Notes

Gas exchange occurs at two sites in the body: in the lungs, where oxygen is picked up and carbon dioxide is released at the respiratory membrane, and at the tissues, where oxygen is released and carbon dioxide is picked up. External respiration is the exchange of gases with the external environment, and occurs in the alveoli of the lungs. Internal respiration is the exchange of gases with the internal environment, and occurs in the tissues. The actual exchange of gases occurs due to simple diffusion. Energy is not required to move oxygen or carbon dioxide across membranes. Instead, these gases follow pressure gradients that allow them to diffuse. The anatomy of the lung maximizes the diffusion of gases: The respiratory membrane is highly permeable to gases; the respiratory and blood capillary membranes are very thin; and there is a large surface area throughout the lungs.

Gaseous Exchange
is the process of swapping one gas for another. It occurs in the alveoli of the lungs. Oxygen diffuses into the capillaries from the air in the alveoli and carbon dioxide diffuses out of the capillaries and into the air in
the alveoli.

Both oxygen and carbon dioxide are transported around the body in the blood – through arteries, veins and capillaries. They bind to hemoglobin in red blood cells although this is more effective with oxygen. Carbon dioxide also dissolves in the plasma or combines with water to form bicarbonate ions.

The main respiratory surface in humans are the alveoli. Alveoli are small air sacs branching off from the bronchioles in the lungs. They are one-cell thick and provide a moist and extremely large surface area for gas exchange to occur. Capillaries carrying deoxygenated blood from the pulmonary artery run across the alveoli - they are also extremely thin so the total distance gases must diffuse across is only
around 2-cells thick.