A Bowman's capsule is part of the filtration system in the kidneys. When blood reaches the kidneys for filtration, it hits this area first, with the capsule separating the blood into two components: a cleaned blood product, and a filtrate that is moved through the nephron, another structure in the kidneys. As the filtrate travels along the nephron, additional impurities are removed, and the filtrate is concentrated into urine for the purpose of expressing waste products and excess water.
Each nephron in the kidneys is attached to its own capsule, and there are hundreds of thousands of nephrons. Functioning kidneys can move the blood around the body about 20 times each day, illustrating just how important these structures are. The system within the kidneys is also very complex, with a number of interconnected structures that work together to filter the blood effectively. The kidneys are truly a marvelous feat of natural engineering, and despite the medical community's best efforts, it has proved impossible to build a fully artificial kidney.
Blood enters the Bowman's capsule through an afferent arteriole, which connects to the glomerulus, a tangled array of tubes. The glomerulus and capsule are often collectively referred to as the renal corpuscle. As the blood is forced through these tubes, impurities and water filter out into the Bowman's capsule. The cleaned blood exits at the vascular pole, while the impurities are moved through the urinary pole so that they can connect with the nephron and start working their way to the ureter.
The blood that leaves the vascular pole connects with the efferent arteriole, a capillary which enters the kidney and wraps around the loop of Henle so that it can reabsorb water and the correct concentration of solutes. At this point, the blood can be sent up to the heart via a network of veins for recirculation.
Red blood cells are not filtered through the nephrons, which is why blood in the urine is a concern, because it means that the kidneys or urinary tract are damaged. Astute observers may note that the composition of urine also changes, depending on dietary factors and the amount of water being consumed. When the kidneys identify impurities that need to be removed, they will express them, and they will also conserve water and minerals that may be useful for the body to have, through the process of osmosis along the nephrons.