At the concave medial margin of each kidney is a vertical cleft, the renal hilum. The renal hilum is the entrance to a space within the kidney, the renal sinus. Structures that serve the kidneys (vessels, nerves, and structures that drain urine from the kidney) enter and exit the renal sinus through the renal hilum. The hilum of the left kidney lies near the transpyloric plane, approximately 5 cm from the median plane. The transpyloric plane passes through the superior pole of the right kidney, which is approximately 2.5 cm lower than the left pole, probably due to the presence of the liver. Posteriorly, the superior parts of the kidneys lie deep to the 11^th and 12^th ribs. The levels of the kidneys change during respiration and with changes in posture. Each kidney moves 2-3 cm in a vertical direction during the movement of the diaphragm that occurs with deep breathing. Because the usual surgical approach to the kidneys is through the posterior abdominal wall, it is helpful to know that the inferior pole of the right kidney is approximately a finger’s breadth superior to the iliac crest.