<span>When there is no oxygen, or the organism cannot use oxygen for respiration.
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Body part, behavioral adaptations, or by whole body descriptions
Permitting the exchange of nutrients and gases between the blood and tissue cells is the primary function of capillaries. Capillaries are the smallest blood vessels , they are only one cell thick, and they act as sites of the transfer of oxygen and other nutrients from the bloodstream to the body tissues. Additionally, they also collect carbon dioxide, waste materials and fluids for return to the veins.
Answer:
Birds and bats have homologous limbs because they are both ultimately derived from terrestrial tetrapods, but their flight mechanisms are only analogous, so their wings are examples of functional convergence.
