CO_2$ rises, it enters the cerebrospinal fluid, alters local hydrogen concentration, and central receptors respond to this shift, sending humoral signals to the respiratory center neurons.