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S E C T I O N 9.1 The Task of Transportation E X P E C TAT I O N S Explain the functions of a transport system in multicellular organisms. Identify the main types of transport system found in animals. Distinguish between an open and closed transport system. Explain how adaptations such as double circulation alter the efficiency of a transport system. Figure 9.1 A jellyfish does not require a specialized internal transport system. Organisms like those shown in Figure 9.1 can rely on their external environments to supply what they need for all their life activities. In most animals, however, the transport system plays a key role in keeping the body in a state of general physical well-being. Its functions include delivering nutrients and oxygen to each and every cell of the body, removing the waste products of cellular processes, and serving as the pathway from one part of the body to another for disease-fighting agents, hormones, and other chemical messengers. In warm-blooded animals, the transport system also assists in the important function of controlling body temperature. The transport system is therefore an essential link among the cells and organs within the body, and between individual cells and the external environment. In Chapter 8 you learned that while all aerobic organisms must exchange gases with their environment, different organisms have evolved different kinds of respiratory systems to meet this challenge. The same is true of transport systems. Almost all multicellular organisms have a vascular system, a system of fluid tissue that plays a role in transporting nutrients and other materials to the cells of the organism. Most animals have a circulatory system, which is a vascular system in which the progress of fluid is controlled by muscle movements so that it follows a specific pattern. A cardiovascular system, found in the higher animals, 282 MHR • Internal Systems and Regulation is a circulatory system in which the vascular fluid is pumped around the body by the action of a specialized organ, the heart. The following paragraphs describe the transport mechanisms found in different kinds of organisms. Internal Transport at Its Simplest A self-sufficient unicellular organism such as Amoeba has no need for an organized transport system. Respiratory gases and nutrients enter the cell through diffusion or active transport across the cell membrane. Once inside the cell, these substances are distributed to the cell’s organelles by the streaming or flowing of cytoplasm. Waste materials diffuse or are carried back across the cell membrane to be released into the environment. As you can see in Figure 9.2, such an organism exchanges the materials it needs directly with the external environment. Some multicellular animals, such as those in the family Cnidaria (which includes hydra and jellyfish), also lack an organized transport system. In these animals, fluid taken in through the mouth enters a body cavity that extends through most of the organism, including the tentacles. Materials are exchanged directly between the fluid in the body cavity and the individual cells of the organism, none of which are more than a few cells away from the body cavity. REWIND For more information about diffusion, turn to Chapter 1, Section 1.3. Although planaria have a higher metabolic rate than cnidaria (and therefore need a faster exchange of materials), planaria can still rely on diffusion to meet their metabolic needs. Nutrients dissolve directly into individual cells from a digestive cavity. This cavity branches and rebranches within the organism to provide a surface for the exchange of materials with individual cells (see Figure 9.3). A separate cavity absorbs waste materials that are then expelled from the animal. They also lack a specialized mechanism for pumping fluid from one body part to another. Instead, fluid containing dissolved respiratory gases, nutrients, and other substances is ingested directly from the external environment. This fluid flows freely within the organism’s body cavities, sometimes assisted by the contraction of cells and muscle fibres as the animal moves. right side of excretory system left side of digestive system substances enter cell by diffusion substances move within cell Figure 9.2 The movement, or streaming, of cytoplasmic fluid helps to distribute materials to the organelles within a cell. None of these organisms have blood, which is a vascular tissue adapted to carry particular substances. MINI Figure 9.3 In the multicellular planarian, the movements of the animal help to stream fluid through a central cavity, from which material can diffuse into and out of its cells. The Specialized Transport System Most multicellular organisms cannot rely on diffusion or active transport to bring materials from the external environment to cells of the internal tissues quickly enough to meet the cells’ metabolic requirements. For such organisms to survive, some means of speeding the exchange of materials between the inside of the organism and the outside world is required. Before organisms could evolve to LAB How Effective Is Diffusion as a Transport Mechanism? In this lab, you will compare the rate of diffusion of a solution into tissues of different sizes. Carefully cut a potato into sections of different shapes and sizes. Select four sections for this experiment. Your thinnest section should be no less than 1 mm wide, and your thickest section no more than 3 cm wide. The sections should be of varying dimensions and volumes. Immerse the potato sections into a solution of potassium permanganate, and leave them for 30 min. At the end of this period, use forceps to remove all the sections. Cut each one in half and measure how far the purple solution penetrated the potato flesh. Record your results in a table. Analyze Calculate the rate of diffusion in each section by dividing the diffusion distance by the time the potato was immersed in the solution. Was the diffusion rate constant for each section? Provide an explanation for any variation. Assuming that the same diffusion rate you measured here could be applied to your body. Use this information to calculate approximately how long it would take for an oxygen molecule absorbed in your lung to reach the fingers of your writing hand (if you had no specialized transport system). Transport and Circulation • MHR 283 sizes more than a few cells thick, adaptations providing for an organized transport system were necessary. The evolution of a transport system also made it possible for larger animals to dedicate specially-adapted organs and structures to specific tasks, and then distribute the products of these tasks to other organs and structures. This, in turn, allowed for the emergence of multicellular organisms capable of undertaking complex activities. The Open Transport System If you dissect a grasshopper, you will find only one blood vessel. This vessel, the aorta, carries blood into the animal’s body cavity. The body cavity is subdivided into a number of sinuses (or chambers) that bring the blood and other body fluids into contact with the internal cells of the animal. The exchange of materials takes place within the sinuses. Co-ordinated movements of body muscles help to move the blood around inside the insect and then back to a sinus surrounding the long, tubular heart. From there, the blood is again pumped through the aorta to the sinuses of the body cavity (see Figure 9.4). In this circulatory system, the blood bathes the cells directly. A transport system in which the blood does not always stay contained within blood vessels is called an open transport system. In an open transport system, fluid can slosh back and forth; therefore it circulates relatively slowly. The result is a system that cannot provide for the rapid delivery of materials around the body. This is one reason why the open transport system is best suited to insects and other arthropods that have relatively small body cavities. Another reason is that (as you saw in Chapter 8) insects have a tracheal respiratory system that delivers oxygen directly from the environment to the animals’ cells. The respiratory and circulatory systems are separate in these animals, so even a very active insect can meet its oxygen requirements despite its relatively slow circulation. BIO FACT Because there is no reason for the blood of grasshoppers and other insects to carry oxygen, it lacks an oxygencarrying respiratory pigment and thus appears clear and watery. The Closed Transport System Animals that rely on the circulatory system to carry respiratory gases need a faster flow of blood than can be provided by an open transport system. These organisms, including the annelids (segmented worms such as earthworms) and vertebrates, have a closed transport system. In this type of system, the blood does not bathe the cells directly, but rather is pumped around the body within a network of vessels. This network includes larger vessels that collect blood for pumping, smaller vessels that distribute the blood throughout the body, and tiny capillaries that provide a surface for the exchange of materials with individual cells. In a closed transport system, blood circulates in only one direction, passing through the animal’s gas exchange system in its cycle through the body. Although all closed transport systems share these features, they differ in ways that result in varying levels of efficiency. The efficiency of a circulatory system is measured by the rate at which heart aorta brain anus mouth intestine nerve cord stomach Figure 9.4 The transport system of a grasshopper. The long, tubular heart extends most of the length of the abdomen. Some insects have additional hearts located at the base of their wings or antennae to help pump blood into these body parts. 284 MHR • Internal Systems and Regulation brain crop gizzard clitellum mouth segments anus aortic arches (hearts) dorsal blood vessel intestine bristles ventral blood vessel Figure 9.5 The earthworm has a relatively simple closed circulatory system. The five aortic arches located near the head link the dorsal and ventral blood vessels. it can transport substances around the body. Factors that affect this efficiency include the composition of the blood, the path of circulation, and the speed of blood flow (usually measured in terms of blood pressure). The circulatory systems of different animals show adaptations related to their unique metabolic requirements. Circulation in Annelids The simplest closed circulatory system is found in annelids. This system consists of two main blood vessels connected by a series of five pairs of heartlike pumps called aortic arches. Blood enters the aortic arches from the dorsal vessel, which (like the arches) contracts to pump the blood forward. From the aortic arches the blood is pumped into the ventral vessel located under the intestinal tract. The blood then flows through a branching series of smaller and smaller vessels to the internal organs and tissues, where the exchange of materials occurs across the thin walls of the capillaries. Some vessels also lead to the animal’s body walls, where the exchange of gases takes place between the capillaries and the external environment. The capillaries lead into larger vessels, which in turn direct the blood into the dorsal vessel and aortic arches to complete the cycle. Dissolved in the blood of the earthworm is a respiratory pigment, a molecule that binds to oxygen. The presence of this pigment allows the blood to pick up and transport more oxygen than would otherwise be possible. The respiratory pigment found in earthworms is the same pigment that is found in the blood of vertebrates. Figure 9.6 Unlike the earthworm, the bird breathes through lungs. It needs a different kind of circulatory system to deliver oxygen around its body. As you saw in Chapter 8, the earthworm uses its entire skin surface to exchange gases. This means that the blood is being oxygenated as it circulates around the animal. For animals with specialized respiratory structures such as gills or lungs, however, a more complex circulatory system is required. This is necessary to ensure that the blood is first passed through the respiratory surface where the exchange of gases takes place, and then is delivered to the rest of the body. In contrast to the circulatory system of the earthworm, which lacks a true heart, all vertebrates have a closed circulatory system in which the regular contractions of a muscular heart force blood past the respiratory surface, through a network of vessels, and into the capillaries where the exchange of materials takes place. Blood travels from the Transport and Circulation • MHR 285 heart to the capillaries through a distinct set of vessels called arteries, and returns from the capillaries to the heart through another set of vessels called veins. The path of circulation and the structure of the heart both play a role in determining the efficiency of the circulatory system in the different animals discussed in the following pages. See Figure 9.7 for a comparison of these systems. Circulation in Fish The heart of a fish has two main chambers and two lesser cavities organized in a row. Blood flows through this series of chambers and is pumped out through a ventral artery to the network of capillaries in the gills, where the exchange of respiratory gases takes place. From the gills the blood travels to a dorsal artery that branches through the body to deliver oxygenated blood. This kind of circulation, in which the blood travels through the heart only once during each complete circuit around the body, is called single circulation. The circulatory system of the fish offers the advantage that all the blood travelling from the heart to the body has been oxygenated in the gills. In this circulation pathway, however, much of the blood pressure generated by the heart is lost as the blood passes through the capillaries in the gills. This happens because the finer tubes of the capillaries offer more resistance to the blood, slowing it down. The relatively slow passage of blood through the rest of the body limits the rate of sinus venosus conus arteriosus oxygen delivery and therefore limits the metabolic rate of the animal. Since fish are cold-blooded, they do not need to generate as much energy as warmblooded animals such as birds and mammals. Circulation in Amphibians Animals in the class Amphibia (which includes frogs, toads, and salamanders) have a heart with a third chamber. The blood is pumped from the heart to the lungs, and then flows back to the heart where it is pumped again before flowing into the arteries that carry blood to the rest of the body. The structure of the heart means that there is some mixing of oxygenated and deoxygenated blood — that is, some of the blood that returns to the heart from the body then flows back to the body without first passing to the lungs. The extra chamber of the heart allows blood to be pumped through the heart twice during every complete cycle of the circulatory system, creating a double circulation. Since only some of the blood is pumped twice in each circuit, the amphibian circulatory system is called an incomplete double circulatory system. The incomplete separation of oxygenated and deoxygenated blood in the heart reduces the efficiency of the respiratory system. Remember, however, that amphibians can respire through their skin as well as through their lungs, so the blood does not have to pass by the lungs to pick up oxygen. lungs body lungs atrium right atrium deoxygenated blood ventral aorta left atrium oxygenated blood ventricle right atrium right ventricle left atrium left ventricle ventricle all deoxygenated blood A The fish has a two-chambered heart and complete, single circulation. B The amphibian has a three-chambered heart and incomplete, double circulation. Figure 9.7 The circulatory systems of a fish, an amphibian, and a mammal. Variations in the structure of the heart and in the path of circulation affect the efficiency of the circulatory system in these different animals. 286 body MHR • Internal Systems and Regulation C The mammal has a four-chambered heart and complete, double circulation. BIO FACT Birds (about 9000 species) and mammals (about 4500 species) are alone among living things in being warmblooded, meaning that their internal temperatures are kept constant. This is possible only because their muscles, which generate heat as they contract, are constantly supplied with richly oxygenated blood by their very efficient respiratory and circulatory systems. Figure 9.8 Adaptations in the circulatory system of mammals provides them with a much higher metabolic rate than is found in fish. Circulation in Birds and Mammals Active, warm-blooded vertebrates have very high energy requirements. Their bodies demand lots of oxygen, and need it delivered quickly. The high level of cellular activity also generates wastes that must be removed quickly from the fluid surrounding individual cells in order to keep those cells healthy. The circulatory systems of birds and mammals show two important adaptations that allow this more efficient circulation. The first is a mechanism to keep oxygenated and deoxygenated blood completely separate, so the blood that flows from the heart to the body contains as much oxygen as possible. The second is a mechanism to maximize blood pressure in order to force blood through the capillaries quickly. SECTION REVIEW 1. K/U Describe the differences in the transport mechanisms found in an amoeba and in a planarian. 2. K/U Explain why an open transport system is well suited to the metabolic needs of an insect. 3. C Using landmarks that your classmates will be familiar with, map out a walking or cycling route that could illustrate a path of single circulation, and another route that could illustrate a path of double circulation. Trade maps with a partner and try to explain how your partner’s two routes compare to the circulatory systems of different animals. 4. All birds and mammals have a heart consisting of four chambers: two atria (singular atrium) and two ventricles. Blood entering the right atrium from the body is pumped into the right ventricle. From there it moves through an artery to the lungs, where the exchange of respiratory gases occurs. The blood loses pressure as it flows through the capillary networks of the lungs — but rather than flowing out to the body, this oxygenated blood is carried back to the heart. It flows into the left atrium, enters the left ventricle, and from there is pumped into the aorta and other arteries that deliver blood to the body. This system is called a closed, complete, double circulatory system. In the sections that follow, you will examine the mammalian transport systems in more detail, using the human system as an example. I The heart rate of mammals is inversely proportional to their size. Accumulate, graph, and analyze data that will either support or discredit this hypothesis. 5. K/U Reptiles have two aorta — one travels down the right side of the body, and the other down the left. In contrast, birds and mammals each have only one aorta; in mammals it is on the left side of the body, and in birds it is on the right. What does this suggest about the evolution of the circulatory system? What selective advantage is offered by the loss of one aorta? 6. MC Amphibian species are sometimes referred to as “indicator species” because changes in their health can provide information about damage to an ecosystem. With reference to the transport of respiratory gases, explain why a frog would be more sensitive than a muskrat to chemical changes in the water of the pond they share. Transport and Circulation • MHR 287