BSCI440 Chapter 15 Notes for Exam 2 Cardiovascular Chapter 15 Blood Flow and the Control of Blood Pressure I The Blood Vessels Arteries are considered the pressure reservoir maintains a continuous driving pressure for blood flow during ventricular relaxation Have thick smooth muscle and lots of elasticity fibrous tissue makes it difficult to stretch the walls of arteries Arterioles downstream from arteries create a high resistance outlet for arterial blood flow Direct distribution of blood flow to individual tissues by selectively II constricting and dilating variable resistance Less elastic and more muscular than arteries Capillaries have leaky epithelium that allows exchange of materials between the plasma interstitial fluid and the cells of the body Distal end venous end of capillaries Veins are the volume reservoir from which blood can be sent to the arterial side of the circulation if blood pressure falls too low More numerous than arteries and have larger diameter Blood flow through pulmonary circuit is the same as blood flow through the systemic circulation Blood vessel contain vascular smooth muscle Vasoconstriction narrows diameter of blood vessel lumen Vasodilation widens diameter of blood vessel lumen contraction of smooth muscle is just like cardiac muscle depends on the entry of Ca2 from the extracellular fluid through Ca2 channels Valves prevent backflow and skeletal muscle compresses veins to force blood toward the heart Respiratory pump and skeletal muscle pump contribute to venous return Blood Pressure When ventricle relaxes and the semilunar valve closes the elastic arterial walls recoil propelling blood forward into smaller arteries and arterioles Arteries sustain driving pressure for blood to keep blood flowing continuously Blood pressure is highest in arteries aorta and lowest in veins vena cava energy is lost due to resistance to flow and friction offered by the vessels 120 mmHg normal systolic pressure 80 mmHg normal diastolic pressure Mean arterial pressure MAP reflects the driving pressure for blood flow If blood pressure falls too low hypotension the driving force for blood flow is unable to overcome opposition by gravity blood flow and oxygen supply to the brain are impaired and person may become dizzy or faint If blood pressure is too high hypertension pressure on the blood vessels may cause weakened areas to rupture and bleed into tissues in brain cerebral hemorrhage stroke Cardiac output and peripheral resistance determine mean arterial pressure Balance of blood flow in and out of arteries If flow in exceeds flow out blood collects in arteries and pressure increases 1 III IV If flow out exceeds flow in pressure decreases Blood flow into the aorta is equal to cardiac output of the left ventricle blood flow out of arteries is determined by peripheral resistance resistance offered by arterioles MAP CO Rarterioles Example CO unchanged but peripheral resistance increases flow into arteries is unchanged but flow out is decreased so blood accumulates in arteries and arterial pressure increases hypertension If arterial blood pressure falls increased sympathetic activity constricts veins decreasing their holding capacity and redistributes blood to arterial side of circulation to raise MAP If blood volume increases blood pressure increase and vice versa Resistance in the Arterioles Myogenic autoregulation automatically adjusts blood flow when smooth muscle fibers in walls of arterioles stretch because of increases blood pressure arterioles constrict increasing resistance and decreasing blood flow Muscle cells stretched mechanically gated channels open cation entry depolarizes cell voltage gated Ca2 channels open Ca2 flows into cell and combines with calmodulin to activate myosin light chain kinase MLCK increases myosin ATPase activity and crossbridge activity contraction Paracrines alter smooth muscle contraction O2 CO2 NO Nitric oxide and adenosine triggers vasodilation The sympathetic branch controls most vascular smooth muscle Tonic discharge of norepinephrine from sympathetic neurons helps maintain myogenic tone of arterioles Norepinephrine binding to receptors on smooth muscle causes vasoconstriction less sympathetic release of norepinephrine means vasodilation of arterioles o Epinephrine from adrenal medulla also binds to receptors reinforcing vasoconstriction but have lower affinity for epinephrine o Diverges blood away from less important organs during fight or flight response Epinephrine binds to 2 receptors found on smooth muscle of heart liver and skeletal muscle arterioles and causes vasodilation fight or flight Regulation of Cardiovascular Function Cardiovascular control center CVCC ensures adequate blood flow to the brain and heart by maintain sufficient MAP can also alter function in organs or tissues while leaving others unaffected Example brain gut communication following a meal increases blood flow to intestinal tract thermoregulatory centers in hypothalamus communicate with CVCC to alter blood flow to skin The baroreceptor reflex controls blood pressure Stretch sensitive mechanoreceptors known as baroreceptors are located in the walls of the carotid arteries and aorta where they can monitor the pressure of blood flowing to the brain carotid baroreceptors and body aortic baroreceptors 2 Baroreceptors are tonically active stretch receptors that fire A P s continuously at normal blood pressures when increased blood pressure in arteries stretches the receptors membrane the firing rate of the receptor increases and vice versa Changes in pressure causes changes in frequency of A P s going to CVCC which integrates sensory input and sends output response carried by both sympathetic and parasympathetic neurons changes CO and peripheral resistance o Peripheral resistance under tonic sympathetic control o Increases sympathetic discharge causes vasoconstriction Baroreceptor response to increases blood pressure increased firing rate activates medullary CVCC increases parasympathetic activity and decreases sympathetic activity slow down heart rate dilate arterioles o CO falls when HR falls o Dilate arterioles lower resistance and allowing more out flow o MAP CO R so decrease in CO and R means overall decrease in Orthostatic hypotension triggers the baroreceptor reflex MAP Lying flat blood is evenly distributed throughout body when you stand blood pulls to lower extremities decreases venous return Less blood is in ventricles at next contraction CO falls