Central venous catheters are inserted with the tip in or close to the superior of inferior vena cava. Blood is pumped from the right side of the heart into the blood vessels of the lung. When blood enters the small capillaries of the lung called the pulmonary capillaries , fresh oxygen enters the blood and carbon dioxide is removed. This is called " gas exchange " or " respiration ".
Because this is the exchange of gases between the atmosphere and the bloodstream, it is also called " external respiration ". When freshly oxygenated blood reaches the capillaries of the tissues, oxygen moves from the blood toward the tissues, and carbon dioxide moves from the tissues toward the blood.
This gas exchange that occur between the blood and the cells of the tissues and organs is called " internal respiration ". Blood vessels have a muscle layer that is able to relax or contract. When we need to increase our blood pressure, the muscle layer contracts and makes the blood vessel diameter smaller.
This is called "vasoconstriction". When the muscle layer of a blood vessel relaxes, the blood vessel diameter becomes larger. This is called "vasodilation". Vasodilation lowers the blood pressure. Drugs that cause the diameter of the blood vessels to change are called vasoactive drugs. Drugs that make blood vessels constrict are used to treat low blood pressure and are called vasocontrictors.
Many one-way venous valves help to move blood through the veins of the lower extremities against the pull of gravity. Blood passing through the veins is under very little pressure and so must be pumped toward the heart by the contraction of skeletal muscles in the legs and by pressure in the abdomen caused by breathing.
Venous valves help to trap blood between muscle contractions or breaths and prevent it from being pulled back down towards the feet by gravity. By: Tim Taylor. It is composed of collagen and elastic fiber connective tissues. This layer allows the vena cava to be strong and flexible. The middle layer is composed of smooth muscle and is called the tunica media. Smooth muscle in this layer allows the venae cavae to receive input from the nervous system.
The inner layer is the tunica initima. This layer has an endothelium lining which secretes molecules that prevent platelets from clumping together and helps blood to move smoothly. Veins in the legs and arms also have valves in the innermost layer that are formed from the infolding of the tunica intima.
The valves are similar in function to heart valves, which prevent blood from flowing backwards. Blood within veins flows under low pressure and often against gravity. Blood is forced through the valves and toward the heart when skeletal muscles in the arms and legs contract. This blood is eventually returned to the heart by the superior and inferior venae cavae. Due to the important role that the superior and inferior venae cavae play in circulation, problems arising with these large veins can have serious consequences.
Since veins have relatively thin walls and the venous system is a low-pressure system, both venae cavae are subject to compression by surrounding tissues that swell. This compression inhibits blood flow and impacts proper heart function. The development of blood clots within the venae cavae can also impede or block blood from returning to the heart. Superior vena cava syndrome is a serious condition that arises from the constriction or obstruction of this vein.
The superior vena cava may become constricted due to enlargement of surrounding tissue or vessels such as the thyroid, thymus, aorta, lymph nodes, and cancerous tissue in the area of the chest and lungs. The swelling may slow or obstruct blood flow to the heart. Superior vena cava syndrome is most often caused by lung cancer and lymphoma. Inferior vena cava syndrome is caused by the obstruction or compression of the inferior vena cava.
This condition results most often from tumors, deep vein thrombosis, congestive heart failure, kidney disease, and pregnancy. Tucker, William D.
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