The semilunar valves are open during ventricular

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blood going back into the atria. The semilunar valves are open during ventricular systole phase to allow the blood to flow into the aorta and pulmonary trunk. Following this phase, the ventricles relax. This is ventricular diastole . The semilunar valves close to stop the blood from flowing back into the ventricles from the aorta and pulmonary trunk. The atria and ventricles once again are in diastole together and the cycle begins again . 5. Blood Vessel Structure and Function Blood Vessels contain a central lumen (cavity) lined with an endothelium, a single layer of flattened epithelial cells. The smooth surface of the endothelium minimizes resistance to the flow of blood. Capillaries The smallest blood vessel, having a diameter only slightly greater than that of a red blood cell.
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Very thing walls, which consist of an endothelium and a surrounding extracellular layer called a basal lamina. The exchange of substances between the blood and interstitial fluid occurs only in capillaries because only there are blood vessel walls thin enough to permit this exchange. Vein A vein has a wall about 1/3 as thick as an artery Two layers of tissue surrounding the endothelium. The outer layer is formed by connective tissue that contains elastic fibers, which allow the vessel to stretch and recoil, and collagen, which provides strength. Because veins convey blood back to the heart at a lower pressure, they do not require thick walls. Veins contain valves, which maintain a unidirectional flow of blood despite the low blood pressure in these vessels. Arteries Two layers of tissue surrounding the endothelium. The outer layer is formed by connective tissue that contains elastic fibers, which allow the vessel to stretch and recoil, and collagen, which provides strength. Walls are thick and strong, accommodating blood pumped at a high pressure by the heart. Walls are also elastic. When the heart relaxes between contractions, the arterial walls recoil, helping maintain, helping maintain blood pressure and flow to capillaries 6. Components and Functions of Blood Vertebrate blood is a connective tissue consisting of 55% plasma and 45% cellular elements Plasma Liquid matrix where cells are suspended Plasma is made up of water, ions, and plasma proteins Dissolved in the plasma are ions and proteins that, together with white blood cells, function in osmotic regulation, transport, and defense. The concentration of ions in plasma directly affects the composition of interstitial fluid, where many of these ions have a vital role in muscle and nerve activity Plasma proteins act as buffers against pH changes and help maintain the osmotic balance between blood and interstitial fluid. Other plasma proteins, such as antibodies, combat viruses and other foreign agents that invade the body.
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