Wet diapers. Yeah. All parents have seen a lot of them. How do the diapers become wet? Obviously it’s because the baby urinated, but what exactly is urination? How is urine formed, and why do the shades of the urine change?
Where does urine come from? The chief organ in the urinary system is the kidneys, a very complex, but extremely fascinating, organ. The kidneys filter nearly 200 liters of fluid from the bloodstream every day, allowing toxins, metabolic wastes, and excess ions to leave the body in urine. It also returns needed substances back to the blood. The kidneys also maintain the proper balance between water and salts, and between acids and bases. Other kidney (I may often refer to the kidney with the term “renal”) functions include gluconeogenesis (the formation of new glucose molecules), producing the hormones renin and erythropoietin (a discussion for a different time), and metabolizing vitamin D to its active form. Kidneys form the urine, which passes into the ureters, then the urinary bladder, and finally through the urethra, where it is excreted from the body.
What is the kidney? The kidneys lie in the back of the abdominal cavity. The right kidney is crowded by the liver and is slightly lower than the left. Here is a diagram of the kidney’s placement in the body:
The adult kidney has a mass of about 5 oz, and its average dimensions are 12cm long, 6cm wide, and 3cm thick. Inside of the kidney, a lot goes on. Here is a diagram of the internal anatomy of the kidney:
The blood vessels of the kidneys are very, very important. Because the kidneys continuously cleanse the blood and adjust its components, they have a rich blood supply. Under normal conditions, the large renal arteries bring one-fourth of total cardiac output (the amount of blood the heart pumps) to the kidneys each minute. Here is a diagram of the blood flow through the kidneys:
Inside each kidney, there are over 1 million tiny structures called nephrons, which are the structural and functional units of the kidneys. Each nephron contains a glomerulus and a renal tubule. The tubule is exceedingly complex. It has four parts. The first part is the glomerular capsule, which completely encases the glomerular apparatus. The rest has three main parts: the proximal convoluted tubule (PCT), the loop of Henle, and the distal convoluted tubule (DCT). Here is a basic diagram of the nephron:
Each part of the tubule plays a different role. To cover everything is beyond the scope of this discussion at the present time, but I will give a basic gist of what each part does. Remember that the goal of the kidneys is to filter and cleanse the blood, make sure blood pressure and volume remain at appropriate levels, and excrete the unwanted substances. Each part contributes to these goals. There is one more thing which is very important, and that is the relationship between the nephron and surrounding blood vessel networks. There are actual two types of nephrons, cortical and juxtulomedullary, and they have different networks. Here is a diagram:
How is urine formed? There are three major processes which occur in the nephrons to form urine: glomerular filtration by the glomeruli, tubular reabsorption by the renal tubules, and tubular secretion by the renal tubules.
Glomerular filtration is the process where hydrostatic pressure forces fluids and solvents through a membrane, meaning from the glomerular capillaries into the tubules. Tubular reabsorption is where most of the contents in the filtrate are reclaimed and flow back into the blood plasma. Without this, all of our blood plasma would be drained away in urine in 30 minutes. Tubular secretion is where unwanted substances are moved back into the tubules to be excreted in urine.
In tubular reabsorption, there are different mechanisms of how substances are reabsorbed back into the blood stream. Each part of the tubule has different mechanisms. Here is a diagram of the mechanisms of the PCT:
The following table gives a very easy tracker of what happens in each part of the nephron:
A very fascinating aspect of the Loop of Henle is that in the descending portion, only water is reabsorbed into the blood. This is because of the presence of aquaporins, which are channels for water. But virtually no solute reabsorption occurs. In the ascending portion, or ascending limb, the opposite is true; solute leaves the filtrate, but not water. Once the filtrate reaches the DCT (distal convoluted tubule), only about 10% of the originally filtered NaCl (sodium chloride) remains, and only about 25% of the water. Reabsorption at this point is mainly dependent on the body’s needs, and is regulated by hormones.
Why does urine have different shades of color? The simple answer to this is that there is dilute urine and concentrated urine. But how the urine becomes dilute or concentrated is a bit more complicated. Without going into too much detail, there are two function of the nephrons called the countercurrent multiplier and countercurrent exchanger. This has to do with what was discussed previously, that different portions of the loop are permeable to different substances.
In the collecting ducts of the nephron (see the first nephron diagram), there are no aquaporins, so no water is reabsorbed. However, if the body is low on water, antidiuretic hormone (ADH) is released by the brain, and ADH causes the insertion of aquaporins into the collecting ducts. If no ADH is released, the filtrate passes through the collecting duct without anything happening to it. This results in dilute urine. But if the body is low on hydration and ADH is released, up to 99% of the water in the filtrate can be reabsorbed. This results in concentrated urine. The yellow color of urine is due to urochrome, a pigment that comes from the body’s destruction of hemoglobin (see the diapers and wipes section for a diagram of the breakdown of hemoglobin). The more concentrated the urine, the deeper the color. So basically, the clearer the liquid, the more hydrated a person is, and the darker the urine, the less hydrated a person is.
I heard from a doctor that the optimum color for urine is pale yellow. Dehydration is not good. But overhydration for long periods of time is also not so good, as a lot of solutes can be lost, which can lead to not so good things happening, such as siezures. So keeping a hydration balance is important.
So next time you change your child’s wet diaper, or use the bathroom yourself, appreciate the incredible complexity of what happened to allow the urine to be formed.