When a kidney removes impurities from the blood does it actually target impurities or does it simply remove water that will bring impurities along with it? Similar to changing 3 gallons of water in a 10 gallon aquarium each day.
This is how I remember learning it in a Freshman Intro Biology class. Almost certainly oversimplified.
The kidney’s don’t have a biologically “programmed” list of all possible impurities to be filtered out. That would be impossible.
Instead, a kidney has a biologically “programmed” list of known ingredients that belong in the blood.
When blood passes through the kidneys, the kidneys will filter out all the known “good” ingredients, along with a substantial amount of water, and all this then gets shunted back into the bloodstream.
Whatever remains of the original bloodstream, along with some amount of water, drains off down into the ureters and into the bladder, to be pissed away later.
That makes good sense, I wonder if it has more to do with the viscosity of the blood rather than the actual chemical makeup.
A reminder to everyone to drink at least 1 glass of water on waking. Why? As we age some if not many begin to lose the ability to sense thirst. Not drinking enough water is very hard on the kidneys.
A friend just lost her 52 year old mother to kidney failure. It was a long and painful death. She was receiving dialysis and was on the UCLA Med Center wait list but they bumped her back because they thought she wasnt doing enough to care for herself. Now its too late.
Both the liver and the kidneys play a role in dealing with extra things in the blood. The specifics depend on the several properties of the impurity - its size, electric charge, solubility and how it is specifically transported in the blood.
The kidney sends blood through long tubes. Each part of the tube has different properties with regard to adding or removing salts, sugars and amino acids. Some parts have the effect of adding or removing water. Some impurities are water soluble and end up in the urine. Others have different chemical groups added in the liver.
How does your kidney know how much of each programmed ingredient to keep?
My understanding is that if you are deficient in something like potassium or sodium, your kidneys hold onto it. If you’ve got too much your kidneys dump it.
Does anyone know how the kidneys can measure the concentration of minerals like this to determine how much to dump?
The kidney doesn’t measure the concentrations as such. It contains many functional tubes with different properties in different places. Some parts contain pumps that use energy to actively exchange certain ions. Others have membranes with different properties so that certain molecules passively flow in one direction.
It might help to look at simpler examples. Blood vessels flow through tissues in the body. Both the blood and the tissues contain water, sodium and other things. Blood flowing from the lungs has lots of oxygen. Things tend to passively move from areas of high concentration to low concentration, if the membranes allow this. So the oxygen in the arteries goes into the tissues, since the concentration of oxygen is higher in the blood vessel than the tissue, and blood vessels permit the flow of oxygen.
Some membranes allow the flow of water. So if the blood has too much sodium (and relatively less water), water might passively flow from the tissue into the saltier blood to even out the sodium concentrations. The situation in blood is further complicated by proteins, which can be though of as holding on to water.
The kidney tube has several parts which act in different ways.
The first part reabsorbs sodium along with glucose, amino acids, chloride, water, uric acid, potassium and phosphate; and secretes ammonia to allow the kidney to secrete hydrogen ions. There are also pumps, requiring energy, that pump sodium out and potassium into the tube. And there are pumps which are stimulated by other chemicals (PTH, angiotensin, antidiuretic hormone and aldosterone) which respectively act on certain parts of the tube to allow for other ions to be excreted or absorbed.
The next part of the tube is impermeable to sodium but not water, which is passively reabsorbed.
The next part is impermeable to water, but uses energy to pump sodium out in exchange for potassium.
The next two parts have different pumps and properties. And the pumps are also affected by various hormones, but only in certain parts of the tube. It gets fairly complicated. But it’s well understood, and you could google the physiology if you really want to know the details (You don’t).
The overall complex of tubes and membranes referred to by Dr Paprika above is known as the Loop of Henle. This Youtube video provides a simplified explanation of the function of the Loop of Henle.
Overall it is really a process of removing the stuff your body wants to keep and keeping that within the kidney while letting the waste products flow through to become concentrated in the urine. So your kidney is not identifying bad stuff to excrete but rather picking the good stuff out to hold onto.
I know almost nothing about kidneys, but thought to pass on some tidbits. Here is a paper claiming to explain why airports and kidneys have similar shapes!
By separating blood into two streams — cleaner blood and urine — the kidneys are reducing entropy. Laws of thermodynamics mean that free energy must be consumed to accomplish that. Although I think the kidneys are fairly efficient thermodynamically, they consume more energy than the brain, and as much as the heart!