Aspiring Doctor? Here’s What You Need To Know About the Kidneys (Part 2)

Welcome to Part 2 of our 2-part series on the kidneys! 

We’ve drawn on our fantastic new collection of CIE Biology IGCSE revision notes to bring you the most up-to-date, teacher-verified content. 

In Part 1, we learnt all about the structure and the function of these crucial little organs, and in this post we are going to explore the fascinating process of selective reabsorption in more detail, as well as looking at how doctors treat patients suffering from kidney failure

So whether you’re an aspiring medic or you’re aiming for top grades in your Biology IGCSE and Biology GCSE exams, this post is one for you! 

Selective reabsorption

You’ll remember from Part 1 that the kidneys play an important role in filtering the blood, and that ultrafiltration is the process by which this occurs.

However, some of the small molecules which are filtered out are actually essential –  in small quantities – to our bodies’ functioning. 

This means that they must be reabsorbed into the blood in controlled amounts in a process called selective reabsorption

This table shows the essential molecules which must be reabsorbed, and the location in the nephron where this occurs. 

Glucose reabsorption

Glucose is essential to power our metabolic activity, so it’s important that our kidneys don’t remove all of it from our bodies.

  • After the glomerular filtrate enters the Bowman’s Capsule, glucose is the first substance to be reabsorbed at the proximal (first) convoluted tubule
  • This takes place by active transport
  • The nephron is adapted for this by having many mitochondria to provide energy for the active transport of glucose molecules
  • Reabsorption of glucose cannot take place anywhere else in the nephron, as the gates that facilitate the active transport of glucose are only found in the proximal convoluted tubule

Look at the diagram below to see how this process works.

Diabetic patients

In a person with a normal blood glucose level, there are enough gates present to remove all of the glucose from the filtrate back into the blood.

People with diabetes cannot control their blood glucose levels and they are often very high, meaning that not all of the glucose filtered out can be reabsorbed into the blood in the proximal convoluted tubule. 

As there is nowhere else for the glucose to be reabsorbed, it continues in the filtrate and ends up in urine – and this is why one of the first tests a doctor may do to check if someone is diabetic is to test their urine for the presence of glucose.

Reabsorption of water and salts

  • As the filtrate drips through the Loop of Henle, necessary salts are reabsorbed back into the blood by diffusion
  • As salts are reabsorbed back into the blood, water follows by osmosis
  • Water is also reabsorbed from the collecting duct in different amounts, depending on how much water the body needs at that time

Your body is incredibly clever – you’ll reabsorb more water when you need it, for example on a very hot day. The process is controlled by hormones and chemicals.

See the diagram above for an illustration of this process!

Kidney dialysis

The kidneys might not work properly for several reasons, including accidents or disease. Humans can survive with one functioning kidney, but if both are damaged then there will quickly be a build-up of toxic wastes in the body, which will be fatal if not removed. 

The usual treatment for someone with kidney failure is dialysis – an artificial method of filtering the blood to remove toxins and excess substances

  • Patients are connected to a dialysis machine which acts as an artificial kidney to remove most of the urea and restore or maintain the water and salt balance of the blood
  • Unfiltered blood is taken from an artery in the arm, pumped into the dialysis machine and then returned to a vein in the arm
  • Inside the machine the blood and dialysis fluid are separated by a partially permeable membrane  – the blood flows in the opposite direction to dialysis fluid, allowing exchange to occur between the two where a concentration gradient exists

    Dialysis fluid contains:
      • a glucose concentration similar to a normal level in blood
      • a concentration of salts similar to a normal level in blood
      • no urea

As the dialysis fluid has no urea in it, there is a large concentration gradient – meaning that urea diffuses across the partially permeable membrane, from the blood to the dialysis fluid.

The dialysis fluid also contains a glucose concentration equal to a normal blood sugar level, this prevents the net movement of glucose across the membrane (as no concentration gradient exists).

As the dialysis fluid contains a salt concentration similar to the ideal blood concentration, movement of salts across the membrane only occurs where there is an imbalance (if the blood is too low in salts, they will diffuse into the blood; if the blood is too high in salts, they will diffuse out of the blood).

Crucially, the fluid in the machine is continually refreshed so that concentration gradients are maintained between the dialysis fluids and the blood, and an anticoagulant is added to blood before it runs through the machine to prevent the blood from clotting and slowing the flow. 

Dialysis may take 3-4 hours to complete and needs to be done several times a week to prevent damage to the body from the buildup of toxic substances in the blood.  This means that patients with kidney failure must spend a lot of time in the hospital! 

Kidney transplants

Kidney transplants – when a pateint recieves a kidney from a donor – are a better long term solution to kidney failure than dialysis; however, there are several disadvantages to kidney transplants, including:

  • Donors won’t have the same antigens on cell surfaces so there will be some immune response to the new kidney (risk of rejection is reduced – but not removed –  by ‘tissue typing’ the donor and the recipient first)
  • This has to be suppressed by taking immunosupressant drugs for the rest of their lives – these can have long term side effects and leave the patient vulnerable to infections
  • There are not enough donors to cope with demand

However, if a healthy, close matched kidney is available, then the benefits of a transplant over dialysis include:

  • The patient has much more freedom as they are not tied to having dialysis several times a week in one place
  • Their diets can be much less restrictive than they are when on dialysis
  • The usse of dialysis machines is very expensive, and so this cost is removed
  • A kidney transplant is a long-term solution whereas dialysis will only work for a limited time

And finally…

That’s it! Our revision series on the kidneys is complete. Well done for sticking through!

Remember – if you’d like to download our full set of revision notes on the Excretory System, you can find them here – just click on the ‘Download PDF’ banner.

If you’re ready to test your knowledge, head straight to our new multiple-choice Topic Questions. 

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