117 Questions
What is the main determinant of extracellular fluid (ECF) volume?
Sodium (Na+)
What accounts for nearly all the osmotically active solute in plasma and interstitial fluid?
Sodium (Na+)
What is the approximate total body content of Na+ in mmol?
4000 mmol
How much of the total body Na+ content is intracellular?
10%
Where is Na+ mainly reabsorbed in the nephron?
Proximal convoluted tubule
Which hormone influences active Na+ absorption in the small intestine and colon?
Aldosterone
What is the primary action of PTH when ionized Ca2+ levels decrease?
Stimulates osteoclast bone resorption to release Ca2+ into the ECF
What is the final step in the manufacture of active vitamin D (calcitriol)?
Renal hydroxylation of 25-hydroxy-calciferol to 1,25-dihydroxycalciferol under the influence of PTH
What is the primary effect of hyperphosphatemia on calcium homeostasis?
Inhibits the renal hydroxylation of vitamin D to 1,25-dihydroxycalciferol
What percentage of circulating calcium is in the biologically active ionized form?
50%
What is one of the mechanisms that may lead to hyperkalemia in organic acidemia?
Activation of H+/K+ exchange
What is the primary cellular action of magnesium (Mg2+) that highlights its diverse clinical applications?
Modulation of ion channel activity
Which cell type in the collecting ducts is responsible for the regulation of potassium (K+) secretion under the influence of aldosterone?
Intercalated cells
How do principal cells influence potassium (K+) secretion in the collecting ducts?
Maintaining low intracellular Na+ concentrations
What is the approximate percentage of total body magnesium found in the extracellular fluid (ECF)?
1%
What is the main reason for hypokalemia associated with diuretics that increase distal tubular Na+ content?
Increased K+ efflux into the tubule
How does acidemia affect the degree of albumin-protein binding of calcium?
Acidemia decreases protein binding and increases the ionized calcium fraction
How do intercalated cells respond in low potassium (K+) settings?
Increased K+ reabsorption at the expense of acid loss
What is the primary role of magnesium (Mg2+) in energy metabolism?
Mg2+ is required for ATP phosphorylation reactions, interacting with the outer two PO43− groups of ATP
Which hormone is essential for maintaining serum calcium concentrations between 4.5 and 5 mEq/L?
Parathyroid hormone (PTH)
What is the role of Ca2+ in coagulation processes?
Linking coagulation factors to platelets
How is cytoplasmic free Ca2+ kept low in cells?
By pumping Ca2+ into the sarcoplasmic reticulum
What is the dominant intracellular cation in the body?
Potassium
What is the primary mechanism involved in excretion of total body excess Na+ mentioned in the text?
Pressure natriuresis
How does low potassium intake combined with chronic action of digitalis-like factor contribute to hypertension?
By inhibiting vascular smooth muscle cell Na+/K+ ATPases
What is the primary role of K+ in excitable tissues according to the text?
Modulating resting membrane potential
Which process involves shifts in K+ between extracellular fluid (ECF) and intracellular fluid (ICF)?
Acute K+ distribution
How does insulin influence intracellular levels of Na+ and K+?
Stimulates Na+/H+ antiporter, leading to intracellular Na+ increase and K+ uptake
How do catecholamines influence K+ handling in muscles?
Stimulate Na+/K+ ATPase activity, leading to increases in intracellular K+
What is the primary function of cell membrane Na+/K+ ATPase according to the text?
To maintain gradients of ions in cells
What is the primary role of Mg2+ in maintaining normal transmembrane electrochemical gradients?
Mg2+ supports the activity of ion-pumping ATPases to stabilize cell membranes and organelles.
How does Mg2+ antagonize the effects of Ca2+ physiologically (physiologic competitive antagonism of Ca2+)?
Mg2+ inhibits L-type Ca2+ channels and modifies membrane potential to prevent Ca2+ influx.
How does Mg2+ antagonize NMDA receptors in the central nervous system?
Mg2+ reduces Ca2+ entry by specific ion channels, thereby inhibiting a diverse array of excitable tissue cellular actions.
What is the primary mechanism of Mg2+ absorption ?
absorbed from the GI tract by a saturable transport system and passive diffusion
What is the main determinant of total body Mg2+ levels?
plasma Mg2+ concentration
How do catecholamines and glucagon affect the intracellular-extracellular balance of magnesium distribution?
Catecholamines, acting by both - and -adrenoreceptors, and glucagon lead to extrusion of magnesium from intracellular stores.
What is the primary function of phosphate in energy metabolism?
ATP synthesis
Which molecule is formed with the help of phosphate?
2,3-diphosphoglycerate (2,3-DPG)
Where is the majority of total body phosphorus stored?
In bone
How is GI uptake of phosphate primarily achieved?
By paracellular diffusion
Which form of inorganic phosphate predominates in the plasma at normal pH?
$HPO_{4}^{2-}$ (divalent)
What happens to postprandial increases in serum phosphate levels according to the text?
They are rapidly dealt with by increased renal excretion
What is the primary regulator of phosphate (PO43-) absorption in the intestine and kidneys?
Calcitriol (1,25-dihydroxycalciferol)
What is the primary role of chloride (Cl-) in the body?
All of the above
How is chloride (Cl-) primarily excreted from the body?
Through renal excretion, primarily in the proximal tubule
Which cells in the distal nephron are responsible for regulated control of chloride (Cl-) excretion?
Intercalated cells
What is the primary action of parathyroid hormone (PTH) on phosphate (PO43-) handling?
Reduces renal PO43- reabsorption
What is the primary mechanism by which gastrointestinal (GI) secretions are formed?
Cellular chloride secretion followed by paracellular sodium and water movement
what is the normal sodium concentration gradient between the intracellular and extracellular compartments
1:15
sodium concentration gradient between the intracellular and extracellular compartments is maintained by: ATPases and is vital for
ATPases
maintaining SODIUM concentration gradient is vital for the function of:
excitable tissues, including action potentials and membrane potential, and for handling of renal solute
daily Na+ intake is
2 to 3 mEq/kg/day at birth
which route is the predominant for Na loss
renal
what is the correct matching for systems involved in the control of circulating volume
atrial volume sensing: ANP release
what is the correct matching for systems involved in the control of circulating volume
atrial volume sensing = ANP release empty = empty hypothalamic osmoreceptor = ADH release juxtaglomerular apparatus = RAA activation
Daily requirements of K?
Term infants require 2 to 3 mEq/kg/day
daily K+ intake is a similar magnitude to ?
entire ECF K+ content
Transmembrane potentials particularly depend on K+ permeability. how Transmembrane potentials achieved ?
achieved when K efflux is equal to K influx
what is the function of skeletal muscle in the presence of hypokalemia
expression of Na+/K+ ATPase is reduced
Other factors that may influence ECF to ICF K+ balance ?
aldosterone
In addition to mechanisms involving aldosterone in a feedback loop, it is likely that feed-forward mechanisms also exist, what is the main function of feed-forward mechanisms.
rapidly modulate renal K+
where its found the calcium major storage
bones
calcium play important role in?
muscular contraction
Increases in cytoplasmic Ca2+ concentration is
key mediator of cell death pathways
the kidney covert 25-hydroxy-calciferol to 1,25-dihydroxycalciferol (calcitriol) under the influence of PTH. wha is the main action of the active vit D(calcitriol) ?
stimulates osteoclastic bone resorption and absorption of Ca2+ from the GI tract.
which other electrolytes can affect Ca2+ homeostasis ?
magnesium
you are in the OR and the surgeon ask if you can take a blood sample to check the calcium level ? what is the best way to take the sample?
take blood sample without tourniquet
what is the primary intracellular anion
magnesium
why the Free ionized Mg2+ levels within the cytoplasm and ECF are low ?
because most of magnesium is sequestered within organelles
Within the plasma, where is the majority of Mg2+ is found
majority is in the biologically active ionized form
magnesium Excretion is via?
GI tract and kidneys
75% of magnesium is freely filtered at the glomerulus, what is the primary tubule for reabsorption.
thick ascending loop of Henle
what is the effect of adrenergic stimulation on magnesium?
decreases in serum Mg2+ concentrations
of total body magnesium, majority found in?
bone
another important function of phosphate in body
buffer system
which type of phosphate normally found in plasma?
inorganic phosphates
which type of phosphate normally found intracellular ?
organic phosphates
typical daily intake of phosphate is
1 g
what is the main mechanism of GI uptake
paracellular diffusion
Plasma inorganic PO43− is freely filtered at the glomerulus, at which tubule occur the majority of reabsorption
proximal tubule
what is the main effect of vitamin D on phosphate absorption
increases GI and renal absorption
what is the main effect of PTH on phosphate absorption
reduces renal reabsorption.
chloride is the second most abundant electrolyte in the extracellular compartment, therefore responsible for?
third of plasma osmolality and two thirds of plasma negative charge.
Most Cl− intake is derived from dietary NaCl, and the GI tract absorbs and secretes large amounts of Cl− in the form of?
gastric hydrochloric acid
how is the GI secretions formed?
paracellular movement of Na+ into the lumen, with water moving down its osmotic gradient
What is the normal range for anion gap in the plasma?
4 to 11 mEq/L
In cases of excess organic acids, what leads to an increase in the anion gap?
Reduction in HCO3− concentration
Which model places changes in plasma HCO3− at the core of plasma acid-base balance?
Anion gap model
Which ions are represented by 'unmeasured' anions in the anion gap model
PO43−, sulfate, and anionic proteins
In the presence of excess Cl− administration, what happens to the anion gap even if HCO3− falls?
It remains normal
According to the Stewart model, which of the following is NOT an independent variable that determines plasma pH?
Bicarbonate (HCO3-)
What is the normal range for the apparent Strong Ion Difference (SID) in plasma?
35-45 mEq/L
How does a reduction in the Strong Ion Difference (SID) affect plasma pH according to the Stewart model?
It leads to a fall in plasma pH.
What is the primary advantage of the Stewart model of acid-base balance compared to the traditional approach?
It is better at explaining acid-base disturbances caused by fluid administration.
intravascular fluid therapy may affect acid-base by?
A + B
What did a meta-analysis of studies comparing saline with balanced perioperative fluid regimes confirm?
Clearance of biochemical abnormalities within the first or second postoperative day in saline groups
What was associated with significant hyperkalemia in a trial involving patients undergoing renal transplants?
Extracellular acidosis due to saline administration
In the emergency department and intensive care settings, which group of patients showed the greatest effect from receiving saline compared to balanced crystalloid?
Medical patients with sepsis
What is the primary cause of hyperchloremic acidosis during high-volume saline infusion?
The high concentration of chloride ions in saline reduces the strong ion difference (SID)
How does lactated Ringer's solution differ from normal saline in terms of its effect on acid-base balance?
The lactate in Lactated Ringer's solution is metabolized, providing an effective SID that counteracts acidosis
Which of the following is a potential deleterious effect of saline-induced hyperchloremic acidosis?
Coagulopathy
According to the Stewart model, what is the primary determinant of plasma pH?
The strong ion difference (SID)
Which of the following solutions has an effective strong ion difference (SID) closest to that of plasma?
Lactated Ringer's solution
What is the primary mechanism by which saline infusion causes a reduction in plasma bicarbonate concentration?
Dilution of bicarbonate ions
IV NaHCO3 to treat metabolic acidosis should be
reserved for the emergency treatment such as severe hyperkalemia and arrhythmias associated with tricyclic antidepressant overdose.
What negative effect does IV HCO3− administration bring due to its significant Na+ content?
Hyperosmolar hypernatremia
What is the major challenge posed by the conversion of HCO3− to CO2 when administered in excess?
Intracellular acidosis
What role does acidosis play during strenuous exercise according to the text?
It helps in O2 offloading to tissues
What is the primary reason for HCO3− administration being disputed in critically ill patients?
It may contribute to hypernatremia
What is one of the potential negative effects of administering IV NaHCO3 to treat metabolic acidosis?
Conversion of HCO3- to CO2, leading to a physiologic challenge for patients with ventilatory impairment
What is one potential reason why the clinical benefit of HCO3- administration may not be apparent in many situations?
Both a and b
What is one of the negative effects of IV HCO3- administration mentioned in the text?
Significant sodium content and osmotic load, leading to potential hyperosmolar hypernatremia and volume overload
What is the main challenge posed by the conversion of administered HCO3- to CO2 according to the text?
Excess CO2 production that must be exhaled, which may present a significant physiologic challenge for patients with ventilatory impairment
when the treatment with bicarbonate should be stoped?
ph above 7.2
In situations in which HCO3− administration is required, the total dose required to correct the base deficit can be calculated using
weight and base deficit
treatment with HCO3− due to metabolic acidosis can lead in certain situations to “overshoot” toward metabolic alkalosis. when dose this can occur ?
renal disease with impaired HCO3− distribution
HCO3− total dose required to correct the base deficit can be calculated using the equation: Dose (mEq) =0.3 × weight(kg) × base deficit(mEq/L).
half of the total calculated dose should be given
Study Notes
Acid-Base Balance
- Stewart's model proposes that plasma pH is dependent on three independent variables:
- pCO2 (plasma CO2 tension)
- Atot (total plasma concentration of nonvolatile buffers)
- SID (strong ion difference)
- SID is the difference between the total charge of plasma-strong cations (Na+, K+, Mg2+, Ca2+) and strong anions (Cl-, lactate, sulfate, and others)
- Normal plasma SID is approximately 42 mEq/L, and reductions in SID lead to a fall in plasma pH
Henderson-Hasselbach Equation
- Represents the HCO3- buffer system
- Plasma HCO3- concentration is an independent determinant of plasma pH
Anion Gap
- Defined as the difference between the most abundant measured cation and anion concentrations in the plasma ([Na+] + [K+]) - ([Cl-] + [HCO3-])
- Normal anion gap is 4 to 11 mEq/L
- In the presence of excess organic acids, the accumulation of unmeasured anions is accompanied by a reduction in HCO3- to buffer the excess H+ ions, leading to an increase in the anion gap
Hyperchloremic Acidosis
- Caused by the administration of fluid with Cl- concentration higher than that of plasma
- Leads to a metabolic acidosis due to the Cl- content
- Can be explained by the Henderson-Hasselbach model (dilution of bicarbonate and a resultant base deficit) or the Stewart model (reduction of apparent SID and therefore reduction of plasma pH)
Bicarbonate Administration
-
Should be reserved for the emergency treatment of select conditions, such as severe hyperkalemia and arrhythmias associated with tricyclic antidepressant overdose
-
Has negative effects, including:
- Carbon dioxide production
- Osmotic load and hyperosmolar hypernatremia
- ECF expansion and volume overload### Sodium Physiology
-
Serum Na+ concentrations are maintained within a tight range (138-142 mEq/L) despite variations in water intake.
-
The excretion of total body excess Na+ relies on inefficient passive mechanisms, particularly the pressure-volume effect.
-
Long-term ingestion of excess salt combined with low potassium ingestion contributes to hypertension.
Potassium Physiology
- K+ is the dominant intracellular cation in the body, with a total body content of approximately 4000 mmol, 98% of which is intracellular.
- The ratio of ICF to ECF K+ balance is vital in maintaining cellular resting membrane potential.
- Daily requirements reflect age and growth, with more K+ required at higher metabolic rates.
- Transmembrane potentials depend on K+ permeability, with K+ egress occurring through ion channels down its concentration gradient.
Acute K+ Distribution
- Acute K+ distribution involves shifts in K+ between the ECF and ICF, performed by ion transport systems under the influence of insulin, catecholamines, and ECF pH.
- The cell membrane Na+/K+ ATPase exports three Na+ for every two K+ imported and is the means by which the gradients of these ions are maintained.
Magnesium Physiology
- Mg2+ helps maintain normal transmembrane electrochemical gradients, effectively stabilizing cell membranes and organelles.
- Mg2+ also antagonizes Ca2+ and N-methyl-d-aspartate (NMDA) receptors within the central nervous system.
- Mg2+ is absorbed from the GI tract by a saturable transport system and passive diffusion, with excretion via the GI tract and kidneys.
Phosphate Physiology
- PO43− is the most abundant intracellular anion and helps form biologic molecules such as ATP, DNA, and RNA, membrane phospholipids, and hydroxyapatite in bone.
- PO43− is required for energy metabolism, cellular signaling, cellular replication, and protein synthesis.
- Normal plasma inorganic phosphates are maintained at 3 to 5 mg/dL, with most intracellular PO43− being organic.
Chloride Physiology
- Chloride (Cl−) has a key role in maintaining plasma osmolality, preserving electrical neutrality, and acid-base status.
- Normal plasma values are 97 to 107 mEq/L, with Cl− responsible for nearly a third of plasma osmolality and two thirds of plasma negative charge.
- Most Cl− intake is derived from dietary NaCl, and the GI tract absorbs and secretes large amounts of Cl−.
Learn about the role of sodium (Na+) as the dominant extracellular cation and its significance in regulating extracellular fluid volume. Explore the concentration gradient between intracellular and extracellular compartments.
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