Editing Heart (section)

====Valves====
{{Main|Heart valves}}
{{multiple image
| align = right
| direction = horizontal
| image1 = 2011 Heart Valves.jpg
| width1 = 230
| caption1 = With the atria and major vessels removed, all four valves are clearly visible.<ref name="CNX2014"/>
| image2 = Diagram_of_the_human_heart_(cropped).svg
| width2 = 184
| caption2 = The heart, showing valves, arteries and veins. The white arrows show the normal direction of blood flow.
}}

[[File:2010 Chordae Tendinae Papillary Muscles.jpg|thumb|Frontal section showing [[papillary muscle]]s attached to the [[tricuspid valve]] on the right and to the [[mitral valve]] on the left via [[chordae tendineae]].<ref name="CNX2014"/>]]
The heart has four valves, which separate its chambers. One valve lies between each atrium and ventricle, and one valve rests at the exit of each ventricle.<ref name=CNX2014/>

The valves between the atria and ventricles are called the atrioventricular valves. Between the right atrium and the right ventricle is the [[tricuspid valve]]. The tricuspid valve has three cusps,{{sfn|Gray's Anatomy|2008|pp=966–967}} which connect to [[chordae tendinae]] and three [[papillary muscle]]s named the anterior, posterior, and septal muscles, after their relative positions.{{sfn|Gray's Anatomy|2008|pp=966–967}} The [[mitral valve]] lies between the left atrium and left ventricle. It is also known as the bicuspid valve due to its having two cusps, an anterior and a posterior cusp. These cusps are also attached via chordae tendinae to two papillary muscles projecting from the ventricular wall.{{sfn|Gray's Anatomy|2008|p=970}}

The papillary muscles extend from the walls of the heart to valves by cartilaginous connections called chordae tendinae. These muscles prevent the valves from falling too far back when they close.<ref>{{cite web|author1=University of Minnesota|title=Papillary Muscles|url=http://www.vhlab.umn.edu/atlas/left-ventricle/papillary-muscles/index.shtml|website=Atlas of Human Cardiac Anatomy|access-date=7 March 2016|url-status=live|archive-url=https://web.archive.org/web/20160317203756/http://www.vhlab.umn.edu/atlas/left-ventricle/papillary-muscles/index.shtml|archive-date=17 March 2016}}</ref> During the relaxation phase of the cardiac cycle, the papillary muscles are also relaxed and the tension on the chordae tendineae is slight. As the heart chambers contract, so do the papillary muscles. This creates tension on the chordae tendineae, helping to hold the cusps of the atrioventricular valves in place and preventing them from being blown back into the atria.<ref name="CNX2014"/>{{efn|Note the muscles do '''not''' cause the valves to open. The pressure difference between the blood in the atria and the ventricles does this.}}{{sfn|Gray's Anatomy|2008|pp=966–967}}

Two additional semilunar valves sit at the exit of each of the ventricles. The [[pulmonary valve]] is located at the base of the [[pulmonary artery]]. This has three cusps which are not attached to any papillary muscles. When the ventricle relaxes blood flows back into the ventricle from the artery and this flow of blood fills the pocket-like valve, pressing against the cusps which close to seal the valve. The semilunar [[aortic valve]] is at the base of the [[aorta]] and also is not attached to papillary muscles. This too has three cusps which close with the pressure of the blood flowing back from the aorta.<ref name="CNX2014"/>