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Bowman's capsule, also known as the glomerular capsule, is a critical structure in the nephron, which is the functional unit of the kidney. Its primary purpose is the filtration of blood plasma. This process occurs as blood enters the glomerulus, a network of capillaries inside Bowman's capsule. The walls of the glomerulus are porous, allowing water, ions, and small molecules (but not larger proteins or blood cells) to pass through into Bowman's capsule. This filtered fluid, known as filtrate, then progresses through the nephron for further processing, ultimately leading to urine formation.

The role of Bowman's capsule in filtration marks the first step in urine production, making it essential for maintaining fluid and electrolyte balance in the body. The other functions listed, such as reabsorption of nutrients, secretion of hormones, and storage of urine, occur in different parts of the nephron or the urinary system, such as the proximal convoluted tubule, renal tubules, and bladder, respectively. Thus, the primary function of Bowman's capsule remains focused on the initial filtration of blood plasma.