The filtrate passes along the loop of Henle, through the distal convulated tubule and into the collecting duct. Hormones control the permeability of these regions, parts are made impermeable to water, ions or urea. The aim of the loop of Henle is to maximise the amount of water retained by producing urine with a high salt concentration. The urine then passes to the bladder via the long tubes called Ureters. It is stored in the bladder until it is released by contraction of the bladder wall and relaxation of the sphincter muscles. The passage linking the bladder to the exterior is called the Urethra.
The table bellow shows the composition of fluids in a healthy kidney (concentrations are given in g per 100cm³ of fluid)
In a kidney which is failing to perform its excretory function, these fluid concentrations may change. A disfunctioning kidney can be the result of a variety of conditions, including bacterial infection, external mechanical injury and/or high blood pressure. If the disease is recognised in its early stages it may be curable through a careful choice of diet. However, if it is allowed to develop, the build up of water, urea and sodium ions in the blood becomes excessive. In this case a more drastic treatment must be undertaken to rectify the problem.
Haemodialysis is a method involving the ‘cleaning’ of a patient’s blood. Blood is withdrawn from the body, through an opening in the vein and passed through a dialysis machine. An anticoagulant is used to prevent clotting.
The machine acts as an artificial kidney, removing all waste products from the patient’s blood. Blood is pumped through a tube of partially permeable membrane, which solutes can diffuse though but blood cells or protein molecules can’t. The fluid surrounding the membrane (dialysate) is made up to have the same concentration as normal tissue fluid. This enables the blood to retain needed substances like salts and glucose as the inward and outward diffusion of these are in equilibrium. Urea and other toxic substances, however, are left out of the dialysate causing them to diffuse out of the blood.
Although Haemodialysis is in efficient process, it is time consuming and can cause disruption to a patient’s life. Treatment is needed three times a week and lasts up to 10 hours each time, an obvious inconvenience. As well as this, strict regulation of diet and fluid intake has to be maintained. On the other hand, haemodialysis enables, an otherwise terminally ill patient, to be kept healthy more or less indefinitely.
Another form of treatment, similar to haemodialysis is peritoneal dialysis. In peritoneal dialysis, the inside lining of ones stomach acts as a natural filter. Wastes are taken out by means of the cleansing fluid dialysate, which is washed in and out of the stomach in cycles. Surgical operation inserts a soft plastic tube (catheter) into the stomach. The dialysate enters the body through this catheter. After the filtering process is finished, the fluid leaves your body through the catheter.
There are two kinds of peritoneal dialysis:
Continuous Ambulatory Peritoneal Dialysis (CAPD)
Continuous Cycling Peritoneal Dialysis (CCPD)
The basic treatment is the same for each. However, the number of treatments and the way the treatments are done make each method different.
CAPD is "continuous," machine-free and takes place whilst a patient continues with life as normal. The treatment is done by the placing of two quarts of cleansing fluid into the stomach, which is later drained out. A plastic bag of cleansing fluid is attached to the catheter. Raising the plastic bag to shoulder level causes gravity to pull the fluid into the stomach. This process must be carried out 3-5 times a day so, again, can prove to be time consuming. Each exchange takes approximately half an hour. The fluid removed after the cleansing process, contains all the waste products from the blood, which have diffused out of the blood and into the fluid.
CCPD differs from CAPD only in that a machine (cycler) delivers and then drains the cleansing fluid. The treatment usually is done at night whilst a patient sleeps whereas CAPD takes place during the day.
CAPD and CCPD have several benefits when compared to haemodialysis. With continuous dialysis, you can control extra fluid more easily, and this may reduce stress on the heart and blood vessels. The patients diet need not be as strictly regulated and it is easier to continue with daily activities.
Like all treatments, there is a down side to this method. Peritoneal dialysis may not cleanse the blood as thoroughly as Haemodialysis. Also, people on peritoneal dialysis are inclined to get hyperlipidemia (high levels of certain fatty substances in the blood) or peritonitis (A stomach infection).
Due to the disadvantages of dialysis, a kidney transplant is perhaps a more desirable method of treatment. This involves the removal of a failed kidney and its replacement with the kidney of a donor. This method provides a long-term solution in which the patient can continue to live a ‘normal’ life. However, transplantation can be subject to many complications. Firstly, the tissue cell types of recipient and donor must be as closely matched as possible, in order to minimise the chances of the kidney being rejected by the body’s immune system. Secondly, once a match has been found, immunosuppressive drugs have to be introduced permanently. This lowers the patient’s immune system, thus reducing their ability to fight infections.
Because the donor’s kidney has to be as closely matched to the recipient as possible, it is difficult to obtain a donor. Also there is a shortage of people willing to donate kidneys.
The majority of people suffering from kidney failure await kidney transplants, whilst continuing with either Haemodialysis or peritoneal dialysis.
