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What are the aims of dialysis?

Fluid removal
Solute removal
Extend life
Improve quality of life


What are the steps of haemodialysis treatment?

1. Blood flows drom the patient to the dialysis machine via a pump

2. Anticoagulant stops the blood from clotting as blood goes around the machine

3. Blood flows through tubes of dialyser

4. 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?



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


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


What are other conditions the patient must follow in order to optimize the effects of dialysis?

Dietary restriction of sodium, potassium and phosphate

Restrict fluid intake

Optimize biochemistry by takinf appropriate medication

Attent all dialysis sessions at prescribed time

Ask how much urine the kidneys produce


What are the reasons behinf controlling sodium, potassium and phosphate intake?

Less likely to develop dangerous variations in electrolytes