Biological Systems MCAT Practice Exam

Which structure in the kidney is responsible for the initial formation of urine through filtration?

1. Collecting duct

2. Glomerulus

3. Afferent arteriole

4. Efferent arteriole

The correct answer is: Glomerulus

The glomerulus is the structure in the kidney responsible for the initial formation of urine through the process of filtration. The glomerulus is a network of capillaries located within the Bowman's capsule, a part of the nephron. Here, blood pressure forces water, ions, and small molecules from the blood into the Bowman's capsule while larger molecules, such as proteins and blood cells, are retained in the bloodstream. This filtrate then moves along the nephron where it undergoes further processing to ultimately become urine. The collecting duct, although crucial in the concentration and passage of urine, does not play a role in the initial filtration of blood. The afferent arteriole carries blood into the glomerulus, but it is not directly involved in filtration. The efferent arteriole carries blood away from the glomerulus and also does not participate in the filtration process. Each of these other structures has a distinct role in kidney function, but the glomerulus is uniquely essential for the initial step in urine formation.