The correct option is Plasma proteins.
Explanation
Urine formation begins with glomerular filtration, a process known as ultrafiltration. Blood is filtered through the capillaries of the glomerulus into Bowman's capsule. The filtration membrane acts as a selective barrier, allowing substances to pass based primarily on molecular size and electrical charge.
Analysis of Options:
- Plasma proteins Plasma proteins (Correct): The glomerular filtration barrier consists of the fenestrated endothelium, the basement membrane, and the filtration slits of podocytes. This barrier prevents the passage of large macromolecules. Plasma proteins (e.g., albumin, globulins) have high molecular weights and are generally negatively charged, which causes them to be repelled by the basement membrane. Consequently, they are retained in the blood capillaries and are not found in the filtrate of a healthy kidney.
- Amino acids Amino acids (Incorrect): Amino acids are small organic molecules that easily pass through the filtration membrane. While they are present in the initial filtrate, they are almost entirely reabsorbed into the blood within the proximal convoluted tubule.
- Glucose Glucose (Incorrect): Glucose is a small molecule that is freely filtered into Bowman's capsule. It is a standard component of the glomerular filtrate, although it is subsequently reabsorbed by the renal tubules under normal physiological conditions.
- Sodium ions Sodium ions (Incorrect): Electrolytes, including sodium ions, are small enough to pass freely through the filtration barrier. They are abundant in the glomerular filtrate.
Key Takeaway: The composition of glomerular filtrate resembles blood plasma but lacks plasma proteins and blood cells. The filtration barrier effectively excludes large macromolecules while allowing water and small solutes (glucose, amino acids, ions) to pass.