Histology, Kidney and Glomerulus

The kidneys have several essential homeostatic functions. These functions include waste removal (NH3), fluid/electrolyte balance, metabolic blood acid-base balance, as well as producing/modifying hormones for blood pressure, calcium/potassium homeostasis, and red blood cell production. The renal corpuscle (filtration unit, which comprises the glomerulus and the surrounding glomerular or Bowman’s capsule) and tubules (reabsorption and excretion) of the kidney perform the majority of these kidney functions. The primary function of the kidney, filtering blood, is in part due to its unique blood flow, with high perfusion autoregulation of flow across the glomerular capillaries over a range of pressures.  The kidneys receive a high proportion of blood, about 20% of cardiac output, thus enabling the filtration of large volumes of blood. Blood flow autoregulates across the filtration capillaries (glomeruli) due to the unique arrangement of blood vessels. The glomerulus, in contrast to the majority of other capillary beds, sits between two arterioles; receiving blood supply from the upstream afferent arteriole, and blood exiting downstream via the efferent arterioles (E for exit). This arrangement allows for precise control of glomerular flow within the glomerulus, and filtration, rate via autoregulatory changes of the diameters of these resistance arterioles (vasodilation/ vasoconstriction).   The primary function of the kidney is to filter blood and form urine. The histological structures of the filtering units of the kidney (renal corpuscles) are crucial for this function. The renal corpuscles are located only in the kidney cortex, with about 1 million per kidney with variation due to race. This unique filtration barrier contains three histological structures: the capillary endothelium of the glomeruli, specialized cells called podocytes, and the fused basement membranes of both of these cells (FIG 1). This filtration barrier allows for the filtration of small molecules such as water, ions, creatinine and glucose, and small proteins (less than). This structure must prevent the filtration of large proteins present in the blood, such as albumin and immunoglobulins.   Any aberrance of this filtration barrier leads to pathological conditions. Indeed, about 90% of end-stage kidney disease is due to glomerular diseases. The primary concern is that once damaged; the kidneys have a limited ability to undergo repair. Indeed, most forms of glomerular disease develop gradually with symptoms only appearing after a significant proportion of the kidney functional units are damaged.