Our Body’s Built-In Filter- Part 3

Hello, internet friends!

It’s been a while since my last post because I took some time off in the countryside spending time cooking great food and bonding with 2 beautiful border collies. But I am ready to get back into revision mode for my upcoming exam in May!

So shall we begin? I finished off my last post with going through how the nephron in the kidneys turn blood into urine

To have a bit of a recap, blood is forced into the glomerulus where fluids in the blood are forced out into the bowman’s capsule. The fluids then travel through a long series of tubules that have different functions of reabsorbing different nutrients back into the blood. The filtered fluids which are now urine,  goes through the collecting duct which then passes out of the kidney, travels down the ureter, and gets collected in the bladder. The urine then leaves the body through the urethra to its final destination, the toilet!

I’ve gone through a lot of information in the past two posts about the urinary system but there is still some more material I would like to go through regarding tiny details of how fluids are pushed out into the glomerulus and how nutrients get reabsorbed into the blood capillaries.

If you read my post from before, you will be aware that blood travels from the renal artery to the afferent arteriole, into the glomerulus. The blood that has not been pushed out into the glomerulus will then exit the glomerulus through the efferent arteriole. Well, it turns out that the efferent arteriole has a smaller lumen than the afferent arteriole!  But why is that? This is because the decrease in the size of the lumen will increase the pressure in the glomerulus which helps the fluids in the blood to get pushed out into the bowman’s capsule.

However, the fluids in the blood don’t just get pushed out into the bowman’s capsule. Because of the three different pressures of the Net Filtration Pressure (NFP), some nutrients that have been pushed out, will get pulled back into the glomerulus. This is the first to producing urine which is called glomerular filtration. The different pressures are the glomerular hydrostatic pressure, osmotic pressure, and capsular hydrostatic pressure. I struggled a bit to understand how these pressures work so I am not 100% confident in explaining this but I will try my best for easy understanding!

Source: APSU Biology

As you can see in the diagram, the GHP pushes fluids out into the bowman’s capsule while the OP and CHP push fluids back into the glomerulus. The OP pushes water in the fluids back into the blood. The CHP works in a similar way but other molecules in the fluid will also get pulled back in. This is where I struggled so, please do understand if you don’t understand what I am about to explain! The CHP is not the same as the OP because this pressure is caused by the GHP. For example, if you were to blow into a bowl of flour, some of the flour will blow back into your face (this has happened a few times for me while baking)! The CHP works in a similar way. Because there is pressure pushing outwards, there is also a bit of pressure that occurs in the opposite way. Or at least that is how I understood it after going through different educational sources to understand how it works. And since the CHP is caused by GHP, the pressure is not as strong. In fact, the OP is not as strong as the GHP either, and this is good because otherwise there would be no fluids to go through the tubules to produce urine!

So to calculate the total NFP, the formula is GHP-(OP+CHP)=NFP which is 10mm Hg(milligrammes of mercury).

If the first step of urine production is glomerular filtration, what is the second step? As we know that the filtered fluids in the bowman’s capsule travel through a long series of tubules to reabsorb nutrients, the second step to urine production is tubular reabsorption. This is when different nutrients from the fluids get reabsorbed back into the blood capillaries, depending on which part of the tubules the fluids are travelling through. But how do the nutrients move from the tubules to the blood capillary? Well, there are two ways. The nutrients can be reabsorbed through a paracellular or transcellular movement. Paracellular movements require nutrients to go in between two tubular cells and the transcellular movement require nutrients to go through a single tubular cell. There are a lot more detail into how the nutrients go through the cells but I have not learnt the specifics yet so I will explain this hopefully next year when I study physiology more in depth!

Source: Unknown

So this is about it for the urinary system! I know it is a lot of information on just one system of the human body so I’m hoping that I will be able to cover each system under 2 blog posts in the future. I have to say, it is like writing an essay for me so it’s quite challenging but it is definitely helping me a lot! And to help myself with my upcoming exam, I’m going to try to type some of the keywords in the urinary system without looking at my notes. Some of the major keywords are: renal artery & vein, afferent & efferent arteriole, glomerulus, bowman’s capsule, distal convoluted tubule (DCT), descending and ascending limbs of loop of Henle, proximal convoluted tubule (PCT), the collecting duct, renal calyxes, renal pyramid, nephrons, kidneys, ureter, bladder, urethra, peristalsis, renal medulla, renal cortex, and the renal pelvis. I may have left out a few words but I’d say these are the major keywords to know if you are studying the urinary system.

I hope that all my explanations have been easy to understand and if you are reading this, please let me know what you think of it! In the next post, I will be explaining about the nervous system . Till then!


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