Final answer:
Vasoconstriction is the constriction of blood vessels due to stimuli like cold, mediated by norepinephrine, leading to decreased peripheral blood flow and increased venous return. Venoconstriction raises blood pressure and flow back to the heart, while fluid retention increases blood volume. Other vasoconstrictors include prostaglandins, activated platelets, leukocytes, and endothelins.
Step-by-step explanation:
Vasoconstriction in Peripheral Vessels
When the body is exposed to cold temperatures, a physiological response known as vasoconstriction occurs. This involves the contraction of blood vessels in the skin, preventing blood from flowing close to the surface and thus reducing heat loss. The neurotransmitter norepinephrine (NE) plays a key role in this process. NE acts as a vasoconstrictor on arteries, leading to a decrease in blood flow to the extremities, while causing veins to constrict, effectively increasing blood flow back to the heart.
This process not only involves the constriction of arteries but also affects venous return to the heart. Venoconstriction increases the pressure inside veins, speeding up the flow of blood back to the heart. Additionally, vasoconstriction leads to the body retaining more fluid via the kidneys, which in turn increases blood volume and pressure. It is important to note that other substances such as prostaglandins, activated platelets, leukocytes, and endothelins can also act as vasoconstrictors.
The body can also engage in a hunting response, which is an alternation between vasoconstriction and vasodilation in extremities that are exposed to cold for prolonged periods. The vasomotor center located in the medulla oblongata is responsible for sending sympathetic stimulation to the smooth muscles of the veins to cause venoconstriction, which is less dramatic than the vasoconstriction seen in smaller arteries and arterioles.