Circulatory systems of vertebrates

Cells, the units of the animal body, need a constant supply of blood. Blood affects the transport of important materials needed for metabolic, synthetic, and degradative activities, supplying energy and materials necessary for growth, repair of worn-out components of cells, reproductive activity, and other functions of the body. Among the many products that blood transports through a system of closed channels are oxygen, nutrients, metabolic wastes, heat, and hormones. The circulatory system links all tissues with one another and with the external environment to and from which many of these materials are transported.

Basically, the circulatory system of vertebrates consists of two parallel systems of blood vessels: One, the arterial system, actively transports blood and its constituents from a central pumping station, the heart; the other, the venous system, passively brings the blood back to the heart. The two systems branch again and again until they ramify all tissues. In the extracellular space of tissues, the finest branches of each system, called arterioles and venules, are connected by means of a network of fine capillaries that allow the movement of blood in one direction, from arterial into the venous system, in which the valves prevent any backflow of blood. A head of pressure generated in the heart pumps the blood in this direction, facilitating the transport of substances as well as their movement and filtration out of the capillary membranes and into the extracellular fluid.

Circulatory Systems of Fish, Amphibians, and Reptiles

The simplest level of organization of the circulatory system of vertebrates is seen in fishes. The heart in fish consists of two chambers, an atrium (auricle) and a ventricle. The oxygen-poor, carbon-dioxide-rich blood returning from the body via a system of veins is first received by an enlarged vein, the sinus venosus, prior to entering the atrium. The atrium empties its blood into the thick-walled, muscular ventricle, which then pumps it into an enlarged artery, the conus arteriosus. The blood then passes through a major arterial trunk, the ventral aorta, going directly to the gills. The arteries in gills branch profusely and are connected via capillaries with other arteries. In the capillary bed, the blood becomes oxygenated and provides nutrients to the tissue. The oxygenated blood then flows to the head and the rest of the body, and from there, returns to the heart through the venous system.

In preparation for their journey to land, ancient aquatic vertebrates had to evolve lungs for aerial breathing and had to evolve a complementary circulatory system. As demands for oxygen for a terrestrial existence increased, greater blood pressure and a new way of oxygenating blood were in order. The atrium became divided into two, the right one receiving the deoxygenated blood returning from the body and the left one receiving the oxygenated blood from the lungs (which replaced gills). The deoxygenated blood, entering the right part of the single ventricle, is pumped into the pulmonary artery, all the way to the lungs. The left part of the ventricle, receiving oxygenated blood from the left atrium, pumps it into the body. This three-chambered heart is present in amphibians and most reptiles. The oxygenated and deoxygenated bloods mix partially in the ventricle. In some amphibians, flaps and partial valves tend to prevent such mixing. Reptiles have a partition between the right and left parts of the ventricle, which is complete in alligators, crocodiles, and turtles. Crocodiles have a four-chambered heart with an additional aorta, which helps them send blood to their stomach, increasing gastric acid production to digest bones.

Circulatory Systems of Birds and Mammals

Later, reptiles, birds, and mammals developed four-chambered hearts. This complete division of the heart into two separate right and left pumps enables birds and mammals to achieve high speeds. One pumping circuit, the pulmonary, receives blood from the body and pumps it to the lungs. The other pumping circuit, the systemic, receives oxygen-rich blood from the lungs and pumps it into the systemic circulation. Valves within the heart prevent the blood from flowing through it in the opposite direction.

The contractile tissue of the heart consists of muscle cells that receive sympathetic and parasympathetic nerve impulses. The vertebrate heart is myogenic—all its muscle cells and fibers possess an inherent capacity to contract (electrically depolarize) rhythmically—however, all these fibers are under the control of a group of specialized heart muscle cells which have a lower threshold for depolarization than other heart muscle cells: the pacemaker. In fish, amphibians, and reptiles, the pacemaker is located in the wall of the sinus venosus (the first heart chamber before the atrium). In higher vertebrates, which lack a sinus venosus, the pacemaker is found in the wall of the atrium and is called the sinoatrial node. The wave of electrical depolarization initiated here is conducted through the atrioventricular node via a special group of fibers called the Bundle of His, which branch out into the ventricular muscle. The depolarization enters and traverses the atrioventricular node only relatively slowly but spreads down the atrioventricular bundle and its branches much more rapidly than it could travel through the ordinary ventricular muscle. This regulates the sequence of contraction of the heart chambers: The atria contract first and the ventricles later, each group of muscles contracting approximately in unison.

Since the pulmonary (right) circuit is much shorter than the systemic circuit, it contains less blood volume and offers less frictional resistance to blood flow. Also, the right ventricle has muscular walls that are less thick than those of the left ventricle, which has to pump large volumes of blood to the entire body via the systemic circuit. After the two ventricles are completely filled (a condition referred to as diastole), they contract simultaneously (called systole). During systole, the maximum arterial pressure is generated, and during diastole (just before systole), arterial pressure decreases to a minimum. The pulmonary side of the heart contains the funnel-shaped valve between the atrium and the ventricle known as the atrioventricular valve, the right one having three flaps, or cusps (and hence named the tricuspid valve), and the left one (the bicuspid or mitral valve) having two. The free edges of these cusps hang down into the ventricular cavities and are anchored by tendon-like cords of connective tissue called chordae tendineae, each of which is attached to the ventricular wall by a lump called a papillary muscle. The pulmonary artery and the aorta originate at the base of the right and left ventricles, respectively, each having a semilunar valve at its origin. Each of these valves opens in the direction of the blood flow and prevents the backflow of blood. The ventricular contraction and the resulting turbulence in the blood produce the long, low-pitched “lub” sound that can be detected with a stethoscope. The sudden closure of the semilunar valves is similarly perceived to emit a relatively short, high-pitched “dup” sound.

The circulatory systems of vertebrates vary to accommodate each animal's specific needs. For example, giraffes' arteries in their head, neck, and upper body are elastic and muscular, which helps fight gravity and bring blood to their brain through a series of one-way valves. The arteries in their lower body are narrow to ensure blood does not gather in their lower half.

Blood Volume and Blood Vessels

The volume of blood pumped by the heart each minute is called the cardiac output or minute volume, whereby the heart beats (contractions) per minute (cardiac stroke rate) eject a typical quantity of blood per beat. This rate is altered by the body’s activity and by the volume of blood returning to the heart from the veins each minute. If the venous blood volume is adequate, then an increase in stroke rate can increase minute volume. The increased stroke rate, however, involves a decrease in the ventricular filling time, and as a result, the ventricles do not fill completely. Thus, the stroke volume is decreased. At rapid heart rates, even the minute volume may be decreased, so it offsets the stroke rate. During systole, the ventricles do not empty completely. A small residual volume of blood remains in them. An increased venous return may cause more complete filling and emptying of the ventricles, thus increasing the cardiac output without changing the stroke rate.

The vessels at various points in the circulatory path differ anatomically and functionally. The great arteries have thick walls heavily lined with smooth muscle and contractile tissue to enable them to transport blood under pressure from the heart to peripheral tissues. The arteries become smaller and thinner-walled as they branch out toward the periphery. The systemic arteries deliver blood to the microcirculatory beds of the tissues and organs. These “capillary beds” consist of microscopic arterioles, capillaries, and venules. The contraction (vasoconstriction) and relaxation (vasodilation) of the smooth muscles in the terminal branches of the arteries play an important role in regulating blood flow in the capillary bed. Control of the arteriole muscles is mediated by sympathetic neurotransmitters, hormones, and local effects. From the arterioles, the blood enters the capillaries, minute vessels whose walls consist of a single layer of cells, facilitating the transfer of oxygen and nutrients to the tissues and the loading of metabolic waste and carbon dioxide, all via the extracellular fluid. Their density depends on the need of the particular tissue for nutrients and oxygen. The capillaries drain into small, thin-walled but muscular vessels called venules, whence the blood begins its return to the heart through the veins. The veins have elastic walls but are without muscles. The venous vasculature serves as a reservoir, storing about 60 percent of the blood.

Studying Vertebrate Circulation

Circulatory systems of vertebrates have been studied since ancient times through dissection and observation of animal and human cadavers: The heart can be cut open to examine its chambers and their structures, and the body wall can be cut open from the ventral side to expose the circulatory organs. Preserved, dissected animals, including fish, amphibians, reptiles, and mammals, are available from suppliers for anatomy students who wish to conduct dissections. The venous systems of these animals are dyed blue, and the arterial systems are dyed red. Plastic models of the circulatory system can be purchased for classroom use.

Scientists are also interested in microcirculation, or circulation at the capillary level. One can fasten a live frog on a frog board and observe the capillaries in the frog’s foot web under a microscope. The movement of the red blood cells into the capillary is observed; it is slow and intermittent. The blood flow is regulated by the central nervous system (the vasomotor center in the medulla), as well as by local conditions (such as levels of carbon dioxide, acidity, histamine, temperature, and inflammation). One can then immerse the foot in hot or cold water and observe the resulting change in blood flow. Histamine can be applied to cause vasodilation, which can be controlled by epinephrine. Drops of dilute hydrochloric acid can be applied to the foot to cause vasodilation and inflammation. Thus, it is clear from the foregoing discussion that the heart and circulatory system are of vital importance to the health of an animal.

Understanding vertebrate circulation across species is important in uncovering the evolutionary processes of prehistoric animals and the ancestors of humans. Scientists have traced evidence of the first circulatory system back 600 million years to the triploblasts using fossil records. For example, the Fuxianhuia protensa, a fossilized arthropod found in the mid-2010s, was discovered with an imprint of its cardiovascular system intact.

Principal Terms

Aorta: The major arterial trunk, into which the left ventricle of the heart pumps its blood for transport to the body

Artery: A blood channel with thick muscular walls which transports blood from the heart to various parts of the body

Atria: The two chambers of the heart, which receive venous blood from the body (via the right atrium) or oxygenated blood from the lungs (left atrium)

Capillaries: The very fine vessels in various tissues, which connect arterioles with venules; it is here that the exchange between blood and the extracellular fluid takes place

Cardiac Output: the amount of blood ejected by the left ventricle into the aorta per minute

Diastole: Relaxation (filling with blood) of the heart chambers

Pacemaker: A specialized group of cardiac muscle cells in the right atrium which initiates the heartbeat; also called the sinoatrial node

Systole: Contraction (emptying of blood) of the heart chambers

Valves: Specialized, thickened groups of muscle cells in the heart chambers, major arterial trunks, arterioles, and veins which prevent backflow of blood

Bibliography

Berne, R., and M. Levy. Physiology. 4th ed., C. V. Mosby, 2006.

Hill, Richard W., and Gordon Wyse. Animal Physiology. 5th ed.. Harper & Row, 2022.

Koeppen, Bruce M., et al. Berne & Levy Physiology. 11th ed., Elsevier, 2024.

Raven, P. H., and G. B. Johnson. Biology. 13th ed., McGraw-Hill, 2023.

Robinson, T. F., et al. “The Heart as a Suction Pump.” Scientific American, vol. 254, June 1986, pp. 84-91. www.scientificamerican.com/article/the-heart-as-a-suction-pump. Accessed 20 Sept. 2024.