<span>They are proteins that are secreted from fat cells to help regulate energy balance.</span>
Capillaries<span> are </span>small<span>, thin </span>blood vessels<span> that connect the </span>arteries<span> and the veins. Their thin walls allow oxygen, nutrients, carbon dioxide and waste products to pass to and from the tissue cells.</span>
RNA contains cytosine and uracil as pyrimidine. DNA has cytosine and thymine.
According to Darwin, evolution occurs because of natural selection.
