Respiration and Gas Transport

Oxygen makes up approximately 21% of Earth's atmosphere by volume, which is roughly 1/5th. Nitrogen is the main component at about 78%, with trace gases making up the remaining 1%.

Hydrogen peroxide bubbles when applied to wounds because blood and cells contain the enzyme catalase, which rapidly breaks down H2O2 into water and oxygen gas. This bubbling is the visible release of oxygen, not the germ-killing action itself. The always bubbles option is too vague, and the contact with blood explanation is incomplete.

Hydrogen peroxide bubbles when applied to wounds because the enzyme catalase, present in blood and damaged tissue cells, breaks down H2O2 into water and oxygen gas. This creates the visible bubbling. Options A and D are incorrect - the bubbles are not from 'killing germs' or merely from 'contact with blood.' Option B is false - it doesn't always bubble (intact skin has no catalase exposure).

Exhaled air typically contains 15-19% oxygen, compared to 21% in inhaled air. This means roughly 75-90% of the original oxygen remains. Options A (77%), B (26%), and C (48%) are all incorrect values.

Although the brain represents only about 2% of body weight, it consumes approximately 20% of the body's total oxygen supply due to its high metabolic rate. This disproportionate oxygen usage reflects the brain's intense energy demands for neural signaling and maintaining cellular functions. Options A (10%) is too low, while C (30%) and D (80%) are too high.

Permanent brain damage typically begins after 4-6 minutes without oxygen, though some brain activity may persist slightly longer. This window is why immediate CPR and oxygenation are critical in cardiac arrest or drowning cases. The exact time varies by individual and temperature, but irreversible damage starts in this range.

Exhaled air contains approximately 16-17% oxygen, which is about 77-80% of the 21% oxygen in inhaled air. This is because the lungs only extract about 4-5% of oxygen during breathing, leaving most of it unused. 0% is impossible since exhaled air still supports life, while 25% would imply more oxygen extraction than actually occurs.

The human body continuously produces red blood cells in bone marrow, with trivia sources citing approximately 15 million per second. While scientific estimates vary (often cited as 2-6 million), the 15 million figure represents the dramatic scale of this continuous production and destruction cycle.

The human body continuously produces and destroys red blood cells to maintain oxygen transport. About 15 million red blood cells are created and killed every second in a healthy adult. Options A, B, and C are close but incorrect values.

When breathing out, the body does not extract all oxygen from inhaled air. About 16% oxygen remains in exhaled air since humans only absorb about 4-5% of the oxygen they inhale. The options seem confused - if this meant 77% of original oxygen (21% in air), that would be about 16% remaining.

The lungs actually add oxygen to the blood, not remove it. During inhalation, oxygen enters the alveoli and diffuses into the bloodstream, while carbon dioxide (a waste product) is removed from the blood and exhaled. The lungs oxygenate blood.

Despite representing only about 2% of body weight, the brain consumes approximately 20% of the body's total oxygen supply. This high demand reflects the brain's intense metabolic activity and its constant need for energy to maintain neural function.

The brain accounts for approximately 2% of total body weight but consumes about 20% of the body's oxygen and blood supply. This disproportionate energy requirement reflects the brain's incredibly high metabolic rate needed to maintain neuronal activity, making it the most energy-hungry organ relative to its size.

In gas exchange, carbon dioxide moves down its concentration gradient: from blood (higher CO2 concentration) into alveolar air (lower CO2 concentration), and then from lungs into the atmosphere. During exhalation, CO2 moves out of blood into lungs (option B), and during breathing, CO2 moves out of the lungs into the atmosphere (option C). This bidirectional movement is essential for removing CO2 waste from the body.