Renal and urinary system

The kidneys are bean shaped, retroperitoneal organs located on either side of the vertebral column extending from vertebrae T12-L3 with the left kidney situated higher than the right kidney. Ribs 11 and 12 are related to the posterior surface of the kidneys. The kidneys are covered by a fibrous capsule and surrounded by renal fat pad. Gerota’s fascia encapsulates the kidney and adrenal gland. The kidneys lie anterior to the quadratus lumborum muscle and lateral to the psoas major.

On cross section, the kidney can be divided into an outer cortex which extends inside as the renal columns of Bertin and an inner medulla which is organized into 5-11 pyramids. The base of the pyramids face the cortex while the tips are directed medially and lead to renal papillae which open into the minor calyces. The renal lobes correspond to the number of pyramids and are composed of a renal pyramid and the cortex adjacent to it , on all sides. Each lobe consists of multiple lobules. Each lobule drains into its corresponding collecting duct.

The functional unit of the kidney is a nephron. The nephron is composed of the renal corpuscle (glomerulus and Bowman’s capsule), proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct. Some collecting ducts are located in the cortex while others are located in the medulla. The medullary collecting ducts merge with larger papillary ducts of Bellini which open into the renal papillae. The minor calyces collect urine from the papillae and lead to bigger, major calyces. The major calyces fuse to form the renal pelvis. Both types of calyces and the renal pelvis are lined by transitional epithelium and have smooth muscle in their walls.

The proximal and distal convoluted tubules and the short loops of Henle of cortical nephrons are located in the cortex. The collecting tubules (connect the DCT to the collecting ducts) and long loops of Henle are in the medulla. Some collecting ducts are located in the cortex while others are in the medulla.

Glomerulus

The glomerulus is a tuft of capillaries, with the afferent arteriole on one end and the efferent arteriole on the other. The glomerular capillaries are continuous, fenestrated capillaries without diaphragms, to aid in filtration of the plasma. The fenestra are about 70-100 nm in diameter. The glomerular basement membrane has type IV collagen, proteoglycans, laminins and fibronectins. They are surrounded by the podocytes or visceral epithelial cells. The glomerular filtration barrier is composed of endothelial cells, basement membrane and podocytes. The mesangial matrix and mesangial cells (pericytes) lie in between the capillaries. The glomerulus is surrounded by the Bowman’s capsule which has an inner visceral layer and outer parietal layer with the Bowman space in between. It then continues as the proximal convoluted tubule.

The proximal convoluted tubule (PCT) is lined by cuboidal epithelium and is connected by tight junctions or zonula occludens. The loop of Henle has four parts - the proximal straight tubule, the thin or descending limb, the thick or ascending limb and the distal straight tubule. The distal convoluted tubule has the macula densa. The collecting duct has three important cell types - principal cells, type A intercalated cells and type B intercalated cells. Principal cells have ADH receptors. Intercalated cells are predominant in the cortical collecting ducts. Type A cells have H+ATPase at the luminal surface while type B cells have H+ATPase in the basolateral membrane.

Vasculature of the kidney

The renal artery and vein supply the kidneys. The renal artery arises from the abdominal aorta at the level of L1-2 vertebrae, just below the origin of the SMA. They run posterior to the renal vein. At the hilum, the renal artery divides into anterior and posterior divisions. In all, each renal artery divides into five segmental arteries. The anterior division divides into upper, middle, lower and apical segmental arteries while the posterior division forms the posterior segmental artery. Segmental arteries further give rise to lobar, interlobar, followed by arcuate and interlobular arteries. The arcuate arteries travel along the base of the renal pyramids at the corticomedullary junction. The interlobular arteries give rise to afferent arterioles that lead to the glomerulus. The efferent arteriole drains the glomerulus.

The efferent arterioles of the cortical nephrons form a peritubular capillary network around the cortical tubules. The efferent arterioles of the juxtamedullary nephrons are arranged in a “hairpin loop” like structure. They give off straight vasa recta that run deep into the medulla. These descending vasa recta form a peritubular network around the medullary tubules. They then join to form the ascending vasa recta. This system is integral to the countercurrent multiplier system.

Accessory renal arteries can be present in 30% of the population, some may cause obstruction of the ureteropelvic junction. Rarely, aberrant renal arteries which enter the renal capsule and not the hilum are seen. Ectopic kidneys may be supplied by renal arteries arising from the celiac, SMA or iliac arteries.

Both renal veins, right and left drain into the IVC. The right renal vein is much shorter than the left. The left renal vein courses anterior to the abdominal aorta and inferior to the SMA. The left adrenal vein, left inferior phrenic vein, left gonadal vein and, in some cases, lumbar veins drain into the left renal vein. On the other hand, the right renal vein typically does not have any such tributaries except an accessory adrenal vein in a few cases. At the renal pelvis, the renal vein is situated anterior to the renal artery.

Stellate veins of the kidney drain into interlobular veins, which drain into the arcuate veins. The arcuate veins will drain into interlobar veins which form the segmental veins that unite into the renal vein. The ascending vasa recta drain into the interlobular and arcuate veins. Anatomical variations are seen in around 30% of the population with respect to the renal veins. Typically, the left kidney is preferred for live organ donation as the longer left renal vein affords easier technical manipulation.