How Does the Fraction of Oxygen in Earth's Atmosphere Today

question 96

Multiple Choice

How does the fraction of oxygen in Earth's atmosphere today compare to what it was 3 billion years ago?

• A) It has significantly declined.
• B) It has significantly increased.
• C) It increased up to 2 billion years ago but has been declining ever since.
• D) It declined up to 2 billion years ago but has been increasing ever since.
• E) It hasn't changed.

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Definitions:

Oxygenated Blood

Blood that has been oxygenated by the lungs and is being transported by the arteries to the tissues throughout the body.

Deoxygenated Blood

Blood that has released its oxygen load to the tissues and contains a higher concentration of carbon dioxide, returning to the heart and then to the lungs for re-oxygenation.

Heart Valves

Structures within the heart that open and close to control the flow of blood through the heart's chambers and into the arteries.

Aorta

Originating from the heart's left ventricle and stretching to the abdomen, it is the body's biggest artery, responsible for delivering oxygen-rich blood to the circulatory system.