Body Fluid Compartments, Renal Physiology, and Micturition

Vocabulary practice flashcards covering body fluid compartments, indicator dilution principles, dehydration/overhydration states, renal clearance, nephron histology and transport, urine concentration mechanisms, acid-base regulation, and the micturition reflex.

Total Body Water (TBW)

The total volume of fluid in the body, accounting for about $60\%$ of total body weight in a normal adult (about $42\,\text{L}$ in a $70\,\text{kg}$ man), which varies with age, sex, and degree of obesity.

Lean Body Mass (LBM)

The fat-free mass of the body (total body weight minus total fat), containing approximately $70\%$ water, related to total body water by the formula $\text{LBM} = \frac{\text{TBW}}{0.7}$.

Insensible Water Loss

Continuous, unnoticed water loss from the body occurring via evaporation through the skin ($400\,\text{ml/day}$) and expired air from the lung alveoli ($300 - 400\,\text{ml/day}$).

Intracellular Fluid (ICF)

The fluid contained within the cell membranes, making up about $40\%$ of body weight ($\approx 28\,\text{L}$ or $\frac{2}{3}$ of TBW) with a $\text{pH}$ of $7.0$, where the major cations are $\text{K}^+$ and $\text{Mg}^{2+}$ and major anions are proteins and organic phosphates.

Extracellular Fluid (ECF)

The fluid outside the cells, making up about $20\%$ of body weight ($\approx 14\,\text{L}$ or $\frac{1}{3}$ of TBW) with a $\text{pH}$ of $7.4$, where the major cation is $\text{Na}^+$ and major anions are $\text{Cl}^-$ and $\text{HCO}_3^-$.

Interstitial Fluid (ISF)

The tissue fluid found in spaces surrounding all cells except blood cells, representing about $\frac{3}{4}$ of ECF volume ($\approx 10.5\,\text{L}$), possessing a composition identical to plasma except for having little protein.

Plasma

The liquid portion of blood measuring about $3\,\text{L}$ (about $\frac{1}{4}$ of ECF), containing albumin and globulins as major plasma proteins, in dynamic equilibrium with interstitial fluid across capillary walls.

Transcellular Fluid

Specialized extracellular fluid collections secreted by epithelial cells (including cerebrospinal, intraocular, pericardial, synovial, and cochlear fluids) totaling about $1 - 2\,\text{L}$ with specialized electrolyte compositions.

Indicator Dilution Technique

A method to measure fluid compartment volumes using the principle $V = \frac{A}{C}$ (or $V = \frac{\text{Amount injected} - \text{Amount excreted}}{C}$), where a known non-toxic marker substance ($A$) is injected and its equilibrated concentration ($C$) is measured.

Iso-osmotic Dehydration

A fluid deficit caused by proportional loss of isotonic fluid (e.g., diarrhea, hemorrhage, vomiting), resulting in decreased ECF volume, no change in ECF or ICF osmolality, and increased plasma protein concentration and hematocrit.

Hyperosmotic Dehydration

A fluid deficit caused by loss of hypotonic fluid (e.g., excessive sweating, fever, decreased water intake), resulting in increased ECF osmolality, water shifting from ICF to ECF, and contraction of both ECF and ICF volumes.

Hypo-osmotic Dehydration

A condition caused by excessive renal $\text{NaCl}$ loss (e.g., Addison's disease/adrenocortical insufficiency), leading to decreased ECF osmolality, fluid shift into cells, decreased ECF volume, and increased ICF volume.

Iso-osmotic Overhydration

Expansion of fluid volume caused by the addition of isotonic fluid (such as an infusion of $0.9\%\,\text{NaCl}$), leading to increased ECF volume and arterial pressure without altering ECF or ICF osmolality.

Hyperosmotic Overhydration

Fluid alteration resulting from excessive intake of hypertonic fluid or salt (e.g., drinking sea water), which raises ECF osmolality and causes water to shift from the ICF into the ECF, reducing ICF volume while expanding ECF volume.

Hyposmotic Overhydration

Condition caused by ingestion of massive water volumes or inappropriate ADH secretion (SIADH), leading to decreased ECF osmolality and a water shift into cells that increases both ECF and ICF volumes.

Edema

An abnormal expansion of the interstitial fluid (ISF) volume caused by an imbalance in Starling forces across capillary membranes or defective lymphatic drainage.

Pitting Edema

A form of extracellular edema where finger pressure displaces mobile interstitial fluid to adjacent areas, leaving an indentation that takes about $30\,\text{seconds}$ to disappear after pressure release.

Non-Pitting Edema

A form of edema where digital pressure leaves no lasting depression because the accumulated fluid within the tissues has coagulated or clotted.

Cortical Nephrons

Nephrons comprising roughly $85\%$ of all nephrons whose renal corpuscles lie within the renal cortex and possess short loops of Henle that penetrate only minimally into the medulla.

Juxtamedullary Nephrons

Nephrons comprising roughly $15\%$ of all nephrons located near the corticomedullary junction, characterized by long loops of Henle descending deep into the medulla that drive the countercurrent concentrating mechanism.

Mesangial Cells

Contractile stellate cells situated between the basal lamina and capillary endothelium of the glomerulus that alter capillary surface area and filtration in response to vasoactive substances.

Renal Clearance

The volume of plasma in milliliters completely cleared of a given substance by the kidneys per minute, calculated as $C_x = \frac{U_x \times V}{P_x}$.

Glomerular Filtration Rate (GFR)

The volume of fluid filtered from the glomerular capillaries into Bowman's space per minute, averaging $125\,\text{ml/min}$ in healthy men and $120\,\text{ml/min}$ in women.

Inulin

An exogenous fructose polysaccharide that is freely filtered but neither reabsorbed nor secreted by the renal tubules, serving as the gold standard indicator for measuring GFR ($125\,\text{ml/min}$).

Para-aminohippuric Acid (PAH)

An organic acid cleared almost completely from renal plasma in a single pass via filtration and tubular secretion, used to measure effective renal plasma flow ($\text{ERPF} = 630\,\text{ml/min}$).

Tubuloglomerular Feedback (TGF)

An intrinsic renal autoregulation mechanism in which the macula densa senses alterations in tubular fluid $\text{NaCl}$ delivery and adjusts afferent arteriolar resistance and renin secretion to stabilize GFR.

Juxtaglomerular Apparatus (JGA)

A regulatory complex composed of macula densa cells of the distal tubule, lacis cells, and renin-secreting juxtaglomerular cells of the afferent arteriole.

Transport Maximum ($T_m$)

The upper limit on the rate (in $\text{mg/min}$) at which a carrier-mediated solute can be actively reabsorbed or secreted by the renal tubules when carrier proteins become saturated.

Renal Threshold for Glucose

The plasma concentration of glucose ($\approx 200\,\text{mg/dl}$) at which the filtered glucose load first exceeds tubular reabsorptive capacity and begins appearing in the urine.

Splay

The curved deviation observed between the actual renal threshold for glucose ($200\,\text{mg/dl}$) and the theoretical saturation threshold ($300\,\text{mg/dl}$), reflecting nephron-to-nephron variation in transport maximum.

Countercurrent Multiplier

The active mechanism centered in the loops of Henle of juxtamedullary nephrons and collecting ducts that deposits solutes in excess of water into the renal medullary interstitium, building a gradient up to $1200 - 1400\,\text{mOsm/L}$.

Countercurrent Exchanger

The passive hairpin vascular arrangement of the vasa recta that traps solutes within the medullary interstitium while delivering nutrients and carrying away reabsorbed water without dissipating the medullary osmotic gradient.

Water Diuresis

Increased urine output featuring hypo-osmolar urine and high free-water excretion resulting from suppression or lack of ADH secretion (e.g., following water consumption, alcohol intake, or in diabetes insipidus).

Osmotic Diuresis

Increased urine volume triggered by non-reabsorbed or poorly reabsorbed filtered solutes (e.g., mannitol, excess glucose in diabetes mellitus) exerting osmotic force in the tubular lumen.

Pressure Diuresis

An increase in urinary fluid excretion caused by elevated arterial pressure washing out the medullary hyperosmotic gradient due to less effective autoregulation in the medullary circulation.

Isohydric Principle

The chemical principle dictating that multiple buffer systems coexisting in a single solution remain in mutual equilibrium with the identical free hydrogen ion concentration ($[\text{H}^+]$).

Titratable Acid

The fraction of excreted urinary hydrogen ions buffered by filtered non-bicarbonate anions (mainly disodium hydrogen phosphate), quantified by the amount of strong base ($\text{NaOH}$) needed to titrate urine $\text{pH}$ back to blood $\text{pH}$ ($7.4$).

Micturition Reflex

An automatic sacral parasympathetic spinal cord reflex ($\text{S}_2 - \text{S}_4$) triggered by stretch receptors at bladder volumes around $300 - 400\,\text{ml}$ that stimulates detrusor muscle contraction and internal urethral sphincter relaxation.

Cystometrogram

A diagnostic graph recording the relationship between intravesical volume and intravesical pressure during incremental fluid filling of the urinary bladder.