Blood flow: Review

In order to grasp the concept of measuring and interpreting hemodynamic values, it is important to understand how blood flowing through the heart is related to the cardiac cycle.

Fluid moves from high pressure to lower pressure. 
Blood within the cardiovascular system adheres to this rule as evidenced by the direction of blood flow. The higher pressure generated by the left heart produces a gradient which moves blood from the left heart, through the body tissues to collect in the right side of the heart.

Diastole:

• The myocardium is relaxed. The atria and ventricles fill passively.
• AV valves allow blood to pass from the atria to the ventricles.
• The aortic and pulmonary artery semilunar valves are closed because the blood in those vessels is at a higher pressure than the ventricles.
• Blood continues to fill atria and ventricles, stretching the compliant heart muscle.

Systole:

• The atria contract and eject the final amount of blood into the ventricles. The atrial contraction contributes only about 10% to the total ventricular volume, while the patient is at rest. If the heart rate is high and the ventricles don't have time to fill completely, atrial systole can contibute as much as 40%.
• Atria relaxation causes atrial pressure to be lower than ventricular pressure.
• High ventricular pressure relative to the atria causes the AV valves to close, preventing backflow while the ventricles contract.
• The ventricles continue to contract, ejecting blood through the semilunar valves out to the lungs and rest of the body.

When the left ventricle (LV) contracts, it generates a systolic blood pressure of 100-140 millimeters of Hg (mm Hg).

• The aortic diastolic pressure is usually 60-90 mm Hg. The LV/aortic pressure gradient causes blood to pass through the aortic valve.
• Blood ejected from the LV to the aorta raises the aortic pressure to nearly equal to the LV pressure.
• A momentary aortic systolic pressure of 100-140 mm Hg is then dissipated across the capillary beds.
• Capillary pressure exceeds that of the venules. The capillary/venule gradient causes blood to flow into the low pressure venous system.
• Low pressure venous blood is returned to the right atrium, aided by skeletal muscle compression, negative intra-thoracic pressure and a multitude of one-way valves that advance the blood toward the vena cavae.

The pressure of blood within the right atrium is the central venous pressure (CVP). 
The blood pressure of the vena cavae is similar to the CVP because there are no valves or flow obstructions between the vena cavae (VC) and the RA. The VC and heart's right side can be viewed as one chamber with a contractile portion at the distal end. The CVP averages between 2-6 millimeters of mercury (mm Hg).

During right ventricular (RV) diastole, the pressure within the RV is between 0-5 mm Hg.
Elasticity and compliance of the ventricular myocardium help generate a lower intraventricular pressure. Lower intraventricular pressure, aided by atrial systole, causes blood to flow across the open atrioventricular AV valve.

Right ventricular systolic pressure is usually from 20-30 mm Hg. 
This exceeds the right atrial pressure. The pressure gradient applies greater pressure to the ventricular side of the AV valve, which causes it to close.

The pulmonary artery (PA) pressure, prior to systole, is normally 8-12 mm Hg. 
During RV systole the PA pressure will rise to equal the RV pressure, usually 20-30 mm Hg. The momentary systolic PA pressure of 20-30 mm Hg is quickly dissepated by the compliance of the pulmonary vascular bed and the PA returns to a diastolic pressure of 8-12 mm Hg.

Blood leaves the pulmonary vasculature at about 4-12 mm Hg, moving into the pulmonary veins. 
The pulmonary veins empty directly into the left atrium . Elasticity and compliance of the ventricular myocardium help generate a slightly lower intraventricular filling pressure. Lower intraventricular pressure, aided by atrial systole, causes blood to fill the left ventricle and stretch the ventricular myofibril to the left ventricle end-diastolic volume (LVEDV).

The LVEDV provides the cardiac myofibril stretching force (preload) that operates the Frank-Staring mechanism.  As Guyton put it, "When the cardiac muscle becomes stretched an extra amount, as it does when extra amounts of blood enter the heart chambers, the stretched muscle contracts with a greatly increased force, thereby automatically pumping the extra blood into the arteries."  Conversely, a reduction of intravascular volume that reduces venous return and LVEDV will reduce stroke volume and cardiac output. 

LV systole generates 100-140 mm Hg.
Aortic pressure during diastole is usually 60-90 mm Hg (afterload). This is the pressure the left ventricle must overcome to open the aortic valve and eject blood into the aorta. The pressure gradient of systolic 100-140/60-90 mm Hg drives blood into the aorta and onward to the rest of the body. The cycle is complete.

References

Guyton A.C. (1986). Textbook of Medical Physiology. London: W. B. Saunders

Tkacs, Nancy C., et al. (2020).  Advanced Physiology and Pathophysiology: Essentials for Clinical Practice. Springer Publishing Company.

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