Dialysis - theory Flashcards
What are the aims of dialysis?
Fluid removal
Solute removal
Extend life
Improve quality of life
What are the steps of haemodialysis treatment?
- Blood flows drom the patient to the dialysis machine via a pump
- Anticoagulant stops the blood from clotting as blood goes around the machine
- Blood flows through tubes of dialyser
- Dialyser acts as a membrane through which diffusion occurs
Why does the blood and dialysate fluid flow in opposing directions?
To maintain the diffusion gradient
What does the haemodialysate solution contain?
Sodium, Potassium, Calcium, Magnesium, Chloride and Bicarbonate
How does the dialysis machine remove excess fluid from patients’ blood?
Ultrafiltration of excess fluid
Hydrostatic presssure gradient created by dialysis machine pushes fluid out of the blood
Why does the blood flow through a pump before it enters the dialysis machine?
To help move blood into the machine
What are the two ways a solute is cleared from the patients’ blood?
Diffusion
Convection
Describe the process by which solute moves via diffusion from the blood into the dialysis machine
Solute moves down a concentration gradient
Small molecules from blood passes through semipermeable membrane into dialysate
Larger and middle-weight molecules are not easily removed - can’t cross the membrane
What factors affect diffusion rate?
Concentration gradient
Molecular weight/ size/ charge of a molecule
Blood/ dialysate flow rate
Temperature
Distance to travel
Describe the mechanism by which solute moves via convection from the blood to the dialysate
Solute is dragged with the fluid into the dialysate
Bulk movement of water and any solute which is small enough to pass through the membrane
What factors affect convection?
Transmembrane pressure
Membrane surface area
Membrane permeability
What are the advantages of convection over diffusion?
More effective at moving substances at higher molecular weight than diffusion
Convective clearance is more similar to the native glomerulus
What is a high-flux dialyser?
A more porous non-cellulosic membrane with increased permeability to large molecules
What are the benefits of a high-flux dialyser?
Better at removing large molecules
Why do patients require many sessions of dialysis throughout the week?
Most products of metabolism are found in the intracellular space
Dialysis only removes solutes in the plasma
So the products of metabolism must move from the intracellular space -> vascular space
This takes time
Can only remove a portion of the solute in one dialysis session - re equilibrates and the patient arrives for the next
