Kidneys are large, bean-shaped organs that are approximately 12 cm long, 6 cm wide, and 3 cm thick in adults.
The kidneys perform a lot of different functions, such as the excretion and removal of metabolic waste and foreign substances through urine.
The kidneys also activate vitamin D when needed, and help maintain the balance of fluid volume, pH, blood pressure, and electrolytes in the body.
The kidneys also secrete important hormones, such as erythropoietin, which increases the production of red blood cells.
The medial border of each kidney has a concave area called the hilum. This is where the ureter exits the kidney and renal artery, renal vein, and lymph vessels enter and exit the kidney.
The functional tissue of the kidney, or parenchyma, has an outer renal cortex and inner renal medulla.
The medulla is organized into cone-like structures called renal pyramids and renal columns in between the pyramids, which are extensions of the renal cortex.
At the junction between the cortex and medulla are millions of functional units called nephrons. Each nephron can be divided into its major parts:
The renal corpuscle, proximal convoluted tubule, loop of henle, distal convoluted tubule, and collecting duct.
Each nephron starts with a renal corpuscle, which is a spherical structure in the cortex that has a diameter of about 200 um.
The corpuscle consists of the glomerulus as well as the surrounding double-layered epithelial capsule called the glomerular or Bowman’s capsule.
The glomerulus is a bundle or tuft of capillaries that supplies the blood that’s filtered to become a fluid called the glomerular filtrate or ultrafiltrate, which typically contains no blood cells or large proteins.
The ultrafiltrate initially drains into the capsular or Bowman’s space, which is actually the space in between the two layers that make up Bowman’s capsule.
The outer layer of the capsule is a parietal layer of flat simple squamous epithelium; and the inner layer is a visceral layer of complex epithelial cells called podocytes.
The podocytes have prominent large oval nuclei and cell bodies that wrap around each of the capillaries. The podocytes form narrow slits between the cells that filter the blood to form the ultrafiltrate.
The glomerulus also has large cells present called mesangial cells that have irregularly shaped nuclei and can be difficult to distinguish from podocytes in a standard slide stained with hematoxylin and eosin; but they’ll often have nuclei that look darker than podocyte nuclei in the image.
The mesangial cells have contractile properties to help regulate the amount of blood flowing to the glomerular capillaries. They’re also involved in immune defense and repair of the glomerulus.
In some images of the renal corpuscle, you’ll also be able to see the vascular pole, which is where the afferent and efferent arterioles enter and exit the glomerulus in order to supply and drain the glomerular capillaries.
The urinary pole is where the ultrafiltrate is drained from the capsular space into the next part of the nephron, the proximal convoluted tubule.
Typically, the proximal convoluted tubule, or PCT for short, can only be seen in cross-sections because of the winding or “convoluted” path it takes through the cortex.