The correct option is Proteins.
Explanation
Ultrafiltration is the initial step in urine formation, occurring in the Malpighian corpuscle (Glomerulus and Bowman's capsule) of the nephron. It is a non-selective process driven by glomerular hydrostatic pressure, where blood is filtered through the filtration membrane.
The filtration membrane consists of three layers: the endothelium of glomerular blood vessels, the basement membrane, and the epithelium of Bowman's capsule (podocytes). This barrier acts as a sieve based on molecular size.
Analysis of Substances:
- Proteins: Plasma proteins (such as albumin and globulins) are macromolecules with high molecular weights. They are too large to pass through the fenestrations of the capillaries and the filtration slits of the podocytes. Therefore, they are retained in the blood and do not enter the Bowman's capsule.
- Glucose, Na+, and Urea: These are small molecules and ions with low molecular weights. They pass freely through the filtration membrane along with water. Consequently, they are found in the glomerular filtrate within the Bowman's capsule.
Key Takeaway: The glomerular filtrate is isotonic to blood plasma and contains all plasma constituents except for plasma proteins and blood cells.