= Pulmonary trunk
The \b[pulmonary trunk] is the common <artery> leaving the <right ventricle> through the <pulmonary valve> before dividing into the vessels supplying the two <lungs>. It is the initial shared segment of the <pulmonary artery> route in the <pulmonary circulation>. The name <artery> describes flow away from the <heart>; the <blood> in the <pulmonary trunk> has not yet acquired <oxygen> in the <lungs>.<How Blood Flows through the Heart by NHLBI>[Heart valves and Adding oxygen to blood]{ref}
At a junction with negligible storage, <conservation of mass> gives
$$
Q_{\rm trunk}=Q_{\rm left}+Q_{\rm right}.
$$
A constructed total <blood flow> of $5$ L/min could therefore divide into $3$ and $2$ L/min. Measuring the total does not establish equal flow to the two <lungs>. In a simple steady model with a common upstream and downstream <pressure>, branch <blood flow> rates depend on their respective <vascular resistance> values; the shared inlet does not force the resistances to be equal.
The <pulmonary trunk> also supplies an important distinction between two measurements. <Right ventricular ejection fraction> describes change in a chamber volume, whereas forward <blood flow> in the <pulmonary trunk> describes transport through a vessel. They agree through <stroke volume> and <heart rate> only under the relevant balance assumptions, including negligible backward flow. An anatomical connection is therefore the starting point for a flow model, rather than a substitute for its assumptions.
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