Describe LOCALLY PRODUCED factors arising from the tissues or from the endothelium which might influence the force of of contraction of vascular smooth muscle for the purpose of matching TISSUE BLOOD FLOW WITH TISSUE METABOLISM.
The specific needs of the tissue nutrition/metabolism are met through DELIVERY ( o2/substrates), REMOVAL ( CO2/H), MAINTENANCE ( proper conc of ions), and TRANSPORTATION ( hormones and other substances)of . Guyton describes a tissue unit – as related to the oxygen demand theory – the unit consists of a metarteriole, a single capillary, and its surrounding tissue. The metabolic/ nutritional needs are met through the cyclic opening and closing of the pre cap sphinc, and metarterioles (vasomotion). Vasomotion is thought to be caused by locally produced vaso-active substances diffusing to the pre capillary sphincter/metarterioles in the tissue unit areas, - thus causing either vaso dilation or vasoconstriction.
Locally produced vasodilator factors are thought to diffuse back to pre capillary sphincters, metarterioles, and arterioles to cause dilation. The number of pre capillary sphincters and that open, and the vasc sm musc around arterioles open and close proprtional to metabolic/nutritional needs of the local tissue.
VASO CONSTRICTORS –
Learning objective - 5
Compare and contrast the PHYSICAL & FUNCTIONAL PROPERTIES of the arterial and venous systems, and describe factors controlling VASCULAR REMODELING in response to changes in transmural pressure and the velocity of flow.
Veins – are thin walled tubes with less muscle and elastic tissue
composed of from inside to out –
Arteries – are thick walled vessels (high pressure) the smooth muscle acts in regulating blood distribution. Arteries are composed of the same tissue as veins only each layer is thicker. The arterial endothelial tissue lack valves.
Capillaries are simple endothelial tubes in which nutrients and gasses can diffuse into or out of tissues in close proximity, in response to simple diffusion and osmotic pressures.
Veins provide a low pressure storage system (resevoir) for blood. Veins (smooth muscle) respond to changes in pressure to by dilating or constricting. The venous system is considered the low pressure system.
Venous return is mediated by the pressure gradient (MSAP – CVP), by pumping of skeletal muscle, and by the respiratory pump. Venous pressure is effected by hydrostatic, pgh, ht function, and the effectiveness of the valves.
Arteries – the arterial walls expand as blood enters them, which decreases the pressure with in them. The arterial walls also recoil to propel blood forward toward the capillaries. Arteries keep pressure changes with in a range to minimize fluctuations, rather than expanding so much to allow pressure to fall near zero, thus the expanding and recoiling. Hydrostatic pressure and pgh affect arterial pressure also.
Transmural pressure can be defined as – the pressure exerted against and through the thickness of the wall of, in this case a vessel.
Velocity of flow can be defined as – speed /sec of movement of, in this case blood, through a vessel in relation to the pressure exerted on the vessel as a result of the velocity of the flow.
Discussion of flow – flow is a motion through a constraint, which is accompanied by deformation of the fluid. In this case we discuss the flow of fluid thru a tube, (diameter is less than its length). Velocity is greatest when the diameter is least, and pressure is at its least at the points of greatest constriction (BERNOULLI). When fluid flows through a tube of varying cross sectional diameters, the velocities of the various segments of fluid, as postulated by the laws of continuity, are greatest where the diameters are least, and the speed is greatest at this point as well. At the point of widest diameter the pressure is the greatest and the speed is the least.
Blood flows from arteries thru caps to veins because pressure is higher in the arteries than in the veins. The blood in the arteries pushes harder than the blood in the veins. It is the difference in pressure that constitutes the driving force for the fluid. The greatest fall in pressure occurs across the arterioles that enter the capillaries. ( in blood circulation the arterioles offer the greatest resistance to flow) this is frictional resistance which is quantified by using the formula (flow = pr diff/resistance). Resistance is determined mainly, by the radius of a tube, and somewhat by the length of the tube. Being that the greatest fall in pressure falls across the arterioles, they serve as a rate limiting area in circulation, and therefore regulate and blood pressure and flow to specific tissues. Arterioles have bands of smooth muscle which are innervated and controlled by nerves, hormones and other mediators, as mentioned in LO4. Through controlling the radius of the arterioles, the body exercises tight control over the flows and pressures in its own circulation.
Factors controlling VASCULAR REMODELING in response to changes in transmural pressure and the velocity of flow. The greater the metabolism in an organ the greater the blood flow ????????(what does this do to pressure in the vessel) ????????. In pathologic conditions, the flow to tissue/organ can be less than is needed in lite of its metabolism. In an effort for the tissue to survive, REMODELING of the tissue occurs so as to bring adequate blood flow to meet metabolic demands. Remodeling ( a structural change) occurs as a result of long term control of blood flow and is accomplished by the release of endothelin, and or angiotension II(vaso constrictors) and NO a vaso dilator. These vaso active substances counter each other to keep each other in check. If a patient developes hypertension the capillaries will undergo changes to maintain a normal perfusion pressure. If a tissue becomes chronically overactive and therefore requires chronically increased amounts of nutrients, the blood supply usually increases with in a few weeks to match the needs of the tissue – that is, unless the circ system has become pathological or too old to respond. REMODELING can be physiologic or pathophysiologic. Angiotensin II causes pathophysiologic changes (to heart and blood vessels) in response to increases in arterial blood pressure.
After Fem Pop arterial bypass graft( vein used in graft) the vein arterializes in response to long term regulated change in vascular tissue, through endothelin ( increases elastin, collagen, and cell growth – hypertrophy), and modified prn by NO.
References are GUYTON, ADRAINI, PHYSIOLOGY COLORING BOOK AND ANATOMY COLORING BOOK. Direct quotes were taken out of these references.
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