It has been observed that hemodialysis patients with high Body Mass Index (BMI) exhibit higher survival chances than those patients with low BMI. It has also been observed that weight loss could lead to lower survival rate. A possible reason for this is that muscle and adipose tissues can alter toxin concentrations in the body. Cronin-Fine et al. (2007) established a mathematical model explaining the relationship between BMI and toxin concentrations which supported clinical data on survival rate and BMI of hemodialysis patients. In 2009, Kappel et al. modified the model to further elaborate the relationship. In this study, the kinetic model developed by Kappel was modified to describe the toxin dynamics in hemodialysis patients considering weight loss.