Kidneys
The kidneys
The kidneys are bean-shaped excretory organs in vertebrates. Part of the urinary system, the kidneys filter wastes (especially urea) from the blood and excrete them, along with water, as urine.
In humans, the kidneys are located in the earlier part of the abdomen. There is one on each side of the spine; the right kidney sits just below the liver, the left below the diaphragm and adjacent to the spleen. Above each kidney is an adrenal gland (also called the suprarenal gland).
Function of the Kidney
The kidneys take care of waste disposal from our body. The kidneys are not the only organs doing this, the liver also takes care of some waste, while even the lungs get rid of some unwanted material (like the alcohol breathed in after a visit to the local bar). Contrary to what some people think, it is impossible for the water that we drink to flow directly to the kidneys. All the liquids that we swallow will be absorbed by the bowels and will get into our body. If it is then discovered that there is too much of it, the excess will be transported via the blood to the kidneys to leave the body as urine.
The kidneys act as a giant sifter with very small holes. The blood pressure in our arteries will make sure that water and small molecules pushes through the holes. Blood cells and bigger molecules like proteins will be too big to pass through the holes and will, fortunately, remain in the blood, that exits the kidney at the other side.
When some waste is too big to fit through the holes a specialized cell in the kidney will detect these molecules, pick them up and drop them in the urine. Other 'non-waste' molecules will be excreted into the urine by accident because they are too small; other specialized cells in the kidney will detect these in the urine, pick them up and drop them into the blood. The kidney is a complex filter system, because there are so many other subsystems either excreting special molecules or absorbing others.
The kidneys are very efficient and will completely clean all blood in less than an hour. It is possible for a human-being to live on one kidney or even one half of a normal kidney would be enough to allow for sufficient waste disposal.
Because the blood flow through each kidney is so huge, the organ is well equipped to monitor blood pressure. If the blood pressure gets too low, the kidney will produce a hormone that marks a signal for other parts of the body, like the heart, to increase the pressure.