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The red blood cells, White blood cells and platelets They are like the parts of a car. Each has a definite function.
Red blood cells carry oxygen. Whites fight infections, that is, viruses and bacteria that attack the body and make us sick. And platelets are responsible for stopping bleeding, as when someone cuts their hands - that is, platelets help with blood clotting. All three are mixed in a liquid substance called plasma. One man has on average 5 million red blood cells per cubic millimeter of blood.
Blood does not walk only on avenues. There are also the streets, which are the venules and arterioles - smaller veins and arteries. And there are still little streets called capillaries. All because blood has to reach every small block of our body, on the most remote periphery.
Look at your hand: It has a lot of veins and arteries under the skin. It is so in your whole body. So when you get a cut - no matter where it is - blood always comes out. Alright, the blood is all over the body. But how much blood, exactly?
It depends on the size of the person. An adult is five liters on average.
The Arteries: Its function is carry oxygenated blood under a high pressure to the tissues, for this reason the arteries have strong vascular walls and the blood flows rapidly in them. Arteries are expandable tubes that have three layers:
- Internal or intimate: formed by endothelial tissue.
- Medium: mainly composed of elastic fibers.
- External or adventitia: composed mainly of fibrous tissue. By the presence of elastic tissue the arteries respond passively to the pressure of the contained blood.
Elastic tissue loses flexibility with old age and so the arteries tend to shrink, becoming crooked and hardened, which causes pressure to change.
The arterioles: These are the last parts of the arteriolar system. Its structure is similar to the arteries, being the middle layer mainly muscular, so it is expected that there are active and non passive changes in its caliber. Therefore, the amount of blood that reaches the capillary layer may increase or decrease in response to tissue needs and sometimes in response to emotional activity. For example: the pallor caused by fear, the coldness of the hands due to apprehension or the facial flush at shame.
The capillaries: Capillaries are composed of only one cape: the endothelium. On average, they are no more than 1mm in length and serve as a connection between arterioles and venules. The function of capillaries is to exchange fluids, nutrients, electrolytes, hormones and other substances between blood and interstitial or tissue fluid. For this function the capillary walls are very thin and permeable to small molecules.
The venules and veins: The venules collect the blood from the capillaries. These come together to form veins. They have three layers like the arteries, but thinner, especially the middle layer. The pressure in them is lower compared to the arteries. Veins act as a conduit for transporting blood from the tissues to the heart but, equally importantly, serve as a fundamental reserve of blood. Veins have a much larger caliber than arteries, and their flow is much slower. These return the blood to the heart against gravity and, therefore, have valves that promote venous return flow to the heart.
Venous congestion that feels hot and tired at the end of a busy day is reduced by placing the feet higher than the trunk.