The autonomic input to the kidney is responsible for regulating blood pressure. The kidneys receive autonomic input from both the parasympathetic and sympathetic nervous systems via the renal plexus. The sympathetic supply originates from spinal cord levels T11-L3. The vagus nerve (cranial nerve X) forms the parasympathetic innervation. The postganglionic fibers follow the arterial blood supply and primarily innervate the cortex but also extend into the medulla.
Alterations in the level of autonomic activity result in physiological changes in renal hemodynamics, as well as the kidneys excretory and secretory function. A decrease in pressure at the carotid sinus due to decreased circulating volume will increase the sympathetic activity of the kidney. A graded increase in sympathetic activity results in a progressive increase in beta-adrenoceptor-mediated renin secretion and an alpha-adrenoceptor mediated decrease in sodium and water excretion as well as vasoconstriction of the kidney. Activation of cardiopulmonary receptors due to stretch and volume expansion lowers sympathetic activity resulting in lower renin levels, an increase in sodium and water excretion, as well as renal vasodilation.
Afferent renal nerves originate from the renal pelvic area at the highest density, but also from the renal cortex. These nerves project to some brain regions including the subfornical organs, the hypothalamus, and brainstem. They are activated by an increase in wall tension. Osmotic pressure and crosstalk occur between the two kidneys to allow the inhibitory renorenal reflex. These afferent nerves travel to T10-T11. During renal injuries (e.g., ischemia, hypoxia, and parenchymal injury), the pain referred dermatomes covering the anterior abdominal wall and flanks.
The embryological development of the kidney is from a cranial to caudal direction. The embryological origin of the kidney is from the intermediate mesoderm, then to the urogenital ridge, nephrogenic cord and finally the urinary collection system.
The earliest kidney is pronephros which appears by four weeks but it nonfunctional. This is followed by the mesonephros which forms the Wolffian duct (mesonephric duct). The mesonephros functions as a temporary kidney for most of the first trimester and will eventually give rise to the male genital system.
The metanephros develops from the mesonephric outgrowth and by five weeks is called the ureteric bud. It is fully functional and canalized by week two of the pregnancy. This orderly progression of kidney development continues till about 34 to 36 weeks of gestation. The metanephros will give rise to the ureter, renal pelvis, calyces and collecting ducts. Most anomalies of the kidney develop at this stage of development.
As the metanephros develops, the ureteropelvic junction fuses with the kidney and is the last to canalize. This is the most common anatomical site of obstruction in the fetus leading to hydronephrosis.
The metanephros grows faster caudally than cranially and then undergoes a rotation which brings the kidneys more medially towards the center of the body. The final location of the adult kidney is at T12-L3, with the right kidney pushed down a little further because of the liver.
The kidneys derived their blood supply from the renal arteries which come off the aorta. This vascular supply is constant in 90% to 95% of cases, but in another 3% to 10%, there may be aberrant arteries. The right renal artery courses under the vena cava to get to the aorta. Both the renal arteries run close together with the renal veins, which drain into the inferior vena cava. The left renal vein is longer and crosses over the aorta to reach the inferior vena cava. For this reason, it is preferred for renal transplantation.
The lymphatic drainage of the left kidney is to the para-aortic lymph nodes whereas the right kidney drains into the interaortocaval and paracaval lymph nodes.
The kidneys are supplied by both the parasympathetic and sympathetic nervous system. The Thoracolumbar nerves original from T10-L1 and provides vasomotor control to the splanchnic nerve, after synapsing with the celiac and renal ganglia. It is important to note that these nerves do not play any role in the maintenance of renal function. This is evident after renal transplantation where the nerves are excised, but the kidney function remains intact. Hormones regulate the kidney function.
Both kidneys are located in the retroperitoneum, just below the diaphragm. The left kidney is usually below the 11 rib extending to L3.
The right kidney is slightly lower and found between the 12 rib and L3.
Located superiorly on each kidney are the adrenal glands. Other nerves that run in the vicinity include the ilioinguinal and iliohypogastric. The kidneys sit on the psoas major and quadratus lumborum muscles.
Accessory or supernumerary renal arteries are the most common type of vascular variation. The majority of these accessory arteries supply the lower pole of the kidney. These vessels may also have an aberrant course like passing anterior to the inferior vena cava (IVC) and over the ureteropelvic junction. They may sometimes obstruct the ureteropelvic junction. Another common anomaly is the left renal vein which may pass under the aorta to reach the inferior vena cava. If not recognized during abdominal aortic aneurysm surgery, it can be torn and result in torrential life-threatening hemorrhage.
Another common anomaly is the left renal vein which may pass under the aorta to reach the inferior vena cava. If not recognized during abdominal aortic aneurysm surgery, it can be torn and result in torrential life-threatening hemorrhage.
The position of the kidneys in the retroperitoneum may also vary. An ectopic kidney may be found in the pelvis when it fails to ascend. Or sometimes the kidney may be malrotated or fused.
In some kidney fusion anomalies, both kidneys may be located on the same side. Some of these abnormalities may also be associated with ureteropelvic junction obstruction and hydronephrosis. Most cases of fused kidneys are, however, discovered as an incidental finding on an imaging study. However, it should be noted that even when the kidneys are ectopic, the adrenal glands usually remain their superior location of the peritoneum, since their embryological origin is different from the kidneys.
In patients with hypertension, there is increased sympathetic activity to the kidneys. In the past renal denervation was done to decrease the sympathetic nerve input and lower blood pressure. Today, the sympathetic nervous system can be blocked by beta blockers. Also, pathological activation of the intrarenal baroreceptors may result from renal artery stenosis or renal ischemia This can also cause an increase in sympathetic nerve activation, hypertension, and hypersecretion of renin.
The variation in the kidney may also exist with the collecting system. Duplication of ureters or anomalies in drainage or connection to the bladder can also occur. Sometimes the ureters may drain into a single bladder orifice or in a duplicated system, the upper pole segment may drain inferomedially into the bladder, and the lower pole segment may drain superolaterally. It is important to be aware of these anomalies during surgery.