Roles of Capillaries In addition to forming the connection between the arteries and veins, capillaries have a vital role in the exchange of gases, nutrients, and metabolic waste products between the blood and the tissue cells. Substances pass through the capillary wall by diffusion, filtration, and osmosis. Oxygen and carbon dioxide move across the capillary wall by diffusion. Fluid movement across a capillary wall is determined by a combination of hydrostatic and osmotic pressure. The net result of the capillary microcirculation created by hydrostatic and osmotic pressure is that substances leave the blood at one end of the capillary and return at the other end. Show
Blood FlowBlood flow refers to the movement of blood through the vessels from arteries to the capillaries and then into the veins. Pressure is a measure of the force that the blood exerts against the vessel walls as it moves the blood through the vessels. Like all fluids, blood flows from a high pressure area to a region with lower pressure. Blood flows in the same direction as the decreasing pressure gradient: arteries to capillaries to veins. The rate, or velocity, of blood flow varies inversely with the total cross-sectional area of the blood vessels. As the total cross-sectional area of the vessels increases, the velocity of flow decreases. Blood flow is slowest in the capillaries, which allows time for exchange of gases and nutrients. Resistance is a force that opposes the flow of a fluid. In blood vessels, most of the resistance is due to vessel diameter. As vessel diameter decreases, the resistance increases and blood flow decreases. Very little pressure remains by the time blood leaves the capillaries and enters the venules. Blood flow through the veins is not the direct result of ventricular contraction. Instead, venous return depends on skeletal muscle action, respiratory movements, and constriction of smooth muscle in venous walls. Pulse and Blood PressurePulse refers to the rhythmic expansion of an artery that is caused by ejection of blood from the ventricle. It can be felt where an artery is close to the surface and rests on something firm. In common usage, the term blood pressure refers to arterial blood pressure, the pressure in the aorta and its branches. Systolic pressure is due to ventricular contraction. Diastolic pressure occurs during cardiac relaxation. Pulse pressure is the difference between systolic pressure and diastolic pressure. Blood pressure is measured with a sphygmomanometer and is recorded as the systolic pressure over the diastolic pressure. Four major factors interact to affect blood pressure: cardiac output, blood volume, peripheral resistance, and viscosity. When these factors increase, blood pressure also increases. Arterial blood pressure is maintained within normal ranges by changes in cardiac output and peripheral resistance. Pressure receptors (barareceptors), located in the walls of the large arteries in the thorax and neck, are important for short-term blood pressure regulation.
<br> </td> <td width="2" bgcolor="#ffcc33"><img src="http://www.vhlab.umn.edu/atlas/graphics/yellowstroke.gif" alt=" " width="2" border="0"></td> <td width="789" valign="top" class="headbkgimage"> <table width="789" border="0" cellpadding="0" cellspacing="0"> <tr> <td colspan="8" align="left"><a target="_blank" href="http://www.vhlab.umn.edu/atlas/physiology-tutorial/index.shtml"><img src="http://www.vhlab.umn.edu/atlas/graphics/PhysTutHeader.gif" alt="Physiology Tutorial" width="789" height="29" border="0"></a></td> </tr> <tr> <td width="64"><a target="_blank" href="http://www.vhlab.umn.edu/atlas/physiology-tutorial/blood.shtml" onmouseout="MM_swapImgRestore()" onmouseover="MM_swapImage('PhysTsub_0161','','/atlas/graphics/PhysTutsub_01-over.gif',1)"><img src="http://www.vhlab.umn.edu/atlas/graphics/PhysTutsub_01.gif" alt="Blood" name="PhysTsub_0161" width="64" height="45" border="0" id="PhysTsub_0161"></a></td> <td width="77"><a target="_blank" href="http://www.vhlab.umn.edu/atlas/physiology-tutorial/blood-vessels.shtml" onmouseout="MM_swapImgRestore()" onmouseover="MM_swapImage('PhysTsub_0261','','/atlas/graphics/PhysTutsub_02-over.gif',1)"><img src="http://www.vhlab.umn.edu/atlas/graphics/PhysTutsub_02.gif" alt="Blood Vessels" name="PhysTsub_0261" width="77" height="45" border="0" id="PhysTsub_0261"></a></td> <td width="67"><a target="_blank" href="http://www.vhlab.umn.edu/atlas/physiology-tutorial/blood-flow.shtml" onmouseout="MM_swapImgRestore()" onmouseover="MM_swapImage('PhysTsub_0361','','/atlas/graphics/PhysTutsub_03-over.gif',1)"><img src="http://www.vhlab.umn.edu/atlas/graphics/PhysTutsub_03.gif" alt="Blood Flow" name="PhysTsub_0361" width="67" height="45" border="0" id="PhysTsub_0361"></a></td> <td width="100"><a target="_blank" href="http://www.vhlab.umn.edu/atlas/physiology-tutorial/the-human-heart.shtml" onmouseout="MM_swapImgRestore()" onmouseover="MM_swapImage('PhysTsub_0461','','/atlas/graphics/PhysTutsub_04-over.gif',1)"><img src="http://www.vhlab.umn.edu/atlas/graphics/PhysTutsub_04.gif" alt="The Human Heart" name="PhysTsub_0461" width="100" height="45" border="0" id="PhysTsub_0461"></a></td> <td width="120"><a target="_blank" href="http://www.vhlab.umn.edu/atlas/physiology-tutorial/cardiovascular-function.shtml" onmouseout="MM_swapImgRestore()" onmouseover="MM_swapImage('PhysTsub_0561','','/atlas/graphics/PhysTutsub_05-over.gif',1)"><img src="http://www.vhlab.umn.edu/atlas/graphics/PhysTutsub_05.gif" alt="Cardiovascular Function" name="PhysTsub_0561" width="120" height="45" border="0" id="PhysTsub_0561"></a></td> <td width="100"><a target="_blank" href="http://www.vhlab.umn.edu/atlas/physiology-tutorial/coronary-circulation.shtml" onmouseout="MM_swapImgRestore()" onmouseover="MM_swapImage('PhysTsub_0661','','/atlas/graphics/PhysTutsub_06-over.gif',1)"><img src="http://www.vhlab.umn.edu/atlas/graphics/PhysTutsub_06-over.gif" alt="Coronary Circulation" name="PhysTsub_0661" width="100" height="45" border="0" id="PhysTsub_0661"></a></td> <td width="100"><a target="_blank" href="http://www.vhlab.umn.edu/atlas/physiology-tutorial/references-and-sources.shtml" onmouseout="MM_swapImgRestore()" onmouseover="MM_swapImage('PhysTsub_0861','','/atlas/graphics/PhysTutsub_08-over.gif',1)"><img src="http://www.vhlab.umn.edu/atlas/graphics/PhysTutsub_08.gif" alt="References and Sources" name="PhysTsub_0861" width="100" height="45" border="0" id="PhysTsub_0861"></a></td> <td width="161" align="left"></td> </tr> </table> <table width="789" border="0" cellpadding="0" cellspacing="0"> <tr valign="top"> <td valign="top" width="779" class="descbkgimage"> <table width="779" border="0"> <tr valign="top"> <td width="15" height="15"></td> <td width="764" height="15"></td> </tr> <tr valign="top"> <td></td> <td> <table border="0" cellspacing="0" cellpadding="0"> <tr valign="top"> <td class="tutorialtext"> <p>In order to sustain viability, it is not possible for nutrients to diffuse from the chambers of the heart through all the layers of cells that make up the heart tissue. Thus, the coronary circulation is responsible for delivering blood to the heart tissue itself (the myocardium). The normal heart functions almost exclusively as an aerobic organ with little capacity for anaerobic metabolism to produce energy. Even during resting conditions, 70 to 80% of the oxygen available within the blood circulating through the coronary vessels is extracted by the myocardium. It then follows that because of the limited ability of the heart to increase oxygen availability by further increasing oxygen extraction, increases in myocardial demand for oxygen (e.g., during exercise or stress) must be met by equivalent increases in coronary blood flow. Myocardial ischemia results when the arterial blood supply fails to meet the needs of the heart muscle, for oxygen and/or metabolic substrates. Even mild cardiac ischemia can result in anginal pain, electrical changes (detected on an electrocardiogram) and the cessation of regional cardiac contractile function. Sustained ischemia within a given myocardial region will most likely result in an infarction.</p> <p>As noted above, as in any microcirculatory bed, the greatest resistance to coronary blood flow occurs in the arterioles. Blood flow through such vessels varies approximately with the fourth power of these vessels' radii; hence, the key regulated variable for the control of coronary blood flow is the degree of constriction or dilatation of coronary arteriolar vascular smooth muscle. As with all systemic vascular beds, the degree of coronary arteriolar smooth muscle tone is normally controlled by multiple independent negative feedback loops. These mechanisms include various neural, hormonal, local non-metabolic and local metabolic regulators. It should be noted that the local metabolic regulators of arteriolar tone are usually the most important for coronary flow regulation; these feedback systems involve oxygen demands of the local cardiac myocytes. In general, at any one point in time, coronary blood flow is determined by integrating all the different controlling feedback loops into a single response (i.e., inducing either arteriolar smooth muscle constriction or dilation). It is also common to consider that some of these feedback loops are in opposition to one another. Interestingly, coronary arteriolar vasodilation from a resting state to one of intense exercise can result in an increase of mean coronary blood flow from approximately 0.5 to 4.0 ml/min/gram. </p> <p>As with all systemic circulatory vascular beds, the aortic or arterial pressure (perfusion pressure) is vital for driving blood through the coronaries, and thus needs to be considered as another important determinant of coronary flow. More specifically, coronary blood flow varies directly with the pressure across the coronary microcirculation, which can be essentially considered as the aortic pressure, since coronary venous pressure is near zero. However, since the coronary circulation perfuses the heart, some very unique determinants for flow through these capillary beds may also occur; during systole, myocardial extravascular compression causes coronary flow to be near zero, yet it is relatively high during diastole (note that this is the opposite of all other vascular beds in the body). </p> <table border="0" cellspacing="0" cellpadding="0"> <tr valign="top"> <td class="tutorialtext"> <p>Oxygenated blood is pumped into the aorta from the left ventricle. This is where it enters the right and left main coronary arteries, and subsequent branching feeds the myocardial tissue of all four chambers of the heart (see Figure 7). The ascending portion of the aorta is where the origins (ostia) of the right and left coronaries reside; specifically, they exit the ascending aorta immediately superior to the aortic valve at the sinus of Valsalva. Blood flow into the coronary arteries is greatest during ventricular diastole when aortic pressure is highest and it is greater than in the coronaries. Typically the right coronary artery courses along the right anterior atrioventricular groove just below the right atrial appendage and along the epicardial surface adjacent to the tricuspid valve annulus. It traverses along the tricuspid annulus until it reaches the posterior surface of the heart, where it then commonly becomes the posterior descending artery and runs toward the apex of the left ventricle. Along its course, a number of branches emerge, most notably those that supply the sinus node and the atrioventricular node; hence blockage of such vessels can lead to conduction abnormalities. Additionally, several marginal branches run to the right ventricular and right atrial epicardial surfaces. The left main coronary artery typically bifurcates quickly upon exiting the ascending aorta into the left circumflex and left anterior descending arteries. The left circumflex artery runs under the left atrial appendage on its way to the lateral wall of the left ventricle. Along the way, it spawns a number of branches that supply the left atrial and left ventricular walls. In some cases, a branch will course behind the aorta to the superior vena cava such that it can supply the sinus node. The left anterior descending artery supplies a major portion of the ventricular septum, including the right and left bundle branches of the myocardial conduction system, and the anterior and apical portions of the left ventricle.</p> </td> <td width="10"></td> <td width="410"> <table width="410" border="1" cellspacing="0" cellpadding="5"> <tr> <td> <p align="center"><img src="http://www.vhlab.umn.edu/atlas/physiology-tutorial/graphics/fig7.gif" alt="Figure 7" width="400" height="406" border="0"></p> <p>Figure 7. Drawing of the coronary arterial circulation in the human heart. The normal human hears does not typically elicit collateralization; each area of myocardium is usually supplied by a single coronary artery. Ao = aorta; LAD = left anterior descending artery; LCx = left circumflex artery; PA = pulmonary artery; RCA = right coronary artery.</p> </td> </tr> </table> </td> </tr> </table> <p>&nbsp;</p> <p>Coronary arteries are so vital to the function of heart; whenever disease states are associated with flow restriction through the coronary arteries, and subsequently the remainder of the coronary circulations (capillaries and veins), the effects on cardiac performance are quite dramatic and often fatal. Coronary artery disease (CAD) is generally defined as the gradual narrowing of the lumen of the coronary arteries due to coronary atherosclerosis. Atherosclerosis is a condition that involves thickening of the arterial walls from cholesterol and fat deposits that build up along the endoluminal surface of the arteries. With severe disease, these plaques may become calcified and so large that they produce stenoses within the vessels, and thus permanently increase the vascular resistance which is normally low. When the walls of the coronary arteries thicken, the cross-sectional area of the arterial lumen decreases; resulting in higher resistance to blood flow through the coronary arteries. This steady decrease in cross-sectional area can eventually lead to complete blockage of the artery. As a result, oxygen and nutrient supply to the myocardium drops below the demand of the myocardium. As the disease progresses, the myocardium downstream from the occluded artery becomes ischemic. Eventually, myocardial infarction (or known as a MI) may occur if the coronary artery disease is not detected and treated in a timely manner.</p> <p>Myocardial ischemia not only impairs the electrical and mechanical function of the heart, but also commonly results in intense, debilitating chest pain known as angina pectoris. However, anginal pain can often be absent in individuals with coronary artery disease when they are resting (or in individuals with early disease stages), but induced during physical exertion or with emotional excitement.</p> </td> </tr> </table> </td> </tr> </table> </td> <td width="10"><img src="http://www.vhlab.umn.edu/atlas/graphics/circles_line_01.gif" alt=" "></td> </tr> </table> </td> </tr> </table> <img src="http://www.vhlab.umn.edu/atlas/graphics/yellowstroke.gif" alt=" " width="980" height="2" border="0"><br> <table width="980" border="0" cellpadding="0" cellspacing="0"> <tr><td> <div align="center"> <small>© 2021 Regents of the University of Minnesota. All rights reserved. The University of Minnesota is an equal opportunity educator and employer. <a target="_blank" href="http://privacy.umn.edu/">Privacy Statement</a></small> </div> <p></p> </td></tr></table> </body> </html></td></tr></table></body></html> Is blood flow through the coronary circulation greatest during systole or diastole?In most tissues, blood flow peaks during ventricular systole due to increased pressure in the aorta and its distal branches. Bloodflow through the coronary vessels, however, is seemingly paradoxical and peaks during ventricular diastole.
During which part of the cardiac cycle is coronary blood flow the greatest?Blood flow into the coronary arteries is greatest during ventricular diastole when aortic pressure is highest and it is greater than in the coronaries.
Why are the coronary arteries perfused during diastole rather than systole?Because these vessels traverse the myocardium, myocardial contraction during systole compresses arterial branches and prevents perfusion. Therefore, coronary perfusion occurs during diastole rather than systole.
Why is coronary blood flow highest when the myocardium is relaxed?This is due to a strong contraction particularly from the left ventricle which compresses the intramuscular vessels. During Diastole the cardiac muscle relaxes, enabling blood to flow through the capillaries with no obstruction.
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Is blood flow through the coronary circulation greatest during systole or diastole explain quizlet?
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