Circulatory systems of invertebrates

Circulatory systems are necessary when the process of diffusion no longer provides an organism with sufficient gas, nutrients, and waste exchange with its environment. Some invertebrate groups, such as sponges, coelenterates, and flatworms, have such thin body walls that diffusion can meet all their needs. For most invertebrates, however, the distance from the organism’s surface to the cells in the interior is too great for diffusion to support their metabolic requirements. A circulatory system is composed of three parts: the pump, the fluid that is pumped, and the vessels in which the fluid is transported. As in the design of invertebrate body plans, there is great diversity in the design of invertebrate circulatory systems.

The Components of the Circulatory System

The pump may be a simple tube lined by muscle fibers. Alternate contraction and relaxation of these muscles produce a peristaltic wave that pushes the blood along. The pump may be a heart: a localizable, discrete organ whose muscle layers are the primary generators of the power that propels blood through the blood vessels. The heart can be a simple muscular enlargement, or it can be complex and multichambered, depending on the evolutionary history and the needs of the organism. Some organisms may have more than one heart.

If the pumped fluid remains within the blood vessels, it is usually called blood. If it leaves the blood vessels to enter cavities surrounded by tissue cells with which it exchanges materials, it is called hemolymph. In the cavities (called lacunae if small and sinuses if large and lined with a membrane), blood mingles with intercellular fluid. Whatever it is called, the fluid is composed of water, solutes (such as salts, sugars, and other nutrients), and, in some cases, cells and formed elements. The blood may also contain a respiratory pigment that helps deliver oxygen to the cells.

Fluid Transport Systems

Blood vessels may extend only a short distance from the heart or provide a continuous path for blood transport. If the blood vessels are incomplete and blood is pumped from arteries into body spaces—sinuses and lacunae—it is an open circulatory system. If the blood vessels are continuous and the blood never leaves these circulatory channels, it is a closed circulatory system. These are generalized terms for convenience and many intermediate cases. Some open systems have membrane-lined sinuses and lacunae, and exchange takes place by diffusion just as it does in the tiniest vessels of closed systems. Blood vessels are categorized according to their function. Arteries carry blood away from the heart, and veins carry blood back to the heart. Closed circulatory systems have tiny tubules called capillaries that connect small arteries to small veins. The exchange of materials between the tissue cells and the blood occurs only in the capillaries.

A few generalities based on physical principles indicate the rules guiding fluid transport. Organisms relying on diffusion alone to circulate nutrients and eliminate wastes are limited to certain shapes and sizes. They are often only two cell layers thick. Organisms using bulk transport of fluid, on the other hand, may be as complex and as differently shaped as plants and animals are.

A fluid transport system is any system in which internal fluid movement reduces diffusion distances—either between points within an organ or between a point within an organism and the external environment. While diffusion is always used for short-distance transport, bulk flow augments diffusion for any long-distance transport. Bulk flow requires a pump and a fluid that must come into intimate contact with tissues and the environment for efficient transfer. Either the tissue layers must be thin (as in open circulatory systems), or the vessels must have a small radius (as in a closed circulatory system). The circulatory fluid should spend the majority of its time in the transfer regions (sinuses and lacunae or capillaries) and not in transit.

Such transport systems use both large and small vessels. Large vessels move fluid from one exchange site to another, and small vessels allow diffusion at the exchange sites. The total cross-sectional area of the small vessels must exceed that of the large vessels so that the flow rate in small vessels will be less than the velocity in large vessels. High-speed pumps in large vessels are preferable to low-speed pumps in small vessels. This means that accessory hearts are only used when the “cost” of operation is not a major factor or when the pump serves some additional functional role—as when active cephalopod mollusks, such as squid and octopods, use accessory hearts to ensure adequate blood flow through their gills.

Closed and Open Circulatory Systems

Circulatory systems are traditionally divided into two categories. Closed systems are those in which the blood is always contained within distinct vessels and is physically separated from the organism’s intercellular fluids. They are usually characteristic of organisms with high metabolic demands. High volumes and high pressures can be maintained in the closed vessels to aid transport and diffusion. Annelids, cephalopod mollusks, and vertebrates usually have closed circulatory systems. In the closed circulatory system of octopuses, squids, and cuttlefish, three hearts exist. Two pump blood to the gills and the other pumps blood to the rest of the body.

Open systems are those that possess large, usually ill-defined, cavities (sinuses if bound by an endothelial layer, lacunae if not) and in which the blood is not physically separated from the intercellular fluids. Arthropods and noncephalopod mollusks have open systems.

Open circulatory systems are not always sluggish, low-pressure arrangements. Some spiders generate sufficient pressure in their open systems to use hydraulic pressure as a substitute for extensor muscles in their legs. In addition, capillaries often exist in open systems, particularly in areas such as the excretory organs and the cerebral ganglion. The major sinus in the foot of the gastropod and bivalve mollusks is not a large, open cavity but a network of channels in a spongy tissue that functions as capillaries. The lack of return vessels in these systems is usually a result of the fact that fluid simply has nowhere to go other than in the direction of low pressure: back to the heart. These volume constraints are sufficient to develop pressure, although this system is incompatible with high pressure and flow rates.

In the two major groups with open circulatory systems, arthropods and noncephalopod mollusks, the circulatory sinuses play an additional role: In bivalve and gastropod mollusks, the hemocoel (main body cavity) functions as a hydrostatic skeleton in locomotion and burrowing. In aquatic arthropods, it serves the same function during molting, when arthropods lose the support of the exoskeleton. In insects, the tracheal system has assumed the respiratory function, and the blood merely delivers nutrients and removes wastes. In large flying insects, the circulatory systems may also have the primary responsibility of removing heat to maintain thoracic temperatures.

Pressure Patterns in Heart Action

To function effectively, the circulatory system must have a regular pattern of pressure increases that will push the blood along through the vessels. The heartbeats that accomplish this may be initiated and maintained by nerves, or they may be self-generated. If nerves initiate the contraction of the heartbeat, the heart is called neurogenic, meaning that nerve impulses generate the depolarization that results in the contraction of the heart’s muscle cells. In these hearts, the heart muscle will not contract without a nerve impulse. Some species with neurogenic hearts are crustacea, horseshoe crabs, some spiders, and scorpions.

Heart muscles that continue to beat even when nervous connections are severed are called myogenic, meaning that the heart muscle contracts without external stimuli. Under these circumstances, the contraction of the muscles may occur at a different rate from that imposed by the nervous system when active. Myogenic hearts are found in mollusks and many insects.

All heart action must be modulated to respond to external and internal conditions, so even myogenic hearts usually receive some innervation. Modulation occurs through the mediation of nerves, hormones, or intrinsic controls in the heart. In the lobster, for example, nerves are crucial to maintaining the best rhythm and amplitude, but neurohormones released from a pericardial organ influence the heart action. Stretching the heart muscle, an intrinsic control, will also increase the vigor and rate of contraction.

In many cases, the structure of the heart and its suspensory ligaments contributes to the functioning of the circulatory system. Values at the openings (ostia) to the heart prevent backflow when the heart contracts. This pulls at the ligaments. Their elasticity pulls back the walls of the heart, creating low pressure that enables the heart to fill on relaxation. In effect, the heart sucks blood from veins to refill itself for the next contraction.

In many species, the contraction of body parts contributes to the circulation of blood. Arthropods have a rigid exoskeleton, so contraction in one part pushes the blood into another segment. In American lobsters, a quick flexion of the abdomen (an important locomotor movement) raises pressure in the abdomen and increases the rate of blood flow to the thorax and into the heart region.

All flow depends upon pressure differences, regardless of whether the circulatory system is open or closed, and there are two kinds of pressure. Background pressure is the pressure that prevails everywhere in the animal. Since pressure differences are responsible for flow, these pressure differences are imposed on the background pressure. If the body changes posture and increases background pressure, the blood pressure must similarly increase to maintain flows at the same level they were before the postural change. The blood pressure gradient and the resistance to flow in the system affect blood flow. The low resistance in open circulatory systems probably permits relatively high rates of blood flow with relatively low pressure. The high blood flow rate compensates for the low oxygen-carrying capacity of the blood.

Open and closed systems differ; neither is necessarily superior. The inherent weakness of the open circulatory system is that the peripheral blood flow cannot be as well controlled as that of closed circulatory systems. Yet, the large sinuses are often subdivided, thereby providing discrete channels of flow, and the peripheral blood flow may be more regular than previously thought. Closed circulatory systems have a flow that is easily controlled. Flow through particular regions can be managed by using muscles to close off certain channels. Cardiac output can be distributed to meet tissue demands. In open systems, this is not possible after the blood leaves the major vessels, although muscle contractions and accessory hearts may influence peripheral flow. Whatever their patterns, the circulatory systems of invertebrates are adequately matched to their needs; they have enabled these creatures to survive and proliferate for millions of years.

Studying Invertebrate Circulation

Methods used to study invertebrate circulatory systems are varied. One basic problem is that 95 percent of all animal species are invertebrates, and many of these creatures are not known and have never been studied. The larger, more common organisms that are easiest to study have been subjected to experimentation. Many invertebrates are difficult to maintain in the laboratory, but techniques to maintain them in good health are being developed and improved. Without these culture techniques, experimenters must use recently caught subjects whose condition is doubtful.

Most knowledge of invertebrate circulatory systems is anatomical. Even a common animal must be described so that a physiologist can apply appropriate techniques to study the functioning of the heart, vessels, and blood. Descriptions are usually derived from dissection and from microscopic study. These painstaking methods have been used on known species for centuries. Larger invertebrates, such as lobsters, crabs, clams, squid, and octopods, have been studied by techniques similar to those used in vertebrate circulation physiology. A heart can be exposed and either attached to a lever that can record its contractions or attached to an electronic force transducer, which can measure the strength of contraction.

The heart is large enough to be punctured for blood samples, and hemolymph can be withdrawn from the larger sinuses. These blood samples can be analyzed for the presence and activity of cells, respiratory pigments, nutrients, and wastes, using ordinary biochemical techniques. The development of microanalytic techniques in biochemistry allows the sampling of body fluids from small insects, worms, and rare organisms that might be damaged by taking larger samples.

In addition, force transducers can be placed along blood vessels and in hemocoels to determine the pressure exerted during flow. Electronic devices can monitor the flow rate by detecting cells or the passage of a dye or magnetic substance. Radioactive tracers can be injected and their path followed. All these less invasive techniques allow the animals to survive longer and to deliver data that can be more reliably interpreted because they are from a healthy subject. The use of fewer organisms in research and the survival of rare creatures are important both to the environment and to the development of a full understanding of these complex systems.

Advantages of Closed Versus Open Systems

Invertebrate circulatory systems occur in two plans, closed or open, and both styles of circulation have benefits for their users. Although open circulatory systems are usually thought to be sluggish and inefficient, they are not necessarily so. Active animals such as crustaceans, cephalopods, and insects have open circulatory systems and metabolic rates (oxygen and nutrient demands) that equal those of the most active invertebrates, including the cephalopods like squid, cuttlefish, and octopods, which have closed circulatory systems.

Nemertean or ribbon worms (phylum Rhynchocoela) are a small group of inconspicuous and inconsequential worms that have an interesting circulatory system. These worms may reach thirty meters in length (although they are only a few millimeters wide), but they have a simple blood system. There may be two or three blood vessels, with connections between them, running the length of the body. The vessels have a layer of muscle in the walls. Contraction of these muscles and the main body muscles move blood—in any direction—along the vessels.

The wormlike animals in the phylum Annelid also have a closed circulatory system. The major blood vessel has pulsatile regions, often called hearts, which drive the blood forward. The pattern of blood vessels, including capillaries, is repeated in each segment of the animal, although most segments do not have “hearts.” Accessory hearts occur in different segments, and the overall pattern of circulation varies greatly among species (and even within a single individual’s many segments).

The velvet worm Euperipatoides rowelli in the Peripatopsidae family has an uncharacteristically but elaborately organized open circulatory system consisting of the vascular system and a minimally impactful lacunar system. These worms breathe through open pores on their skin called tracheae. The vascular system comprises a heart, a pair of antennal arteries, and the suprapharyngeal and supracerebral regions of the anterior aorta. Its heart beats intermittently. This finding helped researchers hypothesize the likely vascular makeup of the ancient ancestor of the Onychophora and Arthropoda.

Arthropods and most mollusks have open circulatory systems; these invertebrates have a well-developed central heart. The heart pumps blood through an extensive arterial network, which may end in capillaries. The blood eventually leaves the blood vessels and enters lacunae. Diffusion of materials takes place in the capillaries or in the lacunae. Sinuses collect the blood for return to the heart. In these organisms, the blood follows an ill-defined path. Contractions of the body musculature affect the speed and volume of blood flow in any region.

The crustacean arthropods are a group having inactive members, which lack a heart and blood vessels entirely, and active members, which have a high level of circulatory system organization. The inactive members pump their body fluid through the sinuses and lacunae using the pressure developed by muscle contraction. Decapod crustaceans, the familiar crabs, have a single-chambered heart whose contraction drives blood into well-defined arteries; the return of blood to the heart from the veins occurs because of the elastic recoil of the ligaments suspending the heart in the pericardial cavity. Part of the beauty of this pattern is that only veins from the gills enter the pericardial cavity. Therefore, only oxygenated blood enters the heart and is pumped into the body. Thus, although there may be body spaces in which flow is indeterminate, the important blood flows are well controlled and can meet the needs of complex and active organisms.

Principal Terms

Blood: The fluid connective tissue within blood vessels that carries raw materials to cells and carries products and wastes from them

Closed Circulation: A circulatory pattern in which blood is always contained within blood vessels

Diffusion: The process whereby a substance moves from an area of greater concentration to one of lesser concentration, as through a cell membrane

Hemolymph: The transport fluid of organisms with open circulation systems in which there is no clear distinction between blood and intercellular tissue fluid

Lacunae: Small spaces among tissue cells through which hemolymph flows in open circulatory systems

Open Circulation: A circulatory pattern in which the blood is not always contained within blood vessels

Sinuses: Larger spaces, thought to represent through channels, for hemolymph in open circulatory systems, sometimes bound by membranes

Tracheal System: The respiratory system of insects and other terrestrial invertebrates; it consists of numerous air-filled tubes with branches extending into tiny channels in direct contact with body cells

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