Water Balance and Kidney Failure

Explained

Controlling the water in your blood, and what happens when the kidneys stop

The kidneys do not decide how much water your body needs. A gland in the brain does, and it tells the kidneys by releasing a hormone. Getting that order of events right is most of this topic.

The loop, in order

The hypothalamus monitors the concentration of the blood passing through it. The receptors that do this are called osmoreceptors.

If the blood is too concentrated, meaning there is too little water in it, the pituitary gland releases ADH into the blood. ADH travels to the kidneys and makes the walls of the collecting ducts more permeable to water. More water is therefore reabsorbed back into the blood, and the urine that leaves is smaller in volume and more concentrated.

If the blood is too dilute, less ADH is released, the collecting ducts become less permeable, less water is reabsorbed, and a larger volume of dilute urine is produced.

Either way the blood concentration returns towards normal, and once it has, the original stimulus is gone and the response stops. That is negative feedback: the response reverses the change that caused it.

The name says what it does

ADH stands for antidiuretic hormone. A diuretic increases urine production, so an antidiuretic reduces it. More ADH means less urine, and the name will remind you of that in the exam if the loop will not.

Alcohol suppresses ADH release, which is why drinking it makes you produce a lot of dilute urine and become dehydrated. Sweating heavily has the opposite effect, because losing water through the skin concentrates the blood and raises ADH.

When the kidneys fail

Waste products build up in the blood, particularly urea, and the water and ion balance is no longer controlled. Untreated, this is fatal. There are two treatments and they are compared often.

Dialysis

Blood is taken from the patient and passed through a machine on one side of a partially permeable membrane, with dialysis fluid flowing on the other side.

The fluid is what makes it work. It contains the same concentration of glucose and of ions as healthy blood plasma, and no urea at all. So there is no concentration gradient for glucose or for useful ions, and they stay in the blood, while urea has a steep gradient and diffuses out. The machine removes the waste without stripping out anything the patient needs.

Dialysis has to be repeated several times a week, each session taking hours. Between sessions the patient must control what they eat and drink carefully, because the waste is building up again in the meantime.

Transplant

A single donated kidney is enough. The patient lives normally between hospital visits and has no dietary restrictions, which is why a transplant is usually preferred where one is available.

The problems are supply and rejection. There are far fewer donor kidneys than there are patients waiting, the donor tissue type has to be matched as closely as possible, and the recipient must take immunosuppressant drugs for as long as the kidney lasts, which leaves them more vulnerable to infection. Transplanted kidneys also do not last forever, so some patients need more than one over a lifetime.

Answering the comparison

Questions on this ask you to evaluate, so give both sides and then commit to one.

Dialysis is available immediately and needs no surgery, but it is time consuming, restrictive and lifelong. A transplant restores normal life, but it depends on finding a match, carries the risk of rejection and requires drugs that suppress the immune system. Say which you would choose and give the reason from the list you have just written.