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toc =Organism: Human Knee=

**Structure**
The human knee is located where the two longest bones in the human body meet. The tibia and the femur. It is also the largest joint in the human body. Because it is a pivotal hinge joint, it allows flexion and extension as well as medial and lateral rotation. In humans, it supports most of the body's weight due to its location on the body. Because of this, the area around it has to be strong.

**Function**
The function of the human knee is to bend the leg. Without our knees, we would not have the ability to walk as easily as we do, or climb stairs. = = =Organ System: Circulatory System= The circulatory system is one of the most important, if not the most important, organ system used for maintaining homeostasis. It circulates blood throughout the body. It works harmoniously with the excretory and nervous systems. The excretory system filters the blood brought to it, which passes through the kidneys to remove excess chemicals in the blood to be passed to urine. Likewise, the nervous system depends on blood to bring oxygen to the brain so that it can function. Without the brain’s specific electrical impulses, a heartbeat would be impossible. Without the kidneys, the blood would become polluted and toxic. The circulatory system is made up of blood vessels, of which arteries supply blood to the limbs and internal organs, and veins take blood from the limbs and internal organs and get it back to the heart.

= = =Organ: Artery= The way in which arteries are organs that belong in the circulatory system is because they have the job of moving blood away from the heart and into various other parts of the body.

Structure
The arteries are made up of three layers. The innermost layer is called the tunica intima layer, the middle is the tunica media layer, and the outer is the tunica externa layer. The tunica intima layer is a very thin layer of endothelial cells (explained below) which is not unlike our skin. It is very important for this layer to be smooth and flexible so that is does at all disrupt the flow of blood. The next layer, the tunica media, is made up of elastic fibers and smooth muscle cells. This is the layer that moves and makes the blood pump. This layer is thicker in arteries than in veins because it has to help the heart by pumping the blood outward. The final layer, tunica externa, is made up of collagen (protein which makes up connective tissues), fibroblasts (cells which make up collagen), and vasa vasorum (tiny blood vessels). This layer holds the whole thing together and nourishes the walls of the arteries. The arteries in our bodies vary in size. The size of an artery depends on exactly what its function is. For example, the biggest arteries are the aorta which are in the heart. They have to be big and strong so that they can force blood to come out of the heart and into the body's circulatory system. However, the smallest arteries, the arterioles, must be small in order to lead blood into small capillaries which are the walls of where oxygen and various other substances are.

Function
The arteries' main function is to move blood away from the heart. The various layers that they are made up of gives them a lot of strength and also flexibility so they can stretch as pressurized blood enters from the heart. They need this much strength in order to be able to shoot blood from your heart to everywhere else in your body. You can even feel your arteries stretching when you feel your pulse.

=Tissues: Blood Vessels (specifically arteries and veins)=

**Arteries**
Note that tunica adventitia means tunica externa. These arteries then feed into capillaries and/or veins. Arteries are large blood vessels that direct blood away from the heart towards the internal organs (lungs, brain, kidneys, liver, intestines, etc.) and extremities (fingers, toes, etc.).

**Veins**
Arteries, capillaries and veins are distinctly different in structure, as their functions are completely different. Veins are solely meant to take blood back to the heart and are equipped with passive valves (which are v-shaped) that are closed by default; when the heart pumps and blood is forced through the veins, the valves are pushed open only to close again when the heartbeat passes and the pressure in the veins decreases to the point where the valves will close themselves again. Arteries and veins also differ in the number of layers they have: they both feature at least two layers, the tunica media and the tunica initma, but arteries feature tunica externa, which is a layer mostly composed of collagen.

Function
Arteries and veins mainly differ on the amount of pressure they have to deal with. Pressure is created by the heart when it pumps, causing blood to flow out of the heart and into arteries, which then supply blood to the internal organs and capillaries. The reason that veins feature two layers as compared to arteries' three is because of this pressure load: the third layer surrounding arteries supports the arteries so that they don't bend or function improperly. For example, think of a garden hose when you turn on the water: it will always jerk with the sudden flow of water. Now consider that arteries are relatively tiny in comparison to the heart and they're supposed to transport all of the blood in a heartbeat through all the arteries and capillaries in your body. That puts a lot of stress on those relatively tiny blood vessels, so the third layer (the tunica externa) anchors the arteries to surrounding tissue so that the recoil of a heartbeat is mostly absorbed by the tissue around the artery.

Veins have a deceptively hard job: they are tasked with sending blood back to the heart so that the circulatory system can work. Unfortunately, by the time blood has traveled to the extremities, the blood has slowed down from all of the flowing that it's been doing. To further complicate things, the pressure exerted by heartbeats on the blood vessels in your extremities is tiny: there isn't enough constant pressure to push the blood back to the heart by new blood because the new blood is just as slow as the old blood that just arrived. Fortunately for us, veins are equipped with those v-shaped valves that I mentioned earlier. This allows each heartbeat to get blood through a certain number of pressure-activated valves so that blood can be pushed and collect at these valves to slowly make its way back to the heart. = = =Cell: Endothelial Cells=

Structure
Endothelial cells are all over the circulatory system as simple squamous cells, which, of course, are epithelial cells. They line the interior of every blood vessel. They make very smooth tissue that allows the blood to flow without a lot of friction or disruptions. They are located inside of the blood vessels so that they can act as a barrier between the blood and its surrounding tissue. This helps to control the passage of materials and movement of white blood cells into and out of the blood stream.

Function
Endothelial cells are meant to reduce the any kind of disturbances that occur in the blood flow so that it can be pumped as far throughout the body as possible. Also, if the blood does not flow smoothly, blood clotting can occur which can lead to death.

= = =Chemical:=

Structure
Blood vessel cells typically contain a small amount of cytoplasm, which is the area of the cell between the nucleus and the cell membrane. This is because they are simple squamous cells. Furthermore, there are many pinocytotic vesicles found on the outside of the cell membranes of blood vessel cells.

Function
The small amount of cytoplasm, along with the pinocytotic vesicles, helps keep the blood vessel cells themselves from absorbing things being carried by blood cells that pass through the blood vessel cells through to the organs and muscles themselves. The vesicles act as containers for chemicals that need to be passed from the blood cells to whatever tissue needs those chemicals.

= = =Sources=

Information

 * "Blood Vessels." //Computing Services for Faculty & Staff//. Web. 04 Apr. 2011. .
 * "Brief Overview of Knee Anatomy and Physiology." //Bob's ACL WWWBoard//. Web. 04 Apr. 2011. .
 * How Veins Work - Modesto Vein Center." //How Veins Work//. Modesto Vein Center. Web. 04 Apr. 2011. .
 * McGeachie, John. "Blue Histology - More about Endothelial Cells." //School of Anatomy & Human Biology Teaching Website - Home//. University of Western Australia, 09 Feb. 1998. Web. 04 Apr. 2011. .
 * "Organelles of Epithelial Cells." //Google Answers//. Google. Web. 04 Apr. 2011. .
 * "September 2010." //International University//. International University. Web. 04 Apr. 2011. .
 * White, Suzanne McCullough. "Structure of an Artery | EHow.com." //EHow | How To Do Just About Everything! | How To Videos & Articles | EHow.com//. Web. 04 Apr. 2011. .

Images

 * http://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookcircSYS.html
 * http://www.stoneclinic.com/knee
 * http://hbcprotocols.com/inflammation2.html
 * http://image.wistatutor.com/content/feed/tvcs/pino_0.JPG