Mechanism of Hemodialysis
During hemodialysis, the blood from the patient is passed through a filter called a hemodialyzer. Hemodialyzers have thousands of small tiny capillaries, through which blood passes at a speed of 150-350 ml/min. These capillaries are made of different chemical materials and are semi-permeable, i.e. they allow small molecules like urea, creatinine, sodium and potassium to pass but do not allow passage of larger molecules like proteins, red blood cells and white blood cells.
Around these capillaries, a fluid called dialysate runs continuously. Dialysate fluid has the same electrolyte concentration as that of a normal body, except urea and creatinine. Moreover it also has bicarbonate or acetate in it. Hemodialysis machines prepare this dialysate fluid by mixing the concentrated dialysate with treated water in a specific ratio.
By the process of diffusion (i.e. movement of molecules across a membrane from an area of high solute concentration to an area of low solute concentration) the waste products like urea and creatinine which are at a higher concentration in the blood, move from the blood to the dialysate. The bicarbonate and acetate present in the dialysate are absorbed in the blood and correct the acidity.
This dialysate around the blood capillaries containing waste products is continuously being replaced with fresh dialysate at a rate of 500-800ml/min to avoid saturation. By applying the negative pressure outside the capillaries, excess water in the blood can be removed and this process is called ultrafiltration.
Mechanism of hemodialysis
So briefly speaking, by means of hemodialysis waste products are removed and bicarbonate or acetate is added to the blood by the process of diffusion and water is removed by the process of ultrafiltration. Urea and creatinine are continuously being formed in the body, so after every few days waste products and excessive water accumulate again and need another session of dialysis.
Normally two to three sessions of hemodialysis are required per week.
Can the functions of native kidneys improve after dialysis?
Dialysis only partially replaces the kidney functions, i.e. with the help of dialysis waste products, excessive fluid and salts accumulated in the body are removed. On its own dialysis does not affect kidney function. Hence if the kidneys are permanently damaged, i.e. patient has been labeled as End Stage Kidney Disease, dialysis will not improve native kidney functions, but will partially replace the functions to keep the patient out of danger.
Patients usually require dialysis at regular intervals (usually twice or thrice a week) for the remainder of his/her life or until successful kidney transplant is done. However if kidneys are not permanently damaged, i.e. the patient is suffering from Acute Renal Failure (ARF) due to any reason and if properly treated there are high chances of recovery of the kidney functions within days to weeks.
During the initial period patients may need dialysis to keep the waste products below a dangerous level. Once the kidney functions start to improve in ARF the need for further dialysis decreases.