Introduction to the Kidney, Renal Physiology, and Contrast

Imaging Anatomy

The kidneys are bean-shaped, paired, retroperitoneal organs. The renal parenchyma has 2 major components: The cortex (outer portion) and the medulla (inner portion). The renal parenchyma is divided in multiple lobes (8-18), each of which is composed of medulla (pyramid) and cortex. Each pyramid drains into a minor calyx. Multiple (2-3) minor calyces converge into a major calyces. The renal pelvis, an extension of the ureter, collects urine from multiple calyces.

The functional unit is the nephron. It is composed of the renal corpuscule (glomerulus and Bowman capsule) and tubules (proximal and distal convoluted tubules and loop of Henle). The filtrate from the distal convoluted ducts passes into collecting ducts that extend to the tip of the medulla (papilla).

Renal arteries branch from the abdominal aorta and the renal veins drain into the vena cava. Accessory renal vessels are common, and correct identification and description is important for surgical planning. The renal lymphatic drainage is into aortocaval lymph nodes.

Embryology and Congenital Malformations

Early in embryologic development, the mesoderm on either side of the dorsal body wall develops into a succession of primitive kidneys. The metanephroi are the definitive functioning kidneys and are induced to form when the ureteric buds penetrate the metanephric blastema, a specialized mesoderm at the caudal end of the embryo. The ureteric bud then bifurcates, which in turn, induces the metanephric blastema to form lobes. As the lobes form, there is a reciprocal inductive effect on the branching of the ureteric bud forming the calyces. As the renal parenchyma develops, it will differentiate into an outer cortex, containing the glomeruli, and the medulla, containing the collecting ducts. The next important event in the development of the kidneys is the ascent from the sacral region, where they are formed to either side of the lumbar spine.

The complexity of the interaction of the metanephric blastema with the ureteric bud, coupled with the ascent of the kidneys, leads to a host of potential congenital renal anomalies, including persistent fetal lobation, prominent columns of Bertin, horseshoe kidneys, renal ectopia, renal duplications, ureteropelvic junction obstruction, and megacalyces.

Principles of Renal Physiology

The main functions of the kidneys are the maintenance of homeostasis, removal of metabolic waste products, and production of urine. The functions are carried out with a combination of glomerular filtration, tubular reabsorption, and tubular secretion. Filtration is conducted in the renal corpuscle (glomerulus, Bowman capsule) in the cortex, while reabsorption and secretion occur in the different tubular components (proximal, distal convoluted tubule, loop of Henle, collecting ducts) located in the cortex and medulla.

Glomerular filtration rate (GFR) is the volume of fluid filtered in the glomerulus per unit of time per unit of body surface area. It is determined by the net filtration rate, capillary permeability, and surface area of the capillary bed. Estimated GFR, an indicator of renal function, can be calculated using the Modification of Diet in Renal Disease equation based on serum creatinine, age, gender, and race.

• Renal function according to eGFR is categorized as stage 1 (eGFR > 90 mL/min/1.73m²), normal renal function; stage 2 (eGFR 60-89 mL/min/1.73m²), mildly reduced renal function; stage 3 (eGFR 30-69 mg/mL/1.73m²), moderately reduced renal function; stage 4 (eGFR 15-29 mL/min/1.73m²), severely reduced renal function; and stage 5 (eGFR < 15 mL/min/1.73m²), end-stage renal failure