Acid-Base Balance

Flashcards covering vocabulary, equations, and physiologic processes related to acid-base balance, including blood buffer systems, renal and pulmonary regulation, and clinical disturbances.

Acid-base balance

Physiologic mechanisms that keep $[H^+]$ of body fluids in a range that supports life, specifically a narrow pH range of $7.35$ to $7.45$.

Volatile acid

An acid in equilibrium with dissolved gas; the only volatile acid of physiologic importance in the body is carbonic acid ($H_2CO_3$).

Fixed (nonvolatile) acids

Acids produced through catabolism (breakdown of proteins) or anaerobic metabolism, such as sulfuric, phosphoric, and lactic acids, which are not in equilibrium with a gas.

Carbonic anhydrase

An intracellular enzyme located mainly in the erythrocyte that catalyzes the $CO_2$ hydration reaction ($CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow HCO_3^- + H^+$).

Isohydric buffering

The process where most $H^+$ produced by the hydration reaction causes no change in pH because hemoglobin ($Hb$) in the erythrocyte immediately buffers the $H^+$.

Equilibrium constant ($K_A$)

Also known as the ionization or dissociation constant, it is a measure of the extent to which acid molecules dissociate in a solution.

Strong acid

An acid molecule that dissociates or ionizes almost completely in an aqueous solution, such as hydrochloric acid ($HCl$).

Weak acid

An acid molecule that ionizes only to a small extent, resulting in a higher concentration of undissociated molecules than products at equilibrium, such as carbonic acid ($H_2CO_3$).

Buffer solution

An aqueous mixture of a weak acid and its conjugate base that resists changes in pH when an acid or a base is added to it.

Conjugate base

The remaining anion (negatively charged ion) portion of an acid molecule that is formed when the weak acid dissociates.

Open buffer system

A system like the bicarbonate system where one component (the product $H_2CO_3$) is broken down into $H_2O$ and $CO_2$ and eliminated by ventilation, allowing buffering activity to continue without reaching equilibrium.

Closed buffer system

A system like the nonbicarbonate system where all components of the acid-base reactions remain in the system, eventually reaching equilibrium and slowing or stopping further buffering activity.

Henderson-Hasselbalch (H-H) equation

A formula used to calculate the blood plasma pH based on the bicarbonate buffer system: $pH = 6.1 + \text{log} \frac{[HCO_3^-]}{PaCO_2 \times 0.03}$.

Excretion

The elimination of substances from the body in the urine by the kidneys.

Secretion

The process by which renal tubule cells actively transport substances into the fluid inside the tubule lumen (the filtrate).

Reabsorption

The active or passive transport of filtrate substances from the tubule lumen back into the tubule cell and the blood of nearby capillaries.

Acidemia

A state where the arterial blood pH is less than $7.35$.

Alkalemia

A state where the arterial blood pH is greater than $7.45$.

Respiratory acidosis

A primary respiratory disturbance produced by hypoventilation, resulting in increased $PaCO_2$ ($> 45$ mm Hg) and decreased arterial pH ($< 7.35$).

Respiratory alkalosis

A primary respiratory disturbance produced by hyperventilation, resulting in decreased $PaCO_2$ ($< 35$ mm Hg) and increased arterial pH ($> 7.45$).

Metabolic acidosis

A nonrespiratory process that decreases plasma $[HCO_3^-]$ ($< 22$ mEq/L) and arterial pH ($< 7.35$), caused by fixed acid buildup or excessive loss of $HCO_3^-$.

Metabolic alkalosis

A nonrespiratory process characterized by increased plasma $[HCO_3^-]$ ($> 26$ mEq/L) or loss of $H^+$ and a high arterial pH ($> 7.45$).

Base excess (BE)

The amount of titratable acid or base per unit volume that needs to be added to a blood sample at a $pCO_2$ of $40$ mmHg and $37^\text{o}C$ to achieve a neutral pH of $7.40$; normal range is $\pm 2$ mEq/L.

Anion gap

The difference between measured cations and measured anions, calculated as $[Na^+] - ([Cl^-] + [HCO3^-])$; the normal range is $8$ to $16$ mEq/L.

Paresthesia

A numbness or tingling sensation in the extremities, which is an early sign of respiratory alkalosis.

Standard bicarbonate

The plasma concentration of $HCO_3^-$ obtained from a blood sample equilibrated at body temperature with a $PCO_2$ of $40$ mm Hg to isolate the metabolic component.

Hypochloremia

A low blood chloride concentration that causes the kidneys to increase $H^+$ and $K^+$ secretion to reabsorb $Na^+$, often leading to metabolic alkalosis.