Embryology: Heart Development

• The heart, derived from the mesoderm, is the first major system to function, appearing in the 3rd week (18-20 days).
• A functioning heart is important for circulation of nutrients, blood, and oxygen, and for waste removal.
• The heart begins to beat in the 4th week (22-23 days).
• Blood circulation occurs in the 4th week of development.

Heart Tube Formation

• The heart tube is the first structure to form.
• Two heart tubes form by day 20.
• These tubes fuse into a single primitive heart tube by day 21, featuring two inlets and outlets.
• By day 22, the fused primitive heart tube develops a primitive ventricle and atrium.

Cardiac Looping

• Looping of the heart occurs from days 23-28.
• Proper looping requires a counterclockwise or rightward direction.

Septation

• Septation, the division of the heart into chambers, begins after cardiac looping at the end of the 4th week and is completed by the 8th week.
• Endocardial cushions separate the heart into right and left, and upper and lower chambers.
• These chambers eventually develop into atria and ventricles.

Atrial Septation

• The ostium primum is the initial opening between the atria.
• The septum primum grows to nearly close this opening.
• Apoptosis in the septum primum creates the ostium secundum at the top of the heart.
• The septum secundum grows downward, overlapping the ostium secundum to form the foramen ovale, which allows one-way blood flow from right to left.

Atrioventricular Septum Formation

• This process begins during the 4th week.
• Endocardial cushions grow towards each other, dividing the atrioventricular canals.
• The tissue thins out to form the atrioventricular (AV) valves.
• The AV valves are derived from endocardial cushions.

Truncus Septation

• During the 5th week, swellings appear in the truncus arteriosus.
• These swellings migrate and rotate to form the aortopulmonary septum.
• The septum divides the truncus into the aorta and pulmonary trunk.
• Remaining swelling tissue thins to form the semilunar valves.

Ventricular Septation

• This process begins during the 4th week as muscular ventricles expand.
• Medial ventricular walls grow towards each other, merging into the interventricular septum.
• The superior membranous portion of the interventricular septum, derived from endocardial cushions, completes the closure of the interventricular foramen.

Blood Flow and Umbilical Vessels

• The umbilical cord forms at week 5, protecting the umbilical arteries and vein.
• The umbilical cord contains two umbilical arteries and one umbilical vein.
• These vessels are embedded within Wharton's jelly, a loose, proteoglycan matrix.

Fetal Blood Circulatory System

• The umbilical vein carries oxygen and nutrients from the mother to the fetus.
• The umbilical arteries carry deoxygenated blood, waste, and CO2 from the fetus to the mother.
• Fetal circulation includes three shunts that bypass the liver and lungs: the foramen ovale, ductus arteriosus, and ductus venosus.
• The foramen ovale allows blood to bypass the right ventricle and lungs.
• The ductus arteriosus shunts blood from the pulmonary artery to the aorta, bypassing the lungs.
• The ductus venosus allows blood to flow from the umbilical vein to the inferior vena cava (IVC) and right atrium, bypassing the liver.

Circulation After Birth

• Clamping the umbilical cord stops blood and oxygen supply, necessitating immediate lung function for blood oxygenation.
• The first breath reduces pulmonary vascular resistance due to oxygen in the lungs and breathing action.
• Clamping the cord increases systemic vascular resistance and improves blood flow to the lungs.

Postnatal Heart Separation

• Increased systemic vascular resistance elevates pressure in the left atrium due to increased blood return from pulmonary veins.
• The higher pressure in the left atrium, compared to the right atrium, presses against the septum primum closing, the foramen ovale.
• High oxygen levels cause the ductus arteriosus to constrict and eventually close.

Newborn Circulatory Milestones

• Foramen ovale closure:
  • Functionally closes immediately.
  • Anatomically closes within 12 months.
• Ductus arteriosus closure:
  • Functionally closes within 12-24 hours.
  • Anatomically closes within 2-3 weeks.
• Ductus venosus constriction/closure:
  • Functionally closes immediately.
  • Anatomically closes within 1-3 months.
• Placental circulation occlusion causes an immediate drop in blood pressure:
  • Systolic BP drops to 46-94.
  • Diastolic BP drops to 24-57.

Abnormalities of Heart Development

• Looping defects:
  • Associated with isomeric cardiac lesions.
  • Dextrocardia involves the heart looping to the left instead of the right, positioning the heart on the opposite side of the body.
  • Related conditions include situs solitus, situs inversus, and situs ambiguus.
• Endocardial cushion/septal defects:
  • Include atrial or ventricular septal defects.
  • Tetralogy of Fallot is a combination of heart defects.
  • Transposition of the great vessels occurs.
• Circulatory system defects:
  • A single umbilical artery instead of two is strongly associated with chromosomal abnormalities and malformations.
• Arterial system defects:
  • Patent ductus arteriosus (PDA)
  • Coarctation of the aorta, characterized by a "pinching" in the aorta.
  • Double aortic arch.
• Venous system defects:
  • Include double or absent inferior vena cava (IVC) or superior vena cava (SVC).
  • Patent Foramen Ovale (PFO).