Circulation Through Special Regions - Coronary Circulation PDF

Circulation Through Special Regions Coronary Circulation Learning Outcomes • Explain the coronary circulation during each cardiac cycle. • Describe the effects of the autonomic nervous system on cardiac blood flow. • Explain the effect of hypoxia on cardiac muscle. • Explain the muscle blood flow...

Circulation Through Special Regions Coronary Circulation Learning Outcomes • Explain the coronary circulation during each cardiac cycle. • Describe the effects of the autonomic nervous system on cardiac blood flow. • Explain the effect of hypoxia on cardiac muscle. • Explain the muscle blood flow at rest and during exercise. • Describe the regulation of cutaneous circulation. Coronary Blood Vessels Coronary arteries • The entire blood supply to the myocardium derives from the right and left coronary arteries, which originate at the root of the aorta behind the cusps of the aortic valves. • Although the anatomy is subject to individual variation, the right coronary artery generally supplies the right ventricle and atrium, and the left coronary artery supplies the left ventricle and atrium. The left coronary artery divides near its origin into two principal branches: • The left circumflex artery sends branches to the left atrium and ventricle, and the left anterior descending artery descends to the apex of the heart and branches to supply the interventricular septum and a portion of the right as well as the left ventricle. • These arteries course over the heart, branching into segments that penetrate the tissue and divide into capillary networks. • Capillary density in histological sections of the human heart exceeds 3000 per square millimeter (skeletal muscle has only ~400 per square millimeter). • The small diameter of cardiac muscle fibers (<20 µm), less than half that of skeletal muscle (~50 µm), facilitates O2 diffusion into the cardiac myocytes, which have a high energetic demand. The right coronary artery branches to form the right marginal artery (RMA) anteriorly. In 80-85% of individuals, it also branches into the posterior interventricular artery posteriorly. • Once blood passes through the capillaries, it collects in venules, which drain outward from the myocardium to converge into the epicardial veins. • These veins empty into the right atrium via the coronary sinus. • Other vascular channels drain directly into the cardiac chambers. These include the thebesian veins, which drain capillary beds within the ventricular wall. • Because the deoxygenated blood carried by the thebesian veins exits predominantly into the ventricles, this blood flow bypasses the pulmonary circulation. • Numerous collateral vessels among branches of the arterial vessels and throughout the venous system act as anastomoses; these provide alternative routes for blood flow should a primary vessel become occluded. The coronary sinus is a wide vein about 2 cm long, which drains most of the venous blood (75% of the total coronary blood flow) from the myocardium (mainly the left ventricle) into the right atrium. Its tributaries are the great cardiac vein, the small cardiac vein, the posterior vein of the left ventricle, and the oblique vein of the left ventricle. Most of the coronary venous blood from the right ventricular muscle returns through small anterior cardiac veins that flow directly into the right atrium, not by way of the coronary sinus. A very small amount (10%) of coronary venous blood also flows back into the heart through very minute thebesian veins, which empty directly into all chambers of the heart. Normal Coronary Blood Flow Averages 5% Of Cardiac Output • The normal coronary blood flow in the resting person averages 70 ml/min/100 g of heart weight, or about 225 ml/min, which is about 4% to 5% of the total cardiac output. • During strenuous exercise, the heart in the young adult increases its cardiac output fourfold to sevenfold, and it pumps this blood against a higher than normal arterial pressure. • Consequently, the work output of the heart under severe conditions may increase 6-fold to 9-fold. • At the same time, the coronary blood flow increases 3-fold to 4-fold to supply the extra nutrients needed by the heart. • This increase is not as much as the increase in workload, which means that the ratio of energy expenditure by the heart to coronary blood flow increases. • Thus, the efficiency of cardiac utilization of energy increases to make up for the relative deficiency of coronary blood supply. Cardiac Muscle Compression Causes Phasic Changes in Coronary Blood Flow During Systole and Diastole • In other systemic vascular beds, blood flow roughly parallels the pressure profile in the aorta, rising in systole and falling in diastole. • However, in coronary circulation, flow is somewhat paradoxical: the coronary capillary blood flow in the left ventricle muscle falls to a low value during systole, which is opposite to flow in vascular beds elsewhere in the body. • The reason for this phenomenon is the strong compression of the intramuscular blood vessels by the left ventricular muscle during systolic contraction. • During diastole, the cardiac muscle relaxes and no longer obstructs blood flow through the left ventricular muscle capillaries, so blood flows rapidly during all of the diastole Figure shows the changes in blood flow through the nutrient capillaries of the left ventricular coronary system in ml/min in the heart during systole and diastole, as extrapolated from studies in experimental animals. • Blood flow through the coronary capillaries of the right ventricle also undergoes phasic changes during the cardiac cycle but, because the force of contraction of the right ventricular muscle is far less than that of the left ventricular muscle, the inverse phasic changes are only partial, in contrast to those in the left ventricular muscle. Blood flow in the left coronary artery may actually reverse transiently in early systole because the force of the left ventricle's isovolumetric contraction compresses the left coronary vessels and the aortic pressure has not yet begun to rise (i.e., the aortic valve is still closed). As aortic pressure increases later during systole, coronary blood flow increases, but never reaches peak values. However, early during diastole, when the relaxed ventricles no longer compress the left coronary vessels and aortic pressure is still high, left coronary flow rises rapidly to extremely high levels. All told, ~80% of total left coronary blood flow occurs during diastole. Regulation of Coronary Blood Flow Local Muscle Metabolism Blood flow through the coronary system is regulated mostly by local arteriolar vasodilation in response to the nutritional needs of cardiac muscle. That is, whenever the vigor of cardiac contraction is increased, the rate of coronary blood flow also increases. Conversely, decreased heart activity is accompanied by decreased coronary flow. This local regulation of coronary blood flow is similar to that which occurs in many other tissues of the body, especially in the skeletal muscles. Oxygen Demand Is a Major Factor in Local Coronary Blood Flow Regulation Blood flow in the coronary arteries usually is regulated almost exactly in proportion to the need of the cardiac musculature for oxygen. Normally, about 70% of the oxygen in the coronary arterial blood is removed as the blood flows through the heart muscle. Because not much oxygen is left, little additional oxygen can be supplied to the heart’s musculature unless the coronary blood flow increases. Fortunately, the coronary blood flow increases almost in direct proportion to any additional metabolic consumption of oxygen by the heart. • The close relationship between coronary blood flow and myocardial O2 consumption indicates that one or more of the products of metabolism cause coronary vasodilation. Factors suspected of playing this role include O2 lack and increased local concentrations of CO2, H+, K+, lactate, prostaglandins, adenine nucleotides, NO, and adenosine. • If oxygen consumption in heart muscle exceeds the rate at which oxygen is supplied by the blood, myocardial hypoxia results. • In response to low tissue oxygen, the myocardial cells release adenosine. Adenosine dilates coronary arterioles in an attempt to bring additional blood flow into the muscle. • Adenosine is not the only vasodilator product that has been identified; others include adenosine phosphate compounds, potassium ions, hydrogen ions, carbon dioxide, prostaglandins, and nitric oxide. Nervous Control Mechanism • Stimulation of the autonomic nerves to the heart can affect coronary blood flow directly and indirectly. • The direct effects result from the action of the nervous transmitter substances acetylcholine from the vagus nerves and norepinephrine from the sympathetic nerves on the coronary vessels. • The indirect effects result from secondary changes in coronary blood flow caused by increased or decreased activity of the heart. • Thus, sympathetic stimulation, which releases norepinephrine from the sympathetic nerves and epinephrine, as well as norepinephrine from the adrenal medullae, increases both heart rate and heart contractility and increases the rate of metabolism of the heart. • In turn, the increased metabolism of the heart sets off local blood flow regulatory mechanisms for dilating the coronary vessels and blood flow increases approximately in proportion to the metabolic needs of the heart muscle. • In contrast, vagal stimulation, with its release of acetylcholine, slows the heart and has a slightly depressive effect on heart contractility. These effects decrease cardiac oxygen consumption and, therefore, indirectly constrict the coronary arteries. • Much more extensive sympathetic innervation of the coronary vessels occurs. • Norepinephrine and epinephrine can have vascular constrictor or vascular dilator effects, depending on the presence or absence of constrictor or dilator receptors in the blood vessel walls. • The constrictor receptors are called alpha receptors, and the dilator receptors are called beta receptors. • Both alpha and beta receptors exist in the coronary vessels. • In general, the epicardial coronary vessels have a preponderance of alpha receptors, whereas the intramuscular arteries may have a preponderance of beta receptors. • Therefore, sympathetic stimulation can, at least theoretically, cause slight overall coronary constriction or dilation, but usually constriction. Coronary Artery Disease (Ischemic Heart Disease) • When flow through a coronary artery is reduced to the point that the myocardium it supplies becomes hypoxic, angina pectoris develops. • If the myocardial ischemia is severe and prolonged, irreversible changes occur in the muscle, and the result is myocardial infarction. • Many individuals have angina only on exertion, and blood flow is normal at rest. Others have more severe restriction of blood flow and have anginal pain at rest as well. • Partially occluded coronary arteries can be constricted further by vasospasm, producing myocardial infarction. • However, it is now clear that the most common cause of myocardial infarction is the rupture of an atherosclerotic plaque, or hemorrhage into it, which triggers the formation of a coronary-occluding clot at the site of the plaque. Skeletal Muscle Circulation • During rest, skeletal muscle blood flow averages 3 to 4 ml/min/100 g of muscle. • During extreme exercise in the well-conditioned athlete, this blood flow can increase 25-to 50-fold, rising to 100 to 200 ml/min/100 g of muscle. • Peak blood flows as high as 400 ml/min/100 g of muscle have been reported for thigh muscles of endurance-trained athletes. Blood Flow During Muscle Contractions. • Figure shows a record of blood flow changes in a calf muscle of a leg during strong rhythmic muscular exercise. • Note that the flow increases and decreases with each muscle contraction. • At the end of the contractions, the blood flow remains high for a few seconds but then returns to normal during the next few minutes. Effects of muscle exercise on blood flow in the calf of a leg during strong rhythmic contraction. The blood flow was much less during contractions than between contractions. • The cause of the lower flow during the muscle contraction phase of exercise is the compression of the blood vessels by the contracted muscle. • During strong tetanic contraction, which causes sustained compression of the blood vessels, the blood flow can be almost stopped, but this also causes rapid weakening of the contraction. Increased Blood Flow in Muscle Capillaries During Exercise. • During rest, some muscle capillaries have little or no flowing blood, but during strenuous exercise, all the capillaries open. This opening of dormant capillaries diminishes the distance that oxygen and other nutrients must diffuse from the capillaries to the contracting muscle fibers; it sometimes contributes a twofold to threefold increased capillary surface area through which oxygen and nutrients can diffuse from the blood to the tissues. Effects of muscle exercise on blood flow in the calf of a leg during strong rhythmic contraction. The blood flow was much less during contractions than between contractions. Cutaneous Circulation • Cutaneous arterioles form a dense network just under the dermis layer of the skin. • Meta-arterioles which arise from the arterioles are relatively high-resistance conduits present between the arterioles and capillaries. • Cutaneous capillaries. The meta-arterioles subdivide into capillary loops, which provide a large surface area for heat exchange. • Venules form an extensive subpapillary venous plexus which holds large quantity of blood and lie parallel to the surface of skin, and play an important role in maintaining the body temperature. • Arteriovenous anastomoses are located in the distal parts of the extremities (hands and feet), nose, lips, and ear lobules. • These vessels serve as shunts and allow blood to bypass the superficial capillary loops and play a major role in the control of body temperature. Regulation Of Cutaneous Blood Flow • The cutaneous blood flow is predominantly regulated by the nervous control. Nerve supply of cutaneous vessels • Sympathetic vasoconstrictor nerves supplying the cutaneous vessels exhibit a sympathetic constrictor discharge under resting conditions. The sympathetic tonic discharge is more marked on A–V anastomoses vessels than the other vessels. • Parasympathetic vasodilator nerves do not supply the cutaneous blood vessels. Vasodilation of cutaneous vessels results due to: • Reduction of sympathetic vasoconstrictor effect, • Local production of bradykinin (a potent vasodilator polypeptide) in sweat glands and • Production of other local vasodilator substances. Neural control mechanisms • The cutaneous blood flow is regulated by the following neural control mechanisms. Hypothalamic control mechanism • The reflex increase or decrease in the sympathetic discharge to cutaneous vessels during thermoregulation is mediated through the temperature regulation centers of the hypothalamus as: • Under resting conditions, i.e. when the person is at thermal equilibrium with the environment (at about 27°C atmospheric temperature), the sympathetic vasoconstrictor fibres have a mild tonic discharge. The tonic sympathetic discharge normally keeps the A–V anastomoses closed. During exposure to heat stress, the tonic sympathetic discharge is reflexly abolished by a hypothalamic mechanism. • Thus the blood flow to the skin is increased by following responses in a chronological sequence: • First of all, A–V anastomoses of hands, feet, and ear lobules dilate due to a reduction in the sympathetic tonic discharge. • Secondly, the rest of the cutaneous vessels dilate due to progressive withdrawal of sympathetic vasoconstrictor activity. • Thirdly, sweat glands get activated due to the cholinergic sympathetic discharge. The bradykinin produced by the secretory activity of the sweat glands acts locally as a powerful vasodilator and increases blood flow to skin. • During exposure to cold stress, via hypothalamic mechanism, there occurs widespread cutaneous vasoconstriction due to increased sympathetic discharge. Baroreceptor-mediated reflex • Cutaneous blood vessels participate in the baroreceptor mediated reflexes during conditions of circulatory stress, such as exercise and hemorrhage. Cortical control mechanism • The emotions affect the cutaneous circulation through the corticohypothalamic pathway. The effects of emotions on cutaneous circulation manifest in the following forms: • Blanching of skin during situations of fear (pale with fear) occurs due to vasoconstriction mediated through cortical mechanism. • Phenomenon of blushing, i.e. emotional embarrassment occurs due to vasodilation of vessels.