Urea, ultrafiltration

Normal urine frequently contains a variety of low-molecular weight substances (urea is a notable example) that can inhibit the enzymes of interest The toxicant may also act as an inhibitor. These substances usually can be removed by dialysis, dilution, Sephadex filtration, ultrafiltration or gel filtration... 

The assessment of adequacy of dialysis treatment for individual patients in the clinical setting includes consideration of the patient s well-being, cardiovascular risk, nutritional status, and degree of achievable ultrafiltration. It also includes estimates of a number of laboratory parameters— such as hemoglobin, phosphate, and albumin— and clearance of the small solutes urea and creatinine. During die following discussion, the term adequacy will refer to small solute clearances obtained from both dialysis and residual renal function (RRF), For practical reasons, HD adequacy is calculated using urea as the small solute. 

Sample preparation Blood. 320. L Whole blood + 450 jtL urea (1 1) + 30 jiL 2 p,g/mL sulfamethazine, shake mechanically for 15 min, filter (Amicon Micropartition System MPS-1) while centrifuging at 4° at 4000 rpm for at least 3 h, inject a 300 pL aliquot of the ultrafiltrate. Urine. Dilute urine if necessary. 320 pL Urine + 30 pL 2 pg/mL sulfamethazine, shake mechanically for 15 min, filter (Amicon Micropartition System MPS-1) while centrifuging at 4° at 4000 rpm for at least 3 h, iiyect a 300 pL aliquot of the ultrafiltrate. 

Blood plasma and ultrafiltrate concentrations of urea and creatinine have been studied with subcutaneous ultrafiltrate probes in diabetic dogs. The dogs had varying degrees of kidney complications secondary to the diabetes. 

In hemodialysis, low molecular metabolic waste such as urea, creatinine, and other toxic substances (solutes up to 6000 mol wt) are removed from the blood of uremia patients by diffusive transport, which is driven by a concentration gradient of blood solutes being dialyzed against a physiological solution. A complimentary process is hemofiltration, in which solutes up to 20,000 mol wt are removed via an ultrafiltration membrane, the transport being caused by a convective transmembrane flux generated by mild hydraulic pressure differences across the membrane. 

DDS has been treated either by addition of osmotically active solute (glucose, glycerol, albumin, urea, fructose, NaQ, mannitol) to the dialysate, or by intravenous infusion of mannitol or glycerol. With the technique of pure ultrafiltration the patient is subjected to ultrafiltration without dialysis. The net result is loss of fluid without the patient undergoing dialysis. Ultrafiltration followed by dialysis does not appear to be associated with DDS (Ronco et al., 1998). Additionally, DDS can be prevented by decreasing the time on dialysis and inCTeasing the frequency of dialysis at the initiation of hemodialysis in patients. Mannitol infusion accompanying the initial... 

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