NCERT Class 7 Science — Chapter 3: Transportation in animals and plants

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What is the primary physiological consequence if oxygenated and deoxygenated blood mix within the heart?

Official previous-year question

Verified 1 Aug 2026.

Options

AThe cardiac muscles cease to function immediately.
BThe blood turns blue due to a lack of haemoglobin.
CThe efficiency of oxygen supply to body cells decreases.
DThe metabolic energy production increases significantly.

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Solution

The correct option is

The efficiency of oxygen supply to body cells decreases.

Explanation

In the circulatory systems of birds and mammals, the heart is divided into four distinct chambers to ensure the complete separation of oxygenated blood (received from the lungs) and deoxygenated blood (received from the body). This separation is essential for maintaining the high metabolic rates required by endothermic (warm-blooded) animals.

Detailed Analysis:

The metabolic energy production increases significantly. is Incorrect: Metabolic energy production relies on aerobic respiration, which requires oxygen. If blood mixes, the oxygen concentration delivered to tissues drops, potentially lowering energy production efficiency rather than increasing it.
The efficiency of oxygen supply to body cells decreases. is Correct: When oxygenated and deoxygenated blood mix, the resulting blood pumped to the body has a lower partial pressure of oxygen compared to pure oxygenated blood. This means body cells receive less oxygen per unit of blood, significantly decreasing the efficiency of oxygen supply. This limits the organism's ability to sustain high energy activities.
The blood turns blue due to a lack of haemoglobin. is Incorrect: The mixing of blood does not affect the quantity or presence of haemoglobin. While the blood may carry less oxygen (leading to a condition called cyanosis where skin appears bluish), the blood itself does not turn blue due to a "lack of haemoglobin."
The cardiac muscles cease to function immediately. is Incorrect: The heart muscle does not stop functioning immediately due to mixing. Many animals, such as amphibians and reptiles, naturally possess hearts where some mixing occurs. In humans, conditions like septal defects allow mixing, which impacts health and stamina but does not cause immediate cessation of cardiac function.

Key Takeaway: The primary physiological advantage of a four-chambered heart is the prevention of blood mixing, which maximizes oxygen delivery efficiency to support high metabolic demands and thermoregulation.

Your note

More from Chapter 3: Transportation in animals and plants

With reference to the composition of blood in humans, consider the following statements: 1. Plasma is the fluid component of blood in which different types of cells are suspended. 2. The red colour of blood is due to a pigment dissolved directly in the plasma. 3. White blood cells (WBC) are responsible for fighting against germs that may enter the body. Which of the statements given above is/are correct?

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Which one of the following best explains the primary function of haemoglobin in the blood?

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Consider the following statements regarding the clotting of blood: 1. A clot is formed to plug a cut and stop bleeding. 2. The formation of a clot is primarily due to the presence of white blood cells. Which of the statements given above is/are correct?

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With reference to blood vessels in the human body, consider the following statements: 1. Arteries always carry carbon dioxide-rich blood from the heart. 2. Veins generally carry carbon dioxide-rich blood from the body to the heart. 3. Arteries have thick elastic walls while veins have thinner walls. Which of the statements given above is/are correct?

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With reference to the Pulmonary Artery, consider the following statements: 1. It carries oxygen-rich blood from the lungs to the heart. 2. It is called an artery because it carries blood away from the heart. 3. It carries oxygenated blood to the kidneys. Which of the statements given above is/are correct?

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The correct option is

The efficiency of oxygen supply to body cells decreases.

Explanation

Detailed Analysis:

The metabolic energy production increases significantly. is Incorrect: Metabolic energy production relies on aerobic respiration, which requires oxygen. If blood mixes, the oxygen concentration delivered to tissues drops, potentially lowering energy production efficiency rather than increasing it.
The efficiency of oxygen supply to body cells decreases. is Correct: When oxygenated and deoxygenated blood mix, the resulting blood pumped to the body has a lower partial pressure of oxygen compared to pure oxygenated blood. This means body cells receive less oxygen per unit of blood, significantly decreasing the efficiency of oxygen supply. This limits the organism's ability to sustain high energy activities.
The blood turns blue due to a lack of haemoglobin. is Incorrect: The mixing of blood does not affect the quantity or presence of haemoglobin. While the blood may carry less oxygen (leading to a condition called cyanosis where skin appears bluish), the blood itself does not turn blue due to a "lack of haemoglobin."
The cardiac muscles cease to function immediately. is Incorrect: The heart muscle does not stop functioning immediately due to mixing. Many animals, such as amphibians and reptiles, naturally possess hearts where some mixing occurs. In humans, conditions like septal defects allow mixing, which impacts health and stamina but does not cause immediate cessation of cardiac function.