The Urinary System – Selective Reabsorption

Last week, we learned about the general filtration process that occurs in our kidneys. This week, we’ll learn just how our kidneys modify the filtrate to produce urine.

The filtrate that is in the renal tubule consists of water as well as other small molecules, like sugars and urea. Some of these molecules, like sugars, can return to the bloodstream in a ‘process’ known as selective reabsorption. It’s called ‘selective’ reabsorption because the bloodstream is picky as to what it absorbs from the tubule. The molecules that leave the tubule enter tiny blood vessels next to the tubule, which are called peritubular capillaries. The molecules can then be carried through the rest of the circulatory system, to provide our body cells with nutrients (if they’re sugars).

Parts of the Kidney

Selective reabsorption mainly occurs in the proximal tubule, which is the beginning of the tubule. The proximal tubule is just after the Bowman’s capsule. Whenever a molecule leaves, it is accompanies by water, which means a lot of water is reabsorbed by the bloodstream in this process.

By returning the molecules to the bloodstream, the remaining filtrate’s composition changes. As water leaves, the concentration of particles in the tubule increases.

Hormones can affect what is reabsorbed in the distal tubule. These hormones are the anitidiuretic hormone (ADH), which is also known as vasopressin, and aldosterone. They’ll be discussed next week when we talk about Concentration and Dilution!

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Blood Types

If you haven’t noticed, this month’s theme is appropriately set to ‘blood’ in honour of Halloween. So we might as well talk about the blood types, or groups, that we hear about so often.

Why do we have different blood types?

While blood is generally the same for everyone, where it contains red cells, white cells, platelets and plasma, there is one key component which differs. This component is a little protein and sugar marker, which is found on the surfaces of our red blood cells. These markers, also known as antigens, act as little flags to your immune system to tell it that the cells are part of your system and not an invader.

This is why blood transfusions can get so complicated; if your immune system recognizes the difference between the markers on your usual blood cells and the blood cells you received from a donor, then your immune system will attack those donor blood cells.

What are the different blood types?

A, B, AB, O

There are three different antigens: A, B, and Rh (which is discussed later).

The presence or absence of these antigens results in four major blood divisions:

    1. Type A – has the “A” antigen.
    2. Type B – has the “B” antigen.
    3. Type AB – has both the “A” and “B” antigens.
    4. Type O – has neither the “A” or “B” antigens.

The fact that Type O blood doesn’t have A or B antigens makes it easier for Type O to be donated to anyone, regardless of their blood types; the immune system cannot recognize Type O blood as an invader because it doesn’t have any antigens for the immune system to latch onto. Type O individuals, however, cannot accept any blood except Type O since its immune system isn’t accustomed to seeing any antigens whatsoever; the presence of an antigen on the donor blood would elicit an immune response.

+, –

The other antigen that might be present on our blood cells is the Rhesus factor (Rh).

If it is present, the main blood divisions (A, B, AB, or O) are assigned a plus sign (+): A+, B+, AB+, O+.

If the Rh factor is absent from the blood cells, they are assigned a minus sign (-): A-, B-, AB-, O-.

O- individuals are universal donors, because they do not carry any antigens which the immune system would be able to attack. AB+ individuals, on the other hand, have all of the possible antigens and therefore are the universal acceptors as their immune system is used to seeing each of the  antigens. This means it won’t attack any of the antigens!

Those are the different blood types. It’s important that you know your own blood type and the blood type of your loved ones in case of emergencies. It’ll be easier for doctors to determine which blood type to provide you!

Donating blood is also a great idea, though it isn’t for everyone. If you are interested in donating blood, contact your local blood donor clinic to check if you meet the requirements! 🙂

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