Second, AVP constricts arterioles, which increases peripheral vascular resistance and raises arterial blood pressure. The internal urethral sphincter is a urethral sphincter muscle which constricts the internal urethral orifice. Norepinephrine constricts blood vessels to increase blood pressure. Activity in the nerve fibers constricts the urethra. Angiotensin II constricts blood vessels, increasing blood pressure. The smooth muscle constricts under parasympathetic innervation and relax under sympathetic innervation. An astringent (sometimes called adstringent) is a chemical that shrinks or constricts body tissues. Endothelin constricts blood vessels and raises blood pressure. The ciliary muscle constricts making the lens thicker, shortening its focal length. Adrenaline constricts blood vessels in the skin and minimizes blood loss from physical trauma. Nicotine results in less blood flow to the fetus because it constricts the blood vessels. Placing pressure on the wound constricts the blood vessels manually, helping to stem blood flow. This grouping of muscles constricts to close the urogenital openings. The nuclear membrane only divides when the waist of the organism constricts. The voluntary, pubococcygeus muscle, part of the levator ani muscle partially constricts the vaginal opening.