Coronal Section of the Kidney
The kidneys are a pair of bean-shaped organs located on either side of the superior posterior abdominal wall. Its lateral border is convex while its medial border is concave. The medial concavity is the point at which the renal neurovascular structures enter and leave the kidneys. These organs accomplish their excretory role by converting nitrogenous waste to urine. It is also an integral part of water and pH homeostasis, blood pressure regulation and red blood cell regulation. In addition, it assists in maintaining the right balance of ions in the body through a series of filtration and reabsorption. Approximately 25% of the total cardiac output is delivered to the kidneys by the renal vessels to be filtered. 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.
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. 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.
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.
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 calyces to be excreted.
Calyces and the Renal Pelvis
The open cavity in the medial concavity of the kidney – the renal sinus – is partly occupied by the calyces and the renal pelvis. The calyces act as conduits for urine to leave the medulla and enter the ureters. There are minor calyces and major calyces. Each minor calyx is a funnel shaped structures that surrounds a renal papilla and collects urine from it through the area cribrosa (sieve-like apparatus). Several minor calyces converge to form one major calyx. The major calyx 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.
Pathologies of the glomerulus most often result from defects in its anatomical structure. If selective filtration is hindered by change in the width of the fenestrae, then larger particles that should be retained (such as blood or protein) will be excreted. Pathogenic renal insults can either be of inflammatory or obstructive aetiology.
Both primary (the kidneys are the only affected organs) and secondary (the kidneys are affected because of an underlying pathology) glomerulonephritis are of major medical concern. Focal segmental glomerulosclerosis (FSGS) is a glomerulopathy that can be either primary or secondary. It involves sclerotic scarring of segments of capillaries (segmental) in some glomerulus (focal). This pathology may or may not produce the nephrotic syndrome (loss of protein in the urine, low blood albumin levels, generalized oedema and high levels of lipids in both blood and urine), azotaemia (nitrogen in blood), hypertension and haematuria (blood in urine). In addition to the glomerular diseases, obstruction of the urinary pathways will eventually initiate pathological processes. The obstructive processes can occur in one or both kidneys and it can be partial or total. Congenital deformities such as stenosis at the ureteropelvic junction and urinary calculi (kidney stones) may lead to renal obstruction. Obstruction will restrict the amount of urine excreted. Consequently, accumulation of waste fluid will result in pain caused by pelvic and calyceal distention. Stagnant urine can also serve as a bacterial breeding ground, causing a bacterial infection that may further impair kidney function.