Circulation
Circulation
We pointed out in the begining to this section that most animals have evolved mechanisms, in addition to simple diffusion, for transporting materials among various regions of the body. For sponges and radiates the water in which they live provides the medium for transport. Water, propelled by ciliary, flagellar, or body movements, passes through channels or compartments to facilitate the movement of food, respiratory gases, and wastes. True circulatory systems— containing vessels through which blood moves—are essential to animals so large or so active that diffusional processes alone cannot supply their oxygen needs. An animal’s shape obviously is important. The flattened and leaflike acoelomate flatworms, even though many are relatively large animals, have no need for a circulatory system because the distance of any body part from the surface is short; respiratory gases and metabolic wastes transfer by simple diffusion.
A circulatory system having a full complement of components— propulsive organ, arterial distribution system, capillaries, and venous reservoir and return system—is fully recognizable in annelid worms. In earthworms (Figure 33-7) there are two main vessels, a dorsal vessel carrying blood toward the head, and a ventral vessel that flows posteriorly, delivering blood throughout the body by way of segmental vessels and a dense capillary network. The dorsal vessel drives the blood forward by peristalsis and thus serves as a heart. Five aortic arches that on each side connect the dorsal and ventral vessels are also contractile and serve as accessory hearts to maintain a steady flow of blood into the ventral vessel. Many smaller segmental vessels that deliver blood to tissue capillaries are actively contractile as well. We see then that there is no localized pump pushing the blood through a system of passive tubes; instead the power of contraction is widely distributed throughout the vascular system.
We pointed out in the begining to this section that most animals have evolved mechanisms, in addition to simple diffusion, for transporting materials among various regions of the body. For sponges and radiates the water in which they live provides the medium for transport. Water, propelled by ciliary, flagellar, or body movements, passes through channels or compartments to facilitate the movement of food, respiratory gases, and wastes. True circulatory systems— containing vessels through which blood moves—are essential to animals so large or so active that diffusional processes alone cannot supply their oxygen needs. An animal’s shape obviously is important. The flattened and leaflike acoelomate flatworms, even though many are relatively large animals, have no need for a circulatory system because the distance of any body part from the surface is short; respiratory gases and metabolic wastes transfer by simple diffusion.
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| Figure 33-7 Blood flow through the closed vascular system of an earthworm. |
A circulatory system having a full complement of components— propulsive organ, arterial distribution system, capillaries, and venous reservoir and return system—is fully recognizable in annelid worms. In earthworms (Figure 33-7) there are two main vessels, a dorsal vessel carrying blood toward the head, and a ventral vessel that flows posteriorly, delivering blood throughout the body by way of segmental vessels and a dense capillary network. The dorsal vessel drives the blood forward by peristalsis and thus serves as a heart. Five aortic arches that on each side connect the dorsal and ventral vessels are also contractile and serve as accessory hearts to maintain a steady flow of blood into the ventral vessel. Many smaller segmental vessels that deliver blood to tissue capillaries are actively contractile as well. We see then that there is no localized pump pushing the blood through a system of passive tubes; instead the power of contraction is widely distributed throughout the vascular system.
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