Blood pressure generally refers to the arterial pressure in the systemic circulation. However, measurement of pressures in the human venous system (Figure 2) and the pulmonary vessels plays an important role in intensive care medicine and is physiologically important in ensuring proper return of blood to the heart, maintaining flow in the closed circulatory system.
Systemic Venous Pressure
Venous pressure is the vascular pressure in a vein or in the atria of the heart. It is much less than arterial pressure, with common values of 5 mm Hg in the right atrium and 8 mm Hg in the left atrium. Variants of venous pressure include:
- Central venous pressure, which is a good approximation of right atrial pressure, which is a major determinant of right ventricular end diastolic volume.
- Jugular venous pressure (JVP) which is the indirectly observed pressure over the venous system. It can be useful in the differentiation of different forms of heart and lung disease (Figure 1).
- Portal venous pressure or the blood pressure in the portal vein. It is normally 5–10 mm Hg.
Pooling and Venous Return
The main role of veins is to facilitate return of blood to the heart. This is assisted by the action of the skeletal-muscle pump, and by the thoracic pump action of breathing during respiration. These mechanical forces increase pressure as normal venous muscular tone is minimal. Standing or sitting for a prolonged period of time can cause low venous return in the absence of the muscle pump, resulting in venous pooling (vascular) and shock. Fainting can occur but usually baroreceptors within the aortic sinuses initiate a baroreflex triggering angiotensin II and norepinephrine release and consequent vasoconstriction and heart rate increases to augment blood flow return. Neurogenic and hypovolemic shock can also cause fainting. In these cases, the smooth muscles surrounding the veins become slack and the veins fill with the majority of the blood in the body, keeping blood away from the brain and causing unconsciousness. Jet pilots wear pressurized suits to help maintain their venous return and blood pressure as high speed maneuvers increase venous pooling in the legs. Pressure suits specifically squeeze the lower extremities increasing venous return to the heart. This ensures that end diastolic volumes are maintained and that the brain will receive adequate blood preventing loss of consciousness.
Vein Structure and Pressures
In general, veins function to return deoxygenated blood to the heart, and are essentially tubes that collapse when their lumens are not filled with blood. The thick outermost layer of a vein is made of connective tissue, called tunica adventitia or tunica externa. Deeper are bands of smooth muscle called tunica media, which are, in general, thin, as veins do not function primarily in a contractile manner. The interior is lined with endothelial cells called tunica intima. Veins often display a lot of anatomical variation compared with arteries within a species and between species. For these reasons, the ability to generate pressure in the veins is significantly compromised compared to arteries. The pressure within the circulatory circuit as a whole is denoted mean arterial pressure (MAP). This value is a function of the cardiac output (total of blood pumped) and the total peripheral resistance (TPR). TPR is primarily a function of the resistance of the systemic circulation. The resistance to flow generated by veins, due to their minimal ability to contract and reduce their diameter means that regulation of blood pressures by veins will be minimal in contrast to muscular vessels, primarily arterioles, that can actively contract, reduce diameter, and increase resistance and pressure. In addition, veins can easily distend or stretch. A vein's ability to increase in diameter in response to a given blood volume also contributes to the very low blood pressures within the venous segment of the circulatory system.