Chapter 22: The Cardiovascular System: Vessels and Circulation
Chapter 22: Cardiovascular System: Development of Vessels and Circulationby John F. Neas
Angiogenesis and Development of Primitive Vascular System
The human egg and yolk sac have little yolk to nourish the developing embryo. Thus, the cardiovascular system is one of the first systems to develop in the embryo, delivering nutrients to the mitotically active cells and disposing of waste products through its association with the maternal blood vessels in the placenta. Blood and blood vessel formation begins as early as 15 to 16 days. The heart and blood vessels develop from mesoderm.
The first blood cells and blood vessels develop early in the third week. The formation of blood vessels begins in the extraembryonic mesoderm of the chorion (somatopleure), connecting stalk (mesenchyme), and the yolk sac wall (splanchnopleure) around the allantoic diverticulum. The main arteries from the yolk sac are the omphalomesenteric arteries whose terminal branches are the vitelline arteries.
Isolated masses and cords of mesenchymal cells in these areas proliferate and form blood islands. The peripheral mesenchymal cells of the blood islands proliferate and flatten to form the endothelial lining of the blood vessels. Intercellular spaces appear between the central cells and become the lumens of the blood vessels. The central cells detached, forming the blood stem cells. The fluid that accumulates between the blood cells becomes blood plasma. Blood plasma and blood cells appear in the blood vessels of the yolk sac and allantois very early. (Blood formation in the embryo begins at about the second month in the liver, spleen, bone marrow, and lymph nodes.) Subsequently, mesenchyme around the endothelium forms connective tissue and smooth muscle components of the tunica intima, media, and externa of the larger blood vessels.
As more blood islands form, isolated areas of endothelial tissue enlarge and unite into a contiguous horseshoe-shaped plexus, the anterior central portion of which is the cardiogenic area. The intraembryonic coelomic cavity located over this region later becomes the pericardial cavity. Channels develop within these plexuses, forming capillaries. Some of the capillaries enlarge into primitive arteries and veins, including the vitelline vessels of the yolk sac and umbilical vessels of the chorion and body stalk. The vitelline veins become confluent in a pair of omphalomesenteric veins that pass from the yolk stalk cephalad in close association with the developing liver and discharge into the sinus venosus.
Blood vessels develop similarly within the embryo from mesenchyme. Some clusters of angiogenetic cells appear bilaterally, parallel and near the midline of the embryonic shield. These clusters canalize into the paired dorsal aortae. These vessels later connect with the horseshoe-shaped plexus that will form the heart tube. Extraembryonic vessels soon establish communication with those in the body of the embryo to create a primitive vascular system, permitting stem blood cells formed in the yolk sac to circulate in the embryonic body.
By the beginning of the fourth week, an extensive network of blood vessels is forming throughout the embryonic body, the heart starts to beat, and blood begins to circulate. Blood is formed and becomes circulated through the vessels by the pumping action of the heart on about the twenty-fifth day after conception.
In the illustrations for this chapter, arteries are colored red and veins blue, regardless of the level of oxygenation of the blood they transport.
The aortae develop by a process similar to that involved in the formation of the endocardial tubes. The ascending aorta develops from the truncus arteriosus. The aorta, from the right pulmonary artery to the left common carotid artery, develops from the aortic sac. The remainder of the aortic arch forms from the left fourth aortic arch artery and the left dorsal aorta. The dorsal aortae fuse to form the descending aorta.
Ventral Aorta (Aortic Sac)
The cephalic end of the truncus arteriosus expands ventral to the pharynx forming the ventral aortae (aortic sac). All aortic arches take origin from the aortic sac. The sac makes a substantial contribution to the pulmonary trunk, arch of the ascending aorta, brachiocephalic artery, and common carotid arteries.
The major arteries develop concurrently with the heart. The most complex and fascinating vascular formation is development of the aortic arches associated with the pharyngeal pouches and arches in the neck region. These embryonic aortic arches arise from the truncus arteriosus, and they fuse to produce the dorsal aorta, the major systemic artery that leaves the heart to distribute blood throughout the adult body. The aortic arches are usually numbered from I to VI to correspond to the pharyngeal arches.
Each of the six pharyngeal arches has an aortic arch artery. The entire series of aortic arches is never present at the same time in mammalian embryos, and the fifth pair of arches is vestigial or absent in human embryos (for example, the first and second pairs of aortic arches disappear before the formation of the sixth). Ultimately, most of the six branchial arch arteries obliterate entirely or partially. The aortic arches transform into the basic adult arterial arrangement during the sixth to the eighth week of embryonic development.
The first and second pairs of arches contribute to a plexus that subsequently becomes the external carotid artery and its branches.
The third pair of arches contributes to the proximal part of the internal carotid arteries.
The left fourth aortic arch contributes to a small part of the aortic arch. The right fourth aortic arch contributes to the proximal part of the right subclavian artery.
The proximal portions of the pulmonary arteries arise as outgrowth of the sixth aortic (pulmonary) arches. Otherwise, the right sixth aortic arch arteries disappear. The distal portions of the left sixth aortic arch arteries persists during intrauterine life as a shunt between the pulmonary trunk and the aorta called the ducts arteriosus. Most of the blood that enters the right atrium bypasses the lungs by passing through the ductus arteriosus to the foramen ovale in the heart. Much of the blood delivered by the venae cavae bypasses the lungs by passing though the foramen ovale and the ductus arteriosus.
Thus, only the left fourth arch participates in the formation of the adult aortic arch that carries blood away from the left ventricle.
The right and left dorsal aortae arise from the paired aortic arches. They fuse caudal to the tenth dorsal intersegmental artery during the fourth week to form the descending aorta.
During the seventh week, the part of the right dorsal aorta caudal to the origin of the primitive subclavian artery degenerates. The remaining part and a remnant of the right fourth aortic arch participate in forming the right subclavian artery. The cranial portion of the left dorsal aorta persists and forms part of the definitive aortic arch. (The aortic arch also includes contributions from the truncus arteriosus, the aortic sac, the left fourth aortic arch, and a short portion of the fused dorsal aortae.) The right dorsal aorta degenerates.
Branches of the Descending Aorta
The descending aorta has unpaired ventral segmental, paired lateral segmental, and dorsal intersegmental branches.
Ventral Segmental Arteries (Allantoic, or Umbilical, Arteries)
Ventral segmental arteries supply the yolk sac, gut, and fetal part of the placenta. The esophageal and bronchial arteries in the thoracic region also develop from ventral segmental arteries.
Only three major segmental arteries that originally supplied the yolk sac and gut remain in the fifth week—the celiac trunk (to the foregut), the superior mesenteric artery from the omphalomesenteric arteries (to the midgut), and the inferior mesenteric artery (to the hindgut).
The allantoic arteries (umbilical arteries) are specialized members of the ventral segmental arteries that terminate in the vascular plexus of the chorionic villi. The stem of the umbilical artery on each side degenerates and a new stem forms by way of an anastomosis with the fifth lumbar dorsal intersegmental artery. This new stem persists as the common iliac artery and gives off branches that become the external and internal iliac arteries, and one or more superior vesicle arteries to the bladder. The umbilical artery becomes the lateral umbilical ligament in the adult.
Lateral Segmental Arteries
The lateral segmental branches of the aorta are pairs of vessels that supply the diaphragm and gonads, including the metanephroi, adrenal glands, and gonads. They become the phrenic, renal, middle suprarenal, and gonadal (testicular or ovarian) arteries of the adult.
Dorsal Intersegmental Arteries
The dorsal intersegmental arteries branch at regular intervals along the aorta between somites to supply the body wall, brain, and spinal cord. The aorta gives off thirty or more pairs of dorsal intersegmental arteries.
The vertebral arteries originate from anastomoses of the first six dorsal intersegmental arteries. These unite superiorly to form the basilar artery. The medial branches of the internal carotid arteries unite with the basilar artery to form the circle of Willis.
The seventh pair of dorsal intersegmental arteries contributes to the subclavian arteries. Thoracic and abdominal dorsal intersegmental arteries become the posterior intercostal and lumbar arteries. The fifth lumbar dorsal intersegmental artery contributes to the arteries of the pelvis and lower limbs.
Arteries of the Extremities
Four or five adjacent dorsal intersegmental arteries create a capillary plexus that supplies the limb bud.
The subclavian arteries develop from the seventh cervical segmental (eleventh intersegmental) arteries with a contribution from the right fourth aortic arch artery. These join a plexus of small blood vessels in the upper limb bud at the site of each adult vessel. The subclavian artery continues as the axillary, brachial, and anterior interosseous arteries. The ulnar artery and the radial artery arise comparatively late as brachial branches to the postaxial and preaxial sides of the extremity. The digital arteries later arise from the radial and ulnar arteries.
Common iliac arteries develop as intersegmental vessels that tap the umbilical arteries. The axial, or sciatic, artery becomes the principal arterial stem of the lower extremity in early stages. It develops as a branch of the umbilical artery (future internal iliac artery). The axial artery terminates in a plexus that produces digital branches.
The femoral artery appears in a plexus on the future ventral aspect of the thigh. The femoral artery is a continuation of the external iliac artery which, in turn, arises as a bud from the common iliac artery. The femoral artery becomes the main blood supply to the lower extremity by annexing with the axial artery and its branches in the popliteal region.
Eventually, the axial artery persists proximally only as the inferior gluteal artery and the artery to the sciatic nerve. The popliteal and peroneal arteries mark its original distal course. The obturator artery arises comparatively late to supply the medial thigh.
The anterior and posterior tibial arteries develop as branches of the popliteal artery. The tibial arteries take over the arteries of the foot.
Vitelline and Umbilical Veins
The vitelline veins return blood from the yolk sac and the gut and develop into the hepatic portal system.
At 4 weeks, the paired umbilical veins return blood from the placenta to capillary networks in the liver. During the fifth week, the right and the proximal portion of the left umbilical veins degenerate so that blood from the placenta travels along a single umbilical vein. At the same time, the left umbilical vein forms anastomoses with the hepatic sinuses and the newly formed ductus venosus. The ductus venosus allows venous blood from the umbilical vein and the portal vein to bypass the liver and flow to the inferior vena cava and sinus venosus. In the adult a remnant of the left umbilical vein persists as the round ligament of the liver (ligamentum teres hepatis) and a remnant of the ductus venosus persists as the ligamentum venosum.
The cardinal veins arise somewhat later but by a process similar to the process that produces the aortae. The precardinal and postcardinal veins return blood from the head and the trunk, respectively. Near the heart, the precardinal and postcardinal veins unite to form common cardinal veins that join the sinus venosus together with the vitelline and umbilical veins.
Numerous large tributary vessels develop from the precardinal veins and converge as cerebral plexuses. Blood passes from the plexuses to the heart through the precardinal and common cardinal veins.
The external jugular veins begin as tributaries near where the precardinal veins enter the common cardinal veins. The precardinal veins become the internal jugular veins and the left brachiocephalic vein forms a connection between them. The right common cardinal vein and the proximal part of the right precardinal vein become the superior vena cava. The left common cardinal vein atrophies, and blood from the left internal jugular vein passes through the left brachiocephalic veins to the superior vena cava.
The postcardinal (posterior cardinal) veins are the most important venous drainage of the caudal body. They atrophy and disappear almost completely and a complex secondary system, the subcardinal, sacrocardinal, and supracardinal veins, takes over venous drainage of the trunk and legs.
The subcardinal veins drain into postcardinal veins. They become united by intersubcardinal anastomoses to form the large median subcardinal venous sinus.
The inferior vena cava develops from the following embryonic structures. (1) The abdominal portion develops from the right sacrocardinal and subcardinal veins. (2) The part dorsal to the liver develops from an anastomosis between the right subcardinal and vitelline veins. (3) The portion from the liver to the heart develops from the persisting proximal portion of the right vitelline vein. (4) The mesenteric portion develops from the subcardinal sinus, the right subcardinal vein, and vascular channels in the mesentery between the liver and the mesonephric kidneys.
Toward the end of the fourth week, angioblasts proliferate around the developing lung buds and the primordium of the common pulmonary vein appears as a blind diverticulum in the dorsal wall of the left atrium. Within a day or two, the common pulmonary vein forms anastomoses with a venous plexus around the lung buds. The pulmonary circulation becomes established by the end of the fifth week.
The common pulmonary vein and its branches absorb into the atrial wall so that there are at first two separate openings of pulmonary veins into the left atrium. Finally, there are four (occasionally six).
Veins of the Extremities
The capillary plexus of the limb buds develop from a peripheral border vein that drains blood distributed to the limb by an axial artery. Permanent veins develop from the border vein along the caudal (ulnar, fibular) margin of the limb bud while the smaller vein on the cranial (radial, tibial) margin atrophies.
The ulnar portion of the border vein persists, forming the subclavian, axillary, and basilic veins at different levels. The subclavian vein eventually drains to the precardinal (internal jugular) vein. The cephalic vein develops secondarily in relationship to the radial border vein, and it later anastomoses with the external jugular vein and finally opens into the axillary vein.
The great saphenous vein arises from the postcardinal vein. It produces the femoral and posterior tibial veins and annexes the fibular border vein. Distally, the border vein develops into the anterior tibial and, likely, the small saphenous veins. Proximally, it greatly reduces to form the inferior gluteal vein.
The circulation of blood through a fetus is necessarily different from that of a newborn infant. Throughout pregnancy, the fetal lungs are nonfunctional and the fetus depends upon the mothers circulatory system for procurement of nutrients and the exchange of gases and metabolic wastes. The capillary exchange between the maternal and fetal circulation occurs within the placenta, a remarkable structure that includes parts of the uterus of the mother during pregnancy and is discharged following delivery as the afterbirth. Accordingly, some unique structures of the fetal circulatory system develop to accommodate the placental arrangement. These structural adaptations of fetal life normally change quickly after birth.
The umbilical cord, a connection between the placenta and the fetal umbilicus, includes one umbilical vein and two umbilical arteries, surrounded by a gelatinous substance called Whartons jelly. Blood rich in oxygen and nutrients flows from the placenta, through the umbilical vein, and toward the inferior surface of the liver. There, the umbilical vein bifurcates into a branch that joins the portal vein and another, the ductus venous, that enters the inferior vena cava. Thus, oxygenated blood mixes with venous blood returning from the lower extremities of the fetus before it enters the heart. The umbilical vein is the only vessel of the fetus that transports fully oxygenated blood.
The inferior vena cava conveys blood to the right atrium of the fetal heart. From the fetal right atrium, most of the blood is directed through the foramen ovale to the left atrium. Blood in the fetal left atrium mixes with a small quantity of blood returning through the pulmonary circulation. The blood then passes into the left ventricle from which it is pumped into the aorta and through the fetal body. Some blood that enters the right atrium passes into the right ventricle and out of the heart through the pulmonary trunk. The nonfunctional lungs of the fetus are collapsed and provide very high resistance to blood flow. Thus, only a small portion of blood continues through the pulmonary circulation. Most of the blood in the pulmonary trunk passes through the ductus arteriosus into the aortic arch where it mixes with blood from the left ventricle. The paired umbilical arteries that arise from the internal iliac arteries return blood to the placenta.
Thus, blood rich in oxygen is transported by the inferior vena cava to the heart and through the foramen ovale and ductus arteriosus to the systemic circulation.