Cities fall under the category of urban ecosystems. urban ecosystems include nature, humans, and animals in a (mostly) human-built environment. energy flow, nutrient cycles, and resource use are all necessary to maintain an urban ecosystem. most urban ecosystems are heterotrophic because they depend on materials and energy from outside their boundaries.

the significance of this reliance on outside resources is ―

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

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blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.