Biochemistry: Water, Osmolality, Electrolytes, and Acid-Base Buffers

Vocabulary flashcards covering water compartments, osmolality, tonicity, electrolytes, acid-base balance, and buffer systems based on the biochemistry lecture.

Hydrolysis reaction

A chemical reaction in which chemical bonds are broken by the direct addition of water ($H_2O$).

Hydrophobic effect

The phenomenon whereby nonpolar substances (such as lipids) do not dissolve well in water, helping phospholipids organize into cell membranes to separate the interior of a cell from its surroundings.

Intracellular fluid (ICF)

The fluid compartment located inside cells, accounting for approximately two-thirds ($\frac{2}{3}$) of total body water (about $28\text{ L}$ in a typical $70\text{-kg}$ adult) and predominated by potassium ($K^+$).

Extracellular fluid (ECF)

The fluid compartment located outside cells, accounting for approximately one-third ($\frac{1}{3}$) of total body water (about $14\text{ L}$ in a typical $70\text{-kg}$ adult); includes interstitial fluid, blood plasma, and smaller specialized compartments, with sodium ($Na^+$) as the predominant cation.

Aquaporins

Specialized membrane channels through which water molecules can cross cell membranes between compartments.

Osmolality

The number of osmotically active dissolved particles per kilogram of solvent, measured in milliosmoles per kilogram ($\text{mOsm/kg}$), with normal plasma values ranging from approximately $275\text{ mOsm/kg}$ to $295\text{ mOsm/kg}$.

Osmolarity

A measure of concentration expressing the number of osmotically active particles per litre of solution.

Osmosis

The net movement of water across a selectively permeable membrane toward the compartment with a higher concentration of non-permeable solute particles.

Tonicity

The property describing the effect of a solution on cell volume, determined mainly by the concentration of effective osmoles that cannot readily enter the cell.

Effective osmoles

Dissolved solute particles that cannot readily cross the cell membrane and therefore exert a sustained osmotic force causing changes in cell volume.

Isotonic solution

A solution that causes no net change in cell volume; an example is approximately $0.9\%$ $NaCl$ solution, in which normal red blood cells maintain their shape.

Hypotonic solution

A solution that causes water to enter the cell, leading the cell to swell and potentially rupture (hemolysis in red blood cells).

Hypertonic solution

A solution that causes water to move out of the cell, resulting in cell shrinkage.

Electrolytes

Substances that form ions when dissolved in water, categorized into positively charged cations and negatively charged anions.

$Na^+/K^+\text{-ATPase}$ (Sodium-potassium pump)

An active transporter that consumes one molecule of ATP to export three $Na^+$ ions out of the cell and import two $K^+$ ions into the cell, maintaining essential trans-membrane ion gradients.

Acid

A substance that can donate a proton ($H^+$).

Base

A substance that can accept a proton ($H^+$).

$$pH$$

A logarithmic measure of the acidity or hydrogen-ion concentration of a solution, calculated as $pH = -\log_{10}[H^+]$.

Acidemia

A clinical state characterized by an arterial blood $pH$ below the normal range, specifically $pH < 7.35$.

Alkalemia

A clinical state characterized by an arterial blood $pH$ above the normal range, specifically $pH > 7.45$.

Buffer

A chemical system, usually composed of a weak acid and its conjugate base, that resists changes in $pH$ when small amounts of acid or base are added.

Carbonic anhydrase

An enzyme, especially important in red blood cells, that accelerates the interconversion between carbon dioxide ($CO_2$) and water ($H_2O$) and carbonic acid ($H_2CO_3$).

Henderson-Hasselbalch equation

An equation that relates the $pH$ of a weak acid buffer to its acid dissociation constant and the ratio of base to acid: $pH = pK_a + \log_{10}\left(\frac{[A^-]}{[HA]}\right)$.

$$pK_a$$

A value describing the intrinsic tendency of a weak acid to donate a proton ($H^+$); equals the $pH$ at which conjugate base and weak acid concentrations are equal.

Chemical buffers (First line of defence)

The body's fastest acid-base defence system, reacting within seconds to bind or release $H^+$ to blunt abrupt $pH$ changes without directly removing acid from the body.

Respiratory regulation (Second line of defence)

The mechanism acting within minutes that controls $CO_2$ levels through changes in alveolar ventilation; increased ventilation expels $CO_2$ to raise $pH$, while reduced ventilation retains $CO_2$ to lower $pH$.

Renal regulation (Third line of defence)

The slower, sustained mechanism operating over hours to days that controls acid-base balance by reabsorbing filtered $HCO_3^-$, excreting $H^+$ (such as ammonium and buffered phosphate), and generating new $HCO_3^-$.

Metabolic acidosis

An acid-base disturbance characterized by a primary reduction in blood bicarbonate ($HCO_3^-$) and lowered $pH$, such as from prolonged diarrhea, often partially compensated by respiratory hyperventilation.

Respiratory acidosis

An acid-base disorder caused by inadequate breathing (hypoventilation) that leads to $CO_2$ accumulation and lowered $pH$, compensated over time by renal retention and synthesis of bicarbonate.

Metabolic alkalosis

An acid-base disorder characterized by an increase in blood bicarbonate ($HCO_3^-$) and elevated $pH$, which can result from prolonged loss of gastric acid through vomiting.