Fluid Compartment Stuff (DSA and Lect) Flashcards
What do derangements of fluid and electorlyte balance result in?
Changes in blood volume and plasma osmotic pressure
Where do starling mechanisms primarily affect tubular reabsorption?
The proximal tubule, where large amounts of H20 and Na+ are reabsorbed.
What determines the fluid flow across capillary walls?
Balance of forces between the hydrostatic pressure gradient and the oncotic pressure gradient
What impact does increased glomerular filtration have on Na+ excretion?
Increases it.
What receptors sense changes in volume and osmolarity of blood?
Volume and osmolality receptors… duh
What effector mechanisms respond to triggering of volume and osmolality receptors?
Neural (Sympathetic discharge), hormonal (ADH and aldosterone) and behavioral (thirst and salt craving)
Extracellular fluid (ECF) and various electrolyte concentrations are homeostatically regulated by negative feedback mechanisms. What is the effector?
The kidney
Where are osmoreceptors located?
•located in the anterior hypothalamus
What do osmoreceptors respond to? What do they do?
–Increase discharge rate in response to a 1% rise in CSF osmolarity and send signals to the “thirst” center
•Results in sensation of thirst and release of ADH
What does not stimulate a response from osmoreceptors?
Increases in glucose or urea
Where do we find volume receptors?
The right atrium
What do volume receptors do in response to increased blood volume? Where do they send signals?
–Increase discharge rate in response to increased blood volume and send signals via the vagus to the medulla
Afferent signals from the volume receptors inhibit what? What else do they signal to?
– the pressor area of the vasomotor center, thereby suppressing sympathetic discharge
–These afferents also reach the hypothalamus to inhibit thirst and ADH secretion
During volume decrease, what do volume receptors do? What must happen before they kick in?
–During volume decrease, this pathway may stimulate thirst and ADH secretion, but volume must drop >10% (less sensitive than osmoreceptors)
What secretes and releases Antidiuretic hormone (ADH/AVP)?
What stimulates ADH/AVP?
–Secreted by the hypothalamus and released by the posterior pituitary
–Stimulated by input from osmoreceptors and volume receptors
What does ADH/AVP do?
–Promotes water reabsorption from DCT and collecting ducts
What stimulates secretion of aldosterone?
–Secretion is stimulated by circulating Ang II (as a result of sympathetic activation), rise in plasma K, fall in plasma Na
What does aldosterone do?
–Promotes reabsorption of Na from DCT and secretion of K
What does volume of distribution (Vd) mean? What is it useful for?
–Apparent volume of body fluid in which the total dose of the drug is distributed at the same concentration as in the plasma. Useful in calculating loading doses.
Where would we expect to find a drug with a Vd > 45 L
•drug widely distributed & bound in body tissues
Where would we expect to find a drug with a Vd < 3?
Only in plasma
A drug with a Vd = 14 L would be found where?
•drug in plasma + interstitial fluid (ECF)
A drug with a Vd = 40-45 L would likely be found in?
The total amount of the bodies water (TBW)
Characterize acidic drugs based on their volume of distribution.
Many acidic drugs (aspirin) are highly protein-bound and have a small apparent Vd
Characterize basic drugs based on their volume of distribution.
–many basic drugs (amphetamine) are extensively taken up by tissues and have an apparent Vd greater than the total body water volume
By what 5 methods do we relieve ourselves of fluid?
Kidneys
Lungs
Feces
Sweat
Skin
Urea contributes to… but not to…
•Urea contributes to osmolality but not to osmotic pressure.
Na+ contributes to… but not at…
•Na+ contributes to osmolality and to osmotic pressure at the cell membrane but not at the capillary wall.
Where are protein levels high?
ICF and intravascular compartments
Protein levels are low in…
Interstitial fluid
What percentage of total plasma protein is albumin?
60%
Plasma proteins (albumin) contribute to oncotic pressure but do not contribute to osmolality. Why?
•since the normal concentration of albumin represents <1 mOsm/kg (and membranes are impermeable to proteins).
Define oncotic pressure
–a type of osmotic pressure generated by large molecules (proteins) in solution, and to which membranes are impermeable.
How can we measure total body water, ECF and plasma volume?
can be measured directly by dilution of injected radioactive substances or dyes
–ICF can then be calculated via TBW-ECF
–Interstitial volume can then be calculated via ECF-plasma volume
•If plasma volume and hematocrit are known, the total blood volume can be calculated by what equation?
TBV = (Plasma Vol)/(1-hematocrit)
Normally what is the balance of ions between ECF and ICF?
–osmolarity in ECF = osmolarity in ICF
How do you estimate plasma osmolality?
•2 x plasma [Na+]
–Or… 2 x [Na+] + glucose/18 + urea/2.8
Why are Na+ and K+ concentrations slightly higher than expected in the vascular space?
the Donnan effect: negative charge of proteins within the vasculature attracts positively charged Na & K ions
What is extracellular osmolarity established by?
Na+ and Cl-
What impact does hypoxia have on ICF Na+?
Since Na-K ATPase pumps shut down in hypoxia the result is increased ICF Na+ and an influx of H20 into the cell.
What is intracellular osmolarity established by?
K+
What is the fluid distribution between EFC and ICF dependent upon?
The osmotic effects of Na+ and Cl-
What is the cell membrane permeable to?
H2O
What is the capillary cell membrane permeable to? What determines the fluid distribution across the capillary barrier?
Water and small ions
Balance between hydrostatic presure and colloid osmotic pressure (starling forces)
What goes down at the arterial end of capillaries in skeletal muscle?
Filtration - Fluid exits capillary since capillary hydrostatic pressure is greater than blood colloidal osmotic pressure.
What goes down at the mid capillary point of the capillary in skeletal muscle?
No net movement of fluid since capillary hydrostatic pressure equals blood colloidal osmotic pressure.
What happens to fluid balance at the venous end of capillaries in skeletal muscle?
Reabsorption - fluid re-enters capillary since capillary hydrostatic pressure is less than blood colloidal osmotic pressure.
What two factors maintain body fluid balance?
–ECF volume & ECF osmolarity
What controls ICF volume?
ECF osmolarity
–Water enters or leaves ECF rapidly to balance osmolarity between ECF & ICF
Define osmotic equilibration
–Movement of water across cell membranes from higher to lower concentration as a result of an osmotic pressure difference (difference in number of solute particles in solution) across the membrane
All water and solutes must first pass through?
ECF
Which compartment should we evaluate changes in first?
ECF
–Fluid distribution between plasma and interstitial fluid (within ECF) is maintained by balance of hydrostatic and osmotic forces across?
capillaries
–Fluid distribution between ECF and ICF is determined by osmotic effect of small solutes across?
cell membrane (highly water permeable, but impermeable to ions)
What do crystalloid fluids contain? Where are they targeted to?
•contain varying concentrations of electrolytes and can stay in ECF or be widely distributed, depending on composition (e.g., normal saline, lactated Ringer’s Solution)
What do colloid fluids contain? Where do they localize?
•contain large proteins and molecules which tend to stay within the vascular space (e.g., dextran, albumin)
Define Isosmotic
•solutions which have the same osmolarity as the ECF
–When added to the ECF, does not change osmolarity, but increases volume
Define hyperosmotic
•solutions having an osmolarity greater than that of the ECF
–When added to the ECF, osmolarity increases and causes water to move from inside the cells to the ECF compartment with a resulting increase in ECF volume and decrease in ICF volume
Hypoosmotic, define
•solutions having an osmolarity less than that of the ECF
–When added to the ECF, decreases osmolarity and water moves into the cells with a resulting increase in both ECF and ICF volume
Define tonicity
•Changes in cell volume due to osmotic equilibrium, with water movement across cell membranes
What does tonicity relate to?
What does it depend on?
- Tonicity of a solution relates to the effect of that extracellular solution on cell volume
- Depends on the concentration of impermeant solutes in the extracellular vs. intracellular fluid
•Distribution of fluid between the ECF and ICF compartments is determined primarily by?
ion distribution
•Distribution in the ECF between the plasma and interstitial compartments is determined primarily by?
balance of hydrostatic vs. oncotic pressures
What are three things edema is caused by?
- Alteration in capillary hemodynamics (altered Starling forces with increased net filtration pressure) - fluid moves from vascular space into the interstitium
- Renal retention of dietary Na+ & H2O, expansion of ECF volume
- Lymph blockage
At what point does edema become apparent?
–Edema does not become apparent until interstitial volume is increased by 2.5-3.0 L. Normal plasma volume is only 3.0 L.
edema fluid is not derived solely from plasma, where else is fluid coming from?
Why is this both good and bad?
–Compensatory renal retention of Na+ & H2O to maintain plasma volume in response to underfilling of the vasculature must occur in this situation to cause edema.
•This renal compensation is appropriate to restore tissue perfusion although it exacerbates edema
(ex: CHF)
In edema what does renal retention of Na+ and H20 result in? (3 items)
§Results in overfilling of the vascular tree
§Inappropriate renal fluid retention
§Usually results in elevated blood pressure, expanded plasma and interstitial volumes
What is non-pitting edema due to?
swollen cells due to increased ICF volume
What is pitting edema due to?
increased interstitial fluid volume
Edema is often treated with?
Diuretics
What are 3 causes of intracellular edema?
–Hyponatremia
–Lack of adequate nutrition to the cells
•Ionic pumps stop working, intracellular Na increases bringing water with it, causing the cells to swell
–Inflammation
•Causes increased cell membrane permeability, causing ions (and therefore water)
What are 2 causes of extracellular edema?
–Abnormal leakage of fluid from the plasma to interstitial spaces across the capillaries
–Failure of the lymphatics to return fluid from the interstitium back into the blood (i.e. lymphedema)
What is the most prompt and effective mechanism in place for correcting increased osmolarity?
Thirst
Where is the thirst center located?
Hypothalamus
What stimulates thirst? What inhibits it?
–Stimulated by osmoreceptors and Ang II, inhibited by impulses from volume receptors
What is salt craving stimulated by? Where is it sensed?
–Evoked by a drop in plasma Na concentration, likely sensed in the amygdala
Sympathetic discharge occurs when?
–Hypovolemia leads to decreased discharge of atrial receptors, resulting in retained Na and H2O
What are the sensors of plasma osmolality?
hypothalamic osmoreceptors
what effectors do hypothalamic osmoreceptors impact?
What is affected?
ADH
Urine osmolality
Thirst
Water intake
What sensors detect effective tissue perfusion (Volume regulators)?
Macula densa
Afferent arteriole
atria
carotid sinus
What effectors are influenced by the sensors that regulate volume?
What do they affect?
RAAS
ANP
ANP-related peptides
Norepi
Antidiuretic hormones
Urinary sodium and thirst
What is the total percentage of body weight that is water?
The percent of body water that is in ICF? In ECF?
60-40-20 rule
60% of weight is water
40% of that is ICF
20% is ECF
What are the two major compartments?
ECF and ICF
What two minor compartments is the ECF divided into?
Plasma and interstitial fluid
There is another body fluid compartment… What is it?
The transcellular compartment. Relatively small, includes cerebrospinal, plueral peritoneal and digestive fluids.
What is hematocrit?
The percentage of blood volume occupied by RBC’s
What type of disturbance is…
Diarrhea and burns?
What impact on…
- ECF volume
- ICF volume
- Osmolarity
- Hematocrit
- Plasma protien
Isoosmotic volume contraction
- ECF volume decreases
- ICF volume does not change
- Osmolarity no change
- hematocrit increases
- plasma protein increases
What type of disturbance is…
sweating/fever/diabetes insipidus?
What impact on…
- ECF volume
- ICF volume
- Osmolarity
- Hematocrit
- Plasma protien
Hyperosmotic volume contraction
- ECF volume drops
- ICF volume drops
- osmolarity increases
- hematocrit = n/c
- plasma protein rises
What type of disturbance is…
adrenal insufficiency?
What impact on…
ECF volume
ICF volume
Osmolarity (ECF)
Hematocrit
Plasma protien
Hypoosmotic volume contraction
- ECF volume drops
- ICF volume increases
- Osmolarity drops
- hematocrit increases
- plasma protein rises
What type of disturbance is…
infusion of isotonic NaCl?
What impact on…
ECF volume
ICF volume
Osmolarity
Hematocrit
Plasma protien
Isoosmotic volume expansion
- ECF volume increases
- ICF does not change
- Osmolarity of ECF does not change
- hematocrit drops
- plasma protein drops
What type of disturbance is…
high NaCl intake?
What impact on…
ECF volume
ICF volume
Osmolarity
Hematocrit
Plasma protien
Hyperosmotic volume expansion
- ECF volume increases
- ICF volume drops
- ECF osmolarity increases
- hematocrit drops
- plasma protein drops
What type of disturbance is…
SIADH (syndrome of inappropriate antidiuretic hormone)?
What impact on…
ECF volume
ICF volume
Osmolarity
Hematocrit
Plasma protien
Hyposmotic volume expansion
- ECF volume increases
- ICF volume increases
- Osmolarity of ECF drops
- Hematocrit does not change
- Plasma protein drops
