Final answer:
In biology, the proportion of the heart's output directed to the brain at rest is approximately 15% of total cardiac output, illustrating the brain's significant and constant demand for oxygen and nutrients.
Step-by-step explanation:
The subject of the question is biology, specifically focusing on the human cardiovascular system and how it functions at rest. At rest, cerebral blood flow is about 15% of total cardiac output in a healthy adult. This percentage represents the proportion of the heart's output of blood directed to the brain, which is essential for maintaining normal brain function.
The brain requires a constant supply of oxygen and nutrients, which is provided by this blood flow, reflecting the high energy demands of the brain even when a person is at rest.