Not 100% sure on this one, but here’s how I approached it: histamine causes arterial dilation (=decreased arteriolar resistance), but all of that blood has to go somewhere since you now have more blood flowing through the arteries and that somewhere is the capillaries (increased capillary hydrostatic pressure). Histamine causes increased permeability of the post-capillary venules (one of Dr. Sattar’s favorite facts) so you’d have increased capillary filtration rate.

Histamine causes arteriole vasodilation, causing a buildup of blood in the capillaries. The increased blood in the capillaries will cause the pressure there to rise. Filtration is dependent on pressure, the higher the pressure, the more the filtration.

Remember blood flow: veins to venules to capillaries to arterioles to arteries

Histamine causes capillary arteriolar dilation (decrease resistance)(increased pressure in the capillary because efferents remain the same size) Histamine also increases capillary permeability.

Histamine causes a decrease in hydrostatic pressure (get this, it’s a vasodilator) but then causes an increase in capillary hydrostatic pressure? (Okay, this is how it causes edema, I guess.) Does anyone get how this actually works?

What they mean by filtration...

Fluid comprised of water and electrolytes, with a very small amount of protein and other macromolecules, normally leaves capillaries and small postcapillary venules by a process called filtration. Filtration is primarily driven by the capillary hydrostatic pressure, and the amount filtered per unit time is additionally influenced by the permeability of the vessel wall (endothelium and basement membrane). The fluid that filters into the tissue flows within the intercellular space (the interstitium) and most of it is reabsorbed at the venular end of capillaries where the hydrostatic pressure is lower. Some of the filtered fluid is taken up by lymphatic vessels and returned to the circulation.