Transport Principles Flashcards
Name TWO (2) factors that may impact the rate of diffusion.
- Molecular size of solutes (smaller molecules will diffuse more easily than larger molecules).
- Blood flow rate and dialysate temperature and flow rate
- Membrane characteristics e.g. surface area, pore size, shape, thickness, charge and number.
What is the semi-permeable membrane in haemodialysis?
An artificial (synthetic) membrane located with the dialyzer.
How are molecules passed through the semi-permeable membrane from the blood?
The concentration of solutes is higher in the blood than it is in the dialysate via diffusion, whish is largely driven by the concentration gradient.
What is the counter current system?
The counter current system is how the concentration gradient is maintained throughout the entire length of the dialyzer. Blood flows through the dialyser in one direction and the dialysate in the opposite direction.
When does ultrafiltration occur?
When fluid moves across a semi-permeable membrane as a result of a pressure gradient being applied across the membrane. Fluid will move from an area of higher pressure to an area of lower pressure.
During haemodialysis, what is the term for the pressure gradient?
Transmembrane Pressure (TMP)
How does TMP occur?
It is the result of positive pressure being exerted on the blood side of the dialysis membrane and negative pressure on the dialysate side of the membrane, resulting in fluid removal from the patient.
What differs convection from diffusion?
Convection is dependent upon the ease at which middle molecules are able to move within the solution across the semi-permeable membrane.
Explain convection.
The process of solute drag or solute movement of middle molecules within a solvent (plasma water) from the blood compartment.
Hi-Flux dialysers allow the combination of diffusion and convection to occur. Why?
They have larger pores that allow for the filtration of solutes with molecular size of 50, 000 daltons.
Name THREE (3) things hydrostatic pressure is dependent on.
- Blood flow
- Haematocrit
- Plasma Oncotic Pressure
- Site of Re-Infusion Fluid
- Resistance to passage
- Surface area of dialyser
Haemodiafiltration and Haemofiltration are both the main forms of renal replacement therapy that use convection. Why is haemodiafiltration more effective of the two?
It combines both diffusion and convection.
True or False.
Decreasing the blood flow rate increases the rate of diffusion of smaller molecules.
FALSE.
INCREASING the blood flow rate increases the rate of diffusion of smaller molecules.
Why does an increased BFR increase the rate of diffusion of smaller molecules?
A faster blood flow will bring more solutes in contact with the membrane. Similarly, increasing the dialysate flow rate will also increase the rate of diffusion.
Why don’t LARGER molecules pass easily through the membrane?
The removal of larger molecules is largely membrane dependent.
