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How Does Blood Move From Arteries To Veins?

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Through capillaries
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Arteries are a type of blood vessel, one of the 5. They are also the largest, and the largest blood vessel is the Aorta. The red blood cells in the blood consist of 55% blood plasma, which contain salts, gases (oxygen from the lungs, C02 from cellular respiration), water, nutrients, and hormones. The 45% is formed elements (red blood cells, white blood cells, and platelets). Red blood cells contain Hemoglobin, which is useful in the respiratory system. White blood cells fight infections, through phagocytosis (engulfing bacteria), and platelets, which help in the formation of blood clotting, so healing wounds. Plasma proteins are very important in the composition of blood, because they are too big to pass through walls of blood vessels, so they exert osmotic pressure, which will make sense in what I am going to explain later. I am going to be as descriptive as I can in my own words, cause I am not sure what you do or do not get.
Anyways, before ANY blood flow can happen, the heart has to distribute the blood. This is how - In the right atrium of the heart, there is an SA node, and an AV node. The medulla oblongata sends an impulse to the SA node, it contracts first, and sends an impulse to the AV node (in the right ventricle), which contracts secondly, and from there, an impulse gets sent to the Purkinje Fibers of the heart, to get the actual ventricles to contract. This is why you don't have to think about beating every second or so for your heart.
The Systemic System (where blood is distributed throughout the entire body, in arteries and veins), is where Dirty, de-oxygenated, un-rich (has poor amounts of nutrients) blood comes into the pulmonary system (the heart). What happens here is from the veins throughout the body, they are carrying this type of blood (deoxygenated) to the heart, via the posterior vena cava, or the anterior vena cava (top part of the heart, and bottom part). From here, they go through the right atrium, down to the left atrium through the AV valve (which prevents the blood from inverting to a previous heart chamber), From the left atrium, the blood goes through the pulmonary trunk, and gets there by the Semi-Lunar Valve (same function as the AV valve), and eventually branches out into the pulmonary arteries. ***The pulmonary arteries take the unclean, deoxygenated, un-rich blood to the lungs, where there are capillaries, and gas exchange occurs so the red blood cells are able to pick up nutrients such as oxygen.

Once the blood is oxygenated, clean, and rich in nutrients, they travel back to the left atrium, via the pulmonary veins. From here, there is an AV valve that separates the atrium from the ventricle yet again, to prevent inversion of the blood. ******From the left atrium, the blood is pumped out to the Aorta (the largest blood vessel / artery in the human body)******

Arteries undergo the highest blood pressure, so the Aorta, undergoing extreme measures of blood pressure, strongly pumps out blood to the entire body.

Here is where we get into it. As blood is flowing through various arteries (and veins going back to the heart), they go through capillaries, where gas-exchange occurs. This is where cells pick up the oxygen required for cellular respiration, which is a cell's form of ATP, as well as other nutrients. The cells also give out their wastes from it (02 + glucose ------> C02 + H20 + Energy). Capillaries are one cell thick, which is a great reason why diffusion of the gases occurs so well. Diffusion is the movement of molecules from a higher to a lower concentration. It doesn't require ATP, because its going from HIGH to LOW, if you think of it that way, down a concentration gradient. Active transport, on the other hand, requires ATP, since it is going from a lower to higher concentration.

Capillaries are the site for exchange of nutrients, gases, and wastes between the tissues and the circulatory system.

On a side note - Before reaching capillaries, arteries branch off into arterioles, which are a branch of arteries, and blood flow slows down. In the arteries (right before hitting the arterioles) the blood pressure is higher then the osmotic pressure (BP > OP), so H20 leaves the blood plasma. After the arterioles, and finally hitting the capillaries, there is an equal net force between blood pressure, and osmotic pressure. In the capillaries, as mentioned before, the cells pick up oxygen, and they give C02 to the blood (hence why the blood loses nutrients, and becomes deoxygenated, since C02 creates that). After the capillaries, they reach venules, and veins (OP > BP). In the veins, the osmotic pressure is higher then the blood pressure, so H20 comes into the blood plasma.

There are major arteries in the body which are ALL supplying oxygenated blood - there is the Subclavian artery (which supplies blood from the aorta to both arms), Iliac Artery (which supplie

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