In this section, you will explore the following questions:

  • What is the difference between an open circulatory system and a closed circulatory system?
  • What are the components of interstitial fluid and hemolymph?
  • What are the differences and similarities among the organization and evolution of different vertebrate circulatory systems?

Connection for AP® Courses

Much of the information in this chapter is not in scope for AP®. However, the chapter is filled with examples that are applicable to concepts we have previously explored. The circulatory system links to all other organ systems. For example, nutrients pass from the digestive system into the blood, and the excretory system rids the blood of wastes. In addition, because heart disease is a leading cause of death in humans, the information is relevant.

Circulatory systems in animals show a distinct pattern of evolution—from none in sponges and jellyfish to open circulatory systems characteristic of insects to closed circulatory systems with hearts, valves, and vessels in vertebrates. Circulatory systems provide body cells with oxygen and nutrients, remove wastes, transports hormones, protect against invaders, and aids in temperature regulation. Although some animals such as the sponge have had no adaptive need to develop a complex circulatory system (they exchange nutrients and wastes directly with the environment), as demands for oxygen increased with terrestrial living, natural selection favored a closed circulatory system in which blood is transported in vessels. To increase efficiency, within closed systems the heart evolved from two chambers in fish (one atrium and one ventricle) to the four-chambered heart (two atria and two ventricles) seen in crocodiles, birds, and mammals.

Information presented and the examples highlighted in the section support concepts outlined in Big Idea 1 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP®Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices.

Big Idea 1 The process of evolution drives the diversity and unity of life.
Enduring Understanding 1.A Change in the genetic makeup of a population over time is evolution.
Essential Knowledge 1.A.1 An adaptation, such as the number of heart chambers, is a genetic variation that is favored by natural selection and provides an advantage to an organism in a particular environment.
Science Practice 2.2 The student can apply mathematical routines to quantities that describe natural phenomena.
Science Practice 5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question.
Learning Objective 1.2 The student is able to evaluate evidence provided by data to qualitatively and quantitatively investigate the role of natural selection in evolution.
Enduring Understanding 1.B Organisms are linked by lines of descent from common ancestry.
Essential Knowledge 1.B.2 Phylogenetic trees and cladogram can represent traits, such as the number of heart chambers in animals, which are derived or lost due to evolution.
Science Practice 3.1 The student can pose scientific questions.
Learning Objective 1.17 The student is able to pose scientific questions about a group of organisms whose relatedness is described by a phylogenetic tree or cladogram in order to (1) identify shared characteristics, (2) make inferences about the evolutionary history of the group, and (3) identify character data that could extend or improve the phylogenetic tree.
Essential Knowledge 1.B.2 Phylogenetic trees and cladogram can represent traits, such as the number of heart chambers in animals, which are derived or lost due to evolution.
Science Practice 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain.
Learning Objective 1.19 The student is able to create a phylogenetic tree or simple cladogram that correctly represents evolutionary history and speciation from a provided data set.

Circulatory System Architecture

The circulatory system is effectively a network of cylindrical vessels: the arteries, veins, and capillaries that emanate from a pump, the heart. In all vertebrate organisms, as well as some invertebrates, this is a closed-loop system, in which the blood is not free in a cavity. In a closed circulatory system, blood is contained inside blood vessels and circulates unidirectionally from the heart around the systemic circulatory route, then returns to the heart again, as illustrated in Figure 31.2a. As opposed to a closed system, arthropods—including insects, crustaceans, and most mollusks—have an open circulatory system, as illustrated in Figure 31.2b. In an open circulatory system, the blood is not enclosed in the blood vessels but is pumped into a cavity called a hemocoel and is called hemolymph because the blood mixes with the interstitial fluid. As the heart beats and the animal moves, the hemolymph circulates around the organs within the body cavity and then reenters the hearts through openings called ostia. This movement allows for gas and nutrient exchange. An open circulatory system does not use as much energy as a closed system to operate or to maintain; however, there is a trade-off with the amount of blood that can be moved to metabolically active organs and tissues that require high levels of oxygen. In fact, one reason that insects with wing spans of up to two feet wide (70 cm) are not around today is probably because they were outcompeted by the arrival of birds 150 million years ago. Birds, having a closed circulatory system, are thought to have moved more agilely, allowing them to get food faster and possibly to prey on the insects.

Examples of a closed and open circulatory system are shown. The closed circulatory system is depicted using a cross section of an earthworm. A purple tube with a C shaped cross section runs down the length of the organism. Two bands around the tube are labeled “Hearts.” A red tube running dorsal (on top of) to the purple tube is labeled “Dorsal blood vessel (main heart)” and a red tube running ventral (underneath) to the purple tube is labeled “Ventral blood vessel.” A honey bee is used to depict the open circulatory system. A red tube with that widens at regularly spaced intervals with holes at those wider spots runs along the dorsal aspect (top) of the honeybee. The wider areas are labeled “Hearts” and the holes are labeled, “Ostia (openings in heart).” The red tube is labeled, “Dorsal blood vessel.” A white arrow starting at the end of the tube at the honeybee head extends downward and along the ventral (bottom) aspect of the honeybee. This area is labeled, “Body cavity.” White arrows branch off this white arrow, extending upward back toward the red tube.
Figure 31.2In (a) closed circulatory systems, the heart pumps blood through vessels that are separate from the interstitial fluid of the body. Most vertebrates and some invertebrates, like this annelid earthworm, have a closed circulatory system. In (b) open circulatory systems, a fluid called hemolymph is pumped through a blood vessel that empties into the body cavity. Hemolymph returns to the blood vessel through openings called ostia. Arthropods like this bee and most mollusks have open circulatory systems.

Circulatory System Variation in Animals

The circulatory system varies from simple systems in invertebrates to more complex systems in vertebrates. The simplest animals, such as the sponges (Porifera) and rotifers (Rotifera), do not need a circulatory system because diffusion allows adequate exchange of water, nutrients, and waste, as well as dissolved gases, as shown in Figure 31.3a. Organisms that are more complex but still only have two layers of cells in their body plan, such as jellies (Cnidaria) and comb jellies (Ctenophora) also use diffusion through their epidermis and internally through the gastrovascular compartment. Both their internal and external tissues are bathed in an aqueous environment and exchange fluids by diffusion on both sides, as illustrated in Figure 31.3b. Exchange of fluids is assisted by the pulsing of the jellyfish body.

Shown are the cross sections of a sponge and a jellyfish. The sponge is concave up, so it has a vase like appearance with a large space in the between. It is made up of multiple sections, each separated with a small opening between the environment and cavity. The jellyfish is concave down, with an upside bowl appearance. It has multiple layers, with a cavity formed by what looks like the union of a small bowl and a large bowl with an opening into its cavity.
Figure 31.3Simple animals consisting of a single cell layer such as the (a) sponge or only a few cell layers such as the (b) jellyfish do not have a circulatory system. Instead, gases, nutrients, and wastes are exchanged by diffusion.

For more complex organisms, diffusion is not efficient for cycling gases, nutrients, and waste effectively through the body; therefore, more complex circulatory systems evolved. Most arthropods and many mollusks have open circulatory systems. In an open system, an elongated beating heart pushes the hemolymph through the body and muscle contractions help to move fluids. The larger more complex crustaceans, including lobsters, have developed arterial-like vessels to push blood through their bodies, and the most active mollusks, such as squids, have evolved a closed circulatory system and are able to move rapidly to catch prey. Closed circulatory systems are a characteristic of vertebrates; however, there are significant differences in the structure of the heart and the circulation of blood between the different vertebrate groups due to adaptation during evolution and associated differences in anatomy. Figure 31.4illustrates the basic circulatory systems of some vertebrates: fish, amphibians, reptiles, and mammals.

Illustration A shows the circulatory system of fish, which have a two-chambered heart with one atrium and one ventricle. Blood in systemic circulation flows from the body into the atrium, then into the ventricle. Blood exiting the heart enters gill circulation, where gases are exchanged by gill capillaries. From the gills blood re-enters systemic circulation, where gases in the body are exchanged by body capillaries. Illustration B shows the circulatory system of amphibians, which have a three-chambered heart with two atriums and one ventricle. Blood in systemic circulation enters the heart, flows into the right atrium, then into the ventricle. Blood leaving the ventricle enters pulmonary and skin circulation. Capillaries in the lung and skin exchange gases, oxygenating the blood. From the lungs and skin blood re-enters the heart through the left atrium. Blood flows into the ventricle, where it mixes with blood from systemic circulation. Blood leaves the ventricle and enters systemic circulation. Illustration C shows the circulatory system of reptiles, which have a four-chambered heart. The right and left ventricle are separated by a septum, but there is no septum separating the right and left atrium, so there is some mixing of blood between these two chambers. Blood from systemic circulation enters the right atrium, then flows from the right ventricle and enters pulmonary circulation, where blood is oxygenated in the lungs. From the lungs blood travels back into the heart through the left atrium. Because the left and right atrium are not separated, some mixing of oxygenated and deoxygenated blood occurs. Blood is pumped into the left ventricle, then into the body. Illustration D shows the circulatory system of mammals, which have a four-chambered heart. Circulation is similar to that of reptiles, but the four chambers are completely separate from one another, which improves efficiency.
Figure 31.4(a) Fish have the simplest circulatory systems of the vertebrates: blood flows unidirectionally from the two-chambered heart through the gills and then the rest of the body. (b) Amphibians have two circulatory routes: one for oxygenation of the blood through the lungs and skin, and the other to take oxygen to the rest of the body. The blood is pumped from a three-chambered heart with two atria and a single ventricle. (c) Reptiles also have two circulatory routes; however, blood is only oxygenated through the lungs. The heart is three chambered, but the ventricles are partially separated so some mixing of oxygenated and deoxygenated blood occurs except in crocodilians and birds. (d) Mammals and birds have the most efficient heart with four chambers that completely separate the oxygenated and deoxygenated blood; it pumps only oxygenated blood through the body and deoxygenated blood to the lungs.

As illustrated in Figure 31.4a Fish have a single circuit for blood flow and a two-chambered heart that has only a single atrium and a single ventricle. The atrium collects blood that has returned from the body and the ventricle pumps the blood to the gills where gas exchange occurs and the blood is re-oxygenated; this is called gill circulation. The blood then continues through the rest of the body before arriving back at the atrium; this is called systemic circulation. This unidirectional flow of blood produces a gradient of oxygenated to deoxygenated blood around the fish’s systemic circuit. The result is a limit in the amount of oxygen that can reach some of the organs and tissues of the body, reducing the overall metabolic capacity of fish.

In amphibians, reptiles, birds, and mammals, blood flow is directed in two circuits: one through the lungs and back to the heart, which is called pulmonary circulation, and the other throughout the rest of the body and its organs including the brain (systemic circulation). In amphibians, gas exchange also occurs through the skin during pulmonary circulation and is referred to as pulmocutaneous circulation.

As shown in Figure 31.4b, amphibians have a three-chambered heart that has two atria and one ventricle rather than the two-chambered heart of fish. The two atria (superior heart chambers) receive blood from the two different circuits (the lungs and the systems), and then there is some mixing of the blood in the heart’s ventricle (inferior heart chamber), which reduces the efficiency of oxygenation. The advantage to this arrangement is that high pressure in the vessels pushes blood to the lungs and body. The mixing is mitigated by a ridge within the ventricle that diverts oxygen-rich blood through the systemic circulatory system and deoxygenated blood to the pulmocutaneous circuit. For this reason, amphibians are often described as having double circulation.

Most reptiles also have a three-chambered heart similar to the amphibian heart that directs blood to the pulmonary and systemic circuits, as shown in Figure 31.4c. The ventricle is divided more effectively by a partial septum, which results in less mixing of oxygenated and deoxygenated blood. Some reptiles (alligators and crocodiles) are the most primitive animals to exhibit a four-chambered heart. Crocodilians have a unique circulatory mechanism where the heart shunts blood from the lungs toward the stomach and other organs during long periods of submergence, for instance, while the animal waits for prey or stays underwater waiting for prey to rot. One adaptation includes two main arteries that leave the same part of the heart: one takes blood to the lungs and the other provides an alternate route to the stomach and other parts of the body. Two other adaptations include a hole in the heart between the two ventricles, called the foramen of Panizza, which allows blood to move from one side of the heart to the other, and specialized connective tissue that slows the blood flow to the lungs. Together these adaptations have made crocodiles and alligators one of the most evolutionarily successful animal groups on earth.

In mammals and birds, the heart is also divided into four chambers: two atria and two ventricles, as illustrated in Figure 31.4d. The oxygenated blood is separated from the deoxygenated blood, which improves the efficiency of double circulation and is probably required for the warm-blooded lifestyle of mammals and birds. The four-chambered heart of birds and mammals evolved independently from a three-chambered heart. The independent evolution of the same or a similar biological trait is referred to as convergent evolution.



What advantages are supplied by closed circulatory systems with chambered hearts in terrestrial vertebrates?