The kidney is an organ with endocrine properties, and the capacity to synthesise and catabolise proteins. However, its fundamental role is to remove fluid and potentially toxic substances by the production of urine as an ultrafiltrate of plasma, and to maintain homeostasis of body protein. These latter functions are the task of the nephron, a microscopic unit comprising the glomerulus, Bowman's capsule, proximal and distal convoluted tubules, and the collecting ducts (Fig. 1.1). The various components of the nephron interact with the systemic circulation, thereby influencing renal handling of plasma proteins. Before discussing this in detail, the structure of the nephron components will be summarised briefly.

The glomerulus is a capillary network composed of a thin layer of endothelial cells, a central region of mesangial cells with surrounding mesangial matrix material, visceral epithelial cells with associated basement membrane, and the parietal epithelial cells of Bowman's capsule with its basement membrane. Bowman's (urinary) space lies between these two epithelial layers, and afferent and efferent arterioles control the capillary blood flow (Fig. 1.1). Glomeruli are innervated by autonomic nerves, and neural control may be particularly important in the larger juxtamedullary glomeruli, close to which is the site of renin secretion.

Molecules in the glomerular ultrafiltrate may traverse the filtration barrier from blood into the urinary space.