What function does the glomerulus serve in the nephron?

The glomerulus, a crucial part of the nephron, serves primarily to filter plasma substances from the blood. It is a network of tiny blood vessels (capillaries) that allows for the selective filtration of blood, where water, ions, glucose, and small molecules are separated from larger molecules and formed elements like blood cells and proteins. This filtration process is essential as it forms the initial filtrate that ultimately becomes urine as it passes through other parts of the nephron.

Each day, the glomeruli filter large volumes of blood, allowing waste products to pass through while retaining the components of blood that are necessary for body function. This selective filtration process is the first step in urine formation, paving the way for subsequent processes like reabsorption and secretion that occur later in the nephron.

The other functions mentioned, such as concentrating urine, reabsorbing water, and secreting ions, happen in different parts of the nephron—mainly in the renal tubules—after the initial filtration by the glomerulus. These processes are vital for maintaining homeostasis, but they do not occur in the glomerulus itself.