KIN 292 1nd Edition Lecture 25 Outline of Last Lecture I 13 6 Cardiac Output and Its Control Outline of Current Lecture I 14 1 Physical Laws Governing Blood Flow and Blood Pressure II 14 2 Overview of the Vasculature III 14 3 Arteries IV 14 4 Arterioles part 1 V 12 6 Smooth Muscle Current Lecture 14 1 Universal Flow Rule Flow rate of a liquid through a pipe is directly proportional to the difference between the pressures at the two ends of the pipe pressure gradient and inversely proportional to the resistance of the pipe Pressure gradient P magnitude of force pushing the blood exerted by blood Resistance R the various factors resisting the flow of liquid in a pipe What are they Flow P R We ll consider P 1st and then R You will see the impact of both in your lab exercises for this week the pressure gradient is what matters not the absolute amount of pressure A gradient must exist throughout the circulatory system to maintain blood flow Heart creates a pressure gradient for bulk flow how much is flowing not how it is distributed Note Pressure in vena cava actually slightly above 0 but is so small that author chooses to ignore it for simplicity So MAP approximately to P Figure 14 3 Mean average Pressures and pressure drops in the pulmonary and systemic circuits at rest These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute o Note P higher in systemic What about flow and resistance comparison in two circuits o CVP central venous pressure in large veins in thoracic cavity RA right atrium Resistance in the Cardiovascular System Factors affecting resistance to flow o Radius of vessel Arterioles and small arteries only vessels that can regulate radius o Length of vessel essentially constant in blood vessels o Viscosity of fluid Blood viscosity depends on amount of RBCs and proteins usually constant R 8 L r4 this is Poiseuille s Law o recall that Flow P R o Putting these together Flow P r4 8 L o Since denominator is constant Flow P r4 Rules governing flow resistance and pressure are the same for individual vessels and for a network of vessels which are the total blood vessels in the entire systemic or pulmonary circuits or an individual organ Regulation of radius of arterioles and small arteries is biggest factor in blood flow o Vasoconstriction Decreased radius increased resistance decreased flow through vessel or network o Vasodilation Increased radius decreased resistance increased flow through vessel of network Relating Pressure Gradients and Resistance in the whole Systemic Circulation Flow P R For systemic circulation Flow cardiac output CO P mean arterial pressure MAP R total peripheral resistance TPR where TPR combined resistance of all blood vessels within the entire systemic circuit which is a vast network of blood vessels Flow P R becomes CO MAP TPR 14 2 Overview of Vasculature Arteries rapidly carry blood away from heart Microcirculation o Arterioles o Capillaries site of exchange o Venules Veins return blood to heart Arteries Pressure reservoir keeps blood flowing after LV ejects it Aorta and large arteries o Thick elastic arterial walls with Low compliance Small increase in blood volume causes a large increase in pressure o Expand as blood enters arteries during systole and Recoil during diastole dicrotic notch in chapter 13 Muscular arteries o Less than 0 1 mm in diameter o Little elastin o Smooth muscle regulates radius helps regulate flow distribution similar to arterioles Arterial Blood Pressure Determination Measurement principles and calculations you will do in lab o Estimates aortic pressure usually measured in brachial artery o The measured BP is shown as systolic pressure diastolic pressure SP DP Example 110 70 o Pulse pressure SP DP Example 110 70 40 mm Hg o MAP SP 2 x DP 3 Example 110 140 3 83 3 mm Hg Arterioles resistant vessels main control of resistance to flow Contain rings of smooth muscle to regulate radius and therefore resistance 12 6 Smooth Muscle Found in internal organs and blood vessels Lacks striations that s how it gets its name Contains actin and myosin but no sarcomeres Myosin ATPase contraction is 10 100 times slower in smooth muscle than in skeletal muscle Dense bodies where thick thin filaments attach to connective tissue inside cell to transmit contractile force Under involuntary control by the autonomic nervous system and certain metabolites Spindle shaped Small approximately 1 10 the size of skeletal muscle Single and multi unit smooth muscle o Single unit smooth muscle Most common Includes arterioles respiratory tract intestinal tract o Electrical signals transmitted to all cells via gap junctions All cells contract as a single unit like cardiac Steps of excitation contraction coupling 1 Most Ca2 comes from outside the cell 2 Through Voltage gated Ca2 channels in plasma membrane 3 Ca2 triggers release of Ca2 from sarcoplasmic reticulum 4 Ca2 binds to calmodulin 5 Ca2 calmodulin activates myosin light chain kinase MLCK 6 MLCK phosphorylates myosin 7 Crossbridge cycling Relaxation o Ca2 removed from cytoplasm as in heart o Phosphatase removes phosphate from myosin Smooth Muscle in Arterioles Tone basal level of contraction Has pacemaker cells that result in low level of contraction which can be modified by various factors regulation of arteriole smooth muscle increases decreases tension compared to tone o Intrinsic control local metabolites that increase blood flow vasodilate to match metabolic needs of cells in the region Most common mechanism is opening K channels causing hyperpolarization thus decreasing chance of reaching threshold potential o Extrinsic control autonomic nervous system and hormones leads to either constriction or dilation depending on what receptor and what organ Most common mechanism is altering MLCK and MLCP Ca sensitivity of contraction determined by MLCP Myosin light chain phosphatase MLCP removes phosphate from myosin Inhibiting MLCP activity results in prolonged phosphorylation of MLC thus increasing contraction at any given Ca concentration This increases Ca sensitivity of contraction Stimulation of MLCP activity decreases phosphorylation of MLC thus favoring relaxation at any given Ca concentration This decreases Ca sensitivity of contraction MLCK and MLCP are highly regulated Regulation of smooth muscle contraction involves changes in Ca activation of contraction and in Ca sensitivity of contraction Contractile