Blood vessels flow through

The technical definition of a tourniquet is any device that is used to prevent blood from flowing through blood vessels below the placement of the tourniquet on either upper or lower limbs. A tourniquet prevents excessive loss of blood from a limb wound with the expectation of saving a life. On the battlefield, the tourniquet in one form or another has been used to control excessive hemorrhaging on nonvital extremities since the Roman Empire days, where a rope or cloth strap was used for toumiqueting a soldier s limb that has suffered a wound. The use of tourniquets has always been as controversial as it has been successful. This controversy results, as will be described later, as much from mistakes made in the application and release of the tourniquet as it does from the primitive design of the most commonly used tourniquets of today. 

SMAs are also used in another application related to cardiac health problems, as vena-cava filters. In some instances, it is desirable to protect a patient against the possibility that a blood clot formed elsewhere in the body will travel through the circulatory system into the heart, where it may cause a heart attack or stroke. Tiny, umbrella-shaped devices made of SMA materials have proven to he effective in such cases. In these devices, called vena-cava filters, the umbrella portion of the device consists of a mesh of tiny wires made of an SMA material. The device is inserted into the circulatory system in the form of a reduced-size (martensite phase), folded-up umbrella. Once in place, it is opened in such a way that the umbrella fills the vessel leading into the heart. The mesh design allows blood to flow through normally but filters out any blood clots that are carried along with the blood. 

Poiseuille was a physician-physiologist interested in the flow of blood through blood vessels in the body. Estimate the viscosity of blood from the fact that blood passes through the aorta of a healthy adult at rest at a rate of about 84 cm sec , with a pressure drop of about 0.98 mmHg m". Use 9 mm as the radius of the aorta for a typical human. 

One of the remarkable features of the heart, which is about the size of a fist, is that the pumping action or contractions (- 70 times/min) of atria and ventricles are simultaneous. Deoxygenated blood is pumped out to the lungs from one side of the heart at the same time oxygenated blood is pumped out from the other side to the aorta and onward through the body. The entire process of blood flow between the atria, ventricles, and the principal vessels is in unison, controlled in part by four one-way valves. The atrioventricular valves, tricuspid on the right and mitral on the left, prevent blood from flowing... 

Coronary vasodilator. A drug that enhances blood flow through the blood vessels of the heart. 

Acute coronary syndromes most often result from a physical disruption of the fibrous cap, either frank cap fracture or superficial endothelial erosion, allowing the blood to make contact with the thrombogenic material in the lipid core or the subendothelial region of the intima. This contact initiates the formation of a thrombus, which can lead to a sudden and dramatic blockade of blood flow through the affected artery. If the thrombus is nonocclusive or transient, it may either be clinically silent or manifest as symptoms characteristic of unstable angina. Importantly, if collateral vessels have previously formed, for example, due to chronic ischemia produced by multi vessel disease, even total occlusion of one coronary artery may not lead to an acute myocardial infarction. 

The nitrates, such as isosorbide (Isordil) and nitroglycerin, have a direct relaxing effect on die smooth muscle layer of blood vessels. The result of diis effect is an increase in the lumen of die artery or arteriole and an increase in the amount of blood flowing through diese vessels. An increased blood flow results in an increase in die oxygen supply to surrounding tissues. 

If pain is present, the primary health care provider may order a narcotic analgesic. Once the clot is dissolved and blood flows freely through the obstructed blood vessel, severe pain usually decreases. 

The adenohypophysis does not have a direct anatomical connection with the hypothalamus therefore, regulation of hormone secretion by way of neuronal signals is not possible. Instead, these two structures are associated by a specialized circulatory system and the secretion of hormones from the adenohypophysis is regulated by hormonal signals from the hypothalamus (see Figure 10.2). Systemic arterial blood is directed first to the hypothalamus. The exchange of materials between the blood and the interstitial fluid of the hypothalamus takes place at the primary capillary plexus. The blood then flows to the adenohypophysis through the hypothalamic-hypophyseal portal veins. Portal veins are blood vessels that connect two capillary beds. The second capillary bed in this system is the secondary capillary plexus located in the adenohypophysis. 

Although skeletal muscle comprises the bulk of muscle tissue in the body, smooth muscle is far more important in terms of homeostasis. Most smooth muscle is found in the walls of tubes and hollow organs. Contraction and relaxation of the smooth muscle in these tissues regulates the movement of substances within them. For example, contraction of the smooth muscle in the wall of a blood vessel narrows the diameter of the vessel and leads to a decrease in the flow of blood through it. Contraction of the smooth muscle in the wall of the stomach exerts pressure on its contents and pushes these substances forward into the small intestine. Smooth muscle functions at a subconscious level and is involuntary. It is innervated by the autonomic nervous system, which regulates its activity. 

The second factor that determines the flow of blood through a vessel is resistance. In contrast to the pressure gradient, blood flow through a vessel... 

Friction also develops as blood contacts the vessel wall while flowing through it. Therefore, the greater the vessel surface area in contact with the blood, the greater the amount of friction developed and the greater is the resistance to blood flow. Two factors determine the vessel surface area length of the vessel and vessel radius. 

Assume two blood vessels of equal length, each has a pressure gradient of 1 mmHg. However, blood vessel A has a radius of 1 mm and blood vessel B has a radius of 2 mm. The flow of blood through vessel A is 1 ml/min and the flow of blood through vessel B is 16 ml/min. Simply doubling vessel radius causes a 16-fold increase in blood flow. 

Ohm s law, which correlates the effects of blood pressure and vascular resistance on blood flow through a vessel (Q = AP/R), may also be applied to blood flow through the entire systemic circulation, or cardiac output ... 

Vascular constriction. The first mechanism to occur is vascular constriction. Immediately after a blood vessel is cut or severed, the vascular smooth muscle automatically constricts. This results in a decrease in the flow of blood through the vessel that helps to limit blood loss. The vasoconstriction is caused by several factors ... 

Localized constriction of the blood vessel, which minimizes further blood flow through the area. 

The main function of vascular smooth muscle is to distribute blood flow through selective vasoconstriction and vasomotion. The latter is clearly associated with oscillations in [Ca2+]j, but it was long thought that tonic contraction was initiated by SR Ca2+ release and maintained by a steady state elevation of [Ca2+] dependent on influx. However, confocal microscopy of intact blood vessels has... 

---