Structure and Function of Arteries, Capillaries, and Veins. The walls of arteries and veins have three layers or tunics: 1  An outer layer (tunica adventitia) of connective tissue with collagen fibers. 2  A middle layer (tunica media) of circularly arranged smooth muscle and elastic fibers.
You might have heard that eating chocolate or greasy foods can cause acne, but these are both just myths. The truth is that food has little to do with breakouts. Acne occurs when the openings of hair follicles become blocked with oil secretions, dead skin cells and bacteria.
These factors have little effect on acne: 1  Greasy foods. Eating greasy food has little to no effect on acne. 2  Dirty skin. Acne isn't caused by dirt. 3  Cosmetics. Cosmetics don't necessarily worsen acne, especially if you use oil-free makeup that doesn't clog pores (noncomedogenics) and remove makeup regularly. 4  Symptoms. Risk factors.