PSL301H Lecture 5: Blood Flow 2022 PDF

PSL301H – Lecture 5: Blood flow How do fluids and nutrients get circulated within the body? How is arteriolar resistance regulated? Silverthorn 7th ed: 440-443, 475-483, 488-493 Silverthorn 8th ed: 436-440, 476-482, 486-492 Blood Vessels: Anatomy Three layers (tunics) Tunic intima...

PSL301H – Lecture 5: Blood flow How do fluids and nutrients get circulated within the body? How is arteriolar resistance regulated? Silverthorn 7th ed: 440-443, 475-483, 488-493 Silverthorn 8th ed: 436-440, 476-482, 486-492 Blood Vessels: Anatomy Three layers (tunics) Tunic intima Endothelium Tunic media Smooth muscle Controlled by sympathetic nervous system Tunic externa Mostly fibrous connective tissue Blood Vessel Structure: Major types High pressure Low pressure Reminder- vessel wall properties of Large Arteries and Large Veins control their function as pressure and volume resevoirs Large Elastic Arteries expand and store energy during ventricular ejection (Windkessel effect) ventricle arteries 1 Ventricle contracts. Arterioles 2 Semilunar valve opens. 1 2 3 Aorta and arteries expand and store pressure in elastic walls. 3 (a) Ventricular contraction Figure 15-4a, steps 1–3 Elastic recoil of arteries keeps blood moving during ventricular relaxation ventricle arteries 1 Isovolumic ventricular relaxation 2 Semilunar valve shuts, preventing 1 flow back into ventricle. 2 3 Elastic recoil of arteries sends blood forward into rest of 3 circulatory system. (b) Ventricular relaxation occurs. Figure 15-4a, steps 1–3 Veins and the Venous Return System Venules drain blood from capillaries, into larger veins Relatively less smooth muscle and connective tissue than arteries valves prevent backflow- series of connected bags. Veins carry about 70% of the body’s blood act as a reservoir during hemorrhage. William Harvey ~1600ad Capillary Beds Capillary beds consist of two types of vessels 1) arteriovenous shunt – directly connects an arteriole to a venule (aka metarteriole) 2) True capillaries – the nutrient exchange vessels a)Oxygen and nutrients diffuse to cells b)Carbon dioxide and metabolic waste products diffuse into blood Only 1 cell layer thick What determines blood/fluid flow in any system? Flow µ DP/R Flow is directly proportional to the driving pressure gradient Flow is inversely proportional to the resistance of the system Fluid Flow through a Tube Depends on the Pressure Gradient Flow is directly proportional to the pressure gradient Flow µ DP The higher the pressure gradient, the greater the fluid flow Fluid flows only if there is a positive pressure gradient (D P). Higher P Flow Lower P Flow P1 P2 P1 - P2 = D P Fluid flow through a tube depends on the pressure gradient Flow depends on the pressure gradient (D P), not on the absolute pressure (P). 100 mm Hg 75 mm Hg Flow D P = 100 - 75 = 25 mm Hg flow is equal: D P is equal 40 mm Hg 15 mm Hg Flow D P = 40 - 15 = 25 mm Hg 125 mm Hg 125 mm Hg Flow flow is ZERO! D P = 125 - 125 = 0 mm Hg Pressure gradient in CV System Blood pressure is greatest in the aorta and decreases as you move through CV system, but always maintaining a positive driving force The heart generates such high aortic pressures The Resistance of a vessel or system affects flow Flow through a tube is inversely proportional to resistance (Flow µ 1/R) If resistance increases, flow decreases If resistance decreases, flow increases Poiseuille’s Law – what determines resistance? R = 8Lh/pr4 or R µ Lh/r4 Resistance is proportional to length (L) of the tube (blood vessel) Resistance increases as length increases Resistance is proportional to viscosity (h), or thickness, of the fluid (blood) Resistance increases as viscosity increases Resistance is inversely proportional to tube radius to the fourth power Resistance decreases as radius increases What happens to R when vessel radius changes? As the radius of a tube decreases, the resistance to flow increases. 1 1 Resistance µ radius4 Flow µ resistance Tube A Tube B Tube A Tube B 1 1 1 1 Rµ Rµ Flow µ Flow µ 1 14 24 1 16 Rµ1 1 Rµ Flow µ 1 Flow µ 16 Radius of A = 1 Radius of B = 2 16 Thinking of a blood vessel: Volume of A = 1 Volume of B = 16 Increase by 25% only: (1.25) =Resistance of 1/2.44 =Flow increase to 244%! Decrease by 25% only: (0.75) =Resistance of 1/0.32 =Flow is decreased to 32% ! Resistance: blood vessels Small change in radius has an enormous effect on resistance to blood flow Vasoconstriction a decrease in blood vessel diameter/radius and decreases blood flow Vasodilation an increase in blood vessel diameter/radius and increases blood flow Example continued: Effect of resistance Flow µ 1/resistance Figure 15-15b Factors that alter arteriolar resistance 1) Myogenic “autoregulation” 2) Paracrines (local) ! Active hyperemia ! Reactive hyperemia 3) Sympathetic control ! SNS: norepinephrine ! Adrenal medulla: epinephrine Active Hyperemia: Locally Mediated Increase in Blood Flow Adenosine Low O2, high CO2 acidic Figure 15-11a Reactive Hyperemia: Locally Mediated Increase in Blood Flow caused by physical blockage (occlusion) System “reacts” to the occlusion - vasodilation Removal of occlusion produces temporary hyperemia Figure 15-11b Sympathetic Regulation: Norepinephrine autonomic control of arteriolar diameter Figure 15-12 GPCRs promote calcium-mediated smooth muscle cell contraction without excitation-contraction coupling Norepinephrine binds alpha-adrenergic receptors PIP2 DAG gb aq aq + PLCb IP3 GDP GTP [Ca2+]i contraction  Calcium oscillations control vascular smooth muscle cell contraction Basal tone- no stimulation Phenylephrine constriction - low % active cells - larger % active cells - low frequency of waves - higher frequency of waves - higher longitudinal velocity of wave Arteriolar diameter affects flow between venous and artery “bags” Flexible filaments allow for large changes in length during contraction Troponin missing! a-actinin Tropomyosin (striated Z-discs) Calponin caldesmon ( ) Cytoskeletal Scaffold (flexible) http://www.le.ac.uk/pa/msc/BotVSMC.pdf ACTIN - 43 kDa globular protein polymerizes into double-helix (forms thin filament) MYOSINS - 2 heavy and 2 pairs of light chains MHC - 200 kDa -the chains form alpha helical rod with 2 heads (Mg2+ -ATPase) MLC1 - essential light chain MLC2 - regulatory light chain http://www.le.ac.uk/pa/msc/BotVSMC.pdf