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All blood enters the heart's right side through two veins: The superior vena cava and the inferior vena cava.
The superior vena cava collects blood from the body's upper half. The inferior vena cava collects blood from the body's lower half. When blood leaves these two veins, it enters the heart's right atrium.
When the right atrium contracts, blood flows through a valve into the heart's right ventricle. When the right ventricle contracts, blood is pumped through a valve into the pulmonary artery and into the lungs where it picks up oxygen. It works this way for an important reason.
Blood returning from the body is poor in oxygen. It has to be full of oxygen before returning to the body. So, the heart's right side pumps blood to the lungs first to gather oxygen before going to the heart's left side where it is sent back to the body full of oxygen.
Blood returns to the heart from the lungs via the pulmonary veins. The blood goes into the left atrium. When the left atrium contracts, blood travels through a valve into the left ventricle. The left ventricle is a very important chamber because it pumps blood through a valve and into the aorta. The aorta - the main artery of the body - receives all the blood that the heart pumps out and sends it to the rest of the body. The left ventricle has a thicker muscle than any other heart chamber. It is thicker because the left ventricle has to pump blood against much higher pressure in the general circulation (the blood pressure).
The heart is a miraculous pump. It works because it is mostly made of muscle. Each tiny cell is able to perform a contraction in one direction. In fact, if they are separated out and given appropriate nutrients, a microscope will allow you to see individual cells still beating! In the heart, these cells are connected together into bundles which contract in an organized fashion when stimulated by an electrical current which moves through the heart with each beat. The heart has four chambers: two atria and two ventricles. The right heart contains the right atrium and the right ventricle. The veins of the body bring used blood back into the right atrium. This venous blood is low in oxygen and looks darker in color, since the oxygen has been partially extracted by the cells of the body. Next, the blood travels through the right ventricle and into the lungs. When the blood comes out of the lungs, it is brighter red since it has higher oxygen content. It enters the left atrium, then the left ventricle, and it is pumped out to the body. So you can think of it as the right heart pumping oxygen-poor blood into the lungs and the left heart pumping oxygen-rich blood to the body. There are four major heart valves. The one-way valves between each atrium and ventricle prevent the blood from being pumped backwards as the ventricle contracts. There is also a valve at the outflow of each ventricle. Those prevent the blood which has already been pumped from being sucked back into the ventricles as they relax. Everyone knows the classic “lub-dub” heartbeat. These two sounds are the valves closing. With each “lub-dub”, or cardiac cycle, the two atria contract together, followed by contraction of the two ventricles. It’s mind-boggling to realize that, with all our technology, we haven’t even come close to creating a pump that can run for 80+ years so efficiently, reliably, and without resting!