Organ systems of the human body interact to maintain a balanced internal environment. as blood flows
through certain organs of the body, the composition of the blood changes because of interactions with those
organs. state one change in the composition of the blood as it flows through the respiratory system. [1]

The changes that occur are; increase in oxygen levels in blood as the blood takes in oxygen, the release of carbon dioxide and also the release of water.

Explanation;
The changes in the blood composition that may occur as the blood flows through the respiratory system include; the blood takes in oxygen as it flows through the respiratory system, thus the oxygen levels goes up or increases. Additionally the blood releases carbon dioxide, which is then exhaled out of the body from the lungs, thus the level of Carbon dioxide in the blood is greatly reduced. Also the blood releases water in form of water vapor which is also exhaled from the lungs together with carbon dioxide. 

During respiration, oxygen moves from the alveoli of lungs into the blood and the carbon dioxide moves from the blood into the alveoli. Blood becomes oxygenated as it flows through the respiratory system.

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The answer is a because - is an direction