Why did circulatory systems evolve

In vertebrates, the lungs are relatively close to the heart in the thoracic cavity. The shorter distance to pump means that the muscle wall on the right side of the heart is not as thick as the left side which must have enough pressure to pump blood all the way to your big toe. The heart muscle is asymmetrical as a result of the distance blood must travel in the pulmonary and systemic circuits. Since the right side of the heart sends blood to the pulmonary circuit it is smaller than the left side which must send blood out to the whole body in the systemic circuit, as shown in Figure In humans, the heart is about the size of a clenched fist; it is divided into four chambers: two atria and two ventricles.

There is one atrium and one ventricle on the right side and one atrium and one ventricle on the left side. The atria are the chambers that receive blood, and the ventricles are the chambers that pump blood. The right atrium receives deoxygenated blood from the superior vena cava , which drains blood from the jugular vein that comes from the brain and from the veins that come from the arms, as well as from the inferior vena cava which drains blood from the veins that come from the lower organs and the legs.

In addition, the right atrium receives blood from the coronary sinus which drains deoxygenated blood from the heart itself. This deoxygenated blood then passes to the right ventricle through the atrioventricular valve or the tricuspid valve , a flap of connective tissue that opens in only one direction to prevent the backflow of blood.

The valve separating the chambers on the left side of the heart valve is called the biscuspid or mitral valve. After it is filled, the right ventricle pumps the blood through the pulmonary arteries, by-passing the semilunar valve or pulmonic valve to the lungs for re-oxygenation.

After blood passes through the pulmonary arteries, the right semilunar valves close preventing the blood from flowing backwards into the right ventricle. The left atrium then receives the oxygen-rich blood from the lungs via the pulmonary veins. This blood passes through the bicuspid valve or mitral valve the atrioventricular valve on the left side of the heart to the left ventricle where the blood is pumped out through aorta , the major artery of the body, taking oxygenated blood to the organs and muscles of the body.

Once blood is pumped out of the left ventricle and into the aorta, the aortic semilunar valve or aortic valve closes preventing blood from flowing backward into the left ventricle. This pattern of pumping is referred to as double circulation and is found in all mammals. The heart is composed of three layers; the epicardium, the myocardium, and the endocardium, illustrated in Figure The inner wall of the heart has a lining called the endocardium. The myocardium consists of the heart muscle cells that make up the middle layer and the bulk of the heart wall.

The outer layer of cells is called the epicardium , of which the second layer is a membranous layered structure called the pericardium that surrounds and protects the heart; it allows enough room for vigorous pumping but also keeps the heart in place to reduce friction between the heart and other structures.

The heart has its own blood vessels that supply the heart muscle with blood. The coronary arteries branch from the aorta and surround the outer surface of the heart like a crown. They diverge into capillaries where the heart muscle is supplied with oxygen before converging again into the coronary veins to take the deoxygenated blood back to the right atrium where the blood will be re-oxygenated through the pulmonary circuit.

The heart muscle will die without a steady supply of blood. Atherosclerosis is the blockage of an artery by the buildup of fatty plaques. Because of the size narrow of the coronary arteries and their function in serving the heart itself, atherosclerosis can be deadly in these arteries.

The slowdown of blood flow and subsequent oxygen deprivation that results from atherosclerosis causes severe pain, known as angina , and complete blockage of the arteries will cause myocardial infarction : the death of cardiac muscle tissue, commonly known as a heart attack.

The main purpose of the heart is to pump blood through the body; it does so in a repeating sequence called the cardiac cycle. The cardiac cycle is the coordination of the filling and emptying of the heart of blood by electrical signals that cause the heart muscles to contract and relax. The human heart beats over , times per day. In each cardiac cycle, the heart contracts systole , pushing out the blood and pumping it through the body; this is followed by a relaxation phase diastole , where the heart fills with blood, as illustrated in Figure The atria contract at the same time, forcing blood through the atrioventricular valves into the ventricles.

Following a brief delay, the ventricles contract at the same time forcing blood through the semilunar valves into the aorta and the artery transporting blood to the lungs via the pulmonary artery. The pumping of the heart is a function of the cardiac muscle cells, or cardiomyocytes, that make up the heart muscle.

Cardiomyocytes , shown in Figure They are self-stimulated for a period of time and isolated cardiomyocytes will beat if given the correct balance of nutrients and electrolytes. The electrical signals and mechanical actions, illustrated in Figure The internal pacemaker starts at the sinoatrial SA node , which is located near the wall of the right atrium.

Electrical charges spontaneously pulse from the SA node causing the two atria to contract in unison. The pulse reaches a second node, called the atrioventricular AV node, between the right atrium and right ventricle where it pauses for approximately 0.

From the AV node, the electrical impulse enters the bundle of His, then to the left and right bundle branches extending through the interventricular septum. Finally, the Purkinje fibers conduct the impulse from the apex of the heart up the ventricular myocardium, and then the ventricles contract.

This pause allows the atria to empty completely into the ventricles before the ventricles pump out the blood. The electrical impulses in the heart produce electrical currents that flow through the body and can be measured on the skin using electrodes.

This information can be observed as an electrocardiogram ECG —a recording of the electrical impulses of the cardiac muscle. The blood from the heart is carried through the body by a complex network of blood vessels Figure Arteries take blood away from the heart.

The main artery is the aorta that branches into major arteries that take blood to different limbs and organs. These major arteries include the carotid artery that takes blood to the brain, the brachial arteries that take blood to the arms, and the thoracic artery that takes blood to the thorax and then into the hepatic, renal, and gastric arteries for the liver, kidney, and stomach, respectively. The iliac artery takes blood to the lower limbs. The major arteries diverge into minor arteries, and then smaller vessels called arterioles , to reach more deeply into the muscles and organs of the body.

Arterioles diverge into capillary beds. Capillary beds contain a large number 10 to of capillaries that branch among the cells and tissues of the body.

Capillaries are narrow-diameter tubes that can fit red blood cells through in single file and are the sites for the exchange of nutrients, waste, and oxygen with tissues at the cellular level. Fluid also crosses into the interstitial space from the capillaries. The capillaries converge again into venules that connect to minor veins that finally connect to major veins that take blood high in carbon dioxide back to the heart.

Veins are blood vessels that bring blood back to the heart. The major veins drain blood from the same organs and limbs that the major arteries supply. Fluid is also brought back to the heart via the lymphatic system.

The structure of the different types of blood vessels reflects their function or layers. There are three distinct layers, or tunics, that form the walls of blood vessels Figure The first tunic is a smooth, inner lining of endothelial cells that are in contact with the red blood cells. The endothelial tunic is continuous with the endocardium of the heart. In capillaries, this single layer of cells is the location of diffusion of oxygen and carbon dioxide between the endothelial cells and red blood cells, as well as the exchange site via endocytosis and exocytosis.

The movement of materials at the site of capillaries is regulated by vasoconstriction , narrowing of the blood vessels, and vasodilation, widening of the blood vessels; this is important in the overall regulation of blood pressure. Veins and arteries both have two further tunics that surround the endothelium: the middle tunic is composed of smooth muscle and the outermost layer is connective tissue collagen and elastic fibers.

The elastic connective tissue stretches and supports the blood vessels, and the smooth muscle layer helps regulate blood flow by altering vascular resistance through vasoconstriction and vasodilation. The arteries have thicker smooth muscle and connective tissue than the veins to accommodate the higher pressure and speed of freshly pumped blood.

The veins are thinner walled as the pressure and rate of flow are much lower. In addition, veins are structurally different than arteries in that veins have valves to prevent the backflow of blood. Because veins have to work against gravity to get blood back to the heart, contraction of skeletal muscle assists with the flow of blood back to the heart. The heart muscle pumps blood through three divisions of the circulatory system: coronary, pulmonary, and systemic. The pumping of the heart is a function of cardiomyocytes, distinctive muscle cells that are striated like skeletal muscle but pump rhythmically and involuntarily like smooth muscle.

The internal pacemaker starts at the sinoatrial node, which is located near the wall of the right atrium. Electrical charges pulse from the SA node causing the two atria to contract in unison; then the pulse reaches the atrioventricular node between the right atrium and right ventricle.

A pause in the electric signal allows the atria to empty completely into the ventricles before the ventricles pump out the blood. The blood from the heart is carried through the body by a complex network of blood vessels; arteries take blood away from the heart, and veins bring blood back to the heart.

Blood pressure BP is the pressure exerted by blood on the walls of a blood vessel that helps to push blood through the body. Mammals and birds have a four-chambered heart with no mixing of the blood and double circulation. Skip to content Chapter The Circulatory System. Learning Objectives By the end of this section, you will be able to: Describe an open and closed circulatory system Describe interstitial fluid and hemolymph Compare and contrast the organization and evolution of the vertebrate circulatory system.

Circulatory System Architecture. Circulatory System Variation in Animals. Simple 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. Exercises Which of the following statements about the circulatory system is false? Blood in the pulmonary vein is deoxygenated. Blood in the inferior vena cava is deoxygenated. Blood in the pulmonary artery is deoxygenated. Blood in the aorta is oxygenated. Which of the following statements about the heart is false? The mitral valve separates the left ventricle from the left atrium. Blood travels through the bicuspid valve to the left atrium.

Both the aortic and the pulmonary valves are semilunar valves. The mitral valve is an atrioventricular valve. Varicose veins are veins that become enlarged because the valves no longer close properly, allowing blood to flow backward. Varicose veins are often most prominent on the legs. Why do you think this is the case? Why are open circulatory systems advantageous to some animals?

Thus the human heart at its different stages of development resembles closely that of fish, amphibians, and reptiles during some stage of their development. If we reconsider the development of the heart, this time in an evolutionary context, then all of this suddenly and dramatically makes sense.

If evolution is true, then fish were the first vertebrates that appeared in the history of life. Thus amphibians evolved from fish, reptiles from amphibians, and mammals from reptiles.

Why reinvent the wheel and develop a complex structure from scratch when it is possible to modify a previous structure to suit instead? When viewed in this light, it becomes quite sensible that the human heart passes through stages during its development which would be similar to those seen in a fish, amphibian, and reptile.

What about the set of arteries leaving the heart? There is a set of six paired arteries that are formed each pair has a left and a right component , connected by a vessel on the belly side of the body called the aortic sac in humans and a pair of longitudinal vessels on the back side each called a dorsal aorta.

Figure 5 at right shows an idealized representation of the six arches. Figure 6 7 below shows them as they actually exist at about six weeks into human development. The six paired vessels are not all present at the same time. The first two pairs at the head end appear first, followed by the last several, which appear in sequence moving toward the hind end of the body.

The first two have disappeared by the time the last ones become prominent. In humans the fifth pair are either rudimentary or do not appear at all, depending on the individual. It turns out that the third, fourth, and sixth pairs of vessels are particularly important. The third pair forms the carotid arteries which supply the head.

The fourth artery on right side withers away, but the artery on the left side is important in forming part of the aorta, which is the main artery leaving the finished heart. The sixth artery on the right side also withers, but the one on the left side is important in the circulation of blood to the lungs Figure 7. Why do the blood vessels emanating from the human heart develop in such a strange way? Why six arches, especially when some subsequently disintegrate?

As we all remember, fish have gills to allow them to exchange gases while living in water. Most fish have six arches at some point in their development, each one containing a blood supply, muscle, cartilage and nerve. Each arch is supplied by an artery called a branchial arch artery, and there are initially six of them, arranged from the head back toward the tail. The heart is on the belly side of the body, and sends blood forward toward the branchial arches.

The blood then passes through the six pairs of arch arteries, and collects in paired dorsal aortae on the other side, finally to go to the rest of the body. At first, in the embryonic fish, these arteries simply carry blood through this region there is no gas exchange yet. In fact, the first two branchial arches and their arteries are diverted to support the development of structures in the head. As the fish matures, slits break through around each of the remaining arches, allowing the flow of water, and the vasculature of arches 3,4,5, and 6 form the functional gills, which then persist throughout life.

The heart now sends blood first toward the gills, through them, and finally away from them, through the mature branchial arch artery vasculature Figure 8. In an amphibian like a frog a process very similar to fish development occurs. Very early on, the branchial arch arteries merely carry blood through the arches, prior to the development of the gills. Then during the tadpole stage the gill vasculature develops from these arch arteries, and the gills become functional.

Finally, as the mature frog develops, the arch arteries are again modified to supply the needs of a land-dwelling animal. The arteries of the 3rd arch supply the head and neck, those of the 4th arch supply the rest of the body, and those of the 6th arch supply circulation to the lungs Figure 8. In reptiles, there is never any functional gill apparatus.

Yet a similar set of branchial arch arteries develops, and supplies various body regions using the same arch arteries as in amphibians Figure 8. We introduced the mammalian pattern earlier in this essay see also Figure 8. Thus at each stage the successor vertebrate type, during its development, will reproduce structures much like those of its series of ancestor organisms during their own early development. They are a classic example of the retention of a structure which has lost its ancestral function i.

This is precisely how we would expect evolution to work. The structural characteristics of the heart and great arterial vessels amongst living vertebrates do not merely possess surface similarities. Two crucial points need to be emphasized here. They are merely connecting pipes. Unlike closed systems where blood and lymph are functionally separated by the endothelium, in truly open systems these two fluids are considered to mix freely and are thus termed hemolymph blood and lymph.

However, the presence or absence of hemolymph does not explicitly define a system as open or closed as comparative physiologists also define hemolymph based on the absence of defined cell lineages red cells, thrombocytes, and leukocytes.

Thus, one may have a cell-lined circulatory system that meets the definition of being closed yet contains hemolymph as seen to varying degrees in the cephalopods and crustaceans [ 4 ]. The definition of open versus closed is therefore based upon histological endothelium and cellular hemolymph terms rather than in physiological terms functional.

Although there are more species of insects than any other group in the world and more individual nematodes, crustaceans exhibit a greater variation in form and diversity than any other animal phylum [ 28 ].

The decapod crustaceans have colonized a wide range of environments from the deep sea through the intertidal zone, and onto land.

During the evolution of the invertebrates a number of key adaptations were responsible for their radiation. In crustaceans, the evolution of a segmental arterial system was a singular event that made the unique adaptive radiation of this group possible and the evolutionary innovation that allowed members of this group to become large and highly mobile [ 29 ].

Historically, the crustacean circulatory system has been considered open. However, during the past two decades our knowledge of the decapod crustacean circulatory system has increased substantially [ 26 , 27 , 29 — 32 ]. The muscular ventricle is housed inside a primer chamber, the pericardial sinus. Heart rate and stroke volume can be controlled independently via nervous input from the cardiac ganglion and CNS or by direct actions of neurohormones on the cardiac muscle [ 29 , 32 ].

This allows rapid modulation of cardiac output resulting in blood pressures that rival those of some fish and amphibians [ 33 , 34 ]. Extrinsic control of cardiac function in vertebrate systems is primarily autonomic sympathetic excitation and parasympathetic inhibition layered upon intrinsic regulatory mechanisms. At the extrinsic level of control, parallel regulatory systems are seen in the neurogenic hearts of decapod crustaceans.

Cardioacceleratory and cardioinhibitory nerves provide input to the cardiac ganglion, modulating the rate and force of myocardial contractions. Additionally, the pericardial organ, an endocrine organ located on the inner wall of the myocardium, releases a variety of neurohormones that can modulate heart rate and cardiac contractility [ 32 ].

Regional blood flow is regulated in closed vertebrate systems by the contraction or relaxation of vascular smooth muscle. Decapod crustaceans do not possess smooth muscle in the artery walls [ 35 ]; instead contraction or relaxation of a pair of muscular cardioarterial valves at the base of each arterial system [ 36 ] controls hemolymph flow through the arteries [ 37 , 38 ]. A variety neurohormones have been shown to control regional hemolymph flow see McGaw and McMahon [ 39 ], Wilkens [ 29 ], McGaw and Reiber [ 26 ] either by direct actions on the cardioarterial valves or by altering downstream resistance of vessels [ 40 , 41 ].

Such ability to modulate cardiac function and regional blood flow rivals that of vertebrate systems [ 42 , 43 ]. In-line with physiological control mechanisms, the anatomy of the system is equally complex. Five arterial systems seven individual vessels originate from the heart, each splitting into smaller arteries and finally into capillary-like vessels that ramify within the tissues.

Some of these vessels are similar in size diameter-wise to those of vertebrate capillaries and form a true closed loop within the brain [ 44 , 45 ] and antennal gland [ 27 ] Figure 5. Nevertheless, decapod crustaceans lack a complete venous system; instead the hemolymph collects in sinuses before flowing into large veins and back to the heart. In part, it is the presence of these sinuses that has defined the system as open.

However, recent evidence has shown them to be more complex than previously described, forming a network of lacunae with a morphology similar to capillaries [ 27 , 47 ], the only difference being the lack of a true endothelial lining.

One hundred and fifty years ago Haeckel [ 48 ] proposed that no unbounded lacunae exist in the crustacean system. Major sinuses are bordered by fibrous connective tissue and the lacunae by basal lamina directly on the organ which they bathe [ 49 ].

The distinction between lacunae and capillary then becomes less distinct, suggesting a more organized structure. Thus, the definition of the open system of decapod crustaceans is really a histological term rather than a functional one. This will clarify some of the confusion associated with the highly complex open systems with a complete series of vessels, versus those that are simple and sluggish with few associated vessels or control mechanisms.

Reiber and Iain J. This is an open access article distributed under the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Article of the Year Award: Outstanding research contributions of , as selected by our Chief Editors. Read the winning articles. Journal overview. Special Issues. Reiber 1 and Iain J. Academic Editor: Stephen Tobe.

Received 03 Jul Accepted 29 Oct Published 26 Jan Figure 1. Solid lines represent defined vessels or a muscular pump or heart.