Why do proteins not pass out of the blood?

Proteins typically do not pass out of the blood due to their large size and the presence of specialized barriers, such as the endothelial cells lining blood vessels, which restrict their movement. Additionally, the blood-brain barrier and glomerular filtration barrier in the kidneys are designed to prevent the leakage of proteins, ensuring that they remain in the bloodstream to perform essential functions. This selective permeability maintains homeostasis and protects the body from potential imbalances that could arise from protein loss.