Larger, more complex animals transport O₂ and CO₂ longer distances to cells within their bodies. These animals rely on bulk flow in addition to diffusion. Bulk flow is the physical movement of fluid (that is, liquids and gases), and the compounds carried by the fluid, over a given distance. Specialized pumps, like the heart, generate the pressure that is required to move the fluid.

Bulk flow occurs in two steps to meet the gas exchange needs of cells in larger animals. The first is ventilation. Ventilation is the movement of the animal’s respiratory medium—water or air—past a specialized respiratory surface. The second is circulation. Circulation is the movement of a specialized body fluid that carries O₂ and CO₂. The circulatory fluid is called hemolymph in invertebrates and blood in vertebrates. This fluid delivers O₂ to cells within different regions of the body and carries CO₂ back to the respiratory exchange surface.

Ventilation and circulation each require a pump to produce a pressure (P) that drives flow (Q) against the resistance to flow (R). The rate of flow is governed by the following simple equation:

Q = P/R

Resistance to flow measures the difficulty of pumping the fluid through a network of chambers and vessels located within the respiratory and circulatory systems. If the resistance to flow doubles, the flow rate is halved. The longer the network of vessels and the narrower the vessels themselves, the greater the vessels’ resistance to the fluid (whether air, water, or blood) moving through them. Consider how much harder it is to blow fluid out of a long thin tube than a short wide one.

In summary, we can view gas exchange in complex multicellular organisms as four steps that are linked in series (Fig. 39.3). To deliver O₂ to the mitochondria in an animal’s cells, (1) fresh air or water moves past the respiratory exchange surface in the process of ventilation (in bulk flow). Ventilation maximizes the concentration of O₂ in the air or water on the outside of the respiratory surface. (2) The buildup of O₂ favors the diffusion of O₂ into the animal across its respiratory surface. (3) Following diffusion into the blood, O₂ is transported by the circulation (in bulk flow) to the tissues. Internal circulation again serves to maximize the concentration of O₂ outside cells. (4) Oxygen then diffuses from the blood across the cell membrane and into the mitochondria, where it burns fuels for ATP production.

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The same four steps occur in reverse to remove CO₂ from the body: CO₂ moves from cells to the blood by diffusion, then is carried in the circulation to the respiratory surface by bulk flow, then moves across the respiratory surface by diffusion, and finally moves out of the animal by bulk flow. Because diffusion and bulk flow are coupled, it is important that each step of transport (whether for a gas, nutrient, or waste) has a similar capacity—that is, the same amount of O₂ enters the blood through diffusion as is transported by the circulation.

Quick Check 2 Those who suffer an asthma attack have difficulty breathing. From the discussion of bulk flow, what do you think happens to these individuals’ airways that makes it difficult for them to breathe?