Chapter 10 Vascular Physiology PDF

Chapter 10 Vascular Physiology Vascular Physiology Objectives Outline the circulatory system Describe the determinants of blood flow in the vascular system Describe the functions of different vessels types Integrate knowledge of heart and vascular functions to describe systemic blood pre...

Chapter 10 Vascular Physiology Vascular Physiology Objectives Outline the circulatory system Describe the determinants of blood flow in the vascular system Describe the functions of different vessels types Integrate knowledge of heart and vascular functions to describe systemic blood pressure control Contents Introduction Blood flow Arteries Arterioles Capillaries Veins Blood Pressure (including cardiac physiology) Circulatory System Overview The three principal components:  the heart (the pump: Chapter 9)  the blood vessels (the pipes: Chapter 10)  the blood (the fluid to be moved: Chapter 11) Affected by:  endocrine system  nervous system  kidneys Blood Flow Blood is constantly reconditioned so composition remains relatively constant Reconditioning organs receive more blood than needed for metabolic needs Digestive organs, kidneys, skin Adjust extra blood to achieve homeostasis Blood Flow Blood flow to other organs can be adjusted according to metabolic needs Brain can least tolerate disrupted supply Distribution of Cardiac Output at Rest Blood Flow Depends on: pressure in system resistance in system Blood Flow Flow rate through a vessel (volume of blood passing through per unit of time) is directly proportional to the pressure gradient and inversely proportional to vascular resistance F = ΔP R F = flow rate of blood through a vessel ΔP = pressure gradient R = resistance of blood vessels Blood Flow Pressure gradient (ΔP) pressure difference between beginning and end of a vessel blood flows from area of higher pressure to area of lower pressure Resistance is measure of opposition of blood flow through a vessel depends on 3 things  blood viscosity, vessel length, vessel radius Relationship of Pressure and Flow Blood Flow Resistance is the hindrance to blood flow and depends on: 1. Fluid properties of blood (viscosity) } 2. Vessel length Vessel Properties 3. Vessel radius Blood Flow - Fluid Properties of Blood Friction between molecules of a flowing fluid greater the friction, the greater the viscosity Blood viscosity determined by number of RBCs Blood Flow - Vessel Properties Longer the vessel, the greater the resistance (R)  but remains constant in the body Vessel radius is the major variable  smaller the vessel, greater the resistance R is proportional to 1 r4 Blood Flow Relationship of Resistance and Flow Doubling radius Decreases resistance x16 Increases flow by x16 Distribution of Cardiac Output at Rest Pressure, Flow and Resistance Pressure (P) is the force exerted (measure in mmHg) Flow (F) is the volume moved - (m)L/min Resistance (R) is how difficult it is for blood to flow between two points at any given pressure difference Pressure, Flow and Resistance Basic equation relating these variables is: F = DP/R OR DP = F x R Defined by anatomy, Defined by anatomy, not oxygen content! not oxygen content! DP Vascular Tree Vascular Tree Summary Closed system of vessels Arteries  carry blood away from heart to tissues Arterioles  smaller branches of arteries Capillaries  smaller branches of arterioles  smallest of vessels across which all exchanges are made with surrounding cells Vascular Tree Summary (continued) Venules  formed when capillaries rejoin  return blood to heart Veins  formed when venules merge  return blood to heart Organization of the Cardiovascular System 2 “loops” in the cardiovascular system:  Systemic  Pulmonary “closed system,” i.e., leaks are bad Vessels on Arterial side Elastic arteries  Pressure reservoirs Muscular arteries Arterioles Capillaries Arteries Arteries Specialized for 2 functions: 1. Serve as rapid-transit passageways for blood from heart to organs  due to large radius, arteries offer little resistance to blood flow Arteries Specialized for 2 functions : 2. Act as pressure reservoir to provide driving force for blood when heart is relaxing Arterial connective tissue contains Collagen fibres provide tensile strength Elastin fibres provide elasticity to arterial walls Arteries as Pressure Reservoirs Blood Pressure Force exerted by blood against a vessel wall Depends on: volume of blood contained within vessel compliance of vessel walls  Arteries as a Pressure Reservoir  “Pressure reservoirs” expand and contract (recoil) as blood is ejected by heart allows blood flow to be continuous during diastole Arteries as a Pressure Reservoir Compliance = Δvolume/Δ pressure The higher the compliance of a structure, the more easily it can be stretched Arteries are often called pressure reservoirs because of the elastic recoil  not as compliant as veins Blood Pressure Force that drives blood around the body Supplies tissues with needs Generated by cardiac muscle contraction Blood Pressure Systolic pressure  peak pressure exerted by ejected blood against vessel walls during cardiac systole  ~ 120 mm Hg Diastolic pressure  minimum pressure in arteries when blood is draining off into vessels downstream  ~ 80 mm Hg  Blood Pressure Direct measure of BP  Blood Pressure  Can be measured indirectly using sphygmomanometer  Korotkoff sounds  Sounds heard when determining blood pressure  Sounds are distinct from heart sounds associated with valve closure Pulse Pressure Pressure difference between systolic and diastolic pressure Example  If blood pressure is 120/80, pulse pressure is 40 mm Hg (120mm Hg – 80mm Hg) Pulse that can be felt in artery lying close to surface of skin is due to pulse pressure Mean Arterial Pressure Average pressure driving blood forward into tissues throughout cardiac cycle Formula for approximating mean arterial pressure Mean arterial pressure = diastolic pressure + ⅓ pulse pressure At 120/80, mean arterial pressure = 80 mm Hg + ⅓ 40 mm Hg = 93 mm Hg Blood Pressure Pressures Throughout the Systemic Circulation Measure BP Vessels on Arterial side Elastic arteries  Pressure reservoirs Arterioles Capillaries Arterioles Arterioles Major resistance vessels Arterioles exist within organs Most of the resistance to blood flow throughout body occurs in arterioles Distribution of Cardiac Output at Rest Pressures Throughout the Systemic Circulation High Resistance Non-pulsatile Arterioles Mechanisms involved in adjusting arteriolar resistance Vasoconstriction narrowing of a vessel Vasodilation  enlargement in circumference and radius of vessel  by relaxation of smooth muscle layer  leads to decreased resistance and increased flow through that vessel Arterioles Vascular Tone Smooth muscle displays a state of partial constriction 2 major factors responsible for vascular tone: myogenic activity of smooth muscle sympathetic fibres continually release norepinephrine Arterioles Only blood supply to brain remains constant Changes within other organs  alter radius of vessels to adjust blood flow to organ Arterioles F = ΔP/R So, if increase resistance by vasoconstriction and keep pressure the same, then flow to a tissue decreases If you need to increase flow to a tissue, then you either increase the pressure or vasodilate to decrease resistance  Same Mean Arterial Pressure Magnitude and Distribution Of the Cardiac Output at Rest and During Moderate Exercise Arteriolar Vasoconstriction and Vasodilation How is organ blood flow adjusted? Intrinsic factors  local environment Extrinsic Factors  neuronal inputs  hormones Arterioles – Intrinsic (local) Factors Local chemical influences on arteriolar radius local metabolic changes histamine release Local physical influences on arteriolar radius  local application of heat or cold  chemical response to shear stress  myogenic response to stretch  Caused by: Myogenic activity Oxygen (O2) Carbon dioxide (CO2) and other metabolites Endothelin Sympathetic stimulation Vasopression; angiotensin II Cold (c) Vasoconstriction (increased contraction of circular smooth muscle in the arteriolar wall, which leads to increased resistance and decreased flow through the vessel)  Caused by: Myogenic activity O2 CO2 and other metabolites Nitric oxide Sympathetic stimulation Histamine release Heat (d) Vasodilation (decreased contraction of circular smooth muscle in the arteriolar wall, which leads to decreased resistance and increased flow through the vessel) Active Hyperemia Increase in metabolic activity increase LOCAL blood flow Arterioles Specific local chemical factors that produce relaxation of arteriolar smooth muscle Decreased O2 Increased CO2 Increased acid Increased K+ Increased osmolarity Adenosine release Prostaglandin release Arterioles Local vasoactive mediators Endothelial cells  release chemical mediators that play key role in locally regulating arteriolar caliber  release locally acting chemical messengers in response to chemical changes in their environment  among best studied local vasoactive mediators is nitric oxide (NO) Function of Endothelial Cells Function of Nitric Oxide Arterioles – Extrinsic Factors Extrinsic control  Accomplished primarily by sympathetic nerve influence  Some by hormones 65 Total Peripheral Resistance - TPR F = DP/R OR DP = F x R DP = F x R MAP = Cardiac output x Total Peripheral Resistance When entire circulation is considered Extrinsic Sympathetic Control of Arteriolar Radius Influence on total peripheral resistance (TPR) Norepinephrine influence on smooth muscle Local controls overriding sympathetic vasoconstriction No parasympathetic innervation to arterioles Arterioles – Extrinsic Factors Cardiovascular control centre In medulla of brain stem Integrating centre for blood pressure regulation Hormones that influence arteriolar radius Sympathetic stimulation of adrenal medulla  Epinephrine and norepinephrine Vasopressin and angiotensin II  Important in controlling fluid balance Vessels on Arterial side Elastic arteries  Pressure reservoirs Arterioles Capillaries Capillaries Capillaries Capillaries are the smallest blood vessels  7 mm in diameter  Thin-walled  Cells