Acid-Base Balance and Buffer Systems in Human Physiology

Acid-Base Balance: Importance and Mechanisms

Introduction to Acid-Base Balance

The regulation of hydrogen ion concentration ([H+]) in body fluids is a critical homeostatic challenge in human physiology. Maintaining acid-base balance is essential for normal cellular function, as even slight deviations in pH can disrupt protein structure and enzyme activity.

• Proteins have complex three-dimensional shapes that are sensitive to changes in pH.

• High-protein diets tend to produce more acids than bases, which can lower blood pH.

• Normal arterial pH is tightly regulated between 7.35 and 7.45.

• Metabolic reactions produce significant amounts of H+; without control mechanisms, body fluids would quickly become acidic and lethal.

Mechanisms used to control acid-base balance include:

• Buffer systems (chemical and physiological)

• Respiratory system

• Urinary system

Chemical Buffer Systems

Overview of Chemical Buffers

Chemical buffers operate by combining a weak acid and its conjugate base, allowing them to resist changes in pH by neutralizing added acids or bases.

• Buffer system: Combination of a weak acid and the anion released by its dissociation (acts as a weak base).

Carbonic Acid-Bicarbonate Buffer System

This is the primary buffer system in extracellular fluid, especially blood plasma. It consists of carbonic acid (H2CO3) as a weak acid and bicarbonate (HCO3-) as a weak base.

• Reaction: $CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-$

• Buffers both organic acids (e.g., lactic acid, pyruvic acid) and fixed acids (e.g., sulfuric acid, phosphoric acid).

• Cannot buffer changes in CO2 concentration directly; not effective for alterations in PCO2.

Phosphate Buffer System

Important in kidney tubules and cytosol, the phosphate buffer system consists of:

• Dihydrogen phosphate (H2PO4-): Weak acid

• Monohydrogen phosphate (HPO42-): Weak base

• Reactions:

  • $HPO_4^{2-} \rightarrow H_2PO_4^-$

  • $OH^- + H_2PO_4^- \rightarrow H_2O + HPO_4^{2-}$

Protein Buffer System

The most abundant buffer system in cytosol and blood plasma. The carboxyl group (-COOH) of amino acids can ionize to its conjugate base (-COO-), allowing proteins to take up free H+ and prevent large pH changes.

• Examples: Hemoglobin and albumin

• In systemic capillary beds, hemoglobin buffers H+ as follows:

  • $Hb-O_2 + H^+ \rightarrow Hb-H + O_2$

Buffer System Action: Visual Summary

The provided diagram illustrates how buffer systems maintain pH by neutralizing added acids or bases. For example, bicarbonate ions buffer acidic solutions, while carbonic acid buffers basic solutions, keeping the pH within a narrow range.

Physiological Buffer Systems

Respiratory System

The respiratory system regulates acid-base balance by controlling the amount of CO2 in the body. CO2 reacts with water to form carbonic acid, so changes in respiration directly affect H+ concentration.

• CO2 levels in body fluids are kept relatively constant by elimination through respiration.

• During increased cellular respiration (e.g., exercise), respiratory rate increases to eliminate excess CO2.

• As CO2 increases, carbonic anhydrase in red blood cells generates H2CO3, which dissociates into H+ (buffered by hemoglobin) and HCO3- (which enters plasma to buffer fixed acids).

Urinary System

The kidneys contribute to acid-base balance by excreting fixed acids and regulating bicarbonate concentration in the blood.

• Kidneys can excrete H+ and reabsorb or secrete HCO3- as needed.

• This process is slower than respiratory compensation but is essential for long-term pH regulation.

Acid-Base Imbalances

Acidosis

Acidosis is a condition in which blood pH falls below 7.35. It depresses synaptic transmission, affecting nervous system function. Severe acidosis (pH < 7) can cause disorientation, loss of consciousness, and death.

Alkalosis

Alkalosis occurs when blood pH rises above 7.45. It leads to over-excitability of the nervous system, causing involuntary spasms, convulsions, and potentially death.

Types of Acid-Base Disturbances

Disturbance	Main Cause	Arterial Blood Gas Findings	Compensation
Metabolic Acidosis	Kidney failure, excess acid production, prolonged diarrhea (loss of HCO3-)	Decreased pH, decreased HCO3-, decreased PCO2 (if compensated)	Respiratory compensation (hyperventilation)
Respiratory Acidosis	Decreased alveolar ventilation, lung disease (emphysema, pneumonia)	Decreased pH, increased PCO2, increased HCO3- (if compensated)	Renal compensation (increased HCO3- reabsorption)
Metabolic Alkalosis	Excess retention of HCO3-, loss of H+ (vomiting, excess aldosterone)	Increased pH, increased HCO3-, increased PCO2 (if compensated)	Respiratory compensation (hypoventilation)
Respiratory Alkalosis	Increased pulmonary ventilation (hyperventilation, high altitude)	Increased pH, decreased PCO2, decreased HCO3- (if compensated)	Renal compensation (decreased HCO3- reabsorption)

Summary: Fluid Balance

Fluid balance is closely linked to acid-base balance, as water and electrolytes are involved in buffering and excretion of acids and bases. Proper hydration and electrolyte management are essential for maintaining homeostasis.

Additional info: Some content was inferred and expanded for clarity and completeness, including definitions, mechanisms, and the summary table of acid-base disturbances.