The Urinary System – Concentration and Dilution

Last time, we talked about how the urinary system allows for our bodies to selectively reabsorbs things that were filtered out of our circulatory system. One of the molecules that can be reabsorbed is water, which is an important molecule for several reasons. In terms of the urinary system, water is important for the concentration and dilution of our urine.

The concentration and the dilution of urine is regulated by 2 hormones: Anti-diuretic hormone and Aldosterone.

The anti-diuretic hormone is released if you’re dehydrated, and literally translates to “against the passing of urine”. In situations where you are dehydrated, your blood volume will be low due to the lack of a sufficient amount of water, resulting in a lower blood pressure. To counteract this, the anti-diuretic acts to reduce the amount of water lost by the body and minimizes how much urine you make and release. This will result in a more concentrated urine that has a less water than usual and is a deeper yellow due to the higher concentration of urea.

Aldosterone is also released when you’re dehydrated. This hormone is responsible for the increase in thirst while also helping your body retain water by increasing the amount of sodium in your body. The more solute there is in your body, the more likely water is to stay with the solute (rules of osmosis, hurray!).

These hormones exert their effects primarily on the distal tubule of the nephron (after the loop of Henle), so that is where the water is reabsorbed!

So when these hormones are released, your urine will be more concentrated because water is retained by your body! Amazing, right?

Next week: We will finally get to talking about the effect of Alcohol on Urine!

Is there something you’d like to learn about? Tell me about it here and I may just do a post about it 🙂

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!

The Urinary System – Filtration

Last week, we talked about the basics of the urinary system, including the organs involved. This week, let’s take a closer look at what happens in the kidney, starting with filtration. The posts will be segmented because there will be a lot of new terms coming your way, and I would hate for you to get overwhelmed! So let’s learn how our bodies makes our pee, step by step.

Parts of the kidney

The basic functional unit within the kidneys is known as the nephron. This consists of all of the numbered portions, starting at the glomerulus (5) and ending at the distal tubule (6). 

The main components involved in filtration are the arteries, and the glomerulus.

Filtration

It is at the Bowman’s capsule where the filtration of blood occurs. The blood from the renal artery (renal means ‘kidney’) flows into a smaller version of an artery, known as the afferent arteriole. The name sounds weird, but ‘afferent’ basically means ‘towards something’. So this arteriole is moving blood towards the glomerulus. So this arteriole can also be referred to as the afferent glomerular arteriole.

The afferent arteriole then branches to form tiny blood vessels known as capillaries. These capillaries form a ball-like structure, which is part of the glomerulus (5). The capillaries are therefore called the glomerular capillaries.

The blood from the capillaries continues to flow into the next vessel, which is another arteriole, named the efferent arteriole. ‘Efferent’ means ‘away from something’, so the blood is moving away from the glomerulus. So this arteriole can also be referred to as the efferent glomerular arteriole.

This movement of the blood increases the amount of pressure within the capillaries, causing fluids to leak out. These fluids pass through a filter-like membrane, known as the glomerular basement membrane. Some substances are too large to pass through the basement membrane and, therefore, continue to circulate through the bloodstream.

The filtrate that goes through the glomerular basement membrane enters the Bowman’s capsule. The space inside the Bowman’s capsule is continuous with the rest of the tubule, which includes the proximal tubule, the loop of Henle and the distal tubule. The filtrate is concentrated and modified within these components of the tubule, which will be discussed next week.

And that is, thankfully, all there is to the filtration process for the urinary system! Next week, we’ll learn about what happens to the filtrate in the tubule.