Neurovascular Supply of the Kidney
The kidneys are bilateral, bean shaped organs that are situated retroperitoneally. The organs lie along the posterior abdominal wall, where they filter blood, maintain ionic homeostasis and produce urine. The body of the liver displaces the right kidney inferiorly, resulting in the left kidney being slightly superior to the right. Along the medial surface, there is a concavity known as the hilum. At this point, the renal arteries enter, and the renal veins and pelvis (beginning of the ureters) leave the kidney. The neural supply from both divisions from the autonomic nervous system also enters the kidney at the hilum. Lymphatic drainage from both the renal cortex (outer layer of kidney) and renal medulla (inner layer of kidney) go to the same group of nodes.
The abdominal aorta gives off many branches, including the renal arteries. The renal arteries branch off perpendicular to the abdominal aorta, travelling posterior to renal veins, nerves and the pancreas. Subsequent to branching from the aorta, the renal artery enters the kidney at the hilum, where it divides into anterior and posterior branches. The posterior division goes on to supply the posterior region of the kidney, while the anterior branch divides further to produce apical, anterior superior, anterior inferior and inferior segmental arteries; each supplying their respective segments. At the level of the minor calyces, the branches of the anterior renal arteries further divides into interlobar arteries that course around the borders of the medullary pyramids. At the base of the pyramids, these arteries are referred to as arcuate arteries. Finally, the arteries enter the nephrons (functional units of the kidneys) as the interlobular arteries, where afferent arterioles bring blood to the glomerulus to be filtered. It should be noted that these arteries neither anastomose nor have accompanying veins.
As the afferent arterioles enter the glomerulus, they form an intricate network of communicating capillaries. The capillaries are lined by a unique fenestrated epithelium (each space being around 70 – 100 nm wide). The fenestration allows selective passage of smaller particles into the renal tubules and keeps larger blood cells in the vessels. The capillaries leave the glomerulus as efferent arterioles, after which they form capillary beds around the nephron’s loop of Henle. In cortical nephrons (loop of Henle does not extend deep into the medulla), the capillary beds are called peritubular capillaries; while in juxtamedullary nephrons (loop of Henle extends deep into the medulla), the capillary beds are called vasa recta. These capillary beds facilitate blood pressure regulation and ionic homeostasis both passively and under hormonal influence.
As the capillaries leave the nephron, they condense to form interlobular veins. Similar to the branches of the renal arteries, the interlobular veins become arcuate veins at the base of the medullary pyramids, then interlobar veins. About five or six interlobar veins join together to form each renal vein. Unlike the branches of the renal arteries, the tributaries of the renal vein communicate with each other.
Superficial lymphatic vessels form a plexus under the renal capsule (thin layer covering the kidneys) known as the subcapsular lymphatic plexus. They, along with medullary lymph vessels, communicate with cortical lymph vessels that travel alongside interlobular, arcuate and interlobar arteries. The renal lymphatics then drain directly to the lumbar lymph trunks (which then drain to the thoracic duct and cisterna chyli) and to para-aortic nodes, including precaval, lumbar and postcaval nodes.
Both the sympathetic and parasympathetic divisions of the autonomic nervous system are responsible for innervating the kidneys. Thoracolumbar outflow from T10 to L1 provide vasomotor supply via the thoracolumbar splanchnic nerve, after synapsing at the renal and coeliac ganglia. Parasympathetic fibers from the vagus nerve as well as fibers from the intermesenteric plexus (S2 to S4) also innervate the kidneys. Afferent fibers conveying pain from the viscera travel along the sympathetic pathway. The afferent fibers that detect pain originating from kidney stones in the renal pelvis or calyces, travel via the coeliac plexus to the sympathetic trunk by route of the splanchnic nerves. The associated nausea and vomiting may result from afferents travelling along the vagal route.
Renal infarction subsequent to occlusion of any of the arterial branches supplying the kidneys is of concern considering that there are no communicating arterial branches. Additionally, the segmental branches of the renal artery are terminal arteries. Furthermore, the medulla is also susceptible to ischemic necrosis because its arterial supply is derived from the efferent arterioles exiting the glomerulus. Therefore, any form of vasculitis can further reduce the already, remarkably low oxygen content of the medulla. Clinicians should take into account the possibility that some cases of hypertension could be resultant of renal artery stenosis. The narrowing of the renal arteries results in ischemia. The kidneys attempt to correct this by producing excess renin, which causes an elevation in blood pressure. This pathology can be corrected surgically. Also of clinical significance are sickle cell associated nephropathies. Individuals with sickle cell disease or the trait are both susceptible to renal insults related to this disease. Haematuria and hyposthenuria (excreted urine with low urea concentration) are typical anomalies associated with the disease.