Filtration Units

PART 1

The Kidneys’ Filtering Units

Nephrons are the structural and functional units of the kidneys. Each kidney contains over one million of these tiny blood-processing units, which carry out the processes that form urine.

Nephrons are very fine blood-filtering tubes, or tubules, about 1-2 inches long. At one end of the tubule is a capsule that contains a cluster of capillaries called the glomerulus, the kidney's filtration barrier. Together, the capsule and glomerulus form the renal corpuscle. Blood flows into and away from the glomerulus through tiny arteries called arterioles. In the glomerulus, fluid filters out of the blood and through the inner wall of the capsule into the nephron tubule. As it does so, certain substances are secreted into it, and others, like water, are selectively reabsorbed from it. The final product in this process is urine.

Blood is filtered through a complex of structures in the glomerulus. The endothelial cells of the glomerulus have fenestrations (pores) large enough that nearly anything smaller than a red blood cell passes through them. Branching epithelial cells cover the glomerular capillaries and form a tightly meshed network of foot processes (footlike parts) that control the filtration of proteins. The clefts or openings between the foot processes are called filtration slits. Through these slits, the filtrate enters the capsular space inside the glomerular capsule (also called the Bowman’s capsule).