Explain how the organ systems work together to warm
up the body on a cold day.

If body temperature rises, blood vessels in the skin dilate, allowing more blood to flow near the skin's surface. ... Conversely, if your body is too cold, blood vessels in the skin contract, and blood flow to the extremities (arms and legs) slows. Muscles contract and relax rapidly, which generates heat to keep you warm.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.