The Circulatory System Part II: The Heart and Circulation of Bloodcontents:1. Location of the Heart2. Structure of the Heart3. The Valves4. Branching Blood Vessels5. The Circulation of Blood* picture of the heart and its parts* picture of the body and some of its organsLocation of the HeartThe center of the circulatory system is the heart, which is the main pumpingmechanism. The heart is made of muscle. The heart is shaped something like a cone,with a pointed bottom and a round top. It is hollow so that it can fill up with blood.An adult’s heart is about the size of a large orange and weighs a little less than apound.The heart is in the middle of the chest. It fits snugly between the two lungs. It is heldin place by the blood vessels that carry the blood to and from its chambers. The heartis tipped somewhat so that there is a little more of it on the left side than on the right.The pointed tip at the bottom of the heart touches the front wall of the chest. Everytime the heart beats it goes “thump” against the chest wall. You can feel the thumps ifyou press there with your hand. You can also listen to them with your ear.* picture of the heart and its parts* picture of the body and some of its organsStructure of the Heart
If you looked inside your heart, you would see that a wall of muscle divides it downthe middle, into a left half and a right half. The muscular wall is called a septum. Theseptum is solid so that blood cannot flow back and forth between the left and righthalves of the heart. Another wall separates the rounded top part of the heart from thecone-shaped bottom part. So there are actually four chambers (spaces) inside theheart. Each top chamber is called an atrium (plural: atria). The bottom chambers arecalled ventricles. The atria are often referred to as holding chambers, while theventricles are called pumping chambers. Thus, each side of the heart forms its ownseparate system, a right heart and a left heart. Each half consists of an atrium and aventricle, and blood can flow from the top chamber to the bottom chamber, orventricle, but not between the two sides.The ValvesBlood can flow from the atria down into the ventricles because there are openings inthe walls that separate them. These openings are called valves because they open inone direction like trapdoors to let the blood pass through. Then they close, so theblood cannot flow backwards into the atria. With this system, blood always flows inonly one direction inside the heart. There are also valves at the bottom of the largearteries that carry blood away from the heart: the aorta and the pulmonary artery.These valves keep the blood from flowing backward into the heart once it has beenpumped out.* picture of the heart and its parts* picture of the body and some of its organsBranching Blood VesselsThe heart is a pump whose walls are made of thick muscle. They can squeeze(contract) to send blood rushing out. The blood does not spill all over the place whenit leaves the heart. Instead, it flows smoothly in tubes called blood vessels. First, theblood flows into tubes called arteries. The arteries leaving the heart are thick tubes.But the arteries soon branch again and again to form smaller and smaller tubes. Thesmallest blood vessels, called capillaries, form a fine network of tiny vesselsthroughout the body. The capillaries have extremely thin walls so that the blood thatthey carry can come into close contact with the body tissues. The tiny red blood cellscan then pass easily through the walls of the capillaries to deliver the oxygen theycarry to nearby cells. As the blood flows through the capillaries, it also collects carbondioxide waste from the body cells. The capillaries containing carbon dioxide return
this used blood to the heart through a different series of branching tubes: Thecapillaries join together to form small veins. The veins, in turn, unite with each otherto form larger veins until the blood from the body is finally collected into the largeveins that empty into the heart. So the blood vessels of the body carry blood in acircle: moving away from the heart in arteries, traveling to various parts of the bodyin capillaries, and going back to the heart in veins. The heart is the pump that makesthis happen.* picture of the heart and its parts* picture of the body and some of its organsThe Circulation of BloodThe human circulatory system is really a two-part system whose purpose is to bringoxygen-bearing blood to all the tissues of the body. When the heart contracts itpushes the blood out into two major loops or cycles. In the systemic loop, the bloodcirculates into the body’s systems, bringing oxygen to all its organs, structures andtissues and collecting carbon dioxide waste. In the pulmonary loop, the bloodcirculates to and from the lungs, to release the carbon dioxide and pick up newoxygen. The systemic cycle is controlled by the left side of the heart, the pulmonarycycle by the right side of the heart. Let’s look at what happens during each cycle:The systemic loop begins when the oxygen-rich blood coming from the lungs entersthe upper left chamber of the heart, the left atrium. As the chamber fills, it pressesopen the mitral valve and the blood flows down into the left ventricle. When theventricles contract during a heartbeat, the blood on the left side is forced intothe aorta. This largest artery of the body is an inch wide. The blood leaving the aortabrings oxygen to all the body’s cells through the network of ever smaller arteries andcapillaries. The used blood from the body returns to the heart through the network ofveins. All of the blood from the body is eventually collected into the two largestveins: the superior vena cava, which receives blood from the upper body, andthe inferior vena cava, which receives blood from the lower body region. Both venaecavae empty the blood into the right atrium of the heart.From here the blood begins its journey through the pulmonary cycle. From the rightatrium the blood descends into the right ventricle through the tricuspid valve. Whenthe ventricle contracts, the blood is pushed into the pulmonary artery that branchesinto two main parts: one going to the left lung, one to the right lung. The fresh,oxygen-rich blood returns to the left atrium of the heart through the pulmonaryveins.
Although the circulatory system is made up of two cycles, both happen at the sametime. The contraction of the heart muscle starts in the two atria, which push the bloodinto the ventricles. Then the walls of the ventricles squeeze together and force theblood out into the arteries: the aorta to the body and the pulmonary artery to thelungs. Afterwards, the heart muscle relaxes, allowing blood to flow in from the veinsand fill the atria again. In healthy people the normal (resting) heart rate is about 72beats per minute, but it can go much higher during strenuous exercise. Scientists haveestimated that it takes about 30 seconds for a given portion of the blood to completethe entire cycle: from lungs to heart to body, back to the heart and out to the lungs.
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