This lesson contains background about the blood vascular system and the heart. Also, the different sizes of capillaries, veins, and arteries, and how they affect blood flow through the system. We will then proceed to talk about the heart's function in the blood vascular system. This will lead into a discussion of heart valves, how they work and what might cause them to fail. Then we will discuss prosthetic heart valves.

This lesson examines the need for artificial heart valves so students will understand what the design requirements for such a device are. Engineers must understand what is required of a device before they build it. capillaries veins arteries prosthetic devices valves vascular system biomedical engineering Identify the difference between capillaries, veins, and arteries. Explain how valves in the heart work. Discuss what a prosthetic valve is and how it works. Explain how engineering contributes to solving problems in the body. The teacher should define what a valve is and then specifically how a one-way valve works. The teacher should specify that it allows flow in only one direction and prevents fluid from flowing back where it came from, if there is a force (e.g. gravity, etc.) that would cause backflow. The teacher should give some examples where this is beneficial (i.e. preventing drainage water from flowing back into drinking water, when donating blood to keep the blood from flowing back into the vein, speaking valves for people on ventilators, etc.). What other things in our everyday lives might act as one-way valves? (some possible answers might include one-way doors and the oxygen regulator on a scuba mask) This will allow the students to brainstorm and sharpen their understanding of what a one-way valve is. Arteries, veins, and capillaries have one main thing in common . . . they are all blood vessels. A vessel in the general sense is a hollow instrument for carrying something. A blood vessel, therefore, is a hollow instrument for carrying blood. The length of all of an average child's blood vessels put together would be over 60,000 miles long. The length of an average adult's blood vessels would be over 100,000 miles long (more than 4 times around the equator). Arteries are the biggest blood vessels in the body. They serve the purpose of bumping the blood directly from the heart to the main dorsal artery. This artery then branches into smaller arteries that help supply each region of the body with freshly oxygenated blood. Arteries are tough on the outside and smooth on the inside. They have to carry blood at relatively high pressures and so they must be able to withstand pressure. Three layers of tissue make up an artery: The outer protective layer of tissue, the muscular middle, and the inner layer of epithelial (skin-like) cells. The muscular middle is elastic and very strong so that it can help the heart pump the blood through contraction and relaxation. The inside wall is smooth so that blood can flow more easily with no obstacles. The arteries deliver the oxygen rich blood to the capillaries. In contrast to arteries, capillaries are very thin, and fragile. They are actually only one epithelial cell thick, in order to aid in the exchange of oxygen and carbon dioxide between the blood being carried and the tissue of the body. The red blood cells release its fresh oxygen to the surrounding tissue. The tissue releases its waste carbon dioxide into the red blood cells. The capillaries then deliver the waste-rich blood to the veins for transport back to the heart and eventually other organs for disposal. Veins resemble arteries in make-up, but are much thinner and weaker because they do not have to carry the blood at such high pressure. Veins have the same three layers, but the layers are thinner, containing less tissue. Also, the middle muscle layer is not as strong as that of arteries. Waste-rich blood flows from the capillaries into the veins. The veins carry the waste-rich blood back to the heart and lungs. Valves inside the veins help make sure that no contaminated blood flows backward to contaminate the oxygen rich blood coming from the heart. These one-way valves act like gates to ensure that blood can only flow in one direction. These valves are also necessary to help blood flow against the force of gravity to travel up your legs, torso, and arms. URL: http://www.sci.mus.mn.us/heart/heart/pumping-f.htm Heart valves are like little doors into and out of your heart that control how much blood gets in and when it comes and goes. The heart contains four of these "doors" and they all only open one way. These are called one-way valves. The purpose of one-way valves is to makes sure the blood only flows in one direction. This ensures that oxygen rich blood is continuously being delivered to the body, while carbon dioxide is continuously being taken out of the body's blood. The heart is divided into four chambers. The two upper chambers are called atria and the two lower chambers are called ventricles. Blood is moved from one chamber to the next when the heart contracts. With each contraction, the valves also open to allow the blood to flow into the next chamber. The valves then shut as the heart expands to prevent blood flow backwards. This allows blood to be moved out of the heart and throughout the body. The waste-rich blood from the body enters the right atrium first. Once this chamber fills with blood, the atrium contracts, forcing the blood down through the tricuspid valve into right ventricle. Next, the ventricle contracts, pushing the blood to the lungs through the pulmonary valve to receive oxygen. The oxygen-rich blood returns to the left atrium of the heart and then it travels to the left ventricle through the mitral valve. From the left ventricle, the blood travels through the aortic valve to the large blood vessel called the aorta. The aorta then distributes blood to the rest of the body. Valve disease can occur when a valve stops functioning correctly. A valve might stop working so that it does not close all the way and allows leakage of blood backward. Alternatively, a valve may not open all the way and thus may keep blood from flowing as well as it could. Both of these problems cause the heart to have to work much harder to carry the same amount of blood through the body. Your heart is about the size of your fist An average adult contains about 5 quarts of blood You heart circulates your blood supply about 1,000 time each day There are two main types of prosthetic valves. The first kind is mechanical, and the second kind is bioprosthetic. URL: http://cape.uwaterloo.ca/che100projects/heart/files/allvalve.jpg Mechanical heart valves have gone through three main evolutions of design since the first one in 1952. Over the years, the evolutions in design have had the main goal of reducing blood clots in the heart and creating more central flow through the valve. The three different designs have been: the caged ball design, the tilting-disc valve, and the bileaflet valve that is primarily used today. The caged ball design uses a metal ball that is held in place by a welded metal cage. When the blood flows one way, it pushes the ball into the cage, which allows blood to flow around the ball and thus through the valve. However, when the blood flows the other way it pushes the ball over the valve opening and prevents blood from flowing backward. The main problem with this design is that the ball prevents blood flow from going centrally through the valve opening like it would in a natural valve. When blood flows non-centrally it causes the heart to work harder to compensate for the momentum lost when the blood has to change direction to flow around the ball. This type of blood flow also tends to cause more damage to red blood cells due to collisions with the ball. When red blood cells are damaged they release blood-clotting ingredients, requiring patients to take lifelong prescriptions of anticoagulants. Taking anticoagulants minimizes the chance of heart attacks, but makes patients very vulnerable to any type of trauma that might cause internal or external bleeding, because the blood would not be able to clot enough to stop the loss of blood. URL: http://cape.uwaterloo.ca/che100projects/heart/files/ballvalve.jpg The tilted-disc design consists of a polymer disc held in place by two welded struts on either side of the disc. The struts are attached in such a way so that the disc only rotates open when the blood is flowing forward. As soon as the blood starts to flow backwards, the disc rotates closed. This design vastly improved on the caged ball design because it allowed improved central flow by opening at an angle of 60 degrees. Yet it still closed at a rate of 70 times/minute and thus prevented backflow of blood. The tilting-disc also allowed decreased mechanical damage to the red blood cells. Thus, blood clotting and infection became less of a problem with this design than with the caged ball design. The new design flaw then became that outlet struts of the tilting-disc would fracture due to fatigue from the repeated slamming of the disc. URL: http://cape.uwaterloo.ca/che100projects/heart/files/convexo.jpg The bileaflet valve consists of two semicircular discs that are attached near the center of the valve on hinges. They swing open to be parallel to the flow of blood and create a rectangular tunnel to help centralize the flow of blood when the blood is flowing forward. When the blood flows back they shut together to cover the circular valve opening. The leaflets are very strong and exhibit excellent biocompatibility, by not damaging tissue or red blood cells and not causing clotting or infection. However, the two semicircular discs do not close completely and allow some leakage when the blood flows backward. Backflow is one of the main reasons prosthetic valves are used in the first place, so these valves are less than ideal. URL: http://cape.uwaterloo.ca/che100projects/heart/files/bileaflet.jpg The most commonly used materials for mechanical valves today are: stainless steel alloys molybdenum alloys pyrolitic carbon (for valve housings and leaflets) silicone polyester (for sewing rings which are attached to the valve and are used to sew the valve into the heart) Mechanical heart valves are good for people that do not want to have a replacement heart valve surgery in the future because they are very durable and typically last the lifetime of the patient. However, they cause an increased risk of blood clotting and thus force the patient to have to take anti-coagulant for the rest of their life, effectively making them borderline hemophiliacs. Bioprosthetic heart valves are also known as prosthetic tissue valves. The design of bioprosthetic valves is much closer to natural heart valves, giving them many advantages over mechanical valves. Bioprosthetic valves do not require patients to take anticoagulants for life, result in better hemodynamics, do not cause damage to blood cells, and do not suffer from many of the structural problems experienced by the mechanical heart valves. Bioprosthetic valves can be divided into two groups: human tissue valves, and animal tissue valves. Human tissue valves can be further subdivided into homografts and autografts. Homografts are human heart valves transplanted from another person. Autografts are valves transplanted from one position to another within the same person. Homografts are valves transplanted from deceased people to recipients. The recipients generally do not have much problem with the body rejecting the valve and do not require immunosuppressive therapy. Once a homograft is donated it must be frozen in liquid nitrogen to preserve it until it is needed for implantation. In cases where the valve implant dimensions closely resemble the patient's valve size, homografts tend to have good blood flow characteristics and durability. Most commonly autografts consist of a patient's pulmonary valve being transplanted to the aortic position to replace a diseased aortic valve. A homograft pulmonary valve is then used to replace the patient's pulmonary valve. This is called the Ross procedure. The Ross Procedure allows the patient the advantage of receiving a living valve to replace the aortic valve. The long term survival and freedom from complications for patients with aortic valve disease are better with the Ross Procedure than any other type of valve replacement. URL: http://cape.uwaterloo.ca/che100projects/heart/files/pigvalve.jpg Animal tissue valves are also referred to as xenografts or heterografts. These valves are usually acquired from animals during commercial meat processing. The leaflet valve tissue of the animals is inspected, and the highest quality leaflet tissues are then preserved. The two most commonly used animal tissues are porcine tissue (valve tissue from a pig) and bovine pericardial tissue (valve tissue from a cow). The most common cause of failure of the bioprosthetic valve is tissue stiffening due to calcium buildup. Calcium buildup causes restricted blood flow and can also tear valve leaflets. Bioprosthetic valves usually last 10-15 years before needing to be replaced due to gradual wear. The future of bioprosthetic valves lies with tissue engineering. The ideal valve would be made up of the patient's own tissues and sculpted to the corrected shape and dimensions for the patient's valve. The condition of being compatible with living tissue or a living system by not being toxic or injurious and not causing immunological rejection A person whose blood has an impaired ability to clot and consequent difficulty in controlling bleeding even after minor injuries A drug used to prevent the formation of blood clots A drug capable of suppressing the immune response which might otherwise result in attacking a foreign implant Relating to the epithelium, the outside layer of cells that covers all the free, open surfaces of the body including the skin, and mucous membranes that communicate with the outside of the body; a membranous cellular tissue that covers a free surface or lines a tube or cavity of an animal body and serves especially to enclose and protect the other parts of the body, to produce secretions and excretions, and to function in assimilation Capillaries are the smallest of blood vessels. They serve to distribute oxygenated blood from arteries to the tissues of the body and to feed deoxygenated blood from the tissues back into the veins A blood vessel that carries blood low in oxygen content from the body back to the heart A vessel that carries blood high in oxygen away from the heart to the body Referring to a prosthesis, an artificial substitute or replacement of a part of the body Any of various mechanical devices by which the flow of liquid (as blood) may be started, stopped, or regulated by a movable part that opens, shuts, or partially obstructs one or more ports or passageways Relating to the blood vessels of the body, which as a group, are referred to as the vascular system; of, relating to, constituting, or affecting a tube or a system of tubes for the conveyance of a body fluid A tube through which the blood circulates throughout the body. No Valve in Vain The definition of blood vessels is hollow instruments that carry blood throughout the body. The differences between capillaries, arteries, and veins in terms of size, make-up, and function. Capillaries are the smallest and most fragile and serve as the vessel for gas exchange between tissue and blood cells. Arteries carry oxygen rich blood into the body and are the largest and strongest vessels. Veins carry oxygen weak blood back to the heart and lungs. Heart valves help control the timing and direction of blood flow through the heart. Heart valves can fail either by not closing properly and allowing backflow of blood, or by not opening completely and thus restricting the flow of blood too much. Prosthetic heart valves are engineering devices that were created to fix broken heart valves in people. There are many different kinds of artificial heart valve. Mechanical heart valves include the caged ball design, the tilted-disc design, and the bileaflet valve design. Bioprosthetic heart valves include both human and animal tissue valves. For human tissue valves you can have either a homograft from a deceased donor, or an autograft in which a valve is taken from one position in the patient themselves and relocated to another position. For animal tissue valves, the most common animals used are pig and cow tissue valves. Mechanical prosthetic valves are good for people who do not want to have to have replacement valve surgery in the future because the are durable and can last for the patient's lifetime. Bioprosthetic valves tend to have better blood flow characterisitcs and do not require the patient to take anticoagulants for long periods of time. However they only last 10 -15 years. Students Should Be Able To: Accurately predict whether fluid would flow faster in veins, capillaries, and arteries. Explain the difference between capillaries, veins, and arteries in terms of size and function. Explain what heart valves do and how they work. Describe different one-way valves and what purposes they serve. List different types of prosthetic heart valves and discuss their pros and cons. Design their own heart valves for experimentation. What are some solutions for faulty heart valves that might be engineered in the future? What other failures of the body has engineering made it possible to repair? Look around your home and find at least 3 things that might act as one-way valves. Do an internet search or go to the library to research different types of prosthetics, how they work, and what they are made out of. Valves and Pumps- A Demonstration - Students will see how valves and pumps work in concert to move blood through the circulatory system. The Heart as a Pump - Students will explore the working of the heart by making comparisons with the actions of a pump. Go With the Flow - Students will name and locate the major areas and structures of the heart and trace the pathway of the blood through the heart, lungs, and body. Great Sites to visit for interactive activities and further information: http://www.sci.mus.mn.us/heart/heart/top.html http://cape.uwaterloo.ca/che100projects/heart/files/testing.htm http://www.stayinginshape.com/4trover/libv/h10.shtml http://sln.fi.edu/biosci/vessels/vessels.html American Heart Association American Heart Association, <http://www.americanheart.org>, 5/5/04 American Heart Association Science Museum of Minnesota Science Museum of Minnesota, <http://www.sci.mus.mn.us/heart/heart/top.html>, 5/5/04 Science Museum of Minnesota Lesson Handout