1625915643-5d53d156c9525bd62bd0d3434ecdc231 (843955), страница 37
Текст из файла (страница 37)
The redundant pathways for blood flow helpto ensure adequate blood flow to the brain in case of injury ordisease, for example due to stenosis of vessels or blockage dueto thrombosis. There is considerable variation in the exact anatomic structure of the circle of Willis between individuals, suchthat fewer than half of the population has the most commonstructure illustrated in many textbooks.in distribution of cerebral blood flow to various parts of thebrain, depending on neuronal activity in those regions.In addition, there are two reflexes that may affect cerebralblood flow:■■SPECIAL CIRCULATIONSBlood flow to various tissues is regulated by local and extrinsicmechanisms, and the importance of these varies among tissues(Fig.
12.8). In addition, there are unique aspects of some circulations. A number of these “special circulations” merit individual consideration.CNS ischemic reflex: If the vasomotor center in themedulla becomes ischemic, strong sympathetic outflowacts on the heart and peripheral circulation to increasearterial blood pressure above the autoregulatory rangefor cerebral blood flow, thus increasing blood flow tothe brain in a “last-ditch” effort to reverse the ischemia.The cerebral circulation is itself not innervated bysympathetic nerves.Cushing reflex: An increase in intracranial pressure,usually associated with traumatic head injury, mayimpede cerebral blood flow. A strong sympatheticoutflow will result, raising arterial pressure in an attemptto overcome the high intracranial pressure that is impeding flow.
This is also a last-ditch mechanism to preservecerebral perfusion. High arterial pressure is accompanied by bradycardia, due to activation of arterialbaroreceptors.Cerebral CirculationThe cerebral circulation is supplied by the arterial circle ofWillis, which is derived from the internal carotid and thevertebral arteries. Because the brain is a vital tissue, housed ina rigid case (the cranium), it is necessary to closely regulateits blood flow. Flow below 35 milliliters per minute (mL/min)causes neuronal dysfunction; high flow may result in cerebraledema and high intracranial pressure, which also producedysfunction.
In a normal individual, cerebral blood flow ismaintained at a constant level of 50 mL/min for mean arterialpressures between 50 mm Hg and 150 mm Hg. This level ismaintained by autoregulation (myogenic regulation).Cerebral blood flow is also under the control of arterial PCO2,although this blood gas does not vary significantly in a healthyperson except during extreme exercise or hyperventilation orhypoventilation. Decreased arterial PCO2 will produce vasoconstriction and reduced cerebral blood flow; high arterialPCO2 levels will cause vasodilation and increased flow. Notethat this response is to arterial PCO2.
Global cerebral bloodflow is not significantly regulated by local metabolic factors(CO2, O2, K+, H+, etc.), but metabolic regulation participatesCoronary CirculationLike any metabolically active tissue, the myocardium receivesits own arterial supply. The coronary circulation is fed by theright and left coronary arteries that originate at the base ofthe aorta (Fig. 12.9). These are also called epicardial arteriesbecause they lie on the surface of the heart (epicardium) andsupply branches into the myocardium. These branches forma very extensive microcirculation, supplying oxygen andnutrients and removing metabolites from the highly metabolically active myocardium. The heart has a very large arteriovenous O2 gradient, even at rest.The left coronary artery gives rise to the left anterior descending (LAD) artery and the circumflex artery; likewise, the rightcoronary artery gives rise to several branches. The venousdrainage of the heart returns blood to the coronary sinus,which empties into the right atrium.
The layers of the heart,from inside to outside, are as follows:■Endocardium: The innermost layer is connective tissue,lined by endothelial cells.The Peripheral Circulation135Brain: Blood flow normally remains constant (autoregulation), althoughregional variation related to brain activity occurs. Little influence ofautonomic nervous system. Local metabolic factors most important forcontrolling blood flow (e.g., H+, CO2, O2, and adenosine increase bloodflow).Coronary arteries: Flowinterrupted during systole. Littleinfluence of autonomic nervoussystem. Local metabolic factorsmost important for controllingblood flow (e.g., adenosine, NO,and decreased O2 increaseblood flow).Lungs: Important to matchperfusion to ventilation. Littleinfluence of autonomic nervoussystem. Changes in O2 tensionmost important in regulatingblood flow (decreased O2decreases blood flow, andincreased O2 increases bloodflow).Liver and splanchnic beds: Bloodflow regulated to facilitatedigestion.
Sympathetic nervesdecrease blood flow, andparasympathetic nerves (lowerGI tract only) increase bloodflow. GI hormones (gastrin,cholecystokinin, secretin)increase blood flow, as doproducts of digestion (glucoseand fatty acids).Kidneys: Blood flow normallymaintained constant(auto-regulation) to maintainnormal glomerular filtration.Sympathetic nerves decreaseblood flow. Circulatinghormones also affect blood flow(A-II and vasopressin decreaseblood flow; ANP increases bloodflow).Right sideof heartLeft sideof heartSkin: Blood flow altered forthermoregulation. Sympatheticnerves decrease blood flow(␣-receptors). Circulatinghormones also affect blood flow.A-II and vasopressin decreaseblood flow.Skeletal muscle: Majordeterminant of total peripheralresistance. Blood flow regulatedin response to exercise.
Basalflow set by sympathetic nerves.Circulating hormones also affectflow (A-II and vasopressindecrease blood flow). Withexercise, release of metabolites(e.g., H+, lactate, CO2,adenosine) increases blood flow.Figure 12.8 Circulation to Special Regions Mechanisms regulating blood flow vary among regionsin the systemic circulation, reflecting the physiological functions and needs of the tissues.136Cardiovascular PhysiologySinoatrial (SA) nodal branchSinoatrial (SA) nodal branchLeft coronary arteryAnterior atrialbranch of rightcoronary arteryCircumflex branch ofleft coronary arteryAortaGreat cardiac veinRightcoronaryarteryAnteriorinterventricular(ant. descending)branch of leftcoronary arteryRightcoronaryarteryMiddlecardiacveinSternocostalaspectAortic Pressure(mm Hg)SmallcardiacveinCoronary sinusSmall cardiacveinDiaphragmaticaspectPosteriorinterventricular(post.
descending)branch of rightcoronary artery12010080400Left coronaryartery300Phasic Coronary Blood Flow(mL/min)Sinoatrial(SA) node2001000Right coronaryartery60402000.20.40.60.81.0Time (seconds)Figure 12.9 Coronary Circulation The coronary arteries supply arterial blood to the muscular wallof the heart. The pressure gradient for flow through the arteries is affected by tissue pressure in the wall ofthe heart during systole, particularly in the left coronary circulation.
Thus, while flow through both right andleft coronary arteries is related to aortic pressure, these compressive forces reduce left coronary flow duringsystole. During diastole, flow is greatly enhanced in the left coronary circulation by the actions of vasodilatormetabolites that build up during systole. The metabolite adenosine is believed to be particularly importantin this enhancement of coronary flow.The Peripheral Circulation■■Myocardium: The muscular layer that forms the bulk ofthe heart wall.
The subendocardium is the inner 1⁄3 ofthe myocardium, and the subepicardium is the outer 1⁄3of the myocardium.Epicardium: The outer, connective tissue layer of theheart. It forms the visceral pericardium. The parietalpericardium is a sac enclosing the heart; the pericardialcavity is the space between the visceral and parietalpericardium.Flow to the myocardium is affected by factors that are in someways different from those regulating flow in other circulations. In particular, coronary blood flow is affected greatly bythe following:■■Compression of the coronary circulation caused bycontraction of myocardium.Powerful metabolic vasodilation during diastole.The pumping action of the left ventricle generates the normalresting arterial pressure of 120/80 mm Hg to supply blood tothe systemic circulation.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
