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The correct answer would be blood vessels.
The circulatory system consists heart, blood vessels, and blood. In humans, heart is a four-chambered muscular organ which pumps blood throughout the life.
The blood vessels are tubular structure which helps in transporting blood throughout the body.
The heart (left ventricle) pumps blood (oxygenated) into aorta, the largest blood vessel. Aorta divides into smaller and smaller vessels and ultimately into capillaries. These capillaries transport blood to each and every cell of the body.
In contrast, veins are the blood vessels which carry blood back from cells to the heart.
Thus, the blank can be correctly filled up with blood vessels.
a researcher conducts an experiment on the secretion of a particular hormone in mice. scientists inject mice with a substance that stimulates the production of the hormone. the scientists then test the levels of hormones produced by the mice. the tool used to measure the hormones consistently detects the levels at 10 points lower than the actual hormone levels in the mice. this tool makes measurements, but the measurements aren't .