Which of the following would NOT move by diffusion across a capillary endothelium to or from the surrounding interstitial fluid and tissues?

A. lipid-soluble substances
B. oxygen
C. carbon dioxide
D. proteins

The correct answer is D. proteins

Explanation:

The endothelium is a tissue that fulfills multiple functions, from vascular homeostasis to those related to the passage of substances to the cells and tissues of the body. Diffusion is a type of passive transport, where all the exchanges between the blood and the cells use the interstitial fluid as an intermediate route. Substances in the blood or interstitial fluid can pass through the walls of a capillary by diffusing through intercellular clefts or transcellular pores (fenestrations), or through endothelial cells (cells that form the circulatory system comprising arteries, veins, and capillaries). Fat-soluble materials such as oxygen (O2), carbon dioxide (CO2), can pass through the capillary walls directly through the lipid bilayer of the plasma membrane of endothelial cells. Most plasma proteins (those that are present in the blood) cannot pass through the walls of the continuous and fenestrated capillaries because they are very large. The gradients for the correct diffusion of respiratory gases, nutrients and waste products are created by the cellular metabolism that consumes some and produces others, leading to increases or decreases in their concentration in the interstitial fluid.

D. proteins

Explanation:

Proteins are macro molecules of  high  molecular size and high ionization,.Therefore they can not pass through the thin,one celled, layer   thick endothelial  wall  of  capillaries.They are restricted  from passing through the endothelial walls to generate oncotic pressure that increase solute potential .so that water and fluid is retain inside the capillaries countering the hydrostatic pressure of the blood which tends to force water out of the capillaries which may lead to Oedema. Thus the restriction through the capillaries wall is physiological.

Although antibodies and certain protein can find their ways into the blood this involved a special signalling and regulatory   mechanism.

Oxygen, Carbon dioxide together with other lipid-soluble substances (alcohol, benzene, have lower molecular  masses and can  therefore cross the capillary enothelium and immediately to reach cells. .