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Capillaries in the lungs play a crucial role in gas exchange. When blood flows from the pulmonary arteries, it is deoxygenated and rich in carbon dioxide, having returned from the body's tissues where it has delivered oxygen and collected carbon dioxide as a waste product.
In the pulmonary capillaries, this deoxygenated blood passes through the thin walls of the capillaries, allowing for the exchange of gases. Oxygen from the inhaled air in the alveoli diffuses into the blood, while carbon dioxide diffuses out of the blood to be exhaled. Therefore, the blood that arrives at the capillaries from the pulmonary arteries is indeed deoxygenated blood.
This understanding is fundamental in the context of pulmonary circulation, which specifically refers to the pathway of blood from the heart to the lungs and back to the heart, highlighting the importance of capillaries in the respiratory system's function.