Changes in the skin help to increase heat loss when body temperature rises
Changes in the skin reduce heat loss when the body cools
The regulation of body temperature involves communication between thermoreceptors, the hypothalamus and effectors to respond to change
Note that vasoconstriction and vasodilation occur in the arterioles that supply the skin capillaries, not the skin capillaries themselves; capillary walls are only one cell thick and do not contain any muscle fibres capable of contracting or relaxing.
Be careful with your use of language; muscles contract, arterioles constrict.
Negative feedback loops involve the monitoring of physiological factors and act to reverse any changes, keeping the factors within normal limits. Information can be transferred via nerve signals, as shown here, or by hormonal signals.
Two corrective mechanisms are involved in the negative feedback loop
The hormone thyroxine acts as a transcription factor by binding to the thyroid hormone receptor; this switches on the gene, allowing it to be transcribed by RNA polymerase
Adrenaline acts by binding to a receptor on cell surface membranes; this activates the second messenger cAMP, leading to a cascade of reactions that affects the activity of the cell, e.g. by influencing transcription factors