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Why is there a structural difference between the bicuspid and tricuspid valve?

Why is there a structural difference between the bicuspid and tricuspid valve?


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There are three cusps in the tricuspid valve and only two in the bicuspid valve? Why is there such a structural difference? Does it have anything to do with that there is oxygenated blood on the left side and deoxygenated on the right?


Heart Valve Structure and Function in Development and Disease

The mature heart valves are made up of highly organized extracellular matrix (ECM) and valve interstitial cells (VIC) surrounded by an endothelial cell layer. The ECM of the valves is stratified into elastin-, proteoglycan- and collagen-rich layers that confer distinct biomechanical properties to the leaflets and supporting structures. Signaling pathways have critical functions in primary valvulogenesis as well as maintenance of valve structure and function over time. Animal models provide powerful tools to study valve development and disease processes. Valve disease is a significant public health problem and increasing evidence implicates aberrant developmental mechanisms underlying pathogenesis. Further studies are necessary to determine regulatory pathway interactions underlying pathogenesis in order to generate new avenues for novel therapeutics.


What causes tricuspid regurgitation?

Tricuspid regurgitation often results from an enlarged lower heart chamber (right ventricle).

Other diseases also may cause tricuspid regurgitation, most commonly infective endocarditis (valve infection), and less commonly, Marfan syndrome, rheumatoid arthritis, rheumatic fever (PDF) (link opens in new window) , injury, carcinoid tumors, and myxomatous degeneration.

Tricuspid regurgitation also is associated with the use of the diet drug &ldquoFen-Phen&rdquo (fenfluramine and phentermine).


Difference Between Atria and Ventricles

Atria vs Ventricles

Atria (pl. atrium) refer to the upper chambers of the heart (2 in number) that receive the impure blood from the veins to send it to the ventricles. On the other hand, ventricles are small cavities or chambers that are present within an organ, usually the left chamber of the heart that accepts blood from the arteries (left atrium) and then contracts to force into the aorta. The right chamber of the heart accepts deoxygenated blood that is carried by the right aorta. There are 4 chambers in the heart and the atria refer to the upper chambers, whereas the ventricles refer to the lower chambers. The right part of our heart has an atrium and one ventricle, while the case is the same for the left side too.

The walls of the ventricles are thicker, while that of the atria are thinner. They however contain valves to pump the blood in and out of the heart. The walls of the heart, including the atria and the ventricles are functional in ensuring effective working of the circulatory system. The walls of the heart are made of 3 layers of tissues ‘“ myocardium, endocardium and epicardium. The function of the right atrium is to receive deoxygenated blood from the veins. Its oxygen has been given to the tissues in return collecting CO2 and tissue waste materials. Deoxygenated blood is transferred from the upper part of the body to the atrium by the SVC or superior vena cava. The IVC or inferior vena cava brings deoxygenated blood from the lower part of the body into the atrium.

The tricuspid valve of the right atrium helps in the storage of blood, for the heart to pump it within the right side of the ventricle for preventing the blood from flowing back, as well as ensure effective cardiac functionality. The function of the left atria is to accept the purified blood from lungs from the pulmonary veins. The mitral or bicuspid valve helps prevent the blood from flowing backwards to the left part of heart until the left side of the atrium pushes blood to the left of the ventricle.

The right ventricle functions by depositing deoxygenated blood that is contained in the right atria. The right ventricle pumps blood into the lungs for purifying it. Of course the purification process is forwarded by the pulmonary valve. Pulmonary arteries transport blood into the lungs. The function of the muscled left ventricles is to receive oxygenated blood that has been pumped within left atria in the body.

Summary:
1. The atria stand for the upper chambers of the heart, while the ventricles are the lower chambers.
2. Atria act as receptors of deoxygenated blood, while ventricles receive blood from the left atria and force it into the aorta.
3. Atria have strictly to do with the inner chambers of the heart, while ventricles may even refer to the interconnected brain cavities.
4. The walls of the atria are thinner with low blood pressure, while those of the ventricles are thicker with high blood pressure.


Conclusions:

Data from this preliminary, registry-based study of propensity-matched patients who underwent TAVR for AS demonstrated that patients with bicuspid versus tricuspid AS had no significant difference in 30-day or 1-year mortality, but had increased 30-day risk for stroke. Because of the potential for selection bias and the absence of a control group treated surgically for bicuspid stenosis, the authors concluded that randomized trials are needed to adequately assess the efficacy and safety of TAVR for bicuspid AS.


What are the Heart Valves?

The heart valves are sets of flaps (leaflets or cusps) that prevents the movement of blood against the direction of flow – from an atrium into a ventricle and then out into the artery (pulmonary artery or aorta).

There are two sets of valves :

  1. Valves between the atria and ventricles known as the atrioventricular (AV) valves
  2. Valves between the ventricles and blood vessels exiting in (pulmonary artery or aorta) known as the semilunar valves.

There are two atrioventricular valves and two semilunar valves :

  • Atrioventricular Valves
  • Tricuspid valve between the right atrium and right ventricle
  • Mitral valve between the left atrium and left ventricle
  • Pulmonary valve (also known as the pulmonic valve) between the right ventricle and pulmonary artery
  • Aortic valve between the left ventricle and aorta


Structure of the Heart

The heart muscle is asymmetrical due to the distance blood must travel in the pulmonary and systemic circuits. Since the right side of the heart sends blood to the pulmonary circuit, it is smaller than the left side which must send blood out to the whole body in the systemic circuit, as shown in Figure 1.

Figure 1. The heart is primarily made of a thick muscle layer, called the myocardium, surrounded by membranes. One-way valves separate the four chambers.

In humans, the heart is about the size of a clenched fist, and it is divided into four chambers: two atria and two ventricles. There is one atrium and one ventricle on the right side and one atrium and one ventricle on the left side. The atria are the chambers that receive blood, and the ventricles are the chambers that pump blood. The right atrium receives deoxygenated blood from the superior vena cava, which drains blood from the jugular vein that comes from the brain and from the veins that come from the arms, as well as from the inferior vena cava which drains blood from the veins that come from the lower organs and the legs.

In addition, the right atrium receives blood from the coronary sinus which drains deoxygenated blood from the heart itself. This deoxygenated blood then passes to the right ventricle through the atrioventricular valve or the tricuspid valve, a flap of connective tissue that opens in only one direction to prevent the backflow of blood. The valve separating the chambers on the left side of the heart valve is called the bicuspid or mitral valve. After it is filled, the right ventricle pumps the blood through the pulmonary arteries, by-passing the semilunar valve (or pulmonic valve) to the lungs for re-oxygenation.

After blood passes through the pulmonary arteries, the right semilunar valves close preventing the blood from flowing backwards into the right ventricle. The left atrium then receives the oxygen-rich blood from the lungs via the pulmonary veins. This blood passes through the bicuspid valve or mitral valve (the atrioventricular valve on the left side of the heart) to the left ventricle where the blood is pumped out through aorta, the major artery of the body, taking oxygenated blood to the organs and muscles of the body. Once blood is pumped out of the left ventricle and into the aorta, the aortic semilunar valve (or aortic valve) closes preventing blood from flowing backward into the left ventricle. This pattern of pumping is referred to as double circulation and is found in all mammals.

Practice Question

Which of the following statements about the heart is false?

  1. The mitral valve separates the left ventricle from the left atrium.
  2. Blood travels through the bicuspid valve to the left atrium.
  3. Both the aortic and the pulmonary valves are semilunar valves.
  4. The mitral valve is an atrioventricular valve.

The heart is composed of three layers the epicardium, the myocardium, and the endocardium, illustrated in Figure 1. The inner wall of the heart has a lining called the endocardium. The myocardium consists of the heart muscle cells that make up the middle layer and the bulk of the heart wall. The outer layer of cells is called the epicardium, of which the second layer is a membranous layered structure called the pericardium that surrounds and protects the heart it allows enough room for vigorous pumping but also keeps the heart in place to reduce friction between the heart and other structures.

Figure 2. Blood vessels of the coronary system, including the coronary arteries and veins, keep the heart musculature oxygenated.

The heart has its own blood vessels that supply the heart muscle with blood (Figure 2). The coronary arteries branch from the aorta and surround the outer surface of the heart like a crown. They diverge into capillaries where the heart muscle is supplied with oxygen before converging again into the coronary veins to take the deoxygenated blood back to the right atrium where the blood will be re-oxygenated through the pulmonary circuit. The heart muscle will die without a steady supply of blood. Atherosclerosis is the blockage of an artery by the buildup of fatty plaques. Because of the size (narrow) of the coronary arteries and their function in serving the heart itself, atherosclerosis can be deadly in these arteries. The slowdown of blood flow and subsequent oxygen deprivation that results from atherosclerosis causes severe pain, known as angina, and complete blockage of the arteries will cause myocardial infarction: the death of cardiac muscle tissue, commonly known as a heart attack.


Valves

The heart consists of four valves to control the flow of blood.   Each valve has the same basic structure although each one is unique down to fine detail.

  •         ATRIO-VENTRICULAR = Tricuspid and Mitral (Bicuspid)
  •         SEMI-LUNAR = Aortic and Pulmonary

The atrio-ventricular (AV) valves prevent the backflow of blood from the ventricles to the atria during systole (contraction). The valves are held in place by Chordae Tendinae, bands of fibrous tissue that attach to the cusps of each valve and to the papillary muscles located in the walls of the ventricles. However it is not the chordae tendinae and the papillary muscles that are responsible for the opening and closure of the valves, but the pressure gradient created across them.

During diastole (relaxation) of the ventricles, the AV valves are open allowing the ventricles to fill with blood from the atria. As the ventricles fill, the intra-ventricular pressure rises and as they enter systole, the AV valves are forced to close due to the pressure gradient. The closure of these valves creates the ‘lub’ sound or S1 phase of the heart sound. On the right side of the heart the AV valve is called the Tricuspid valve. On the left, it is called the Mitral or the Bicuspid valve.

  •     Mitral valve allows blood to flow from the left atrium to the left ventricle. It has two cusps (bicuspid).
  •     Tricuspid valve allows blood to flow from the right atrium to the right ventricle and has three cusps.

The rising ventricular pressure forces the semi-lunar valves to open. These include the aortic valve and the pulmonary valve leading to the aorta and pulmonary trunk respectively. After ventricular systole and the ventricles relax again, the pressure drops rapidly and the semi-lunar valves close. Closure of these valves creates the second heart sound (S2) or ‘dub’ sound of a heartbeat.


Anatomy and Physiology: The Four Chambers

So why do you need four chambers if three worked just fine for frogs and lizards? Humans, and indeed all mammals (not to mention birds!), are endothermic (warm blooded). Warm bloodedness requires a great deal of oxygen, for the oxygen is used to generate both ATP and heat. A four-chambered heart is an enormous evolutionary advantage over a three-chambered heart. To understand this, you need to look at the chambers and the circuits together.

Medical Records

Some babies are born with a ventricular septal defect, which means an opening between the left and right ventricles, which means that their hearts are acting like three-chambered hearts. Surgery to correct the defect is necessary in order for the child to live a normal life.

Remember the fish, with an atrium to receive blood from the body, and a ventricle to pump it out again? Well, with a three-chambered heart there are two ventricles and one atrium. The two atria emphasize a higher degree of separation between two of the circuits: the pulmonary circuit and the systemic circuit. At this point you need to start thinking of the heart in terms of left and right. The right atrium receives deoxygenated blood (low in O2, and high in CO2) from the systemic circuit, and the left atrium receives oxygenated blood (high in O2, and low in CO2) from the pulmonary circuit.

Medical Records

Don't forget that left and right in all these discussions always means the patient's left and right, which means you need to pay attention to whether any diagrams are in anterior or posterior view!

This advance was only so good, however, because both atria pump the blood to the single ventricle. In a three-chambered heart the blood pumped out of the ventricle is a mixture of both oxygenated and deoxygenated blood. This blood is pumped out to both the pulmonary and the systemic circuit (in truth, because it is pumped right back to the tissues of the heart, it really goes to all three circuits). For ectothermic (cold blooded) animals that is plenty of oxygen, but it's just not enough for you.

Birds and mammals evolved a ventricular septum, turning one ventricle into two. The result is the evolution of entirely separate pulmonary and systemic circuits (see Figure 11.2). The blood sent to the lungs is completely deoxygenated, and the blood pumped out to the rest of the body is fully oxygenated. The evolution of two ventricles, making a four-chambered heart, doubled the amount of O2 being sent to the tissues. The amount of food and waste in the blood going to the systemic circuit is not so cut and dried (see Cardiovascular and Lymphatic Circulation).

In the human heart the right atrium sends deoxygenated blood from the body to the right ventricle, which then pumps it to the lungs (pulmonary circuit). The left atrium sends oxygenated blood from the lungs to the left ventricle, which then pumps it to the body (systemic circuit).

Figure 11.2 The human heart has four chambers, which equally separate the right and left sides of the heart, maximizing the oxygen content of the blood being sent to the systemic circuit. (LifeART1989-2001, Lippincott Williams & Wilkins)

Blood Vessels and Chambers

When you look at the orientation of the heart at the bottom of the thoracic cavity (see The Respitory System to learn about the pericardium) you will see that, rather than being straight up and down, the heart is at an angle, and a bit twisted (kind of like me!). This is due in part to making room for the liver, and in part to the location of the many blood vessels that attach to the heart.

Figure 11.2 shows the blood vessels connected to the heart, but you may find the flowchart in Figure 11.3 a bit easier to understand. Don't forget that the blood flow in the pulmonary and systemic circuits is continuous, meaning that blood from one circuit moves on immediately to the other circuit. Next, the central location of the heart means that blood going to the lungs needs to be pumped both left and right, and blood going to the body needs to be pumped both up and down. Thinking in terms of opposites will help you to remember the vessels.

Figure 11.3 This flowchart illustrates the flow of blood, in terms of opposite directions, to and from both the systemic and pulmonary circuit. (Michael J. Vieira Lazaroff)

Remember, there is no particular place where all of this starts, given that the circuits are continuous. Let's start with the oxygenated blood in the arteries of the systemic circuit, leaving the left ventricle of the heart via the aorta. Immediately after leaving the top of the heart, all blood vessels enter and leave through the top of the heart, the aorta arches downward to send blood to the lower body. At the top of the arch there are three large branches that go to the upper body in this way, the systemic circuit is divided in two.

Crash Cart

A common mistake is to define arteries as vessels carrying oxygenated blood, and veins as vessels carrying deoxygenated blood. Although this is generally true, there are two important exceptions, because of the true definitions of arteries carrying blood away, and veins carrying blood to the heart. The two exceptions, which make perfect sense, both involve the pulmonary circuit: The pulmonary arteries carry deoxygenated blood to the lungs to be oxygenated, and the pulmonary veins carry the newly oxygenated blood away from the heart!

After picking up and delivering various materials in the capillaries of the upper and lower body, becoming deoxygenated in the process, the veins drain into the largest veins in the body, the superior vena cava and the inferior vena cava. Anyone who works with quadrupedal animals should know that those same vessels are called the anterior and posterior venae cavae (plural for vena cava). The venae cavae drain into the upper and lower portions of the right atrium since the right atrium is in the upper third of the heart, this inferior vena cava is still considered attached to the top of the heart.

As the right atrium contracts, the blood must pass through a valve between the atrium and the ventricle. This valve is called the tricuspid valve (for its three cusps or flaps), or the right atrioventricular (AV) valve. Once the blood is pumped out of the right ventricle, the right AV valve prevents backflow into the right atrium. The contraction of the right ventricle does pump the blood through another valve, the pulmonary semilunar valve (named for its half moon shape), and into the pulmonary trunk. Just as the aorta splits, so does the pulmonary trunk, but this time the blood splits into the left and right pulmonary artery, in order to go to both lungs. (To see what happens next, take a deep breath and read up on the respiratory system in The Respitory System.)

Flex Your Muscles

A good way to remember the difference between the two atrioventricular valves?the tricuspid (right AV valve) and the bicuspid (left AV valve)?is to think about the dissolved gases in the blood as it passes through those valves. The deoxygenated blood passing through the tricuspid valve contains CO2, which contains three atoms (tri = three), and the oxygenated blood passing through the bicuspid valve contains O2, which contains two atoms (bi = two). A pretty cool coincidence, considering the valves were named because of their structure!

Blood returning to the heart always returns from separate vessels, whereas blood leaving the heart always leaves from a single vessel and then splits to go in opposite directions. Having vessels in pairs makes sense, but single vessels leaving the heart? Why? Think about the shape of the heart. The cone shape of the apex gives a hint about the way the heart contracts. The contraction of the ventricles, which happens simultaneously, narrows the lumen of the ventricles, as well as shortening the length of the ventricles, which pumps the blood up! It is more efficient, in ensuring the equal flow to both lungs, for example, to have the blood leave one vessel, only to split later.

Oxygenated blood returns from the two lungs through the pulmonary veins, which attach to opposite sides of the left atrium. The rest of the trip is almost the same as on the right side: the left atrium pumps the blood through the left AV valve (or bicuspid valve) into the left ventricle, and the ventricle pumps the blood through the aortic semilunar valve into the aorta.

Just as the ventricular walls are thicker than the atrial walls (because of the difference in the distance the blood is pumped), the left ventricle, which has to pump to the entire body, has thicker walls than the right ventricle, which pumps blood only to the neighboring lungs. The thick left ventricular walls also provide a greater pressure on the left AV valve with each ventricular contraction. This valve, also called the mitral valve, can sometimes bulge into the left atrium, which is called mitral valve prolapse.

To help prevent such prolapses, there are fibrous, tendon-like cords called chordae tendineae. These connective tissue cords support the valve whenever the ventricles contract. Every time a ventricle contracts, there must be enough pressure in the contraction to exceed the pressure in the pulmonary trunk or the aorta, and thus push through the semilunar valves. This puts a great strain on the AV valves, so in addition to the chordae tendineae, there are small muscles attached to the bottom of the chordae tendineae, called papillary muscles, that contract whenever the ventricles contract.

So just one question: why are there no valves where the blood enters the atria? There are two reasons for this. The first is that the blood in veins returning to the heart is at extremely low pressure, so low that it could not easily push through the closed valves already in the veins. The other reason involves the weaker contraction of the atria. The atria contract when the ventricles are relaxed, which means that the lower pressure of the ventricles at that point will make it easier for the blood to flow in that direction than backward into the veins that are filled with blood.

Excerpted from The Complete Idiot's Guide to Anatomy and Physiology 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.


Why is there a structural difference between the bicuspid and tricuspid valve? - Biology

The aortic valve separates the left ventricular outflow tract from the ascending aorta. The aortic valve has also been called the left semilunar valve and the left arterial valve and has three leaflets, or cusps: the left coronary cusp, the right coronary cusp, and the non-coronary cusp. The inlets to the coronary artery system can be found within the sinus of Valsalva, superior to the the leaflet attachments and inferior to the sinotubular junction. The left coronary ostium is found midway between the commissures of the left coronary cusp, and almost immediately branches into the anterior interventricular branch and the circumflex branch. The right coronary ostium is found above the right coronary cusp and gives rise to the right coronary artery. The final cusp is named the non-coronary cusp and is positioned posteriorly relative to the other two cusps.

The mitral valve is also called the bicuspid valve and the left atrioventricular valve. As the name bicuspid valve may suggest, the mitral valve is considered to have two primary leaflets: the anterior and posterior leaflets. The anterior leaflet has also been called the septal, medial, or aortic leaflet, while the posterior leaflet is also referred to as the lateral, marginal, or mural leaflet. Each leaflet is then further broken down into scallops divided by commissures, or zones of apposition. Due to the high variability of leaflet and scallop anatomy, and an alphanumeric nomenclature has been proposed by Carpentier that breaks the leaflets into regions. Three regions are found on the anterior leaflet (A1-A3) with opposing regions on the posterior leaflet (P1-P3). The subvalvular apparatus of the mitral valve consists of chordae tendinae attaching to the anterior and posterior papillary muscles of the left ventricle.

The relative positions of the aortic, mitral, pulmonary, and tricuspid valves are shown in the diagram of the heart at the center of the figure. The aortic valve has three cusps: the left coronary cusp (LCC), the right coronary cusp (RCC), and the non-coronary cusp (NCC). The mitral valve has an alphanumeric nomenclature that numbers from the anterior to the posterior, with respect to the heart, and attaches an A or a P in front of the anterior or posterior leaflets, respectively (A1-A3, P1-P3). The pulmonary valve has three cusps: the anterior cusp (AC), the left cusp (LC), and the right cusp (RC). The tricuspid valve has three leaflets named the anterior (A), septal (S), and posterior (P).

The pulmonary valve separates the right ventricular outflow tract of the right ventricle from the pulmonary trunk. The pulmonary valve can also be referred to as the pulmonic valve, the right semilunar valve, and the right arterial valve. Its three leaflets, or cusps, are difficult to name because of the oblique angle of the valve. Its nomenclature is therefore derived based on the nomenclature of the aortic valve, which lies in proximity to it. The two leaflets attached to the septum are named the left and right leaflets, and correspond to the right and left leaflets of the aortic valve, which they face. The third leaflet is called the anterior leaflet or the non-coronary leaflet (to maintain the nomenclature of the aortic valve).

The tricuspid valve, also called the right atrioventricular valve, gets its name because it is generally considered to have three leaflets: the anterior, posterior and septal leaflets. Of these, the anterior, also called the infundibular or anterosuperior, leaflet is typically the largest. The posterior leaflet is also referred to as the inferior or marginal leaflet and the septal leaflet is also referred to as the medial leaflet. Terminating on the ventricular side of the tricuspid valve leaflets, the chordae tendinae are connected to three papillary muscles in the right ventricle. In humans, the three papillary muscles of the right ventricle have highly variable anatomy. The anterior papillary muscle is usually the most prominent, with the moderator band terminating at its head. The moderator band typically originates from the septal papillary muscle. The septal papillary muscle is normally the least prominent, and is missing 21.4% of the time.