In this lesson we’re going to look in a little more detail about the structure and function of the kidneys. But I’ll give you a little disclaimer – the detailed explanations of all the functions of the kidneys are going to be in their own lessons still so that we can give them the attention they deserve!
So let me just remind you what we talked about in the urinary system anatomy lesson. The kidneys are in the abdominal cavity and they’re plastered against the posterior abdominal wall by the parietal peritoneum – so they are considered retroperitoneal. Here are the major functions of the kidneys. The one most people know already – the formation and excretion of urine. We also see the kidneys playing a huge role in fluid & electrolyte, and acid-base balance, as well as helping to regulate and maintain blood pressure. The kidneys also produce a hormone called erythropoietin that tells the bone marrow to make blood cells. And the kidneys will also help to activate vitamin D. These major functions are all covered in individual lessons. The blood pressure regulation can be found in the lesson called the renin angiotensin aldosterone system. Make sure you check those out individually to really understand the details of how the kidneys work.

So, let’s talk about structure. Externally, the kidneys are about the size of your fist. The lateral sides are convex and the medial sides are concave and have a little notch in them called the Hilum. The Hilum is where all the blood vessels and the ureters come in and out of the kidneys. Our kidneys are also surrounded by a fat pad called the perirenal fat pad that is there to help cushion and protect the kidneys. And the outer surface of the kidneys are covered with a white fibrous connective tissue layer called the renal capsule. So that’s the external structure of the kidneys.

In this image you can really see the internal structure in more detail. The outer layer in the kidneys is called the renal cortex. It contains renal corpuscles and the upper portions of the nephron. The nephron is the functional unit of the kidneys and we’re going to look at that in just a second. The inner area of the kidneys is called the renal medulla. This contains the renal columns, which is where the blood vessels flow through to the cortex, and the renal pyramids, which has lower portions of the nephron as well as our urine collecting ducts. Those collecting ducts all dump into the minor calyx or calyces plural, these smaller tubes that collect the urine – then those fuse together to form the major calyces like you see here. Those major calyces then fuse together into the renal pelvis and take the urine there so that it can be transferred to the ureters to go to the bladder. So the urine is created in the nephrons in the cortex and medulla, transferred to the minor and major calyces, then to the renal pelvis and ureter to the bladder.

So, now let’s look at the nephron. The nephron is the functional unit of the kidneys – this is where all the magic happens in the kidneys. The first place we start to see urine being created is the glomerulus, which is a tuft of capillaries that sits inside Bowman’s capsule right here. Then the urine process continues into the proximal convoluted tubule, then through the loop of henle down here, then into the distal convoluted tubule. From there, the urine is collected in the collecting ducts. Now to make it a little clearer, I like this little drawing of everything stretched out. So it comes in through the glomerulus into bowman’s capsule, into the proximal convoluted tubule, loop of henle, distal convoluted tubule, then the collecting duct. And, as you can see over here – we have many nephrons that dump into the same collecting duct. So remember we said the renal cortex contains the upper portions of the nephron – so basically all of our tubules are in the cortex. Then longer loops of henle and the collecting ducts are found in the medulla. If you go to the lessons on formation and excretion of urine and the fluid and electrolyte balance lesson, you’ll really see the major roles of the nephron in the kidneys.

Last thing I want to talk about quickly is the blood flow through the kidneys. This is kind of a simplified drawing of that. So – the renal artery comes off the abdominal aorta and into the kidneys – as it splits down it becomes the afferent arteriole and enters the glomerulus. Then we exit the glomerulus through the efferent arteriole and go into the peritubular capillaries. You can’t really tell here, but the peritubular capillaries are a network of capillaries that wrap all around the proximal and distal convoluted tubules AND the loop of henle. Then those capillaries will converge into venules and veins and into the renal vein to exit the kidneys and then dump into the inferior vena cava to return to the heart. This network of blood vessels is how our kidneys can help with all of the regulation that they do.
So, let’s recap. The kidneys are responsible for urine creation, fluid and electrolyte and acid base balance, and blood pressure regulation, as well as hormone and vitamin D production Remember these functions all have their own lessons so make sure you check those out. The kidneys are … well.. kidney shaped and they’re retroperitoneal and have a notch called the hilum on the medial side. Internally, we see the renal cortex on the outside, then the renal medulla, and we see urine dump into the minor and major calyces and the renal pelvis before emptying into the ureter. Remember that the nephron is the major functional unit of the kidneys and it’s where all the magic happens! In terms of blood flow, the renal artery comes in and feeds the afferent arteriole, which enters the glomerulus and comes out as the efferent arteriole, then to the peritubular capillaries. That network of capillaries comes back together and eventually leaves the kidneys as the renal vein.
Again, make sure you check out all the individual lessons on the different functions of the kidneys and all the resources attached to this lesson. Now, go out and be your best self today. And, as always, happy nursing!