Chapter 49

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University of Toronto Scarborough
Biological Sciences
Kamini Persaud

49.1 Why Do Animals Need a Circulatory System? A circulatory system consists of a muscular pump (the heart), a fluid (blood), and a series of conduits (blood vessels) through which the fluid can be pumped around the body. Heart, blood, and vessels are also known collectively as a cardiovascular system. The function of circulatory systems is to transport things around the body. Circulatory systems transport: heat, hormones, respiratory gases, blood cells, platelets, elements of the immune system ,nutrients ,and waste products. These all seem like important tasks, so why do so many species not have circulatory systems? Some animals do not have circulatory systems Single-celled organisms serve all of their needs through direct exchanges with the environment. Such organisms are mostly found in aquatic environments or very moist terrestrial environments. Similarly, for multicellular organisms a circulatory system is unnecessary if all of their cells are close enough to the external environment that nutrients, respiratory gases, and wastes can diffuse between the cells and the environment. Small aquatic invertebrates have structures and body shapes that permit direct exchanges between cells and environment. Many of these animals have flattened, thin body shapes that maximize the amount of surface area that is in contact with the external environment and minimize the diffusion path length for exchanges between the cells and the external environment. The cells of some other aquatic invertebrates are served by highly branched central cavities called gastrovascular systems that essentially bring the external environment into the animal. All the cells of a sponge, for example, are in contact with, or very close to, the water that surrounds the animal and circulates through its central cavity. Very small animals without circulatory systems can maintain high levels of metabolism and activity, but bigger animals without circulatory systems such as sponges, coelenterates, and flatworms tend to be inactive, slow, or even sedentary. Larger, active animals need circulatory systems. Larger and more active animals must support the metabolism of their cells by delivering nutrients to them and taking wastes away from them with circulatory systems. The critical environment for each cell is the extracellular fluid that surrounds it. All of the cells nutrientsoxygen, fuel, essential moleculescomes from that fluid, and the waste products of the cell go into that fluid. In many animals, the extracellular fluid is continuous with the fluid in the circulatory system. The vessels of these animals empty their fluid directly into the tissues. At other locations the extracellular fluid flows back into the circulatory system to be pumped back out again. These circulatory systems are called open. In contrast, closed circulatory systems completely contain the circulating fluid, blood, in a continuous system of vessels. However, even in closed circulatory systems, liquid and low-molecular-weight solutes are exchanged between the blood and the extracellular fluids surrounding the cells of the body. In fact, the term extracellular fluid refers to both the fluid in the circulatory system and the fluid between the cells of the body. To distinguish the two, we refer to the fluid in the circulatory system as the blood plasma and the fluid around the cells as interstitial fluid. A normal 70 kilogram person has a total extracellular fluid volume of about 14 liters. A little more than a quarter of it, about 3 liters, is the blood plasma; the rest is interstitial fluid. Circulatory systems carry materials to and from all regions of the body to maintain the optimum composition of the interstitial fluids, which in turn serve the needs of the cells. Importantly, circulatory systems speed up the delivery of nutrients and deliver them preferentially to the most active tissues. Open circulatory systems move extracellular fluid In open circulatory systems extracellular fluid squeezes through intercellular spaces as the animal moves. A muscular pump usually assists the distribution of the fluid in these systems. The contractions of this simple heart propel the extracellular fluid through vessels leading to different regions of the body, but the fluid leaves those vessels to trickle through the tissues and eventually return to the heart. Open circulatory systems are found in arthropods, mollusks, and some other invertebrate groups. In the generalized arthropod the fluid returns to the heart through valved openings called ostia. In mollusks, open vessels aid in the return of extracellular fluid to the heart. Closed circulatory systems circulate blood through a system of blood vessels In a closed circulatory system, a system of vessels keeps circulating blood separate from the interstitial fluid. Blood is pumped through thisvascular system by one or more muscular hearts, and some components of the blood never leave the vessels. Closed circulatory systems characterize vertebrates, annelids, and some other invertebrate groups. A simple example of a closed circulatory system is that of the earthworm. One large ventral vessel carries blood from its anterior end to its posterior end. Smaller vessels branch off and transport the blood to even smaller vessels serving the tissues in each segment of the worms body. In the smallest vessels, respiratory gases, nutrients, and metabolic wastes diffuse between the blood and the interstitial fluid. The blood then flows from these vessels into larger vessels that lead into one large dorsal vessel, which carries the blood from the posterior to the anterior end of the body. Five pairs of muscular vessels connect the large dorsal and ventral vessels in the anterior end, thus completing the circuit. The dorsal vessel and the five connecting vessels serve as hearts for the earthworm; their contractions keep the blood circulating. The direction of circulation is determined by one-way valves in the dorsal and connecting vessels. Closed circulatory systems have several advantages over open systems: Fluid can flow more rapidly through vessels than through intercellular spaces, and can therefore transport nutrients and wastes to and from tissues more rapidly. By changing resistance in the vessels, closed systems can be selective in directing blood to specific tissues. Specialized cells and large molecules that aid in the transport of hormones and nutrients can be kept within the vessels, but can drop their cargo in the tissues where it is needed. It seems logical to accept the premise that in all but very small animals, closed circulatory systems can support higher levels of metabolic activity than open systems can. But we then have to ask: How do highly active insect species achieve high levels of metabolic output with their open circulatory systems? The reason is that insects do not depend on their circulatory systems for respiratory gas exchange. Recall from the last chapter that respiratory gas exchange in insects is through a system of air-filled tubes. 49.1 RECAP Circulatory systems consist of a pump and an open or closed set of vessels through which is pumped a fluid that transports oxygen, nutrients, wastes, and a variety of other substances. 49.2 How Have Vertebrate Circulatory Systems Evolved? Vertebrates have closed circulatory systems and hearts with two or more chambers. When a heart chamber contracts, it squeezes the blood, putting it under pressure. Blood then flows out of the heart and into vessels where pressure is lower. Valves prevent the backflow of blood as the heart cycles between contraction and relaxation. As circulatory systems become more complex, the blood that flows to the gas exchange organs becomes more completely separated from the blood that flows to the rest of the body. In fish, the phylogenetically oldest vertebrates, blood is pumped from the heart to the gills and then to the tissues of the body and back to the heart. In birds and mammals, the phylogenetically youngest vertebrates, blood is pumped from the heart to the lungs and back to the heart in a pulmonary circuit, and then from the heart to the rest of the body and back to the heart in a systemic circuit. We will trace the evolution of the separation of the circulation into two circuits. The closed vascular system of vertebrates begins with vessels called arteries that carry blood away from the heart. Arteries give rise to smaller vessels called arterioles, which feed blood into capillary beds. Capillaries are the tiny, thin-walled vessels where materials are exchanged between the blood and the tissue fluid. Small vessels called venules drain capillary beds. The venules join together to form larger vessels called veins, which deliver blood back to the heart. arteries arterioles capillary beds venules veins Fish have two-chambered hearts The fish heart has two chambers. An atrium (plural atria) receives blood from the body and pumps it into a more muscular chamber, the ventricle. The ventricle pumps the blood to the gills, where gases are exchanged. Blood leaving the gills collects in a large dorsal artery, the aorta, which distributes blood to smaller arteries and arterioles leading to all the organs and tissues of the body. In the tissues, blood flows through beds of tiny capillaries, collects in venules and veins, and eventually returns to the atrium of the heart. Most of the pressure imparted to the blood by the contraction of the ventricle is dissipated by the high resistance of the narrow spaces in the gill lamellae through which blood flows. As a result, blood leaving the gills and entering the aorta is under low pressure, limiting the maximum capacity of the fish circulatory sy
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