National League for Nursing (NLN PAX) Practice Exam

When blood is filtered in the kidney, why do red blood cells and plasma proteins not pass through the blood capillaries?

1. They are too large to filter

2. They are needed by the body

3. They are actively transported back

4. They have a higher density

The correct answer is: They are needed by the body

Blood filtration in the kidneys occurs primarily in the glomeruli, where a selective barrier separates the blood from the filtrate. Red blood cells and plasma proteins do not pass through the capillary walls during this filtration process mainly because they are too large to filter through the glomerular membrane. The glomerular capillaries are designed to allow small molecules, water, and electrolytes to pass while restraining the passage of larger entities such as red blood cells and plasma proteins. While they are indeed critical for the body's functions, the primary reason they remain in the bloodstream has to do with their size, not their necessity. The filtration membrane has specialized pores and structures that create a selective barrier, thus ensuring that vital components like red blood cells and plasma proteins are retained within circulation to maintain blood volume and osmotic pressure.