Fig. 1. Anatomy of the glomerulus.

The glomerulus is the blood-filtering unit of the kidney. Each glomerulus drains the filtrate into its own tubule, and the glomerulus and its tubules together constitute the functional unit of the kidney, the nephron. The vascular part of each glomerulus includes an afferent arteriole, an efferent arteriole and a capillary network inside the glomerulus, where the filtration occurs under conditions of high perfusion pressure and shear stress. The capillary network is held together by mesenchymal cells, known as mesangial cells, and a matrix, which regulate capillary tension. Parts of the glomerular filtrate pass through the mesangium; hence, circulating antigens and immunoglobulins can get trapped there. Glomerular capillaries are characterized by a fenestrated endothelium covered with glycocalyx and attached to the glomerular basement membrane. At the outer aspect of the glomerular capillaries, podocytes attach to the glomerular basement membrane. Podocytes are specialized epithelial cells with neuron-like primary and secondary foot processes interdigitated with the respective secondary foot processes of neighbouring podocytes. Between podocyte foot processes is the slit diaphragm, which covers a large area of the filtration barrier and is essential for preventing the passage of serum proteins such as albumin into the filtrate. Water, ions and other small solutes cross the filtration barrier through pores in the slit diaphragm. Inflammatory processes in the glomerulus typically alter the barrier function and cause leakage of serum proteins and frequently also of intact blood cells into the urine.