The kidneys are two very important reddy-brown organs which are found in the abdomen. They are probably most famous for making urine, but they actually have a number of functions. They are retroperitoneal, which means they are found behind a membrane in the abdomen called the peritoneum. The blood supply comes to each kidney via an artery (the left and right renal arteries), and returns to the vena cava via a vein (the left and right renal veins).
One way of thinking about the function of the kidney is to think about them as an organ that keeps everything in balance. The body is in a delecate state, and everything needs to be kept as steady as possible in order to prevent illness, or even death. There are a number of ways that the kidney does this, but it's probably easiest to think of its function in two ways.
The first is to act a bit like a filter for the blood. As much as 20% of the blood that the heart pumps out every minute will go through the kidneys, constantly being filtered to get rid of anything which shouldn't be there, and keeping the things which should. In fact, although the body can cope with one, humans usually have two kidneys to make sure the blood is properly filtered. Of course, this also means that any damage to the kidneys can cause things to leak out into the urine, or for things which should be filtered out to stay in.
As well as this, the kidneys are there to produce certain chemicals - urine, as already mentioned, but also certain hormones like Vitamin D, renin and erythropoietin.
As already mentioned, the kidneys lie in the retroperitoneum - i.e. close to the back, near the spine. In fact, they lie between the levels of T12 and L3 of the vertebral column, which means they're towards the lower end of your back. Each kidney has an adrenal gland sitting on top of it, and a ureter running out of it, leading to the (urinary) bladder.
The right kidney sits slightly lower down in the abdomen because the liver is just above it and effectively pushes it down. On the left hand side, you have the spleen. There are also muscles in the area around the kidneys - the diaphragm muscle running over the top of the kidneys, separating the abdomen from the thorax, and the quadratus lumborum muscle on the posterior aspect of the kidneys.
The kidney itself focuses in on the intent on one side - a 'concavity' called the renal hilum. This is where all the blood vessels come in and go out, and where the ureter leaves. This is also the point called the renal pelvis, and where the ureter leaves the kidney is where all the urine tubes leading out of the kidney converge. It's a bit like lots and lots of water slides in the swimming pool all leading into one pool before going on in one huge slide down to the bladder.
The kidney itself is made up of an outer section called the cortex, and an inner section called the medulla (often separated into the outer and inner medulla). This becomes important when the kidney is trying to make sure that it's filtration is working properly, as explained a bit later on.
The blood comes in through the renal artery, and leaves through the renal vein. Because the kidneys lie on either side of the main blood vessels (aorta and inferior vena cava), the length of the artery or vein depends on which side you're on. So the left renal artery is shorter than the right renal artery, because the right renal artery has to pass behind the inferior vena cava to get to the kidney. Conversely, the left renal vein is longer than the right renal vein, because the left renal vein has to pass over the front of the aorta in order to get to the inferior vena cava.
The kidney is made up of a huge number of individual tubes that ultimately all meet together to form the main renal pelvis and ureter. Each tiny individual tube is called a nephron, and is made up of the corpuscle and the tubule leading away from it. Over all, there are in the region of 1 to 1.5 million nephrons in each human kidney!
The corpuscle is the part where the glomerular apparatus is surrounded by Bowman's capsule. The glomerular apparatus, or glomerulus, is the part at the beginning of the tube where the arteriole coming into the kidney forms a capillary bed, or series of looped capillaries which has a huge surface area. It is through these kidneys that the kidney squeezes out anything that is going to be filtered out into Bowman's capsule. Leading away from Bowman's capsule is the tubule, which is split up into different named parts according to how far along you have got. The reason for this is clear when you see that each part has a slightly different part to play in its function, with different salts diffusing in and out of the tubule depending on the part that we're looking at.
The individual parts of the nephron are shown in the picture on the right. As the picture shows, different parts of the nephron can be found in different parts of the kidney, with the tubule stretching from the corte into the inner medulla. Again, the reason that this is important will become clear when you find out the diffusion of salts that needs to happen at each part. The osmolarity (i.e. the concentration of salts) in each area determines how much will diffuse.
The picture on the right actually doesn't give a fair impression of just how small the glomerulus is in relation to the rest of the nephron. A nephron usually measures roughly 5cm long, while the glomerulus itself is only 0.2mm in diameter. However, it works extremely hard to get filter blood coming through, and between all the nephrons in the human kidney, each day they get through roughly 800 litres of plasma (which means they filter the blood about 250 times a day).
It's hard to think of how impressive that is when we're talking on such a small scale, so let's just think about that for a second. If the glomerulus were actually the size of a beach ball, the nephron would be roughly the height of Big Ben clock tower in London, and the kidneys would be filtering enough water every day to fill an Olympic sized swimming pool!
How do you make sure that the right things are filtered, and you keep in place everything that you need? It's a difficult task, because you don't know what might be in the blood - in the same way that the liver has quite a hard job making sure that it cleans properly. The kidney is well designed for the task!
The basic arrangement is that everything small gets squeezed out of the specialised capillary bed called the glomerulus at the beginning. Then, as it works its way along the nephron tubule, differences in concentration between the blood and the nephron, combined with specific transporters, get everything that is needed back into the blood.
As you work along the tubule, the electrolytes (e.g. sodium, potassium) are swapped into or out of the urine in order to keep concentrations at the appropriate level. Meanwhile, a clever system that relies on carbonic acid as a buffer is used to control the acidity of blood. Finally, reabsorption of fluid at the distal tubule is an important mechanism for controlling blood pressure.
The end result is that you get clean blood, and all the waste products dissolved in the urine. Genius! Of course, all this is a very complicated system that has barely been touched on here. In fact, it is dealt with in considerably more detail in the article on kidney function.
I would say that the kidney's main function is filtering, but it's also really important for a number of other functions, notably production of three key hormones.
The first is renin. Renin is an enzyme which is released by the kidney at the glomerulus, at a specific part of the glomerulus where the blood pressure is measured. This special machinery (called the juxtaglomerular apparatus) measures the blood pressure and releases renin when the blood pressure is low in order to increase the blood pressure again. The way that it achieves this is explained more thoroughly in the section on the renin-angiotensin system.
The second is calcitriol or activated vitamin D - 1,25-dihydroxycholecalciferol to be precise. Vitamin D is produced in the skin or obtained from food, and then undergoes chemical reactions to get it to calcitrol. The first of these reactions is in the liver, the second is in the kidneys. By producing activated vitamin D, the kidneys are involved in the action of Vitamin D - which is to stop calcium levels getting too low in the blood, and to help get it into bones. If calcium levels get too low, muscle contractions can be messed up and bones don't develop properly. This is why people with kidney failure can get problems with their bones.
The third is erythropoietin (also known as Epo). This is a hormone which is produced when the body thinks it doesn't have enough red blood cells, and it causes more red blood cells to be produced. The way the body knows there aren't enough blood cells is when it realises that there isn't enough oxygenated blood flowing around; not enough oxygenated blood indicates not enough blood, which suggests we need more. Of course, the kidney can be tricked into making more erythropoietin even if there is enough blood if there's a chronic problem with the lungs (e.g. COPD), and so some people with chronic lung disease end up with an excess of erythropoietin and an excess of red blood cells - or polycythaemia, as it's known. Because it makes more blood cells which increases the amount of oxygen which can be carried around, some athletes use it to try to improve their performance.