Critical Care Medicine-Renal, Electrolyte and Acid Base Disorders>>>>>Renal Replacement Therapy
Question 3#

Which of the following techniques does hemodialysis employ for solute clearance?

A. Diffusion
B. Convection
C. Osmosis
D. Both diffusion and convection

Correct Answer is A


Correct Answer: A

The goals of RRT could be one or a combination of the following—solute and fluid clearance, normalization of electrolytes, and acid-base status. This could be achieved through diffusion or convection, depending on the modality of RRT.

In diffusion, which is employed in hemodialysis, blood and the dialysate fluid flow in a countercurrent fashion on either side of the semipermeable membrane of the hemofilter. The driving force that moves solutes across the semipermeable membrane is the solute concentration gradient. Diffusion is effective in removing small molecules, such as potassium, ammonium, and creatinine (<20 kDa). It is less efficient in removing larger solutes and water.

In hemofiltration, a convective process is utilized wherein solutes and water are transported across the membrane by a pressure differential. Pressure forces water and consequently “drags” solutes with it from the blood compartment to the so-called effluent compartment. The permeability coefficient of the membrane and the difference in pressure between both sides of the membrane determine the amount of fluid and solutes transported across the membrane. With a large amount of fluid removed, crystalloid replacement is given back to the patient to restore circulating volume.


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