Describe how the structure of the renal corpuscle facilitates ultrafiltration.

In the human body, ultrafiltration is the non-specific filtration of blood at a molecular level under high pressure. Renal corpuscles are the starting point of nephrons (the functional units of the kidney), and are located in the cortex of the kidney. Each consists of a glomerulus and a bowman’s capsule. As blood enters the kidney via an afferent arteriole (a branch of the renal artery) it enters a knot-like tuft called the glomerulus. This glomerulus is encased by a Bowman’s capsule - a cup like structure lined with a parietal layer of squamous epithelium and a visceral layer of podocytes. Podocytes have cellular processes, called pedicles, which wrap around the glomerular capillaries. In between the capillary endothelium and the pedicles is a glycoprotein matrix called the basement membrane. The afferent arteriole has a wider diameter than the efferent arteriole, therefore, when blood flows through the glomerulus, a high hydrostatic pressure is exerted on the endothelial walls of the capillaries. Fortunately, the walls have fenestrations (tiny gaps), which allow blood plasma to flow through into the basement membrane. However, the basement membrane is a mesh, with gaps only big enough to allow molecules with a relative formula mass equal to/ or less than 69,000 into the capsule. This means plasma and its constituent colloid molecules such as glucose, amino aci.ds and salts can pass through, but not larger molecules such as proteins and cells. Once through, gaps between the pedicles allow glomerular filtrate to enter the Bowman’s space and further through the nephron. In conclusion there are 3 structural adaptations: the difference in diameter between the afferent and efferent arteriole which generates the pressure gradient; the fenestrations in the capillary wall and in between the pedicles of the podocytes; the mesh-like structure of the basement membrane which selectively allows smaller molecules into the bowman's capsule.