Collin County Community College

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1 Collin County Community College BIOL Anatomy & Physiology WEEK 6 Blood Vessels 1 Anatomy of Blood Vessels Walls of blood vessels contain 3 distinct layers : Tunica intima innermost layer includes endothelial lining with underlying C.T. Tunica media Concentric sheets of smooth muscle in a frame work of loose C.T. Tunica externa Also called tunica adventitia Collagen fibers with scattered elastic fibers 2 1

2 Anatomy of Blood Vessels 3 Anatomy of Blood Vessels Compared to veins, arteries Have thicker walls Have more smooth muscle and elastic fibers Are more resilient Compared to veins, arteries Undergo changes in diameter o Vasoconstriction decreases the size of the lumen o Vasodilation increases the size of the lumen Classified as either elastic (conducting) or muscular (distribution) Small arteries (internal diameter of 30 um or less) are called arterioles 4 2

3 Types of Blood Vessels Large arteries or elastic arteries Tunica media has many elastic fibers Big walls withstand pressure Medium arteries or muscular arteries Tunica media has larger smooth muscle content Important for resistance control Arterioles Smaller version of muscular arteries They become the capillaries Capillaries Only blood vessels whose walls are permeable They only have a tunica interna 5 Capillaries Consist out of an endothelial tube inside a basal lamina These vessels Form networks Surround muscle fibers Radiate through connective tissue Weave throughout active tissues Capillaries have two basic structures Continuous Fenestrated 6 3

4 Capillaries The endothelial lining forms a complete, uninterrupted lining One cell may wrap all the way around the lumen Permits diffusion of water, small solutes and fat soluble material 7 Capillaries The endothelial lining contains windows and pores. Permits fast exchange of water and larger molecules. Located for example in choroid plexus, certain endocrine organs Number and size of pores varies with the organ 8 4

5 Capillary Beds An interconnected network of vessels consisting of Collateral arteries feeding an arteriole Meta-arterioles Arteriovenous anastomoses Capillaries Venules 9 Capillary Beds The beds have precapillary sphincters that control the blood flow through the bed. The cycling of contraction and relaxation of smooth muscles that changes the blood flow through the beds is called VASOMOTION. 10 5

6 Veins Collect blood from all tissues and organs and return it to the heart Are classified according to size Venules Medium-sized veins Large veins Venules and medium-sized veins contain valves Prevent backflow of blood 11 Veins Veins run between muscles; Contraction of muscles has a milking effect, squeezing blood upwards. The one-way valves operate like elevator stages. 12 6

7 13 Summary of Blood Vessels 14 7

8 Blood Flow Dynamics Since Blood is a fluid, it obeys the physical principles of fluid dynamics. Blood flow, velocity and resistance all come into play in determining Cardiac Output, Blood Pressure and the Capillary Exchange Phenomena. Needless to say is that these events are under neuronal and hormonal control! 15 Hemodynamics HEMO-DYNAMICS is the study of motion of the blood within the cardiovascular system. To generate motion of a fluid one requires the following components A fluid A force A conduit Blood Blood-pressure Blood-vessels 16 8

9 Hemodynamics The heart generates the needed force by contracting against a closed system. A force exerted by a fluid against the walls of the container holding the fluid is a Pressure Force Boyle s Law : Pressure x Volume = C te For a volume system that is flexible (such as air) this means that, if the volume is decreased, the pressure of the system will increase. P 1 x V 1 = P 2 x V 2 P 1 P 2 V 2 = 0.5 V 1 V 1 V 2 P 2 = 2 P 1 17 Hemodynamics Pressure makes fluids move. The heart generates this pressure. However, two other conditions need to be met for a fluid to move. 1) There needs to be an open conduit system for fluids to move into. Pressure within a closed system only builds up potential energy ( = hydrostatic pressure ). 18 9

10 Hemodynamics To decrease the volume of a container usually implies applying some force to the system. That force will be transmitted to the inside, generating the pressure inside. If that volume contains a fluid ( being non compressable) the pressure will increase even faster. The heart generates pressure by contracting the heart muscles against the blood filled ventricles within a closed system (like pressing on a water filled balloon). Once pressure is build up, the fluid (blood) will squirt out when an opening becomes available. 19 Hemodynamics 2) What makes a fluid move is not the absolute pressure but the Pressure Gradient (Δ P) Fluids flow from an area of high pressure to an area of low pressure. = Δ P Flow (Q) ~ Δ P where Δ P = P 2 - P

11 Hemodynamics Note : Pressure in heart and blood vessels is measured in millimeters of mercury pressure ( mm Hg, 1 mm Hg = 1 torr) 21 Hemodynamics 22 11

12 Hemodynamics 23 Flow (Q) Hemodynamics Definitions movement of a liquid expressed as volume per time unit ( ml/min ) Velocity (V) speed at which a fluid flows expressed as distance per time unit ( cm/min) V = cm/min V = (cm /min) x (cm 2 /cm 2 ) V = cm 3 /(min x cm 2 ) cm 3 = ml V = ml /(min x cm 2 ) V = Q / cm 2 Velocity = flow per cross sectional area 24 12

13 V = Q / cm 2 Translation? velocity is the Flow per area velocity is equal to the flow of a fluid that passes through the cross sectional area of a conduit (vessel) Correlation? Hemodynamics Definitions velocity and flow are directly proportional if cross sectional area does not change if flow is constant, Velocity will increase if cross sectional area decreases if velocity stays constant, Flow will change in the same way as cross sectional area does. 25 Hemodynamics 26 13

14 Hemodynamics Definitions Bernoulli's Principle 27 Factors that Influence Flow Friction is one of the main factors that influences flow. In the CVS, this friction is called resistance (R) and it opposes flow. Poiseuille s relationship Q (flow) ~ 1/R R = [8/π ] x [L. η /r 4 ] L = length of vessel r = radius of vessel η = viscosity of fluid 28 14

15 Factors that Influence Flow Poiseuille s relationship R = [8/π ] x [L. η /r 4 ] Q (flow) ~ 1/R Translation? the longer the vessel, the higher the resistance, the lower the flow the higher the viscosity, the higher the resistance, the lower the flow Resistance is inversely proportional with 4 th power of the radius of the vessel. Thus if the radius doubles, R decreases by a factor 16 and flow increases by a factor Factors that Influence Flow Though the radius of tube B is just half the radius of tube A, its resistance to flow is 16-times greater. Thus, the rate of flow in tube B is only 1/16 th the rate of flow in tube A

16 Blood Flow = Cardiac Output Flow (Q) ~ Δ P where Δ P = P 2 - P 1 Q (flow) ~ 1/R Flow = Δ P /R Cardiac Output = Δ P /R Cardiac Output = Blood Pressure / SVR Blood Pressure = CO x SVR SVR = systemic vascular resistance 31 Questions! If BP = CO x SVR and R increases with length of a vessel increases when diameter decreases Then this implies that BP will increase the further away we move from the heart. Is this indeed so? And if not so, what is going on? 32 16

17 Answer! Once the blood leaves the heart, there is not just one blood vessel into which the blood flows. There is an increasing amount of branching that occurs. The total Resistance (Rt) of a branching network becomes much smaller that the resistance of one tube of similar total cross sectional area. Most resistance occurs in the arterioles, capillaries and venules. The veins and arteries are too big to offer any major contribution to SVR. 33 Blood Flow = Cardiac Output Blood Pressure = CO x Vascular Resistance The pumping of the heart plus the blood volume provides the pressure in our system. The less blood volume, the lower the pressure head. Reduced pumping, reduced pressure! If any of the parameters changes, homeostatic systems come into play to adjust the system and maintain blood pressure. The quickest adjustments is by altering vascular resistance! 34 17

18 Arterial Blood Flow and Blood Pressure Blood flow = volume of blood that flows per unit of time (ml/min) = cardiac output CO = Δ P /R CO = BP / SVR SVR = systemic vascular resistance Blood velocity = distance blood travels per unit of time (cm/min) = blood flow per cross sectional area 35 Arterial Blood Flow and Blood Pressure Pressure in a container, such as a ventricle or blood vessel, is determined by the volume in the container the stretchability of the container The easier the container can be stretched, the more volume can be added without increasing pressure. This is referred to as the compliance of a vessel. Compliance = Δ Volume / Δ Pressure A high compliance thus means that a small increase in pressure results in a large increase in volume or that a large volume increase goes along with a small pressure increase 36 18

19 Arterial Blood Flow and Blood Pressure Compliance = Δ Volume / Δ Pressure Veins have a much larger compliance compared to arteries. Increased contraction of the tunica media decreases compliance in both veins and arteries. 37 Arterial Blood Flow and Blood Pressure Elastic arteries ( such as the aorta) have relative low compliance They are elastic in nature and expand when the heart contracts and expels blood into these arteries 38 19

20 Arterial Blood Flow and Blood Pressure When the heart goes in diastole, potential energy is stored in these arteries and they recoil, pushing the blood down the systemic circulation With age, elasticity decreases and compliance increases ( stiffening of the arteries). 39 Arterial Blood Flow and Blood Pressure The systole/diastole aspect of the heart and the elastic nature of the arteries creates a pulsating pressure fluctuations. = Systolic - Diastolic pressure 40 20