Mediastinum Anatomy and Fetal Circulation

Posted on Dec 2, 2024 in Biology

Mediastinum

  • Occupied by the tissue mass between the two lung cavities. Covered by the mediastinal parietal pleura on each side and contains all thoracic viscera and structures except the lungs.
  • Extends from the upper chest opening to the diaphragm inferiorly, and from the sternum and costal cartilages anteriorly to the thoracic vertebral bodies posteriorly.
  • High mobility due to mostly hollow visceral structures held by loose connective tissue, often infiltrated with fat.

Superior Mediastinum (Manubrium to T4/T5)

  • Extends downward from the upper chest opening to the horizontal plane, including the sternal angle anteriorly and crossing the junction (approximately Disk IV) of T4 and T5 vertebrae posteriorly to the transverse plane of the chest.

Lower Mediastinum

  • Situated between the transverse plane of the chest and the diaphragm, divided by the pericardium:

Anterior

Front of the pericardium, behind the sternum. Example: Thymus.

Middle

Pericardium, heart, and roots of its large vessels (part of the ascending aorta, pulmonary artery, and superior vena cava).

Posterior

Posterior to the diaphragm and pericardium, and anterior to the mid-lower thoracic vertebrae. Example: Esophagus, Thoracic Aorta.

Fetal Circulation

Oxygenated blood from the mother’s placenta comes through the umbilical vein, bypassing the liver via the venous shunt (only a few branches nourish the liver). The umbilical vein then drains into the inferior vena cava, entering the right atrium. Since the lungs are non-functional, most blood passes to the left atrium through the foramen ovale, then to the left ventricle. A small portion goes to the right ventricle to nourish the lungs, with some shunted back into the aorta. Blood from the left ventricle enters the aorta and returns to the placenta via the umbilical artery for oxygenation.

Adrenaline Injection

For adrenaline injection, target between the 4th and 5th ribs, close to the outer side.

In trauma, the right ventricle is most exposed and susceptible to impact due to its anatomical position (posterior to the sternum).