The kidneys are the major excretory organs of the body. They excrete the organic wastes urea, uric acid and creatinine. They regulate the volume of blood (and hence blood pressure) by increasing or decreasing the volume of urine produced to match the body’s requirement for water. They help regulate blood pH by excreting hydronium ions and by reabsorbing and producing bicarbonate ions. They maintain blood osmolarity at ~290 mosml/L and individual ions (electrolytes) at their healthy concentrations. The kidneys produce the enzyme renin, the hormone erythropoietin and the “vitamin” calcitriol. In turn the physiological functions of the kidney are influenced by ADH, aldosterone, parathyroid hormone, ANP, BNP and angiotensin II. The nephron is the blood processing functional unit of the kidney. The nephron consists of a glomerulus and a renal tubule. The glomerulus is a spherical capillary bed that is supplied with blood via the afferent arteriole and is drained by the efferent arteriole. The renal tubule consists of the “Bowman’s” capsule (which surrounds the glomerulus and receives the fluid filtered from the glomerulus) and the proximal convoluted tubule, the descending and ascending limbs of the loop of Henle and the distal convoluted tubule. A collecting duct receives filtrate from several nephrons and delivers it to a calyx. Macroscopically, the kidney consists of a superficial cortex (where all of the glomeruli are located) and the deeper medulla. The medulla consists of renal “pyramids” separated by renal “columns”. The renal tubules extend into the pyramids and the collecting ducts pass through them to deliver urine from their apex (the papilla) into a tube called a minor calyx. Minor calyces join to from major calyces which then expand into the renal pelvis. The pelvis is drained by a ureter that delivers urine to the storage bladder. The efferent arteriole feeds into the peritubular capillary bed which surrounds the PCT and the DCT of the nephron and is located in the cortex. A small percentage of the blood from the efferent arteriole also flows into another capillary bed known as the vasa recta which surrounds the loop of Henle and extends into the pyramids of the medulla. The interlobar arteries are located in the renal columns and deliver blood from the renal artery to the arcuate arteries, which lie superficial to the bases of the pyramids, and then to the cortical radiate arteries which in turn supply the afferent arterioles which enter the glomeruli.