038 Body fluid compartments Flashcards
What % of body weight is made up of total body water?
60%
How much of body water is intracellular?
40%
How much of extracellular fluid consists of plasma?
20%
What markers can be used to measure volume of total body water?
3H2O (tritiated water)
What marker can be used to measure extracellular fluid?
Inulin or thiosulphate
Which marker can be used to measure plasma?
Evans blue, or plasma proteins labelled isotopically e.g. 125l albumin
Why is urea solution not isotonic?
Urea is not an effective osmole
What is the issue with using inulin to measure extracellular fluid?
It can enter the intracellular part and can also be lost in urine.
What is the issue of using evans blue to measure intracellular fluid?
It has a short half life so you must measure it quickly.
What is the difference in plasma composition between extracellular and intracellular fluid?
There are proteins and organic phosphates intracellularly. This gives intracellular compartments a slight negative charge which is balanced by high levels of Cl- and HCO3- extracellularly.
What is the difference in body fluid compositions between interstitial and plasma compartments?
Interstitial fluids have low protein, and since half the Ca2+ in plasma is protein bound, the Ca2+ levels are lower in interstitial compartments.
What is the osmolality of plasma?
295 mOsmol/kg
Which ions contribute most to plasma osmolality?
Na+, K+, HCO3-, Cl-
What will occur to body fluid composition during rapid ingestion of water?
Even distribution between compartments in their proportions 60% intracellular
What occurs when there is rapid ingestion of 0.9% saline?
This is isotonic. Most of the water will remain in the extracellular fluid as Na+ won’t go through the membrane. (This helps increasing blood pressure without lysing cells)
What occurs when there is IV infusion of 1L 5% albumin?
This will only be retained in the plasma, so plasma retains all of the water. It does not enter the interstitial fluid.
What is elephantitis
Blockage in the lymphatics
How does hemorrhage change fluid movement?
There is reduced capillary hydrostatic pressure. This means more fluid enters the capillary, so there is an increased blood volume at the expense of interstitial volume.
How does congestive heart failure change fluid movement?
There is increased venous pressure, increasing hydrostatic pressure. It takes longer before oncotic pressure >hydrostatic pressure. This causes more fluid entering the tissues, causing oedema.
How does liver failure change fluid movement?
Liver failure causes reduced plasma protein concentration This causes reduced oncotic pressure that draws fluid inwards, so hydrostatic pressure remains higher for a longer period of time along the capillary. More fluid enters tissues to cause oedema.
What is the most abundant protein in plasma?
Albumin
What is the relationship between oncotic and hydrostatic pressures in pulmonary circulation?
Hydrostatic pressures are lower then oncotic pressures, allowing alveoli to be clear of fluid.
How does inflammation result in local oedema?
Capillary permeability to protein draws water in.
What factors lead to ankle oedema during prolonged standing?
Reduced venous return due to venous hydrostatic pooling causes increased venous pressure and hence increased capillary pressure. Fluid moves out of the capillaries.
Why does left ventricular failure often lead to pulmonary oedema? Explain the implications for respiration
There is increased pressure in the pulmonary capillaries causing fluid to move out rather than in. Pulmonary oedema leads to inefficent gas exchange across alveoli.
In hypotonic solution, what occurs to the red blood cells?
cell lysis occurs as water moves in
In a hypertonic solution, what occurs to the red blood cells?
Cells shrink as water moves out.
Why isnt urea an effective osmole?
Urea is able to move into the cell along with water, causing cell to burst. Hence an iso-osmotic solution of urea is not necessarily isotonic.
Normally, filtration at arteriole end exceeds reabsorption at venule end. Why doesnt oedema occur?
The difference of about 2-4L is returned to circulation via the lymphatic system and doesnt accumulate in the interstitium.
What is transcellular fluid and give 2 examples?
Transcellular fluid is the portion of total body water contained within epithelial lined spaces. e.g. aqueous humor, synovial, cerebrospinal
What is the 60:40:20 ratio?
Of total body weight:
60% is water (42L)
40% is intracellular (28L)
20% is extracellular (14L)
