Question

What are capillary walls and how can WBC ca squeeze out through it?

Answer 1

Normally, blood cells do NOT pass through capillary walls. Capillaries are 'leaky' meaning that plasma and anything dissolved in the plasma will leak through between endothelial cells. Blood cells are just too big to fit. Oxygen, water, and other chemicals pass through the capillary wall.

However, some capillaries called sinusoidal capillaries (found in the liver, bone marrow and other places) have larger openings between endothelial cells and can allow white and red blood cells to pass through.

So the key is: BETWEEN the endothelial cells, not through the cells.