| MadSci Network: General Biology |
Dialysis is a process whereby impurities and toxins are removed from the body by a machine. This procedure is done on patients whose kidneys are not working to perform this function properly. In general, the kidneys (each person has two, one left and one right) filter the blood. All day long, the heart pumps blood through the kidneys. Much of the material in the blood, including proteins and cells, are to large to pass through the filter system of the kidney, and remain in the bloodstream. Small molecules do pass through the kidney's filter system. These include water, dissolved salts (called ions), and other metabolites. Much of these materials are reabsorbed by the kidney into the bloodstream, to keep the body working properly (the salt concentration of the blood must be kept just right at all times). Any materials that are not needed are sent out of the kidney as urine. Urine travels from the kidney via the ureters to the bladder, where it is temporarily stored until we go to the bathroom. Here are a few links on how kidneys work, which discuss this process in much greater detail. They are very good, and I recommend that you check them out. http://www.clark.net/pub/nhp/med/kidney/basics.html http://nephron.com/htkw.html Dialysis is a way for a machine to perform the same function as the kidney, when someone's kidneys are failing. If a person is having kidney failure, and does not get dialysis, the body will not be able to remove waste products from the blood, or be able to maintain proper salt concentration of the blood. The person will get very sick, and can eventually die. Two types of dialysis exist. One is called hemodialysis. In this procedure, blood is removed from a vein, passed into a machine which filters out wastes, and then returned to the person via another vein. This procedure usually requires a few hours at a time to fully filter the blood, and is required usually a few times per week. Inside of the dialysis machine, there exists a membrane that is semipermeable. What this means is that only molecules below a certain size are allowed to pass through the membrane. Blood flows on one side of the membrane, and a special fluid flows on the other side of the membrane. Blood cells and large molecules don't pass through the membrane, and so they stay in the blood and are returned to the person. Small waste molecules and salts pass through the membrane into the solution on the other side. The molecules will move from the side which has the higher concentration to the side with the lower concentration. The waste molecules are initially in higher concentraion on the blood side, and so they leave the blood and pass across to the other side. This is how the wastes are removed from the blood. Another form of dialysis is called peritoneal dialysis. The idea behind this procedure is the same as for hemodialysis, but instead of using a machine which contains the semipermeable membrane, doctors make use of the peritoneum, a semipermeable membrane which is present around all of your abdomenal organs. By inserting the special dialysis fluid into the belly but outside of the peritoneal membrane, wastes will flow across this membrane and out into the dialysis fluid. The dialysis fluid is then drained out of the belly, taking with it the wastes. This procedure has the advantage that a person can do it at home, without the need for a machine or a doctor. The major problems with this procedure are that the waste products are not removed as completely as with hemodialysis, and that infections sometimes arise in the belly, which can be very dangerous. Some links about dialysis are as follows: http://nephron.com/dialysishg.html http://adam.excite.com/info/?id=003421 Hope this helps. Good luck with your case study, and for your friend who will be on dialysis. BRIAN EDELSON edelsonb@medicine.wustl.edu
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