Systems Physiology Lecture 4 March 1 2 Organ and tissue blood flow and its local control Organ tissues receive blood flow according to their needs Tissue needs for blood flow are largely determined by ratio of oxygen supply to oxygen demand. Metabolic, myogenic, neurogenic, and hemodynamic mechanisms influence an organ tissue ? s rate of flow Myogenic ability of muscle cel ls to stretch/de-stretch Neurogenic Local blood flow phenomena include: reactive hyperemia, pressure flow autoregulation, active (exercise and functional) hyperemia Anterior/posterior views of the major coronary va s cula ture of the human (mammalian) heart -Landmarks of the heart -Epicardium: outermost layer of epithelial cells on the external surface of the ventricles/atria. -Subpericardium: Anything below the epicardium -Myocardium: Muscle layer between the epicardium an d endocardium -Endocardium: endothelial cells that line the ventricular and atrial chambers -Midmyocardium: Muscle cells that lie midway between the epicardium and endocardium -Of the total amount of blood flow going through the tis s ues of the heart, about 80-85% go through the left main coronary artery and about 15% go through the right main coronary artery. -The left main coronary artery bifurcates into a left anterior descending (LAD) and circumflex branch. -Circumflex branch - ca rries blood flow from the left main coronary artery to the posterior surface of the left ventricle -The great cardiac vein t e rminat e s inside the vena cava where it joins with the right atrium. - The opening of this vein is called the coronary sinus (an ori fice where most of the venous drainage returns to the atrium) Physiologic concept of myocardial oxygen supply and demand -Supply determined by: - Arterial oxygen (leads to myocardial oxygen supply) - Coronary blood flow (leads to myocardial supply) - Vasodilator metabolite is right between myocardial supply and demand (leads to coronary blood flow) -Myocit e s work more vigorously to compensat e for incre ase O2 demand and same-rate myocardial supply - Co ronary arteries dilate to accommodate more blood flow - Supply of oxygen increases - Ratio is reestablished -Content (sum of how much oxygen bound to hemoglobin and how much oxygen is dissolved in solution) of oxygen in arteriole blood -Coronary blood flow (what volume of blood is passing through the LAD and r ight coronary artery -Demand determined by: -Myocardial metabolic rate (leads to myocardial oxygen demand) -Heart rate -Ventricular contractility -Wall tension -This scheme has been called t he adenosine hypothesis ( after Rober t B e rne who first enunciated it in the 1960's and beyond) - Take home message: There' s a ratio between supply and demand and in homeostasis. - The ratio is normally around one when the tissue is demanding/consuming the amount of blood being supplied. -In some cases homeostasis is los t, leading to the heart having to respond to achieve homeostasis. Influence of cardiac cycle on coronary blood flow (CBF, left) and influence of CBF on oxygen use (MVO2, right) - Myocardial oxygen consumption and coronary blood flow are interdependent. - Single cardia c cycle - Aortic blood pressure in the left main coronary artery - And the right main coronary artery - Tension in the wall o f the myocardium extravascular compression -During the systolic phase, when the force is the greatest, the left coronary artery is at its lowest point of blood flow - Systolic pressure and development of systolic pre s sure d i astoly - Majority of blood flow occurs during the diastolic phase of the cardiac cycle -Right corona ry artery sees most blood flow The right ventricle is like an appendage to the left ventricle. An experiment revealing the influences of extra v ascular compression on pulsatile CBF - When the heart fibrillates , it can't contract can't generate force - The oxygen demand is now reduced since the heart is no longer contracting The oxygen that was being us ed when the heart was contracting is no longer being used coronary sinus oxygen goes up - Sympathetic nerve stimulation on those relationships Stimulation coronary blood flow drops - Sympathetic nerves release norepinephrine which stimulates art e rioles. - The norep inephrine also activates myocardium, causing vigorous contraction, elevating b lood flow, causing increase of oxygen demand - The heart rate goes up, blood flow increases, despite sympathetic nerves being stimulated - 2 situations lead to vasodilation: - Neurogenic mediated coronary vasoconstriction - Simultaneous positive inotropic/positive chronotropic demand Vasodilation always has priority over vasoconstriction. Despite nerve stimulation, vasodilation wi n s. Summary Organs and tissues have an intrinsic ability to regulate blood supply Blood flow based on ratio of oxygen demand and supply E. G. when heart experience positive inotropy/chronotoropy, oxygen demand increase s ; so mu s t oxygen supply Oxygen extraction vs. hyperemia as mechanisms for obtaining increase in oxygen supply - Extract more oxygen - Experience coronary hyperemia -Hyperemia: excess flow in coronary circulation by increased demand for oxygen Local blood flow phenomena Reactive hyperemia ? blood flow responds to some intervention suddenly Hyperemia excess blood flow Pressure flow autoregulation - Expresses the relationship between blood pressure and blood flow - Autoregulation the ability of an organ to regulate its own blood supply independe nt of extraorgan influences ( including nerves) Active ( functional , exercise) hyperemia ? in a more controlled action Blood flow increases due to function of the organ (ex. B lood flow increases to the stomach after eating) Coronary reactive hyperemia - Happens in the heart - Similar responses occur in brain, skeleton and skin -C u taneous reactive hyperemia During the period of resting with head against hand, the blood vesse ls are compressed between bones of the hand and the skull. The blood supply to the tissue around the head is limited. The metabolic activity of those tissues are maintained tissues build up an oxygen deficit. Adeno s ine and other vasodilators are release d from those cells. Relax the smooth muscles so the blood supply can enter a relaxed set of vasculature When the hand is removed, more blood flow occurs in less time. -LAD, blood flow drops to zero due to a clamp (occlusion) , leading to an oxygen deficit adenosine release -When the clamp is released, blood flow quickly returns to base level and overshoots. After a return to baseline, metabolites are wash ed out. R eactive hyperemia in perfused skeletal muscle preparation Blood pressure in the femoral artery Blood flow through the femoral artery - The longer the period of occlusion of the blood supply becomes, the greater the duration of hyperemia. -There is a direct correlation between disruption o f blood flow and period of recovery of blood flow. - If blood flow drops to 0, why doesn't pressure in the vessel downstream of the occlusion drop to 0? There are branches downstream from the clamp that still have blood supply flowing through them. Hypoxic environment stimulates a n giogenesis ( growth of collateral blood vessels in the heart ). Hypoxic environments include high altitudes (mountains etc.) Though the left ventricle benefits, the right ventricle experiences hypertension. Pressure flow autoregulation in the skeletal muscle vasculature - The a b ility of an organ to remain relative ly constant despite changes in p erfusion in pressure -Initially, increase of blood flow occurs when there ? s an increase in pressure Blood flow changes as we change pressure - T here's a biphasic response Passive flow response - E levat e the pressure and this pressure distends the vasculature - Vasculature co n tracts in response to this distension - As it contracts, pressure increases back to normal distension Summary Reactive hyperemia a metabolic/myogenic response Duration of hyperemia corresponds to duration of occlusion (damage occurs after 30-40 minutes) Pressure-flow autoregulation, two phases (passive then active) Occurs in sever al organs; multiple mechanisms involved Active hypremia in skeletal muscle - Sending voltage through a motor nerve causes p hasic contractions (twitching) - No occlusion of blood flow before the stimulation Influence of muscle activity on blood flow (active exercise functional hyperemia ) Skeletal muscle makes up a lot of body mass When the muscles are active, they require more than normal cardiac output other parts of the body rece ive less cardiac output During resting, the kidney (renal system) and gut take up about 50% of cardiac output. During exercise, kidney and gut are deprived of blood flow During exercise, skeletal muscles extract more oxygen Alveol ar oxygen uptake as a function of oxygen delivery by peripheral tissues Fit organisms have more maximal oxygen uptake and maximal oxygen delivery (demand)
Want to see the other 5 page(s) in Merrill - Lecture 4?JOIN TODAY FOR FREE!