Human kidneys play an integral role in the development of primary or secondary immunologic diseases. As a major filtering organ, the kidneys, which represent about 0.5 percent of the human body mass, receive 20 percent of the total cardiac output. The enormous blood flow (1 L/min) to the renal microcirculation exceeds that observed in other major vascular organs (heart, liver, and brain). Urine is produced after a complex process of glomerular filtration, tubular transport, and reabsorption at a rate of 1 ml/min. Cellular elements involved in immunity thereby have a high probability of interacting with glomerular and tubular cells that may or may not cause renal disease.
Sufficient knowledge of the anatomy and histology of the kidney is vital in understanding the pathogenesis of renal diseases. The renal corpuscle, or glomerulus (Figure 17.1), is composed of capillary tuft lined by a thin layer of endothelial cells; central region of mesangial cells and its surrounding matrix; and visceral and parietal epithelial cells with their respective basement membranes. The glomerulus is primarily responsible for production of ultrafilrate from the circulating plasma. The filtration barrier between the bloodstream and urinary space is made up by the fenestrated endothelium, the glomerular basement membrane (GBM), and the slit pores seen between the foot proc esses of the visceral epithelium (Figure 17.2).
The endothelial cells form as initial barriers to cellular elements of the blood (red blood cells, leucocytes, and platelets) in reaching the subendothelial space. The endothelial cells produce nitric oxide (a vasodilator) and endothelin-1 (a potent vasoconstrictor), chemical substances implicated in inflammatory processes.