Parasympathetic nerves, on the other hand, innervate salivary glands, gastrointestinal glands, and vaginal erectile tissue, causing vasodilation. Sympathetic activation raises cardiac output, systemic vascular resistance (including arteries and veins), and arterial blood pressure. It also causes dilation of the venous sinuses in the brain and spinal cord, which can lead to increased intracranial and intradural pressure. Blood vessels are controlled by both sympathetic and parasympathetic nerves. If the sympathic nervous system is activated, then the blood vessels constrict to cause pain or to make room for blood if there is a cut or some other damage to the skin. If the parasympathic nervous system is activated, then the muscles around the blood vessel relax so that more blood flows through the vessel. So, basically, blood pressure is controlled by two systems: the sympathetic nervous system and the parasympathetic nervous system. The type of control depends on what part of our body needs blood flow or not. For example, if we need more blood flow into an injured part of the body, then we activate the sympathetic nervous system. This causes muscles all over our body to tighten up, including the muscle walls of our blood vessels. This makes it harder for blood to reach those parts of our body that need it most - the muscles repair themselves faster now that there's less bleeding.