Vertebrates have the most highly evolved circulatory system in the animal kingdom
The circulatory system performs a variety of functions including:
Lymphatic system: drains fluids that accumulate in the tissues (tissue fluids), which are first collected by lymphatic capillaries, which pass into lymphatic vessels and then empty into the venous system
The system is also highly adaptable - you can graft veins to other locations (such as in a heart bypass) or tie off a vessel without seriously inconveniencing the system
Blood and blood vessels
Blood is a fluid tissue containing cellular elements that are derived from mesoderm. There are two primary components to blood:
Blood vessels have three layers of tissues (Fig. 12.1, p. 423)
Veins channel blood to the heart, while arteries channel blood away from the heart
Capillaries, the intermediaries between the arteries and the veins, are generally composed only of endothelium because they are where most of the diffusion in the circulatory system occurs - the junction at the capillaries marks an anastomosis, or a peripheral union between the blood vessels
The embryonic heart is formed from the splanchnic layer of the mesoderm. When first developed, the heart is composed of two layers (Fig. 12.8, p. 429):
• the myocardium forms the muscular part of the heart, containing cardiac muscle fibers.
• from the ventricle, the blood passes along a series of semilunar valves into the muscular conus arteriosus, and finally into the arterial system
Homeotherms are characterized by having a four-chambered heart with a double circuit pump
Coronary circulation is necessary to supply the metabolic needs of the cardiac muscle of mammals because it is larger in size than the two or three-chambered heart
The arterial channels are vessels that comprise the initial functioning system of the embryo, and is basically the same for all vertebrates (Figure 19-4, p. 676 in text).
Aortic arches of fishes
Afferent branchial arteries lead into the gills from the aortic arches.
Blood then flows through the gills through collector loops, and the oxygenated blood travels into the efferent branchial arteries, which continue into the dorsal aorta.
Rostrally the dorsal aorta branches into the internal carotid arteries that supply oxygenated blood the head.
Caudally the dorsal aorta continues into the caudal artery, from which the following arteries offshoot:
Gonadal (supply the gonads)
Renal (supply the kidneys)
Intersegmental (associated with the myomeres)
Subclavian (leads to the branchial arteries)
Iliac (leads to the femoral arteries)
Tetrapods also lack the first and second aortic arches.
The carotid system of tetrapods carries blood to the head and is derived from the third aortic arches. It is composed of the common carotid arteries, which branch into
The left branch of the fourth aortic arch becomes the arch of the aorta in mammals, while the right one becomes the subclavian arteries.
external carotid arteries, which supply the throat and the ventral part of the head internal carotid arteries, which supply the brain and the rest of the head.
The sixth aortic arch becomes the pulmonary arteries, which are derived from a common pulmonary trunk on the aorta.
The dorsal aorta is the large median longitudinal artery that extends posteriorly and eventually branches into the caudal artery.
Other branches of the dorsal aorta include:
The initial pattern of the venous channels comprises three
The anterior veins are derived from the cardinal system.
In tetrapods, the anterior veins are composed by the internal and external
The jugular veins unite with the subclavian vein and lead to the superior (cranial) vena cava, which also receives blood from the coronary veins (that first drain into the coronary sinus).
Hepatic portal system
The hepatic portal system is derived from the subintestinal system. The hepatic portal vein receives blood from the gut region.
Inside the liver the vein breaks up into hepatic sinusoids, where the blood comes into contact with hepatic cells and phagocytic cells. Harmful materials are detoxified and removed from the blood in the liver.
Posterior to the liver the blood is collected into the hepatic vein, which joins with the caudal vena cava.
Renal portal system
The renal portal system is derived from the posterior cardinal veins.
Blood from the posterior part of the body flows into the renal portal veins, which passes into the caudal vena cava.
The renal portal system is found only in fishes, amphibians, reptiles and birds. Thus, mammals have no renal portal system. All that remains in mammals is the azygous vein, which is an unpaired vein that drains most of the intercostal space on both sides of the mammalian thorax.
The posterior veins contain veins that are derived from one or all three initial systems (subintestinal, cardinal, abdominal). Adult birds and mammals lack the abdominal system, but as fetuses possessed two parts of the system (allantoic or umbilical veins).
Circulation in the mammalian fetus
Fetal mammals possess shunts between the pulmonary and systemic circuits because the placenta, not the lungs, is the site for gas exchange
Blood returns to the fetus from the placenta via the umbilical vein, enters the ductus venosus in the liver, and then passes into the caudal vena cava. The blood then passes through the foramen ovale (located in the septum between the right and left halves of the heart) and into the left atrium (thus bypassing the pulmonary circuit).
Since fetal lungs are not inflated, the pulmonary circuit is bypassed by the ductus arteriosus (a remnant of the sixth aortic arch) which joins to the aorta.
At birth, lungs are inflated and the pulmonary circuit becomes more important in gas exchange. The pressure due to the flow of the blood from the lungs causes closure of the foramen ovale, leaving a grown-over region called the fossa ovalis. Due to lack of use as a shunt the ductus arteriosus closes and is filled with connective tissue to become the ligamentum arteriosum.
The purpose of the lymphatic system is to drain fluids that accumulate in the tissues and empty into the venous system.
Although Chondricthyes and other primitive fishes lack a true lymphatic system, they do have some vessels that help to drain the tissues (called the hemolymphatic system) that seems to be a precursor of the true lymphatic system.
Components of the tetrapod lymphatic system include lymphatic capillaries to drain the tissues.
Across the vertebrate classes there is significant variation in the drainage of lymphatic capillaries into common larger ducts (Figure 19-20, p. 705 in text).
Amphibians and reptiles have three primary lymph vessels (subcutaneous, subvertebral, visceral) as well as lymph hearts, which are segmentally arranged masses containing smooth muscles that help propel lymph through the lymphatic system.
In birds and mammals the subvertebral ducts called thoracic ducts. Mammals also have the cisterna chyli, which is a sac that receives lymph from the abdominal viscera and caudal parts of the body. Mammals and birds also have lymph nodes, found in the neck, armpits and groin. Lymph nodes are the site of convergence for lymphatic vessels.
Other parts of the lymphatic system include the tonsils (lingual, pharyngeal and palatine), Peyer's patches (patches of lymphatic tissue in the small intestine of mammals), the vermiform appendix, and the bursa of Fabricius (a pouch in the cloaca of birds that contains lymphatic tissue.
The thymus is a lymphatic organ that produces T lymphocytes, which are involved in the humoral immune response.
Anastomosis - peripheral union between the blood vessels
Bursa of Fabricius - pouch in the cloaca of birds that contains lymphatic tissue Cisterna chyli - a sac that receives lymph from the abdominal viscera and caudal parts of the body in mammals
Hemopoietic tissue - a tissue in which blood cells are formed
Homeostasis - the condition in which a constant internal environment is maintained despite factors that tend to destabilize it
Peyer's patches - patches of lymphatic tissue in the small intestine of mammals