I’m going to try to finish writing about the urinary system in this post today but there is quite a lot more material to cover so we’ll see how it goes!
So where were we… I finished going through the basic functions and anatomy of the main organs of the urinary system last time. In this post, I will focus more on the details of how the kidneys filter out our blood to urine. It is quite a fascinating subject in my opinion so this will be fun going through all the details again! You will appreciate your wee so much more after this post (I hope)!!😆
If you have read my earlier post, you should know that the key organ of the urinary system, the star of the show, are the bean-shaped kidneys. The kidneys are where blood is filtered out into the urine, maintaining our homoeostasis in blood pressure, blood pH, water, electrolytes, and red blood cell production. The kidneys also eliminate waste products such as nitrogenous wastes, toxins, and drugs through the urine.
The nephron is the functional unit of the kidneys and that is where all the filtration occurs and there are about a million nephrons per kidney! If I had to very briefly explain how the filtration occurs in each nephron, I would say that blood is forced through very narrow blood vessels where some of the fluid gets squeezed into a tube. That fluid then travels through a long tube giving away its nutrients if necessary, which results in urine. That is basically how urine is produced but there is so much more than that!
Firstly, let’s begin with blood entering the nephron. I’m not sure if I mentioned this before but anything “renal” has to do with the kidneys. Why did I say that? It is because the blood enters the nephron via the renal artery. It then goes through the afferent arteriole entering the glomerulus. The rest of the blood then exits the glomerulus through the efferent arteriole.
However, when blood reaches the glomerulus, some fluids in the blood are pushed out into the bowman’s capsule where the filtration begins. As you can see in the diagram below, the filtered fluids start travelling from the bowman’s capsule through a long tube which has different parts and names to it. The fluids travel through the proximal convoluted tubule (PCT), down the descending limb of the loop of Henle, up the ascending limb of the loop of Henle, through the distal convoluted tubule (DCT), and finally to the collecting duct.
As the filtered fluids travel through the different tubes with different functions, the fluids get filtered even more as it reaches the collecting duct. Starting from where the fluids first get filtered, the bowman’s capsule is responsible for catching the fluids from the glomerulus.
Then the next stop is the PCT. This is where most of the water in the fluid, bicarbonate ions, all of the glucose, amino acids, and most of the sodium, chlorine, and potassion ions are reabsorbed into to the blood capillaries depending on the current status of the body. If the water concentration in the blood capillaries is lower than the water concentration in the PCT, water will get reabsorbed to the blood capillaries through passive transport, which does not require any energy. However, when all of the glucose and amino acids molecules including some of the ions get reabsorbed, it is not through passive transport but rather active transport. This requires energy in the form of ATP which is why the simple cuboidal epithelial cells of the PCT have lots of mitochondria. Since there is a lot of reabsorption that occurs in the PCT, the inner lining of the cells have lots of tiny hair-like extensions called villi. The villi are there to help increase the surface area of which reabsorption can take place. And in the cell membranes of the PCT, there are tiny channels or holes to let the reabsorbed material into the cells so it can eventually reach the blood capillaries. There are two ways in which this happens, but I will explain that towards the end.
Phew!💦That was one long paragraph on such a small part of the nephron! You might want a short break from reading this as there is still a lot to go through!
Okie Dokie, let’s move on!
After travelling through the PCT, the fluids reach the loop of Henle. Since the loop of Henle is always in the shape of a ‘U’, the two parts are called the descending (going down) and ascending(going up) limbs of the loop of Henle. The loop of Henle also has simple epithelial cells like the PCT but it has fewer villi. I’m guessing this is because the loop of Henle has fewer substances it reabsorbs so there is no need for lots of villi. In the descending limb, water again will get reabsorbed depending on the body’s needs, decreasing the water concentration in the fluids. Since the water concentration has decreased in the descending limb, when the fluids reach the ascending limb, there is a higher concentration of sodium, chlorine, potassium, magnesium, and calcium ions in the tubule. This allows the magnesium and calcium ions get reabsorbed through passive transport, while the sodium, chlorine, and potassium ions get reabsorbed through active transport. The methods of which the ions get reabsorbed are different but doing so, the water and ion concentration in the fluids become more neutral.
Once the fluids travel through the loop of Henle, the fluids reach the DCT. The cells of the DCT again have the same type of cells as the other tubules mentioned before but they do not have villi. Once the fluids reach the DCT, there aren’t many substances available to be reabsorbed than hormones and some last minute reabsorption of water. The hormones that are reabsorbed are aldosterone -a steroid hormone, the antidiuretic hormone, and the parathyroid hormone which are both peptide hormones. These are all hormones that are related to water regulation in the body. The reabsorption of hormones will depend on the body’s current status and is a very complicated process so I will write about it when I learn about it!😅
Once the fluids have finished going through the DCT, it’s urine now! Whoop whoop! The freshly made urine travels to the collecting duct where it receives urine from other neighbouring nephrons. Also at this stage, if the body needs even more water, some water can be reabsorbed for the last time before it travels to the renal pyramid, to the major calyxes, and down the ureter, to the bladder.
So that is it for making urine in the nephron! But whoah wait, I’m not done yet! There are still some more to come on the urinary system but since I don’t want to bore you to death, I will finish up on the urinary system on my next post. Not sure when my next post will be since tomorrow is the last day of lectures for me before I will be taking 5days off for the Easter holidays! But I hope you learnt something new from this post and Happy Easter!!🐰🐰