Body Fluids and Circulation

Arteries carry blood away from the heart under high pressure, so they have thick, muscular and elastic walls and a narrow lumen.
Veins carry blood towards the heart under low pressure, so they have thin walls, a wide lumen and valves.
The pulmonary artery (carrying deoxygenated blood to the lungs) is an artery, so its wall is thicker than that of the pulmonary vein.
Both have an endothelium lining.
So the pulmonary artery differs in that its wall is thick.

White blood cells (leucocytes) are nucleated and are produced mainly in the red bone marrow (some lymphocytes mature in the thymus).
They can change shape and squeeze out through the walls of blood capillaries into the tissues. This is called diapedesis, and it lets them reach sites of infection.
A deficiency of WBCs (leucopenia) lowers immunity; leukaemia (a cancer) is associated with an excess of abnormal WBCs.
So the true statement is that WBCs can squeeze through blood capillaries.