Human Embryology: Heart Development

Embryological Development of the Heart

• Heart development begins in the 3rd week when the embryo requires its own blood supply
• Two heart tubes form from the mesoderm and fuse to form a single primitive heart tube
• The primitive heart tube expands and forms structures of the fully developed heart
• By day 22, the primitive heart begins to beat and bend, and the heart tube becomes partitioned into four chambers

Position of the Heart

• The heart is located in the mediastinum, the central space in the thoracic cavity between the two lungs
• The heart is oriented with the right side more anterior and the left side more posterior
• The apex of the heart points towards the left, with approximately 2/3 of the heart sitting to the left of the midline
• The base of the heart sits opposite the apex and is directed posteriorly, also referred to as the posterior surface of the heart

Structure and Function of the Pericardium and Heart Wall

• The pericardium is the protective sac around the heart that keeps it in place within the thorax
• The pericardium has two parts: the outer fibrous pericardium and the inner serous pericardium
• The serous pericardium has an outer parietal layer and an inner visceral layer, with a potential space in between containing serous fluid
• The visceral layer of the serous pericardium lines the outer surface of the heart directly, forming the outer layer of the heart wall, also called the epicardium
• The heart wall has three layers: an outer epicardium, a middle myocardium, and an inner endocardium

External Features of the Heart

• The heart has four chambers: a left and right atrium and a left and right ventricle
• The two atria are the "receiving" chambers, receiving blood from either the lungs or the body
• The ventricles are the "pumping" chambers, pumping blood to either the lungs or the body

Internal Features of the Heart - Right Atrium

• The right atrium receives deoxygenated blood from the body through three main veins: the superior vena cava, the inferior vena cava, and the coronary sinus
• The right atrium has internal features including the fossa ovalis, the interatrial septum, and the pectinate muscles

Internal Features of the Heart - Right Ventricle

• Deoxygenated blood from the right atrium flows into the right ventricle through the right atrioventricular orifice
• The right ventricle has a valve called the right atrioventricular valve
• Other features of the right ventricle include the chordae tendineae, papillary muscles, trabeculae carneae, and the interventricular septum
• From the right ventricle, deoxygenated blood is pumped to the lungs to be oxygenated via the pulmonary trunk, guarded by the pulmonary semilunar valve

External Features of the Heart - Posterior and Inferior Views

• The posterior surface of the heart is formed by the left atrium
• The inferior surface of the heart sits on the diaphragm and is also called the diaphragmatic surface
• The posterior interventricular sulcus runs between the two ventricles on the inferior surface of the heart

Embryological Development of the Heart

• Heart development begins in the 3rd week when the embryo requires its own blood supply
• Two heart tubes form from the mesoderm and fuse to form a single primitive heart tube
• The primitive heart tube expands and forms structures of the fully developed heart
• By day 22, the primitive heart begins to beat and bend, and the heart tube becomes partitioned into four chambers

Position of the Heart

• The heart is located in the mediastinum, the central space in the thoracic cavity between the two lungs
• The heart is oriented with the right side more anterior and the left side more posterior
• The apex of the heart points towards the left, with approximately 2/3 of the heart sitting to the left of the midline
• The base of the heart sits opposite the apex and is directed posteriorly, also referred to as the posterior surface of the heart

Structure and Function of the Pericardium and Heart Wall

• The pericardium is the protective sac around the heart that keeps it in place within the thorax
• The pericardium has two parts: the outer fibrous pericardium and the inner serous pericardium
• The serous pericardium has an outer parietal layer and an inner visceral layer, with a potential space in between containing serous fluid
• The visceral layer of the serous pericardium lines the outer surface of the heart directly, forming the outer layer of the heart wall, also called the epicardium
• The heart wall has three layers: an outer epicardium, a middle myocardium, and an inner endocardium

External Features of the Heart

• The heart has four chambers: a left and right atrium and a left and right ventricle
• The two atria are the "receiving" chambers, receiving blood from either the lungs or the body
• The ventricles are the "pumping" chambers, pumping blood to either the lungs or the body

Internal Features of the Heart - Right Atrium

• The right atrium receives deoxygenated blood from the body through three main veins: the superior vena cava, the inferior vena cava, and the coronary sinus
• The right atrium has internal features including the fossa ovalis, the interatrial septum, and the pectinate muscles

Internal Features of the Heart - Right Ventricle

• Deoxygenated blood from the right atrium flows into the right ventricle through the right atrioventricular orifice
• The right ventricle has a valve called the right atrioventricular valve
• Other features of the right ventricle include the chordae tendineae, papillary muscles, trabeculae carneae, and the interventricular septum
• From the right ventricle, deoxygenated blood is pumped to the lungs to be oxygenated via the pulmonary trunk, guarded by the pulmonary semilunar valve

External Features of the Heart - Posterior and Inferior Views

• The posterior surface of the heart is formed by the left atrium
• The inferior surface of the heart sits on the diaphragm and is also called the diaphragmatic surface
• The posterior interventricular sulcus runs between the two ventricles on the inferior surface of the heart

Embryological Development of the Heart

• The heart develops from the mesoderm, one of the three primary germ layers of the embryo.
• The development of the heart begins in the third week of development, when the embryo reaches a size where it requires its own blood supply.
• Two heart tubes form from the mesoderm and then fuse to form a single primitive heart tube.
• The primitive heart tube forms several expansions, which ultimately give rise to the structures of the fully developed heart.
• By day 22 of development, the primitive heart begins to beat and begins to bend and fold on itself.
• In utero, the heart has two openings, foramen ovale and ductus arteriosus, which allow blood to bypass the lungs, as the lungs are not functional until after birth.

Position of the Heart

• The heart is located in the mediastinum, the central space in the thoracic cavity between the two lungs.
• The heart is orientated in such a way that the right side of the heart is more anterior, while the left side of the heart is more posterior.
• The apex of the heart points towards the left, and approximately 2/3 of the heart sits to the left of the midline.
• The base of the heart sits opposite to the apex and is directed posteriorly, also referred to as the posterior surface of the heart.
• The heart sits on its inferior surface, also referred to as the diaphragmatic surface, which is fused with the central tendon of the diaphragm.

Pericardium and Heart Wall

• The pericardium is the protective sac around the heart, which not only protects the heart but also keeps it in place within the thorax.
• The pericardium has two parts: the outer fibrous pericardium, which is fused with the central tendon of the diaphragm, and the inner serous pericardium.
• The serous pericardium has an outer parietal layer and an inner visceral layer, with a potential space in between them containing a thin film of serous fluid.
• The visceral layer of the serous pericardium lines the outer surface of the heart directly, forming the outer layer of the heart wall, also called the epicardium.
• The heart wall is composed of three layers: an outer epicardium, a middle myocardium, and an inner endocardium.

External Features of the Heart

• The heart has four chambers: a left and right atrium and a left and right ventricle.
• The atria are the "receiving" chambers, receiving blood from either the lungs or the body, while the ventricles are the "pumping" chambers, pumping blood to either the lungs or the body.