Cardiovascular System Chapter 1: The Double Circulation

The Heart Chambers and the Double Circulation

The Systemic Circulation

• The heart is divided into a right side and a left side.
• The left side pumps blood to the various systems of the body, but not to the lungs.
• The main artery to leave the left side of the heart is the aorta, which carries oxygenated blood.

Right Atrium

• The right atrium has an earlobe or auricle that lies in front of the 1st part of the aorta.
• The anterior wall of the right atrium is composed of muscular ridges called musculi pectinati.
• These ridges arise from a vertical ridge called the crista terminalis.
• The cristae terminalis has an important part of the electrical conducting system of the heart, the sinu-atrial node.
• The wall behind the crista terminalis is smooth and is known as the sinus venarum.
• There are three openings into the right atrium: SVC, IVC, and the coronary sinus.

Right Ventricle

• The region of the septal wall close to the outflow tracts of the heart is smooth, and this reflects the membranous portion of the fetal heart septum.
• In the adult, this is called the conus arteriosus or infundibulum.
• The infundibulum serves to direct the blood out of the right ventricle via the pulmonary valve.
• Ventricular septal defects (VSDs) can occur in this area if it fails to develop properly.

Atrial Septal Defects (ASD's)

• ASD's can also occur and are fairly common, with approximately 20% of the population having a small patent foramen ovale called a probe patency.
• More serious conditions include large deficiencies in the septum and hence a large patent foramen ovale.
• Sometimes a complete absence of the inter-atrial septum is seen, which is called common atrium syndrome.

Tetralogy of Fallot

• A persistent interventricular foramen is usually associated with a malformation of the aorta and pulmonary trunk.
• One such condition is known as tetralogy of Fallot, which occurs when the developing outflow tracts divide unequally.
• This condition is characterized by a narrow pulmonary trunk (called a pulmonary infundibular stenosis), a defect in the interventricular septum, an “over-riding” aorta, and a hypertrophied right ventricle.

Left Atrium

• The left atrium receives the pulmonary veins, carrying oxygenated blood from the lungs.
• It is entirely smooth-walled internally, except for the auricle.
• The auricle is the only remnant of the original atrial chamber in the embryo.

Left Heart

• The valve between the left atrium and the left ventricle is the mitral valve, which is a bicuspid valve.
• The anatomical features of the left ventricle are similar to those of the right, except the left ventricle is larger and has thicker walls.
• There is no moderator band in the left ventricle.
• There are usually only two papillary muscles, and it lacks septal papillae that are a feature of the right ventricle.

Fetal Circulation

• Blood is oxygenated in the placenta.
• At birth, the lung beds open suddenly as the first breath is taken.
• The pulmonary system pressure drops, and this triggers a closure of both the foramen ovale and ductus arteriosus.
• The foramen ovale is then known as the fossa ovalis, and the ductus arteriosus shrivels and fibroses to form the ligamentum arteriosum.

Patent Fetal Circulation in Newborn

• Occasionally, the foramen ovale fails to close, which is known as “patent foramen ovale” if large, or “probe patency” if small.
• When this occurs, the pressure gradient is such that the systemic system is at a higher pressure than the pulmonary system, and blood flows from left to right.
• This can cause oxygenated blood to re-enter the pulmonary system instead of being distributed to the body tissues that need it.