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.


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.


What’s up with hair? I mean, we see a bunch of commercials telling us how to take care of it and how to impress other people with it, but what is it made up of exactly?

What is hair?

Hair is actually composed of four components: the follicle, the shaft, the inner and outer sheaths.

The follicle

The follicle is a tube-like structure located in our skin and it has two components: the papilla and the bulb. The papilla contains itty bitty blood vessels (called capillaries) which provide nutrients to a bundle of cells. This bundle of cells is referred to as the bulb. The cells in the bulb actually divide every 23-72 hours, which is faster than any other cell in our body!

The shaft

The shaft is the part of the hair that is visible to us. It is composed of three layers dead, hard protein called keratin. The innermost layer is called the medulla and isn’t always present. The middle layer is the cortex, which makes up the majority of the shaft,  and the outermost layer is the cuticle. The cuticle is formed by overlapping scales, somewhat like a roof.

The inner and outer sheaths

The follicle is surrounded by two sheaths, an inner and outer sheath, which help protect and mold the hair. The inner sheath ends just above the opening of the sebaceous gland (Remember those? They provide oil [sebum] to our skin and are found in the dermis layer of our skin!) and follows the rest of the shaft. The outer sheath encloses both the inner sheath and the follicle, and it ends just below the sebaceous gland.

And that’s the structure of hair! The follicle is similar to the roots of a plant in that it provides the nutrients and allows the hair to stay in the skin. Around the follicle are two sheaths, which wrap around it to add more stability and strength. Finally, the part we see everyday is known as the shaft of the hair. That’s all to it.

In the next few weeks, we’ll explore different topics involving hair like goosebumps,  curly hair, the colour of hair, and greying hair! There will be a few random topics sprinkled in there too before things get too hairy.

Yes, that just happened.

Brannon, H. 2006. The biology of hair. <> August 7th, 2013.

Have a suggestion? Why not place it here?