Coronal section of the kidney

Renal cortex

Just deep to the thin fibrous layer of the renal capsule is the renal cortex. The renal cortex is a highly vascularized area; interlobar arteries and veins and the interlobular vessels can be observed histologically. An outline of the journey of waste matter from the renal arteries to the renal pelvis can be better appreciated when the kidney is viewed in a coronal section.

The interlobular vessels indirectly provide arterial supply to the renal cortex. They give off afferent arterioles, which enter the glomerular apparatus then leave as efferent arterioles after which they move on to supply the renal cortex.

The renal corpuscle (which consists of the glomerulus and the glomerular (Bowman’s) capsule), proximal and distal convoluted tubules also reside in this area of the kidney. The distal ascending and proximal descending limbs of the loops of Henle of cortical nephrons, as well as the collecting tubules (not considered a part of the nephron) can also be seen in the cortex. In between the medullary pyramids, the cortex extends towards the renal pelvis as the renal columns.

Renal corpuscle

As previously mentioned, the renal corpuscle consists of the glomerulus and the Bowman’s capsule. The glomerulus is an anastomosing capillary bed formed from afferent arterioles of the interlobular arteries. The capillary walls are fenestrated and surrounded by a double-layered filtration membrane. The visceral filtration membrane has a unique endothelium that is formed by podocytes that intertwine with each other, forming a size-selective filtration layer.

The parietal layer of the filtration membrane is simple squamous , without fenestration. The cavity formed between the two layers is continuous with the lumen of the proximal convoluted tubules. The fenestrations are typically about 70 - 100 nm wide, thus restricting larger particles from leaving the capillary bed. Urea and other solutes that are able to pass through the fenestra form the filtrate. Based on physiological need, the filtrate is further processed along the renal tubules in the medulla.

Renal medulla

Numerous striated pyramidal structures found throughout substance of the kidneys are called the renal medulla. The loops of Henle of juxtaglomerular nephrons extend into these pyramids and are surrounded by capillary beds (vasa recta) that facilitate water and ionic reabsorption.

Also found in the medulla are collecting tubules of varying sizes. From a histological perspective, the collecting tubules are larger than the loops of Henle and lightly stained with cuboidal cells; while the loops of Henle contain cuboidal cells that are stained darker than the collecting duct cells. Several collecting tubules drain into a collecting duct.

The apex of the renal medulla, or the renal papilla, contains the end portion of the collecting ducts. Here they are referred to as the papillary ducts. The ascending limb of the distal and the descending limb of the proximal tubules are also found in the papilla. The papillary ducts then drain urine into the minor calices to be excreted.

Calices and renal pelvis

The open cavity in the medial concavity of the kidney – the renal sinus – is partly occupied by the calices and the renal pelvis. The calices act as conduits for urine to leave the medulla and enter the ureters.

There are both minor and major calices. Each minor calix is a funnel shaped structures that surrounds a renal papilla and collects urine from it through the cribriform area (sieve-like apparatus). Several minor calices converge to form one major calix. The major calices (of which there are usually three) then facilitates the passage of urine to the renal pelvis (the beginning of the ureters). The renal pelvis is an empty cavity that exits the medial border of the kidney posterior to the renal neurovascular structures.To master the anatomy of the kidney and its related structures, take a look at the following resources: