Abstract: The invention provides an apparatus and method for plasma separation which is designed so that during the process of separating blood into plasma and corpuscular components by means of plasma separation membranes, if the rate of plasma permeation through the membranes is decreased so that the trans-membrane pressure differential exceeds a predetermined value, the number of revolutions of a plasma pump is adjusted downward to permit restoration of a trans-membrane pressure differential lower than the predetermined value. The rate of plasma discharge is gradually decreased according to the varying permeability of the membranes to plasma, whereby a most effective performance of the membranes can be assured. The apparatus and method can be employed in practicing various blood treatment methods wherein blood is separated into plasma and corpuscular fractions to remove the plasma containing pathogenic factors or to remove the pathogenic factors from the plasma.

Abstract: The invention provides a blood treatment apparatus realizing double-step-membrane filtration, namely separation of blood into plasma and a corpuscular fraction and separation of high-molecular-weight substances (e.g. gamma-globulin) in the plasma from low-molecular-weight substances (e.g. albumin). A method of treating blood is also provided.The apparatus and method are effective e.g. in the treatment of blood of patients with peripheral circulatory insufficiency due to arteriosclerosis and of patients with rheumatoid arthritis, which is an autoimmune disease.