The workhorses of the urinary system are the kidneys which are the twin, bean-shaped organs in your body that clear harmful substances by filtering blood - like a water purification plant that helps clean the drinking water for a city.
Blood filtration happens inside the over a million nephrons scattered inside each kidney, and each nephron is made up of a renal corpuscle and a renal tubule.
So let’s zoom in on the renal corpuscle, which is where blood filtration starts.
The renal corpuscle is made up of the glomerulus - a tiny bed of capillaries - and the Bowman’s capsule surrounding the glomerulus.
Between the glomerulus and Bowman’s capsule there’s a space called Bowman’s space.
Blood gets to the glomerulus through the afferent arteriole, but interestingly enough, once the blood leaves the glomerulus, it doesn’t enter into venules.
Instead the glomerulus funnels blood into efferent arterioles which divide into capillaries a second time.
These capillaries are called peritubular capillaries - because they are arranged around the renal tubule.
Now, the first step in blood filtration happens at the glomerular filtration barrier.
The glomerular filtration barrier is made up of three layers and together they separate the blood inside the glomerular capillaries from the fluid inside Bowman’s capsule.
They work like a sieve, allowing water and some solutes in the plasma like sodium, to pass into Bowman’s space, while keeping red blood cells and plasma proteins in the blood.
Starting from the capillary lumen, the first layer of the glomerular filtration barrier is the endothelium, made up of glomerular capillary endothelial cells.
These cells have fenestrations, which are like pores in the cell themselves, tiny spots where the cytoplasm isn’t filled in so that solutes and proteins can pass right through. But the fenestration are tiny so they block red blood cells from passing through.
Blood minus red blood cells is plasma - so plasma gets to the second layer of the glomerular filtration barrier, which is the basement membrane.