How does oxygen intake at birth affect pressure in the right atrium?

• A. It increases pulmonary vascular resistance
• B. It leads to a further decrease in right atrial pressure
• C. It causes right atrial pressure to remain unchanged
• D. It decreases pressure in the right atrium

Answer: (D)

At birth, the transition from fetal to neonatal life is marked by significant changes in physiology, especially regarding the cardiovascular system. One of the critical changes involves the intake of oxygen. In the fetus, the lungs are filled with fluid, and oxygen is supplied via the placenta, with the right atrium receiving a mixture of deoxygenated blood from the body and oxygenated blood from the placenta.

When the newborn takes its first breath, the lungs expand, filling with air. This exposure to oxygen stimulates several physiological responses. Importantly, the expansion of the lungs significantly reduces pulmonary vascular resistance because the lung blood vessels dilate in response to increased oxygen levels. This decrease in resistance allows more blood to flow into the lungs for oxygenation, subsequently increasing the oxygen content in the blood.

As a result of this increased blood flow into the lungs, there is a corresponding decrease in the volume of blood returning to the right atrium. Since the right atrium is receiving less blood due to the redistribution of pulmonary flow, the pressure within the right atrium decreases. This transition is essential not only for proper oxygenation but also for the closure of the foramen ovale, which is an important step in normal circulatory adaptation at birth.

Therefore, oxygen