**4. The physiology and function of pulmonary veins**

In the species that have two atriums, two ventricles, and execute oxygen exchange via the lungs, the oxygenated blood is pumped from the aorta and sent into tissues. The oxygen, nutrients and metabolic products diffuse and exchange in the capillaries that converge and form the vein. Vena cava collect all the venous blood and return to right atrium, right ventricle and lungs. After oxygenation in the lung, these fresh, oxygen-rich blood is returned into left atrium via pulmonary veins, therefore complete the cycle of blood circulation.

Before we go deeper into more understanding of the pulmonary veins, there is an important concept that should be explained first. The cardiovascular system has several functions that are all indispensable to keep the body works normally. Maintaining the systemic arterial pressure is the first priority of the cardiovascular system, it means that the systemic arterial pressure is the last one that the decompensation occurs. The second one is to keep the cardiac output at an adequate level that can provide enough blood flow to the peripheral tissues. Maintaining the normal capillary pressure is the last priority, and therefore it is the reason that the first sign of heart failure is commonly those that associate with congestion [11]. In the cases of pulmonary vein abnormalities, although the pathophysiological mechanisms are different among diagnosis, the loss of normal capillary and venous pressure is often the end result of the developmental disorders. Patient is commonly presented to the clinic because of signs related to congestion. Therefore, we will discuss the pulmonary venous pressure in the next paragraph.

In the fetus, the pressure of the pulmonary system is higher compared to after birth because of very high pulmonary vascular resistance and resultant low pulmonary blood flow (only account for 10 to 15% of right heart stroke volume). The pulmonary vascular resistance falls after birth, and the pressure of pulmonary system drops to a lower level than the systemic circulation in normal setting [12]. In an experiment that studying normal dogs with light sedation, the mean pulmonary venous pressure (17.1 ± 6.5 mm Hg) is consistently slightly higher than mean left atrial pressure (13.4 ± 6.3 mm Hg), which is almost the same with mean pulmonary wedge pressure (13.3 ± 6.2 mm Hg). Considering that the lungs are a large organ that occupy the thorax cavity, the pulmonary venous pressure between locations that differ from altitude (distance from left atrium) is vary [13]. Generally, the pulmonary veins share the similar intravascular pressure with left atrium because there is no valve between them.

During ventricular systole and early diastole, the blood in the pulmonary veins flow into left atrium, and part of blood in the left atrium would regurgitates back into

#### *Pulmonary Vein: Embryology, Anatomy, Function and Disease DOI: http://dx.doi.org/10.5772/intechopen.100051*

pulmonary veins when the atrial active pumping that corresponds to the ventricular late filling phase. The changes of pulmonary venous profile among different cardiac cycle can be record by the echocardiographic Doppler examination [14]. It is therefore reasonable that any reason that elevates pressure of the left atrium has the potential to increase the pulmonary venous pressure, because of the higher impedance of draining blood forward and larger regurgitated volume from the high-pressured left atrium.

Another important characteristic of vessel that we cannot forget when we are discussing the hemodynamic is the vascular distensibility and compliance. Distensibility is an ability of vessel whose volume can increase or decrease for every increase or decrease intravascular pressure, and the compliance is equal to distensibility times the volume of blood in the given portion of the circulation. Because of the different wall constitution between veins and arteries, the distensibility of veins is about eight times larger than that of arteries. That is, the venous system can conserve more blood and only has slightly elevation of the intravascular pressure [15]. The pulmonary veins have similar distensibility to the systemic veins, meaning that the pulmonary venous pressure would not exceed the normal range before large amount of blood is congested in the pulmonary capillary and veins.

Various congenital and acquired cardiovascular diseases that affecting pulmonary veins themselves and the left atrium could lead to the congestion of pulmonary veins. They can be simply classified into conditions that cause obstruction or pulmonary overcirculation. Occlusions of one or more pulmonary veins, and the divided left atrium (like the CTS) are examples that pulmonary venous blood flow has difficulties to get through obstacles in its normal pathway and therefore causing high pressure to the rest part of pulmonary veins. In addition, pulmonary overcirculation caused by intra- or extra-cardiac left to right shunting (atrial and ventricular septal defects, patent foramen ovale, patent ductus arteriosus, and anomalous pulmonary venous connection and so on) also has the potential to causes pulmonary congestion because of larger than normal volume that circulates the pulmonary vasculature. Among them, CTS, TAPVC and PAPVC are three of the good examples that is closely related to the development of pulmonary veins. We will discuss these diseases in the following sections.
