Circulation and Gas Exchange
Overview: Trading with the Environment
Every organism must exchange materials and energy with its environment, and this exchange ultimately
occurs at the cellular level.
The resources that they need, such as nutrients and oxygen, move across the plasma membrane
to the cytoplasm.
Metabolic wastes, such as carbon dioxide, move out of the cell.
Concept 42.1 Circulatory systems reflect phylogeny
Diffusion alone is not adequate for transporting substances over long distances in animals
Bulk fluid movement in the circulatory system, powered by the heart, quickly carries the oxygen-rich
blood to all parts of the body.
As the blood streams through the tissues within microscopic vessels called capillaries, chemicals
are exchanged between blood and the interstitial fluid that bathes the cells.
Most invertebrates have a gastrovascular cavity or a circulatory system for internal transport.
A body wall only two cells thick encloses a central gastrovascular cavity that serves for both
digestion and for diffusion of substances throughout the body.
The products of digestion in the gastrovascular cavity are directly available to the cells of the inner
layer, and it is only a short distance to diffuse to the cells of the outer layer.
In more complex animals, two types of circulatory systems that overcome the limitations of diffusion
have evolved: open circulatory systems and closed circulatory systems.
Both have a circulatory fluid
a set of tubes
and a muscular pump (the
The heart powers circulation by using metabolic power to elevate the hydrostatic
pressure of the blood
which then flows down a pressure gradient
through its circuit back to the heart.
In insects, other arthropods, and most molluscs, blood bathes organs directly in an
There is no distinction between blood and interstitial fluid, collectively called
One or more hearts pump the hemolymph into interconnected sinuses surrounding the organs, allowing
exchange between hemolymph and body cells.
When the heart contracts, it pumps hemolymph through vessels out into
When the heart relaxes, it draws hemolymph into the circulatory system through pores called ostia.
Body movements that squeeze the sinuses help circulate the hemolymph.
closed circulatory system,
found in earthworms, squid, octopuses, and vertebrates, blood is
confined to vessels and is distinct from interstitial fluid.
One or more hearts pump blood into large vessels that branch into smaller ones coursing through
Materials are exchanged by diffusion between the blood and the interstitial fluid bathing the cells.
The fact that open and closed circulatory systems are both widespread in the animal kingdom suggests