Forming glomerular filtrate
Urine formation begins with ultrafiltration. Blood flowing through the glomerular capillaries is under high hydrostatic pressure.
The afferent arteriole is wider than the efferent arteriole. This difference helps maintain the high pressure inside the glomerulus.
The pressure forces small components of the blood plasma across the filtration barrier into Bowman's capsule, producing glomerular filtrate.
| Enter the glomerular filtrate | Remain in the blood |
|---|---|
| water | blood cells |
| glucose | large proteins |
| amino acids | |
| urea | |
| small inorganic ions, including , and |
Blood cells and large plasma proteins are too large to cross the filtration barrier, so they stay within the circulation.
Exam Tip: Link the vessel diameters to the mechanism: a wider afferent arteriole and narrower efferent arteriole help create high glomerular pressure, which forces small molecules into Bowman's capsule while cells and large proteins remain in the blood.