How does kidney dialysis work? Kidney dialysis works by filtering the blood to remove excess water and toxins in cases where the kidneys have failed.
Most people have two kidneys, although we can function perfectly on only one. The second one is basically an evolutionary backup in case one of them fails. This is called bilateral symmetry and is also why we have two lungs. A lot of our organs come in pairs because we are generally symmetrical. We have two eyes, two ears, and, even though you might not think it, we have two hearts and two brains. The brain has a left hemisphere and a right hemisphere, and the heart starts as two tubular organs that fuse into one during our embryonic phase. Everything that is part of the circulatory system comes in pairs, while everything that is part of the digestive tract is singular. The liver is part of the digestive tract, which is why we only have one. Although even the liver has two lobes and we can survive without one of them. The exceptions to this rule are the lungs and the spleen. The lungs are an outgrowth of the digestive tract (they develop from the foregut), but we have two, and the spleen is part of the circulatory system, but we only have one. The quirks of evolution.
The kidneys are basically organs for filtering blood. They take in blood from renal arteries, filter it, and return it to renal veins. The difference between veins and arteries is that arteries carry blood away from the heart and veins return blood to the heart. The kidneys take out the blood on its way from the heart and clean it before it goes back to the heart.
The kidneys filter blood using filtering units called nephrons. There are about a million of these in each kidney, which allows the kidneys to filter about 120 ml of blood every minute. The blood enters the kidneys and is diverted down smaller and smaller blood vessels until it enters a nephron. Inside each nephron is a filter called a glomerulus. This is a collection of very tiny blood vessels. The walls are very thin and as the blood moves through the blood vessels, small molecules, waste, fluids, and water pass through the walls. Larger molecules, like the blood cells and proteins, cannot pass through the thin wall and stay in the blood. On the other side of the glomerulus is a long tubule, which sorts out what to keep and what to return. There is a blood vessel next to the tubule and this passes necessary minerals and nutrients into the tubule. The tubule keeps toxins and waste, and passes the necessary minerals, nutrients, and most of the water back into the blood vessels, where it goes to the vein and continues on to the heart. The tubule connects to the collecting duct system and the excess fluid and toxins are sent to the bladder where they will be removed as urine.
This process is what dialysis has to emulate. Kidneys can fail for many reasons. When they do, waste products such as urea build up, and the body can’t keep fluid and salts in balance. People become very unwell from what’s often called uremia, and without treatment, it can be fatal. Without functioning kidneys, a person can only live a few weeks before they succumb to the poisons. If this happens and a kidney transplant isn’t an option, or until a kidney becomes available, dialysis must be carried out. The idea is simple. A needle is inserted into an artery, usually in the arm, and blood flows out through a tube. The blood enters a dialysis machine, which is an artificial kidney, and is filtered. The filtered blood enters another tube and returns to the body through a needle into a vein. Because the kidneys are constantly cleaning the blood, dialysis must be carried out at least three times a week and sometimes as often as every day. It can take up to five hours at a time. It is a big thing, but it can save lives. People on dialysis can live for years, and often long enough for a kidney to become available.
The dialysis machine works on the principle of osmosis. Osmosis is the natural movement of water molecules across a semipermeable membrane from an area of high concentration to an area of lower concentration. Blood enters the machine and reaches the semipermeable membrane. On the other side of the membrane is a dialysis fluid, which is a lower concentration than the blood. The membrane has lots of tiny pores, just like the glomerulus in the kidneys, and fluids, water, and smaller particles can pass through, while the large particles can’t. The dialysis fluid flows in the opposite direction to the blood because that increases the concentration difference between the two fluids and makes more toxins and fluids from the blood cross over. The cleaned blood then returns to the body. Dialysis can increase the risk of infection and can lead to reduced circulation, but it is far better than the alternative. And this is what I learned today.
Sources
https://www.niddk.nih.gov/health-information/kidney-disease/kidneys-how-they-work
https://my.clevelandclinic.org/health/treatments/14618-dialysis
https://en.wikipedia.org/wiki/Kidney_dialysis
https://en.wikipedia.org/wiki/Countercurrent_exchange
https://en.wikipedia.org/wiki/Kidney
https://biology.stackexchange.com/questions/100928/why-do-we-have-two-kidneys-but-one-liver
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