The kidneys are a pair of bean-shaped organs located on either side of the spine, just below the rib cage. These organs filter waste products and excess fluid from the blood. This process helps maintain a healthy balance of water, salts, and minerals in the body, necessary for nerves, muscles, and other tissues to function properly. Filtration is the initial step in this cleansing process.
The Kidney’s Tiny Filters
Each kidney contains approximately one million microscopic filtering units called nephrons. These nephrons are the structural and functional components responsible for processing blood and forming urine. Within these structures, blood purification begins as nephrons remove waste and excess water.
The Filtration Powerhouse
The primary site for blood filtration in the kidney is the renal corpuscle. Each renal corpuscle is situated in the outer region of the kidney, the renal cortex. This compact unit consists of two main parts: a cluster of tiny blood vessels called the glomerulus and a cup-shaped sac surrounding it, Bowman’s capsule.
The glomerulus is a network of capillaries where blood from the renal artery enters for filtration. Bowman’s capsule, a double-walled structure, encases this capillary tuft and collects the filtered fluid. The inner layer of Bowman’s capsule associates with the glomerular capillaries, forming a filtration barrier. This arrangement allows fluid and small solutes to transfer from the blood into the capsule’s space.
How Blood Becomes Filtrate
Blood enters the glomerulus under high pressure, maintained by the differing diameters of the arterioles leading into and out of it. This pressure forces water and small dissolved substances from the blood capillaries through a specialized filtration barrier into Bowman’s capsule. This process is called ultrafiltration due to its selectivity.
The filtration barrier, located between the glomerulus and Bowman’s capsule, consists of three layers. These include fenestrated endothelial cells lining the glomerular capillaries, a basement membrane, and specialized epithelial cells of Bowman’s capsule called podocytes. Podocytes have foot-like processes that interdigitate, creating filtration slits. This barrier allows water and small solutes to pass through, while preventing larger components like blood cells and most proteins from entering the capsule.
What Passes Through the Filter
The fluid collected in Bowman’s capsule after initial filtration is called glomerular filtrate. This filtrate is essentially blood plasma stripped of its large proteins and blood cells. Its composition includes dissolved substances such as water, salts, glucose, amino acids, and nitrogenous waste products like urea and creatinine. This initial filtrate is not yet urine; it will undergo further modification as it travels through the rest of the nephron’s tubular system.