Pressure, Force and Elasticity Lecture Notes (Oct 2024) PDF

Lecture 6: Pressure, Force and Elasticity Dr Isabel Hwang Senior Lecturer Division of Education School of Biomedical Sciences, Faculty of Medicine, CUHK Email: [email protected] Email: [email protected] Email: [email protected] Office number: 3943...

Lecture 6: Pressure, Force and Elasticity Dr Isabel Hwang Senior Lecturer Division of Education School of Biomedical Sciences, Faculty of Medicine, CUHK Email: [email protected] Email: [email protected] Email: [email protected] Office number: 3943 6795 Important Notice: These slides contain copyright materials. Access is limited to students of MEDF1011 unless otherwise specified. Copyright © 2016 The Chinese University of Hong Kong 1 Lecture Outline The flow rate equation Types of blood flow (laminar versus turbulent) The pulmonary and systemic circuits The Poiseuille’s law Factors affecting resistance (R) to blood flow Blood pressure and the importance of having a (arterial) blood pressure Compliance and elastance of arteries and veins Physiological significance of elastic recoil in an elastic artery during a cardiac cycle Factors that affect venous return (VR) Copyright © 2024 The Chinese University of Hong Kong 2 Pre-class assignment on Blackboard Micromodules 4 & 5 3 Introduction The rate of blood flow through many tissues in the human body is controlled mainly in response to tissue demand for nutrients and A near-linear increase oxygen E.g. during a dynamic exercise. Reference: https://cvphysiology.com/blood-flow/bf005 Our circulatory system (also called cardiovascular system) comprises the heart (pump), blood vessels and blood – Both heart and blood vessels are controlled constantly to create pressure gradient (P) that drives to different local organs. 4 Blood flow through a blood vessel is determined by two factors P 1. Pressure difference (i.e. gradient, P) of blood between the two ends of a vessel It denotes the force that pushes the blood through the vessel 2. Resistance to blood flow Hindrance to blood flow, is a measure of the friction that impedes flow 5 Driving force of blood flow The ohm’s law (the flow rate equation): P is generated by cardiac (ventricular) contractions and its magnitude can be modified through homeostatic control through hormones, nervous systems, etc. 6 The flow rate equation Flow (F) is the volume Pressure gradient moved and it is ( P) is measured in measured in mL/min mmHg or L/min Resistance (R) cannot be measured directly and is measured in mmHg/ml/min or mmHg/L/min Flow (F) is directly proportional to the pressure difference (P) between two points and inversely proportional to the resistance (R). This equation applies to the CV system in which blood (fluid) moves by bulk flow Bulk flow refers to movement of fluid or gases from region of higher pressure to one of lower pressure 7 P is the not absolute pressure at any point in the vascular system but the difference in pressure between the relevant points A positive pressure gradient is a must to drive blood flow 88 The importance of blood pressure to the human body Adequate blood pressure (in arteries) is essential to maintain or drive blood flow. Organs such as the brain and the heart are critically dependent on steady arterial blood supply (perfusion) to function normally. Reduced blood flow to organ → reduced delivery of glucose and oxygen → death of brain (stroke) and heart cells (heart attack) Atherosclerosis is thickening or hardening of the arteries caused by a buildup of plaque in the inner lining of an artery 9 Two types of blood flow (laminar vs turbulent) Laminar flow Turbulent flow Flow is streamlined Blood flow becomes too great Blood flow at a steady state and/or passes by an obstructed through a long, smooth blood vessel, rough surface vessel Flow becomes turbulent or Velocity of flow in the center of disordered (not streamlined) the vessel is greater (parabolic) Tend to cause audible sound Tend to be silent (bruit) in damaged or blocked blood vessel 10 Turbulence in leaky or stenotic (narrowed) cardiac valves 11 The cardiovascular system The circulatory system is a closed-loop system meaning that blood is contained within the vascular system – It functions through two circuits (systemic and pulmonary) 12 The pulmonary and systemic circuits both originate and terminate in the heart Pulmonary circuit Right ventricle → lungs→ left atrium Systemic circuit Left ventricle → peripheral organs/tissues → right atrium In both circuits, blood vessels carrying blood away from the heart is called arteries; those carrying blood from body organs and tissues back toward the heart are called veins. 13 Pressure gradients (P) in the pulmonary circuit P = (Pressure in pulmonary arteries − Pressure in pulmonary veins) = (15 − 0) mmHg = 15 mmHg closed loop direction of blood flow 14 Pressure gradients in the systemic system P = (Pressure in the aorta) − (Pressure in vena cava) = (85 − 0) mmHg = 85 mmHg = mean systemic arterial pressure/ mean arterial pressure (MAP) 15 Blood flow (F) is the same across both circuits ✓ Blood flow ✓ Blood flow generated generated by right by left ventricle ventricle (Fpulmonary) (Fsystemic) Fpulmonary = Fsystemic 16 Resistance in the CV system vary between the two circuits Q. What does it mean when flow is the same but P is different? The P in the systemic circuit is much greater than the P in the pulmonary circuit but blood flow across both circuits is equal in normal condition. 17 Resistance (R) in the CV system The Poiseuille’s law: Applies only to fluid moving smoothly through a cylindrical tube (i.e. laminar flow only). L = length of blood vessel r = internal radius of blood vessel η = fluid viscosity 8/ = a mathematical constant 18 Effect of tube radius on resistance A given volume of fluid is exposed to far more wall surface area and frictional resistance to blood flow in a smaller tube. 19 Effect of tube radius on flow Decreasing the radius of a tube two fold increases its resistance 16-fold. If P is held constant in this example, flow through the tube decreases 16-fold because F = P/R. 20 Resistance in the CV system There are 3 factors that affect resistance to flow Radius of vessel (r) Length of vessel (L) Viscosity of fluid () This is the biggest Total number of Is the thickness of contributor to blood vessels blood which minute-to-minute (usually constant) depends on control of Obesity increases amount of red resistance in the number and thus blood cells (RBC) vascular system. blood vessel length and proteins Relaxed/dilated (usually stable and blood vessels constant) decrease resistance Anemia reduces  Constricted vessels Decrease in increase resistance temperature and Atherosclerosis severe dehydration reduces radius increase  21 Five types of blood vessels in the vascular system Veins act as reservoir of Arteries transport high- blood and transport low- pressure blood from the heart pressure blood from to smaller arteries and venules to the heart arterioles Capillaries allow exchange between blood and interstitial (tissue) fluid Venules connect capillaries Arterioles connect arteries with veins and capillaries 22 Resistance in the CV system The effect of arteriolar radius on resistance and blood flow Internal radius of arterioles (and small arteries) modified through contraction and relaxation of smooth muscle cell layer 23 Example: what happen to resistance (R) if smooth muscle cells in the arteriole contract? Smooth muscles of arteriole contract Constriction of the arteriole (vasoconstriction) Radius (r) of arteriole is reduced Resistance (R) to blood flow is increased 24 Resistance in the CV system Total peripheral resistance (TPR) is the combined resistance of all blood vessels (especially arterioles) within the systemic circuit only Resistance across a network of blood vessels depends on resistance of all blood vessels Blood flow through network varies with resistance Arteries supply tissues and organs in parallel circuits Changes in resistance in these circuits determines relative blood flow to the local tissue/organ 25 Arteries and veins have difference compliance (or elastance) Direction of blood flow Arteries Arterioles Capillaries Venules Veins Compliance (C) Elastance (E) Describe how easily a lumen of a Also called elasticity, is the blood vessel expands when it is tendency of blood vessels to recoil filled with a volume of blood. toward original dimensions as blood pressure falls Compliance =  Volume /  Pressure Elastance =  Pressure /  Volume 26 26 Period of ventricular contraction Period of ventricular relaxation 27 Elastic recoil Elastic recoil Is the tendency of the artery to return to its original shape/length once stretched (e.g. by blood) During diastole, the arterial walls recoil, helping to smooth the flow of blood through the vessel  the elastic recoil helps to push the blood forward through the artery as well as maintain arterial pressure between systolic and diastolic phases 28  Veins hold most of blood in body (60- 70%) and are also called capacitance vessels 29 Venous return Is return of blood flow to right atrium via veins Controls end-diastolic volume (EDV) and thus stroke volume (SV) and cardiac output (CO) – improving venous return improve EDV, SV and CO Dependent on: 1. Blood volume and venous pressure Can occur in 2. Skeletal muscle pumps resting state 3. Pressure drop during inhalation *4. Venoconstriction due to sympathetic stimulation * Veins do not normally constrict unless it is stimulated by the sympathetic nerve 30 The skeletal muscle pump and one-way valves work together 31 Venous return cannot be facilitated by skeletal muscle pump alone 32 The respiratory pump creates a sucking effect towards the heart direction 33 A deep breath can enhance your venous return Unequal pressure during inhalation creates an upward “sucking” effect that pulls blood towards the heart. 34 Real-life example: physiological significance of modified compliance in veins Stress Sympathetic Reduced venous Venoconstriction compliance activation Enhanced venous return Increased venous pressure Increased blood flow to EDV SV CO MAP right atrium 35 Factors affecting venous pressure and, therefore, mean arterial pressure (MAP) To be elaborated in the next lecture 36 If we just consider the systemic circuit, F = P / R P = F x R MAP = CO x TPR 37 Homeostatic regulation of MAP Requires Cooperation of heart, blood vessels, and kidneys Supervision by the brain (controlling/integrating center) Changes in one variable will be quickly compensated for by changes in other variables 38 Short-term and long-term regulation of MAP Covered in Health Sciences I To be partly covered in Health Sciences II 39 The baroreceptor reflex is a short-term regulation of MAP Baroreceptors are located in carotid arteries and aortic arch, also walls of large arteries of neck and thorax – Functions through negative feedback and integrated by the medulla oblongata in the brainstem baroreceptor reflex  MAP baroreceptor reflex  MAP  MAP  MAP 40   Venous return From previous lecture  Atrial pressure  End-diastolic pressure CO = HR x SV 41 Learning outcomes To recall the flow equation and recognise how blood flow is affected by both pressure gradient and resistance To reproduce the Poiseuille’s law and summarise the 3 key variables in the equation Describe why radius or diameter of arterioles if the main physiological contributor to resistance of blood flow Identify differences between laminar and turbulent flow in a blood vessel Define compliance and elastance of a blood vessel Describe why the contractile activity of smooth muscle in arterioles would affect resistance to blood flow Define venous return and summarise factors that help improve venous return Describe how compliance in a vein can be changed through sympathetic stimulation Write a flow diagram to show how an improvement of venous return can increase mean arterial pressure (MAP) in a person at rest or under stress (hint: a stress condition will lead to sympathetic stimulation in a person) Copyright © 2024 Access is limited to students of MEDF1011 unless otherwise specified 42 Required reading: Basic Concepts in biomedical sciences I Chapter 6, Pressure, fluids, gases and breathing Page 122-153 43 43  The end of this lecture Copyright © 2024 The Chinese University of Hong Kong 44