Dialysis blocks the passage of larger substances (for example, red blood cells, large proteins). This replicates the filtering process that takes place in the kidneys when the blood enters the kidneys and the larger substances are separated from the smaller ones in the glomerulus. The two main types of dialysis, hemodialysis and peritoneal dialysis, remove wastes and excess water from the blood in different ways. Hemodialysis removes wastes and water by circulating blood outside the body through an external filter, called a dialyzer, that contains a semipermeable membrane. The blood flows in one direction and the dialysate flows in the opposite.
Blood is thicker than water der Glîchezære. The 13th-century Heidelberg manuscript reads in part, "ouch hoer ich sagen, das sippe blůt von wazzere niht verdirbet" (lines 265-266). In English it reads, "I also hear it said, kin-blood is not spoiled by water." which may in part refer to distance not changing familial ties or duties, due to the high seas being tamed. In 1412, the English priest John Lydgate observed in "Troy Book", "For naturally blood will be of kind / Drawn-to blood, where he may it find." By 1670, the modern version was included in John Ray's collected "Proverbs", and later appeared in Sir