Primitive Heart TubeThe development of the heart begins with the formation of the primitive heart tube following the folding of the embryo during the end of the third week. Show
Firstly, lateral folding creates the heart tube by bringing together two precursor regions, then cephalocaudal folding positions the heart tube in the future thorax. Initially, the heart tube is suspended within the pericardial cavity by a membrane; this subsequently degenerates to allow for further growth. From superior to inferior, the primitive heart tube is comprised of six regions:
The heart tube continues to elongate, and begins looping at around day 23 of development. The bulbus cordis moves ventrally, caudally, and to the right (forward, down and right), and the caudal portion – the primitive ventricle – moves dorsally, cranially and to the left (backwards, up and left). This process produces a shape that is much closer to the fully developed heart. By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons Fig 1 – The process of heart tube looping. AtriaAt the fourth week, the sinus venosus is responsible for the inflow of blood to the primitive heart, and empties into the primitive atrium. It receives venous blood from the right and left sinus horns. Over time, the venous return shifts to the right side of the heart, causing the left sinus horn to recede and form the coronary sinus (responsible for the drainage of venous blood from the heart itself). The enlarged right sinus horn is absorbed by the growing right atrium and eventually forms part of the inferior vena cava in the adult. In the left atrium, a similar process occurs with the pulmonary veins. The four pulmonary veins are incorporated into the left atrium, forming the smooth inflow portion of the left atrium and the oblique pericardial sinus. Aortic ArchesThe early arterial system begins as a bilaterally symmetrical system of arched vessels, which then undergo extensive remodeling to create the major arteries that exit the heart. The derivatives of the aortic arches in the adult are as follows:
Each of the arches has a corresponding nerve during development. The most important of these is the recurrent laryngeal nerve (a branch from CN X) – which is associated with the 6th arch:
The long course of the left recurrent laryngeal nerve is clinically relevant, as it is susceptible to pathology in the chest (e.g. compression by an aortic aneurysm). Fig 2 – The aortic arches and their major derivatives. Septation of the HeartSeptation of the heart into right and left channels occurs first, when endocardial cushions developing in the atrioventricular region expand to divide the heart. AtrialThe septation of the primitive atrium involves the formation of two septa and three ‘holes’. Firstly, the septum primum forms and extends down towards the fused endocardial cushions to split the atrium into two. The ostium primum is a hole present before the septum primum completes fusion with endocardial cushions. Before the ostium primum is closed a second hole, the ostium secundum, appears within the septum primum. Following this a second septum, the septum secundum, grows with a hole known as the foramen ovale present. The presence of both the ostium secundum and foramen ovale allows a right to left shunt to be present in the developing heart. The timing of this process is carefully controlled. At all times, at least one hole is present in the septa to allow communication between the left and right atria. This allows blood to be shunted to the left side of the heart, bypassing the non-functional lungs. By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons Fig 3 – Septation of the atria and ventricles. VentricularThe interventricular septum of the ventricles has two components; one muscular and one membranous. The muscular portion forms much of the septum and grows up from the floor of the ventricles towards the fused endocardial cushions, but a small gap, the primary interventricular foramen, remains. This gap is filled by the membranous portion of the interventricular septum, which is comprised of connective tissue derived from endocardial cushions. Outflow TractEndocardial cushions also appear within the truncus arteriosus which grow towards each other. As they grow towards each other they twist around each other and form a spiral septum, dividing the outflow tract into left and right sides. Circulatory ShuntsIn the fetal circulation, vascular shunts are required to bypass the liver and non-functioning lungs. The lungs are bypassed by two separate shunts, firstly the foramen ovale between the two atria, which is responsible for bypassing the majority of the circulation. Any blood that does not pass through the foramen ovale enters the pulmonary trunk, which is linked to the distal arch of aorta by the ductus arteriosus. These two separate shunts allow the circulation to bypass the lungs. The oxygenated blood entering the fetus also needs to bypass the primitive liver, this ensures that enough oxygen reaches the developing brain. This is achieved by passage through the ductus venosus, which is estimated to shunt around 30% of umbilical blood directly to the inferior vena cava. At birth, these shunts need to close to allow the normal adult circulation to be established:
The circulatory shunts are summarised in the table below:
By OpenStax College [CC BY 3.0], via Wikimedia Commons Fig 4 – Fetal shunts and their closure after birth Clinical Relevance – Tetralogy of FallotTetralogy of Fallot is a congenital heart defect in which there are four abnormalities found:
It is the most common cyanotic congenital heart defect and is often diagnosed prenatally using echocardiography. Treatment takes the form of surgery during the first year of life, in which the pulmonary valve can be widened and the ventricular septal defect repaired. If the condition is not treated, Tetralogy of Fallot results in progressive worsening of the right ventricular hypertrophy, eventually leading to heart failure. What is the purpose of the intrauterine circulatory system?The direction of flow of the intrauterine circulation helps to maximize oxygen delivery to the developing brain and heart. Although blood from the ductus venosus and inferior vena cava (IVC) merges near the fetal heart, blood from each vessel is directed separately within the heart3.
What is the function of the circulatory system Quizlet?Your circulatory system is vital to your survival. Its function is to distribute blood and other nutrients to all your body’s organs and tissues. The small blood vessels called capillaries facilitate the exchange of oxygen and nutrients between your blood and the cells in your body.
How does the circulatory system adapt to the transition from intrauterine to extrauterine?The transition from intrauterine to extrauterine life requires a rapid adaptation of multiple organ systems. Separation from the placental circulation results in increased systemic vascular resistance, while initiation of ventilation lowers pulmonary vascular resistance.
Do humans have a closed or open circulatory system?Humans have a closed circulatory system. The human circulatory system functions to transport blood and oxygen from the lungs to the various tissues of the body. The human heart consists of four chambers – two ventricles and two auricles. Organs or Parts of Circulatory System
Is one of the first systems to become functional in intrauterine life?The circulatory system is one of the first systems to become functional in intrauterine life? The length of a pregnancy is commonly measured from the first day of the last menstrual period?
Which of the following adaptations of the circulatory system occurs shortly after birth?Which of the following adaptations of the circulatory system occurs shortly after birth? The foramen ovale closes and seals, blocking blood between the atria from mixing. The ductus arteriosus collapses and stops pulmonary trunk blood from mixing with aortic arch blood.
What organ of the pregnant woman is central to the exchange of nutrients for waste products with the fetus?Placenta. An organ shaped like a flat cake. It only grows during pregnancy. The fetus takes in oxygen, nutrients, and other substances from the placenta and gets rid of carbon dioxide and other wastes.
Which of the following tasks is the most important task of the first trimester of pregnancy quizlet?Which of the following tasks is the most important task of the first trimester of pregnancy? Rationale: Before a fetus moves, adjusting to pregnancy is a primary task; later, adjusting to having a baby becomes the primary task. As a pregnant woman lies on the examining table, she grows very short of breath and dizzy.
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