Final answer:
Deoxygenated blood arrives at the lungs, undergoes gas exchange in the alveolar capillaries, is oxygenated, and then transported back to the heart. This process involves diffusion and the binding of oxygen to hemoglobin in red blood cells.
Step-by-step explanation:
Gas Exchange in the Lungs
When deoxygenated blood reaches the lungs via the pulmonary artery, it enters a network of capillaries surrounding the alveoli. Here, external respiration occurs, where oxygen from the alveoli diffuses into the capillaries and carbon dioxide is expelled from the blood into the alveoli.
The inhaled oxygen enters the alveoli, and owing to a higher concentration in the alveoli compared to the blood, it diffuses into the bloodstream through the respiratory membrane.
Oxygen transport is mainly facilitated by hemoglobin found in red blood cells (RBCs). Oxygen binds with hemoglobin and forms oxygenated blood which then returns to the heart through the pulmonary veins, ready to be circulated throughout the body.
Meanwhile, carbon dioxide produced as a byproduct of cellular respiration within the body cells is transported back to the lungs, diffuses into the alveoli and is exhaled out of the body, completing the respiration cycle.