Why is the blood that flows from the lungs toward the heart a brighter red than the blood that flows the other direction?
the presence of pigments found in the lungs makes it bright red.
the presence of oxygen–bound hemoglobin makes it bright red.
the presence of carbon–dioxide–bound hemoglobin makes it bright red.

The blood that flows from the lungs toward the heart is brighter red than the blood that flows in the other direction because of the presence of oxygen-bound hemoglobin.

Explanation:

The blood is an important fluid in the body of higher animals that transport and distribute nutrients and essential materials round the body as well as remove toxic wastes from their various sources in the body to the sites where they will be disposed out of the body.

One of the most important material that the blood transport round the various cells and tissues of the body is oxygen. Oxygen is important for aerobic respiration, a catabolic process during which the energy locked up in the food that organisms take is unlocked and made available for various body processes.

Bloods are usually oxygenated in the lungs when an organism breathes-in oxygen. The oxygenated blood then travels to the heart where it is pumped round the body through the aorta to smaller arteries. While traveling round the body, oxygen diffuses into the various cells and tissues that requires it and excretory product such as diffuses the other way round.

De-oxygenated bloods do not return into the arteries. Rather, they enter the veins and then find their way into the lung where they become oxygenated again and the cycle becomes repeated.

Hence, the blood entering the heart from the lung contains oxygen which is usually bonded to the hemoglobin component of the red blood cell, giving it a brighter red colour compared to the blood entering the lung from the veins which has been depleted of oxygen.

The reason for the brighter red colour is the presence of oxygen-bound hemoglobin.

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a. the production of carbon dioxide through the process of photosynthesis