The circulatory and respiratory systems work as an integrated “gas-exchange and distribution” team: the lungs swap gases with the blood, and the heart and blood vessels move that gas-rich blood to every part of the body. Key points

• Gas exchange in the lungs: In the lungs, air reaches tiny air sacs called alveoli. Oxygen from inhaled air diffuses across the alveolar and capillary walls into the blood, while carbon dioxide diffuses from the blood into the alveoli to be exhaled. This process is driven by concentration gradients and thin, moist surfaces that facilitate diffusion.
• Blood transport: Oxygenated blood leaves the lungs via pulmonary veins and returns to the heart, where the left side pumps it through the systemic circulation to supply tissues throughout the body. Deoxygenated blood returns to the right side of the heart from the body and is sent back to the lungs for reoxygenation.
• Cardiac roles: The heart functions as two sides in series. The right atrium and ventricle handle the pulmonary circuit (to and from the lungs), while the left atrium and ventricle manage the systemic circuit (to the rest of the body). Each heartbeat propels blood through its respective loop.
• Gas exchange and cellular respiration: Oxygen delivered by the bloodstream is used by cells to produce energy (via cellular respiration). Waste carbon dioxide from metabolism is transported back to the lungs for removal.
• Integration and regulation: Breathing rate and depth adjust in response to carbon dioxide, oxygen, and pH levels in the blood, helping to match oxygen uptake with metabolic demand. The circulatory system adjusts blood flow to prioritize tissues with higher needs, such as exercising muscles.

How they fit together in simple terms

• Inhalation brings in oxygen-rich air to the lungs.
• Oxygen diffuses into the blood in the lungs; carbon dioxide diffuses out into the air to be exhaled.
• The heart pumps the oxygenated blood to body tissues; cells use the oxygen to produce energy and generate carbon dioxide as a waste product.
• The blood returns carbon dioxide-rich and oxygen-poor to the heart, which sends it back to the lungs to restart the cycle.

Common to both systems

• Thin barriers: Gas exchange relies on very thin membranes to speed diffusion.
• Blood flow and ventilation coupling: Regions with higher blood flow often have higher ventilation to maximize gas exchange efficiency.
• Health impact: Problems in one system can burden the other (for example, lung diseases can strain the heart, and heart failure can cause fluid build-up in the lungs).

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