We talk about things like heart health, taking care of your digestive system and things like that, but we never really think about the urinary system unless its telling us we need to pull over at the next rest stop. Yet, the urinary system is an important part of keeping your body in homeostasis. It clears the blood of waste and balances fluids and electrolytes. The kidneys also regulates blood pressure, secretes hormones and activates Vitamin D so we can use it. The urinary system consists of a pair of kidneys, pair of ureters, the bladder and the urethra. We’re going to take a look over the next couple of weeks at how this incredibly, and often forgotten, part of your body works!

Starting with the kidney. This will be just a sort of overview of the anatomy because the kidney gets super complex super fast. We pretty much all know what it basically looks like. Yeah, like the bean! The kidney is even sort of a reddish color. They sit on either side of our spinal column in the retroperitoneal space, so in the back of the abdominal cavity. On the inside, where the bean sort of folds in, is the hilum. Hilum in medical terms basically means where things go into something. So, nerves, blood vessels, lymphatics, ureters, etc. Outside of that is the renal pelvis, which has two or three major calyces which also branch into several minor calyces.

The capsule of the kidney itself has two layers – an outer dense irregular connective tissue and an inner layer which has myofibroblasts (like a smooth muscle mixed crossed with a fibroblast).

One layer deep of the capsule is the internal stroma. Here, we have reticular fibers and collagenous fibers.

So that’s like the outer shell. Inside the kidney, we divide it into the cortex (the outer layer) and the medulla (the inner part). Of course, those get subdivided, too, because…y’know…medicine.

The cortex has renal corpuscles, convoluted and straight tubules, collecting tubules and collecting ducts. These are part of the filtration system of the kidney. They also have an extensive vascular supply. The tissue itself of the cortex is characterized by vertical striations called medullary rays. Those are straight tubules and collecting ducts from the medulla. In the medullary layer, we see straight tubules, collecting ducts, and vasa recta (capillary networks). The vasa recta running next to the tubules is what allows the kidney to send products back into the blood so it can regulate the concentration of urine.

When you look inside of a kidney, you can see all these pyramidal looking structures with the peak being near the hilum. These are made from the tubules. That pyramid, along with the cortex above it makes up a renal lobe. On either side of the pyramid are renal columns. This portion is actually an embryological remnant of when the original renal lobes we make were first fusing together. At the tip of the pyramid is the papilla, which drains to a minor calyx which in turn drains to the major calyx, which in turn become the renal pelvis. This is how urine is getting out so it can drain into the ureters and then into the bladder.

Okay, so those lobules have interlobular arteries on either side of them. Those give rise to a series of glomeruli, each one has an afferent (towards) glomerular arteriole, a capillary network and an efferent (away) glomerular arteriole. We need all these in order to filter the blood. That capillary network is called a glomerulus and is surrounded by a Bowman’s capsule. So, blood and waste products are filtered out in the glomerular network, through the Bowman’s capsule and into the (uriniferous) tubules. Each kidney is rocking somewhere around 1.3 million of these tubules, each one consisting of the nephron we just described and collecting ducts.

The nephron has the renal corpuscle and a long renal tubule. The renal tubule has its own set of parts, the proximal (first) convoluted tubule, the loop of Henle, and the distal (farther) convoluted tubule which empties into the collecting tubule. These tubules dip down into the pyramids which has increasing degrees of pressure causing wastes to move in and out. How that works is totally a different post. It gets complicated quick.

Outside of that, it’s important to know that the epithelium of the nephron has fenestrations or slits that allow things to flow out and go into the tubules. Those tubules also have podicytes, which are also a special type of epithelium, with long projections that interweave with each other. So, it’s kind of like fingers interlaced together. This also helps regulate filtration.

Let’s leave it here for now and we’ll tackle some more stuff next week!