All this and your heart weighs about a pound!

Blood Circulation in the Heart

The Right Side

Deoxygenated blood returns from the superior and inferior branches of the vena cava (see Figure), and drains into the right atrium which is normally at a very low pressure. During diastole, the relaxation phase of the cardiac cycle, the pressure in the right ventricle drops to near zero. The pressure gradient formed between the right atrium and ventricle, plus slight contraction by the atrium, causes blood to flow into the ventricle. As the ventricle fills, the blood passes through the tricuspid valve, pushing its leaflets aside. As systole, the pumping phase, starts, the ventricle starts to contract, increasing intraventricular pressure. This causes the leaflets of the tricuspid valve to snap shut, and the cusps of the pulmonary valve to open. Blood then flows out of the ventricle through the pulmonary artery and on to the lungs. As the ventricle relaxes, intraventricular pressure drops below the pressure in the pulmonary artery, causing the pulmonary valve to close. In this way, blood returns from the body to the right side of the heart and is pumped to the lungs for gas exchange.



The Left Side

The story on the left side of the heart is practically identical to the right, but the names change. Oxygenated blood from the lungs arrives at the left atrium from the pulmonary vein. During diastole, blood flows through the mitral valve into the left ventricle. As systole begins, intraventricular pressure rises, shutting the mitral valve and opening the aortic valve. Blood flows out the aorta to feed the systemic circulation. As the ventricle again begins to relax, the aortic valve snaps shut and the ventricle begins to fill.

While the basic geometry and function of the two sides of the heart are very similar, they have different jobs. The right supplies blood only to the lungs, while the left must pump blood to the rest of the body. It is not surprising that the left side of the heart must develop pressures ten times higher than those on the right side. This must certainly contribute to the fact that heart valves on the left side of the heart are most often affected by disease.

Background on Heart Valves and Their Function

Basic Heart Physiology (Cont'd)

The Valves

There are four valves in the heart, one at the exit of each chamber. In order of blood flow they are the Tricuspid (right atrium), Pulmonary (right ventricle), Mitral (left atrium) and Aortic (left ventricle). Due to the higher pressure gradients, the mitral and aortic valves are usually affected most by disease. We have focused our research efforts on the aortic valve, particularly porcine and human valves.

The aortic valve is one of the two semi-lunar valves, named for the shape of its cusps. The cusps are cup-like, passive soft tissue structures attached to the wall of the aorta, in a region called the aortic root. The root and a cut-out schematic of the aortic valve are shown below. As the ventricles contract during systole, the three cusps are pushed aside, towards the walls of the aorta and blood flows through the valve to the body. As the ventricles relax, the pressure in the left ventricle drops, and blood begins to flow backwards. This causes the cusps to snap together, preventing regurgitation.

This figure shows the aortic valve in its normal orientation with one cusp removed. The left ventricle is below, and the aorta is above. Blood flows up during systole. Another feature to note is the presence of three aortic sinuses, one for each cusp. These are indentations, or bulges, in the aortic root which change the fluid dynamics of the valve significantly. It is believed that the sinuses cause the formation of vortices that aid in valve closure. The two coronary arteries branch out from two of the sinuses, termed the left coronary sinus and right coronary sinus. It's not a surprise that the other sinus is called the non-coronary or posterior sinus. The cusps are named in a similar manner.

This view shows the aortic valve cut open longitudinally to show all three cusps and both coronary arteries. The orientation is the same as for the previous diagram. Notice the posterior leaflet of the Mitral valve at the bottom. The anterior mitral leaflet is not shown. This schematic is of a porcine aortic valve, and you can tell because one of the only anatomical differences between human and porcine hearts is visible here. The gray area at the bottom of the right coronary cusp represents myocardium. In the pig, this muscle contacts the cusp, forming a muscular shelf. Humans have no such muscular shelf.

If we take a closer look at an aortic cusp, we can identify some of it's features. Along the top of the cusp is the free edge, the part of the cusp that 'flaps in the breeze' during blood flow. Just in from the free edge along the upper portion of the cusp is the coaptation region which is the portion that contacts the neighbouring cusps. The curved bottom portion connects the cusp to the aortic wall. The regions where the free edge meets the aorta are called the commissures. The corpus arantii(or nodulus of Arantus) is a large collagenous mass in the coaptation region which supposedly aids in valve closure and reduces regurgitation.

The cusp is a complex, multi-layer structure. The two main structural layers are the fibrosa that covers the whole cusp and the ventricularis that covers all but the coaptation region. The fibrosa's structure is oriented in a circumferential direction, corresponding to left-to-right in the diagram above. The ventricularis is less highly organized.

In cross-section, the cusp has three distinct layers, the fibrosa, spongiosa and ventricularis. The fibrosa is on the top surface of the cusp if looking down from the aorta, with the ventricularis on the bottom. The fibrosa is considered to be the primary structural layer and contains a large amount of collagen organized into large bundles and fibers which are oriented in a circumferential direction. This directionality results in a structure that is considerably stiffer in the circumferential direction than the radial. A matrix of elastin surrounds the collagen bundles to maintain the valve's microstructure during unloading. Collagen and elastin are connective tissue proteins found in most parts of the body. Collagen is formed in long fibrils and is the primary component of tendons and ligaments and is both strong and stiff. Elastin, as the name implies, is considerably less stiff than collagen. It occurs in large quantities in blood vessels, notably arteries.

The ventricularis, while less organized than the fibrosa, contains a significant amount of collagen and elastin. However, because the collagen is not oriented in any specific direction, it tends to be less stiff than the ventricularis.

The spongiosa is perhaps the least studied layer because it is more of a gap between the other two than a definable structure on its own. It is primarily water, but also contains glycosaminoglycans (GAG's) and small amounts of collagen and elastin which connect the fibrosa and ventricularis together.