Question

Scientists currently believe that the main barrier to the filtration of proteins into the glomerular capsule is the a. capillary fenestrae. b. basement membrane. c. slit diaphragm. d. macula densa.

Short answer

The main barrier to the filtration of proteins into the glomerular capsule is the slit diaphragm (Option C). It prevents the passage of larger molecules like proteins, allowing only smaller molecules to pass through.

Step-by-step solution

Understanding Glomerular Filtration Barriers

The filtration process in the glomerulus takes place across three barriers: the capillary fenestrae, the basement membrane, and the slit diaphragm. These barriers ensure selective filtration of molecules from the blood into the glomerular capsule.

Option A: Capillary Fenestrae

Capillary fenestrae are small pores in the endothelial cells of glomerular capillaries. These pores allow the passage of water, ions, glucose, amino acids, and other small molecules but not larger molecules like proteins and blood cells.

Option B: Basement Membrane

The basement membrane sits between the capillary fenestrae and the podocytes, which form the slit diaphragm. It is composed of extracellular matrix and acts as a size-selective barrier, allowing smaller molecules to pass through, while larger molecules, like proteins, are generally excluded.

Option C: Slit Diaphragm

The slit diaphragm is formed by the interdigitating podocyte foot processes, which wrap around the capillary loops. The slit diaphragm serves as a final barrier for filtration and prevents the passage of larger molecules like proteins.

Option D: Macula Densa

Macula densa is a group of specialized cells in the distal convoluted tubule that comes into contact with the afferent arteriole, regulating glomerular filtration rate and blood pressure. However, it is not involved in the filtration process itself.

The Main Barrier

Among the provided options, the main barrier to the filtration of proteins into the glomerular capsule would be the slit diaphragm (Option C). It prevents the passage of larger molecules like proteins, allowing only smaller molecules to pass through.