Kidneys

The kidneys are retroperitoneal organs located on the posterior abdominal wall at the level of T12-L3 vertebrae (Figure 36.1). Occupying a region either side of the vertebral column, the right kidney lies slightly lower than the left. The kidneys have some mobility, being able to move up to 2–3 cm during respiration and as a result of changes in body positioning (e.g. moving from erect to supine position). The kidneys have anterior and posterior surfaces, medial and lateral margins, and also superior and inferior poles (Figure 36.2). On the medial concave margin is a vertical slit called the renal hilum, which is the region where vessels, nerves and the renal pelvis enter/leave the kidney. The superomedial aspect of each kidney makes contact with the suprarenal (adrenal) gland. These two organs function independently, with a fascial septum separating them. The kidneys and suprarenal glands are surrounded by a protective layer of fat (perinephric fat), which in turn is enclosed by a layer of renal fascia. The kidneys receive their blood supply directly from the abdominal aorta, via the renal arteries (Figures 36.1 and 36.2). Close to the hilum, the right and left renal arteries divide into approximately five segmental arteries to supply the kidney. Venous blood drains through the renal veins to empty into the inferior vena cava (Figure 36.1). Lymphatic drainage of the kidney is to the lumbar lymph nodes. Innervation is from sympathetic, parasympathetic and visceral afferent fibres via the renal plexus.

Each kidney is composed of an outer cortex and an inner medulla. The medulla is composed of renal pyramids (approximately 10–18), which have an apex and base. The cortex extends to the base of the pyramid and between the pyramids, forming the renal columns (Figure 36.3). The cortex and medulla are composed of nephrons, which are the functional units of the kidney. Each nephron has a glomerulus (situated in the cortex) and a long tubule (loop of Henle) that extends into the medulla of the kidney, before uniting with a collecting tubule. Numerous blood vessels (vasa recta) are also found in the medulla (Figure 36.3).