How does the glomerulus filter blood in the kidney?

The glomerulus filters blood in the kidney by allowing small molecules to pass through its capillary walls.

The glomerulus, a network of tiny blood vessels or capillaries, is the first part of the nephron, which is the functional unit of the kidney. It plays a crucial role in the kidney's function of filtering waste products, excess substances, and small molecules from the blood.

Blood enters the glomerulus under high pressure, which is maintained by the difference in size between the afferent arteriole (bringing blood to the glomerulus) and the efferent arteriole (taking blood away). This pressure forces small molecules such as water, glucose, amino acids, urea, and ions to filter out of the blood and into the Bowman's capsule, the next part of the nephron. This process is known as ultrafiltration.

The walls of the glomerular capillaries are uniquely structured to facilitate this filtration process. They are composed of a single layer of endothelial cells, which have large fenestrations or pores. These pores are permeable to small molecules but prevent larger components like blood cells and proteins from passing through. This ensures that these vital components are not lost from the blood during filtration.

The filtered fluid, or filtrate, then moves into the renal tubule of the nephron, where reabsorption and secretion processes occur to further refine the composition of the urine. Any useful substances like glucose and certain ions are reabsorbed back into the blood, while waste products are secreted into the filtrate.

In summary, the glomerulus is a critical component of the kidney's filtration system. It uses a combination of high blood pressure and specialised capillary walls to filter small molecules from the blood, initiating the process of urine formation.