The main job of the cardiovascular system is to keep the blood moving, and to help facilitate that - blood pressure and more importantly is kept under tight control.
A major way the body does that is through a set of hormones that make up the renin- angiotensin- aldosterone system.
But, first things first.
Everything starts in the kidney.
Now, within each kidney, blood from the renal artery flows into smaller and smaller arteries, eventually reaching the tiniest of arterioles called the afferent arterioles.
After the afferent arteriole, blood moves into a tiny capillary bed called the glomerulus.
The glomerulus is part of the functional unit of the kidney, called the nephron.
There's about 1 million nephrons in each kidney, and each of them consists of a renal corpuscle - made up of the glomerulus and the Bowman’s capsule surrounding it - and a renal tubule.
The renal corpuscle is where blood filtration starts.
Interestingly, once the blood leaves the glomerulus, it does not 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 renal tubule is made up of a proximal convoluted tubule, the nephron loop - also known as the loop of Henle - which has an ascending and a descending limb - and finally the distal convoluted tubule.
As filtrate makes its way through the renal tubule, waste and molecules like ions and water are exchanged between the tubule until, finally, urine is formed.
At the same time, the peritubular capillaries reunite to form larger and larger venous vessels.
The veins follow the path of the arteries, but in reverse - so they keep uniting until they finally form the large renal vein, which exits the kidney and drains into the inferior vena cava.
Okay - now if we zoom into the wall of the afferent arterioles, we’ll find a very special kind of smooth muscle cells, called juxtaglomerular cells, because they’re next to or “juxta” the glomerulus.