Lesson: Put Your Heart into EngineeringContributed by: Techtronics Program, Pratt School of Engineering, Duke University
Educational Standards :
Learning Objectives (Return to Contents)
After this lesson, students should be able to:
Introduction/Motivation (Return to Contents)
What is a valve? (Define what a valve is and, specifically, how a one-way valve works.) One-way valves allow flow in only one direction and prevent fluid from flowing back where it came from, should there be a force that would cause backflow (such as gravity).
What might be examples where this idea of "flowing one way" is beneficial? (Listen to student ideas.) Examples might be preventing drainage water from flowing back into drinking water, or when donating blood to keep the blood from flowing back into the vein, or speaking valves for people on ventilators.)
What other things in our everyday lives might act as one-way valves? (Possible answers: One-way doors, the oxygen regulator on a scuba mask, plumbing valves that regulate water flow at faucets. Give students time to brainstorm ideas to sharpen their understanding of what a one-way valve is and what it does.)
Lesson Background & Concepts for Teachers (Return to Contents)
Arteries, Capillaries and Veins, Oh My!
Arteries, veins and capillaries have one main thing in common: they are all blood vessels. In a general sense, a vessel is a hollow instrument for carrying something. A blood vessel, therefore, is a hollow instrument for carrying blood.
Arteries are the biggest blood vessels in the body. They serve the purpose of pumping 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. Since they carry blood at relatively high pressures, they must be durable to 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 oxygen-rich blood to the capillaries.
In contrast to arteries, capillaries are very thin and fragile. They are only one epithelial cell thick in order to effectively aid in the exchange of oxygen and carbon dioxide between the blood being carried and the body's oxygen-starved tissue. The red blood cells release fresh oxygen to the surrounding tissue, and the tissue releases its carbon dioxide waste into the red blood cells. The capillaries then deliver this 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 backwards 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.
If a computer/projector is available, show students the animated image at: http://www.sci.mus.mn.us/heart/heart/pumping-f.htm. It provides an excellent depiction of the movement of blood through the heart.
What Are Heart Valves?
Heart valves are similar to little doors in 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 make sure the blood only flows in one direction, which 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.
How Do Heart Valves Work?
The heart is divided into four chambers. The two upper chambers are called atria (or atrium when referring to one) 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 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.
What Can Go Wrong with a Heart Valve?
Valve disease can occur when a valve stops functioning properly. A valve might not close all the way, which allows leakage of blood backwards. Alternatively, a valve may not open all the way, keeping blood from flowing as well as it might otherwise. Both of these problems cause the heart to work much harder to carry the same amount of blood through the body.
Prosthetic Heart Valves
The two main types of prosthetic valves are mechanical and bioprosthetic.
Mechanical Heart Valves
Since first designed in 1952, mechanical heart valves have gone through many design evolutions. Over the years, improvements helped to reduce blood clots in the heart and create more central flow through the valve. Three main different designs are the caged ball valve, the tilting-disc valve, and the bileaflet, which is the valve that is primarily used today.
Caged Ball Design
The caged ball valve design uses a metal ball held in place by a welded metal cage (see Figure 1). When the blood flows one way, it pushes the ball into the cage, which allows blood to flow around the ball and through the valve. However, when the blood flows the other way, it pushes the ball over the valve opening and prevents blood from flowing backwards. 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 anti-coagulants. Taking anti-coagulants minimizes the chance of heart attacks, but makes patients vulnerable to any type of trauma that might cause internal or external bleeding, because the their blood is unable to clot well-enough to stop rapid blood loss.
The tilting-disc valve 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 permits improved central flow by opening at a 60° angle. Yet, it still closed at a rate of 70 times per minute and thus prevented blood backflow. 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 design flaw of the tilting-disc, however, is that the outlet struts of the disc would fracture due to fatigue from the repeated slamming of the disc, causing valve failure.
The bileaflet valve consists of two semicircular discs attached near the center of the valve with hinges. These discs 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 backwards, the discs come together to cover the circular valve opening. The leaflets are very strong, exhibiting excellent biocompatibility in that they do not damage tissue or red blood cells and do not cause clotting or infection. However, the two semicircular discs do not close completely and still allow some leakage when the blood flows backwards. Backflow is one of the main reasons doctors have switched to using prosthetic valves because mechanical valves can be less than ideal.
Currently, the most commonly used materials for mechanical valves are:
Pros and Cons of Mechanical Heart Valves
Mechanical heart valves are good long-term solution because they are durable and typically last a patient's lifetime, which generally eliminates the need for a second valve surgery. However, they cause an increased risk of blood clotting and thus force patients to take anti-coagulant medications for the rest of their lives, essentially creating blood clotting risks for them.
Bioprosthetic Heart Valves
Bioprosthetic heart valves are also known as prosthetic tissue valves. The design of bioprosthetic valves is much more similar to that of natural heart valves, giving them many advantages over mechanical valves. Bioprosthetic valves do not require patients to take anti-coagulants for life. Additionally, these valves result in better hemodynamics (blood movement), do not cause damage to blood cells, and do not experience many of the structural problems found in mechanical heart valves. Bioprosthetic valves are of two types: human tissue valves, and animal tissue valves.
Human Tissue Valves
The two types of human tissue valves are homografts and autografts. Homografts are human heart valves transplanted from other people. 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 problems with the body rejecting the valve and, thus do not require immunosuppressive therapy (medicines to prevent the body from rejecting a transplanted organ). Once a homograft is donated, it is frozen in liquid nitrogen to preserve it until it is needed for implantation. In cases in which the valve implant dimensions are a good match and closely resemble the patient's valve size, homografts resulte in 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 original pulmonary valve. This is called the Ross Procedure, and the advantage is that the patients receive living valves to replace the aortic valves. Both the long term survival rate and the fact that there is minimal risk for complications for patients with aortic valve disease make the Ross Procedure a more viable option than any other type of valve replacement.
Animal Tissue Valves
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 (pig) and bovine pericardial tissue (cow).
The most common cause of failure of the bioprosthetic valve is tissue stiffening due to calcium build-up. Calcium build-up causes restricted blood flow and can also tear valve leaflets. Due to gradual wear, bioprosthetic valves usually need to be replaced after 10-15 years.
Engineering currently plays a huge role in the design of artificial valves. Specifically, the future of bioprosthetic valves lies with tissue engineering. The ideal valve would be composed of a patient's own tissues and sculpted to the ideal shape and dimensions for the patient. With rapid advances in engineering and technology, this idea may not be too far off.
Vocabulary/Definitions (Return to Contents)
Associated Activities (Return to Contents)
Lesson Closure (Return to Contents)
Attachments (Return to Contents)
Assessment (Return to Contents)
Evaluate to make sure that students are able to:
Lesson Extension Activities (Return to Contents)
What are some solutions for faulty heart valves that might be engineered in the future?
What other failures of the body have engineering inventions made it possible to repair?
Look around your home and find at least three things that might act as one-way valves.
Research different types of prosthetics, their material components, and how they work. .
Extension Activities from the Science Museum of Minnesota
Additional Multimedia Support (Return to Contents)
The following websites offer interactive activities and attritional information on heart valves:
References (Return to Contents)
American Heart Association. Accessed May 5, 2004. http://www.americanheart.org
Science Museum of Minnesota. Accessed May 5, 2004. http://www.sci.mus.mn.us/heart/heart/top.html
ContributorsEmily McDowell, primary content creator, Alice Hammer, supplementary content creator
Copyright© 2004 by Engineering K-Ph.D. Program, Pratt School of Engineering, Duke University
(Including copyrighted works from other educational institutions and/or U.S. government agencies; all rights reserved.)
Supporting Program (Return to Contents)Techtronics Program, Pratt School of Engineering, Duke University
Last Modified: February 25, 2013