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BLOOD VESSELS 1

     Can animals exist without a cardiovascular system and its blood vessels?  Yes.  There is no cardiovascular system in sponges, cnidarians (jellyfish, hydra, corals), and many worms (such as flatworms and roundworms)—these animals are small enough to supply food and gases to their cells and remove wastes through diffusion alone.  The digestive cavity helps distribute materials throughout the organism and in some organisms, such as flatworms, the gastric cavity is highly branched to further increase this ability.  Flatworms also make enhance the effectiveness of diffusion by being flat, providing a greater surface area for diffusion to occur.  A cardiovascular system is not necessary for animal life and the first steps of animal evolution occurred without one.

HYDRAhydra
Note that the digestive system in planaria (stained black in the images below) is highly branched so that nutrients can be distributed throughout the body.
PLANARIA PLANARIA

     There are a variety of circulatory mechanisms which exist in eukaryotes.  Eukaryotic cells (including most, if not all, animal cells) create currents inside the cell through movements of the cytoskeleton which distribute materials throughout the cell.  Some animals, particularly sponges and cnidarians, circulate materials by controlling the movement of the water outside the body and its movement through body channels.  Skeletal muscle contraction, especially during movement, can cause fluid to circulate through body cavities.  Higher animals possess a circulatory system, although its blood vessels may be closed or open (in open circulatory systems, blood vessels communicate with the body cavity).  Vertebrates possess separate lymphatic vessels which communicate with the veins (Prosser, 1973).

     Some worms do possess a circulatory system and nemertine worms may be the most primitive animals to possess one.  While nemertine worms are similar to flatworms and have sometimes been classified with them, more recent studies suggest an affinity with the pseudocoelomates (Harris).    Although most nemertines possess colorless blood, some possess hemoglobin as a respiratory pigment.  In spite of the presence of hemoglobin in some nemertines, the circulatory system of nemertines primarily functions to distribute the nutrients absorbed from the digestive tract since it is too deep to efficiently exchange oxygen with the environment (Barrington). 

      The circulatory system of nemertines is a closed system in which blood only flows through blood vessels and spaces known as lacunae (Hickman).  Some nemertines, such as Cephalothrix, possess the simplest type of circulatory system consisting of only a pair of longitudinal vessels which are joined at both ends.  The organization of blood vessels in nemertines can vary and only the genera Hubrechtella and Hubrechtia possess a median dorsal vessel. (Turbeville, 1986, p. 3).  The larger vessels may possess muscles in their walls and these contractile vessels move the blood since there is no heart (Hickman, Barrington).  This muscle can consist of both circular and longitudinal muscle fibers.  In Cephalothrix, the epithelial cells which line a blood lacuna possess myofilaments, forming a myoepithelium.  Two palaeonemertines, Carinoma tremaphoros and Tubulanus rhabdotus, possess cilia in their vessels (Turbeville, 1986, p. 20).

    Are the blood vessels of nemertines homologous to the blood vessels of all higher organisms?  It may be that nemertine blood vessels are modified portions of the coleomic cavity and distinct from those of higher animals.  The cells which line invertebrate coelomic cavities frequently have junctions and cilia or rudimentary cilia (including those found in hemichordates, echinoderms, and annelids).  These junctions and cilia are also observed in the blood vessels of some nemertines (Turbeville, 1986, p. 135).  Invertebrates do not possess a complete endothelial lining around their system of blood vessels and, where an endothelial lining is present surrounding sections of blood vessels, its structure is different than that observed in nemertines. (Turbeville, 1986, p. 137)

     Lophophorates, whose ancestors may have diverged near the split of coelomates into protostomes and deuterostomes, possess a closed circulatory system with 2 main vessels, (1 dorsal, 1 ventral) which are connected through a plexus of smaller vessels.  There is no heart and muscle contractions in blood vessels propel the blood.  The blood flows directionally through the major blood vessels—the dorsal vessel is afferent (moving blood away from the heart) and the ventral vessel is efferent (Hickman).   Protostomes do not possess a single type of circulatory system: circulatory systems may be open (as in most arthropods and molluscs), closed (as in annelids), or nonexistent (Priapuloidea, Sipunculoidea, and Bryozoa among the protostomes and Chaetognatha among the deuterostomes) (Beklemishev 2, p. 356).  Open circulatory systems have the advantage of maintaining low blood pressure and thus the risk of blood loss after injury is less.  

     The circulatory systems of most invertebrates are not closed systems, as seen in the vertebrates.  Blood vessels are typically not lined by cells and, in groups where there is a cellular lining (such as certain mollusks and echinoderms), the lining is incomplete.   Coelomate animals (such as hemichordates, echinoderms, and some annelids) often use their coeloms for distribution of materials, forming a coelomic circulatory system, supplementing the functions of the cardiovascular system.  In coelomates other than chordates, the respiratory pigments are found primarily in the coleom (Beklemishev 2, 356). The disadvantage of a coelomic circulatory system is that internal body cavities are separated from other body regions and fluid does not cross the septa (although in some animals such as leeches these barriers may be lost). (Turbeville, 1986, p. 2). 

     Open circulatory systems are present in the primitive chordates as well.     Hemichordates possess contractile blood vessels (in addition to the heart) to pump blood through their open circulatory system and sinuses which drain into the heart.  Many spaces are present (Benito, form Harrison 1997; Hickman). The circulatory system of urochordates is open, lacking an endothelial lining that completely surrounds vessels or the heart lining.  (Burighel, from Harrison, 1997, p. 261).  In tunicates, a few vessels connect the blood which is primarily located in large sinuses.  In Amphioxus, blood cells may leave the blood vessels and travel through the sinuses of the connective tissue, thus retaining characteristics of an open circulatory system. The smaller vessels and sinuses are continuous with connective tissue (Ruppert, from Harrison, 1997, p. 445-52).   Jawless fish also possess this sinus system and thus have larger blood volumes and lower blood pressures than the jawed vertebrates. (Webster, 1974, p. 4).  The percentage of body weight composed by the blood drops from 30-40% in tunicates to 16% in hagfish to 5% in jawed fish. (Webster, 1974, p. 57; Forster, 2001).

     In Amphioxus, blood vessels develop which all craniates will retain, at least as embryos.  In all vertebrate embryos, a primordial circulatory system develops in which blood travels from the heart cranially through a ventral aorta and, after passing through an aortic arch, proceeds caudally through the dorsal aorta to the yolk sac and then back to the heart.  In bony fish and amphibians, the veins which develop from the GI tract and much of the body feed into the yolk sac before returning to the heart.  In amniotes, these veins bypass the yolk sac to return blood to the heart (Torrey).  This change corresponds to the increase in the size of the yolk in the ancestral amniotes.

     The hagfish circulatory system is unique among craniates: they have the largest blood volume, the lowest blood pressure, a large percentage of the blood is retained in extensive venous sinuses, the greatest number of venous propulsors, and the greatest dependence of the heart on anaerobic metabolism (Forster, 1997).  Hagfish have the lowest metabolic rates among craniates.  The low blood pressure (the lowest known in craniates) and the similarity in ionic concentration between the extracellular fluid and seawater conserves energy (Forster, 1997).  The caudal “heart” provides pressure to contribute to venous return but depends on the contraction of surrounding skeletal muscles.  While the blood of the central circulation has a low hematocrit, 13.5%, that of the sinuses has an even lower hematocrit of 4.3%, reducing the pressure that the caudal and cardinal hearts must generate (Forster, 1997).  There are separate venous sinouses through which venous blood passes rather than traveling directly through veins back to the heart.  The peribranchial system empties into the cardinal anastomoses close to the sinus venosus (Forster, 1997). 

     The endothelial lining of hagfish blood vessels resemble the vessels of Amphioxus and the lymphatic vessels of mammals.   Hagfish vessel linings seem intermediate between those of Amphioxus and those of higher vertebrates in their endothelial nature, the presence of open junctions, and the lack of fenestrae.  The capillary endothelium of sharks also shares characteristics found in mammalian lymphatics rather than mammalian capillaries.  Comparative anatomy suggests that blood vessels have evolved through several stages, beginning with an interrupted basement membrane lining only to a second stage represented by a lining made of separated endothelial cells and then finally an endothelial lining with many open junctions to an endothelial lining in which cells are joined by closed junctions.  This progression is observed in comparing mammalian lymphatic vessels as the smallest vessels form larger and larger vessels and also when comparing the progression of vessels from primitive chordates through mammals.  These changes seem to have occurred in response to the need for higher blood pressures in more complex chordates (Casley-Smith, 1975). 

     The blood sinus-system in jawless fish may function as a type of hydrostatic skeleton.  The blood sinus system in hagfish is more extensive than in lampreys (Tsuneki, 1993).   Lampreys also possess a large number of venous sinuses.   (Webster, 1974, p. 55)  In sharks, there are a number of sinuses in the venous system.  The cardinal veins draining the head are more similar to an interconnected system of sinuses than to standard blood vessels (Torrey, 1979).

LAMPREY LARVA

LAMPREY LARVA

LAMPREY

SINUS

SINUS
Below is an image of the artery wall of a frog with the prominent layer of smooth muscle.
ARTERY ARTERY ANDVEIN

AORTIC ARCHES

     In hemichordates, there is a concentration of blood vessels around the pharnygeal slits but most of the gas exchange is still performed by the skin.  The gill bars are more important in the filtration of food.  Thus, the pharyngeal blood vessels in tunicates may not be involved in respiration but merely function in servicing the gill bars (Benito, form Harrison 1997, p. 59).   In urochordates, the vessels to the pharnygeal slits are essential for gas exchange.  In Amphioxus and in the primitive vertebrate condition, a single ventral blood vessel produces a series of aortic arches to carry deoxygenated blood to these pharyngeal arches (and their gills) for gas exchange.  These arches then bring the oxygenated blood to the dorsal aorta (or, primitively, a pair of dorsal aortae).  In Amphioxus, there are a large number of aortic arches. 

AORTIC ARCHES IN AMPHIOXUS

AORTIC ARCHES IN AMPHIOXUS

AORTIC ARCHES IN AMPHIOXUS
     Hagfish may have as many as 15 aortic arches, some fossil jawless fish possessed 10, and lampreys possess 8 (Kardong).  Gnathostomes typically have only 6, at least as embryos.  In the jawed fish the pharyngeal arches do not form pouches for the gills as in the hagfish below  (Romer)
HAGFISH HAGFISH
HAGFISH
In the picture below, the efferent portion of the aortic arches are fusing with the dorsal aorta in the shark.

SHARK

The following image is of a shark’s aortic arches.

SHARK AORTIC ARCHES

SHARK

SHARK

SHARK

     In lungfish and amphibians, the pulmonary artery develops as a branch of aortic arch VI.  Thus the blood in the sixth aortic arch can (after passing the gills in lungfish) either travel through the pulmonary artery to the heart or mix with oxygenated blood in the dorsal aortae.  This connection (through a vessel named the ductus arteriosus) is retained even in adult salamanders which presents the anatomical dilemma of emptying deoxygenated blood into systemic arteries.  Although lungfish and amphibians have mechanisms for decreasing the potential disadvantages of such an anatomical setup (Kardong), amniotes develop pulmonary vessels which go to the lungs only, forming a pulmonary circuit.   In adult frogs, the sixth aortic arch supplies both the lungs and skin as the pulmocutaneous artery. 

 

 

Human embryos develop aortic arches which are later remodeled to form the blood vessels found in the adult.

PHARYNGELA POUCHES HUMAN HEART

2) BLOOD VESSELS OF THE HEAD

     External and internal carotid arteries have existed since the evolution of jawed fish and are present in modern sharks (Bohensky).  However, the common carotid artery which unites them in humans has a more complicated history.  The common carotid is absent in fish, due to the presence of the aortic arches.  When amphibians undergo metamorphosis, the aortic arches are reorganized and the common carotid forms from the ventral aorta which formerly connected aortic arches III and IV.  The internal carotid, together with aortic arch III and part of the dorsal aorta, joins it and the carotid body forms at the terminus of the common carotid (Kardong, Romer).  In some amphibians and even some reptiles, the carotid duct remains to unite the portion of the internal carotid derived from the dorsal aorta with the remainder of the dorsal aorta (Romer).  Tetrapods possess a swollen region (called a carotid body) where the common carotid branches where chemoreceptors and baroreceptors important in the regulation of heartrate and breathing are located (Weichert, 1970). The carotid bodies of higher vertebrates are homologous to the sensory cells located in diverse sites in fish and amphibians (Milsom, 2007).

     In Amphioxus and all higher chordates, a pair of anterior cardinal veins develops to drain blood from the head and all vertebrate embryos retain these primitive vessels.  Lamprey circulation possesses a cephalic circle, as does that of jawed vertebrates (Weichert, 1970, p.560).  Primitive vertebrates possess lateral head veins which drain into them.  There is some modification of the lateral head veins in both archosaurs (crocodiles and birds) and mammals in which the blood is drained through the braincase, forming the internal jugular vein (which may fuse with the external jugular to form the common jugular).  The jugular veins and subclavian veins fuse to form anterior vena cavae.  In many mammals, the embryonic left common cardinal vein degenerates and the blood from the head drains into the remaining right cardinal vein, called the anterior (superior) vena cava.  Catarrhine primates return blood from the brain through the internal jugular as opposed to the external jugular and vertebral veins typical of other mammals. (Kimbel, 1984).  In rhesus monkeys, the anterior meningeal artery may be a homolog of lacrimal meningeal artery in apes (Falk, 1992).

     A. africanus and H. habilis possessed the transverse-sigmoid sinus drainage system most common in apes, including humans.  A. afarensis and A. boisei have enlarged occipital-marginal sinuses (Kimbel, 1984).  Occipial-marginal sinus systems do occur in modern human populations and in Pleistocene populations of Homo (Kimbel, 1984).  O/M drainage in A. afarensis may have been linked to upright posture.  Other alternate routes present in low frequencies among humans are multiple hypoglossal canals and foramina for emissary veins.  African apes have low frequencies of several alternate channels including the occipital-marginal system typical of humans  (Falk, 1986; Falk, 1983).

TURTLE

TURTLE

CAT

CAT

MONKEY

MONKEY

Note the paired anterior vena cavae in the opossum and the single vena cava in the cat and monkey.

OPOSSUM

OPOSSUM

OPOSSUM
OPOSSUM OPOSSUM
OPOSSUM VENA CAVAE
OPOSSUM
OPOSSUM