Vascular Physiology Lecture 2 PDF

Vascular physiology Lecture 2 By Dr. Asem Alkhalaileh  Concept 3 : Blood viscosity Blood viscosity Definitions and units Isaac Newton described viscosity in 1713 as internal friction to flow in a fluid or lack of slipperiness. Viscosity reflects to the thickness of fluid, and it ind...

Vascular physiology Lecture 2 By Dr. Asem Alkhalaileh  Concept 3 : Blood viscosity Blood viscosity Definitions and units Isaac Newton described viscosity in 1713 as internal friction to flow in a fluid or lack of slipperiness. Viscosity reflects to the thickness of fluid, and it indicates resistance to flow. It can be measure in vitro by viscometer Unit of viscosity Poise (after Poiseuille). A fluid of 1 Poise viscosity has a force of 1 dyne/cm2 of contact between layers when flowing with a velocity gradient of 1 cm/sec. The poise (P) is the unit of dynamic viscosity in the gram –centimeter-second The analogous unit in the International System of Units is the pascal-second (Pa·s): Blood viscosity Relative viscosity Relative viscosity is a more often used term, and it refers to the viscosity of fluid relative to viscosity of water at body temperature (37° C) Viscosity of water at 21 °C is 0.01 poise or 1 centipoise. Viscosity of water at body temperature (37 °C) is 0.695 centipoise Plasma has a viscosity of 1.2 centipoise at 37°C Plasma has a relative viscosity of 1.7. Blood has a viscosity of 2.8-3.8 centipoise at 37°C Blood (plasma plus cells) has a relative viscosity of about 4-5. Blood viscosity Factors affecting viscosity ❑ Several factors affecting viscosity including: 1. Blood composition changes: ①RBC mass ②Plasma protein ③Cells deformation ④Clotting mechanisms 2. Temperature 3. Shear rate or blood flow velocity gradient. 4. Plasma skimming 5. Diameter of blood vessel (Fahraeus-Lindqvist Effect) Blood viscosity Factors affecting viscosity (RBC mass) An increase the number of RBC, Hematocrit (or Packed cell volume), and hemoglobin all will increase viscosity. ❑ Examples: a. anemia decrease viscosity b. polycythemia increases viscosity Those factors considered as most important factors that increase viscosity The most important factor of these is the RBC because each RBC exerts frictional drag against adjacent cells and against the wall of the blood vessel. When the hematocrit rises to 60 or 70%, which it often does in polycythemia, the blood viscosity can become as great as 8-10 times that of water, and its flow through blood vessels is greatly retarded Blood viscosity Factors affecting viscosity (Plasma proteins) An increase in plasma proteins will increase viscosity. Changes in Plasma protein (such as hyper-gamma-globulin-emia) has less effect on viscosity than RBC changes. Blood viscosity Factors affecting viscosity (Cells deformation) Viscosity increase in hereditary spherocytosis and sickle cell Blood viscosity Factors affecting viscosity (Clotting mechanisms) If clotting mechanisms are stimulated in the blood, platelet aggregation and interactions with plasma proteins occur. This leads to entrapment of red cells and clot formation, which dramatically increase blood viscosity. All these factors will explain the higher viscosity of blood than water. Blood viscosity Factors affecting viscosity (Temperature) A decrease in body temperature will increase blood viscosity Blood viscosity increase 2% for each oneº C decrease in temperature. When the hand kept in ice water regional blood viscosity show a threefold increase. Blood viscosity Factors affecting viscosity (Blood flow velocity) Viscosity of the blood decrease as the shear rate or the blood flow velocity increase and vice versa. Shear rate is the rate of change of velocity at which one layer of fluid passes over an adjacent layer. Newtonian fluids such as Water, air, alcohol, glycerol, and thin motor oil and plasma, their viscosity remains the same whether they are flowing fast or slowly. Non- Newtonian fluids such as blood, saliva, semen, mucus, and synovial fluid its viscosity changes with its velocity. Blood viscosity Factors affecting viscosity (Blood flow velocity) A. At high blood flow velocity such as: exercise and during systole blood viscosity decrease. This is because red cells tend to collect in the center of the lumen of a vessel and move with their long axis parallel to the direction of flow where flow rate is fastest leaving cell free zone of plasma at periphery, an arrangement known as axial streaming. Axial streaming ►reduces the viscosity ► reduces resistance to flow. B. At low blood flow velocity such as: increased vessel diameters or downstream from an obstruction, in diastole and circulatory shock blood viscosity increase This is because low blood flow velocity increases molecular interactions to occur between red cells with each others and with plasma proteins This can cause red cells to stick together and form chains of RBCs (rouleax formation) within the microcirculation, which increases the blood viscosity. Blood viscosity Factors affecting viscosity (Plasma skimming) The RBC tend to accumulate along the axis of the blood vessel so there is a decrease of hematocrit as blood approaching the micro-vessel. The phase separation due to axial migration affects the cellular content of blood flowing into the side branches of blood vessels. A branch originating from a vessel of higher order fed mainly by the marginal stream (which contains plasma) of the higher order vessel so receives blood with a lower hematocrit. ❑ This is called plasma skimming which has the following effects: ❖ Larger branches with higher flow rates receive relatively more RBC ►higher hematocrit blood ❖ Smaller breaches with low flow rates receive relatively less RBC ►lesser hematocrit blood This explains why the hematocrit of capillary blood is about 25% less than the whole– body hematocrit.  Blood viscosity Factors affecting viscosity (Diameter) The Fahraeus-Lindqvist effect (1931): There is a 'decrease in hematocrit and viscosity as the tube's diameter decreases Although only with a tube diameter of between 10 and 300 μm = 0.01 and 0.3 mm) this effect will disappear after the tube diameter reaches 0.5 mm and above. Cell free marginal layer model is a mathematical model try to explain Fahraeus-Lindqvist effect which states: “The RBCs move over to the center of the vessel, leaving a thin layer (approximately 3 μm) of plasma near the wall of the vessel (a plasma cell-free layer) which reduces viscosity and resistance to flow” The methods used to measure the blood flow are: ❶Electro-magnetic flow meters. The electromagnetic flowmeter in based on the principle that when a vessel containing blood (a conductor) is placed between the two poles of a magnet, voltage is generated in the blood flowing through the magnetic field. The magnitude of the voltage is proportionate to the volume of flow and can be measured with an appropriately placed electrode on the surface of the vessel. ❷ Ultrasonic Doppler Flowmeter Ultrasonic flowmeter can be applied to the outside of the vessel Ultrasonic flowmeter contains a prop with minute piezoelectric crystal transmits ultrasound along the flowing blood. A portion of the sound is reflected by the red blood cells in the flowing blood. The reflected ultrasound waves then travel backward from the blood cells toward the crystal. This effect is called the Doppler effect. Like the electromagnetic flowmeter, the ultrasonic Doppler flowmeter can record rapid, pulsatile changes in flow, as well as steady flow. ❸Venous occlusion plethysmography: Venous occlusion plethysmography is a simple but crude method of measuring blood flow through the limbs. It is principle based on the principle that if venous return of a region (part) is suddenly obstructed, that part increases in size due to arterial flow. The increase in size is equivalent to the blood flow to To that that part. part Overview: Concept 1: Blood flow: a new equation Concept 2: Resistance: More details Concept 3 : Compliance and elasticity  Concept 1: Blood flow: a new equation Blood flow: a new equation Q = ∆P/R Q = flow or cardiac output (mL/min) or (L/min) R = resistance ∆P = P1-P2 = pressure gradient (mm Hg) not the absolute pressure in the vessel The “pressure gradient” along the vessel is the pressure difference of the blood between the two ends of the vessel because the Blood flows from a high-pressure area to a region with lower pressure. Pressure gradient pushes the blood through the vessel Pressure is a measure of the force that the blood exerts against the vessel walls as it moves the blood through the vessels. Blood flow: a new equation Effective perfusion pressure, which is (the mean intra-luminal pressure at the arterial end minus the mean intra-luminal pressure at the venous end) which is the pressure needed to overcome resistance The highest pressure is in the heart and decrease as distance increase from the heart The heart (ventricle) > arteries > arterioles > capillaries > venules > veins > heart (atria) Blood flow: a new equation Blood pressure in elastic arteries are pulsatile (Blood pressure rises and falls) Systolic Pressure: Peak pressure exerted by ejected blood against vessel walls during cardiac systole (ventricular contraction) Diastolic Pressure: Minimum pressure in arteries when blood is draining off into vessels downstream and lowest level of arterial pressure during ventricular cycle Due to the two types of pressure (systolic and diastolic blood pressure) a pulse is created, which is the difference between these two pressures  Blood flow: a new equation Flow and perfusion pressure Flow is directly proportion with “pressure gradient”. There is a linear relationship between flow and perfusion pressure in a rigid tube There is a curve relationship between flow and perfusion pressure in a distensible blood vessel (aorta). When measuring pressure on a rigid or glass tube, it is different from measuring on a distensible tube(more like a blood vessel) -as when the blood vessel increases in size, there will be a resistance force to it unlike on the tube it would just increase in size or break -as the blood vessel (or elastic as an example) will increase in size due to increasing blood flow. The resistance will also increase. Hence blood flow and perfusion pressure will not have a straight linear relationship as shown in the tube, but a curve relationship like the dispensable tube Blood flow: a new equation Flow and perfusion pressure Critical closing pressure Since the flow-pressure relationship in a rigid tube is linear, so flow will cease only if the pressure is zero. However, in a blood vessel the flow ceases when the blood pressure is 20 mm of Hg or even more. The pressure value at which the vessel collapses, its lumen closes, and flow ceases is called critical closing pressure. Blood flow: a new equation Flow and perfusion pressure The blood flow ceases when the blood pressure falls below the critical closing pressure because: ❶ Certain amount of intramural pressure is essentially required to push the RBC (with average diameter of 7.5μm through the capillaries (average diameter 5μm). ❷ Further, the tissue pressure exerted over the vessels also causes their collapse. Therefore, certain amount of intramural pressure is must to counteract the tissue pressure and thus to keep the vessels patent and to maintain the blood flow. Blood flow: a new equation Flow and perfusion pressure ❑ Values of critical closing pressure When whole blood is flowing through the vessels the average value of critical closing pressure is 20 mm Hg. When plasma is flowing the value of critical closing pressure is about 5-10 mm Hg. On sympathetic stimulation, value of critical closing pressure increase to 60 mmHg On sympathetic inhibition, value of critical closing pressure falls to zero mm of Hg. Blood flow: a new equation Flow and perfusion pressure Sympathetic stimulation and other vasoconstrictors decrease the cross sectional area of a vessel, so they alter the flow because (flow= area X velocity). Thus, inhibition of sympathetic activity increases the blood flow twofold or more because it greatly dilates the vessels Conversely, very strong sympathetic stimulation constrict the vessels much that blood flow occasionally decreases to as low as zero for a few seconds despite high arterial pressure. Blood flow: a new equation Flow and resistance Flow is inversely proportional with resistance Resistance is the impediment to blood flow in a vessel Resistance cannot be measure by any direct means. Resistance must be calculated from measurements of blood flow and pressure difference between two points in the vessel  Blood flow: a new equation Flow and resistance The major mechanism for changing blood flow in the cardiovascular system is by changing the resistance of blood vessels, particularly the arterioles. The total resistance of the circulation sometimes called Total peripheral resistance because the highest resistance found in arterioles, which are peripherally located. Atrial resistance is referred to as the total resistance as it is the collected resistance from all of the previous blood vessels(artery ,arterioles , capillaries ,veinioles, veins) COMBINED  Blood flow: a new equation Flow and resistance The units used for resistance are: dyn·s/cm5, pascal·s/m3, mmHg·min/L The rate of blood flow through the entire circulatory system = The rate of blood pumping by the heart = Recap We have said that the resistance can’t be measured, but theoretically calculated The cardiac output Hence, there are specific total resistances calculated in specific areas ❑ Total resistance of the body in the adult human being is found in the 1. Systemic circulation 2. Pulmonary circulation 3. Local resistance: Blood flow: a new equation Flow and resistance: in the systemic circulation ❑ Systemic circulation The pressure difference from the systemic arteries to the systemic veins is about 100 mm Hg Blood flow through the entire circulatory system = the cardiac output= 100 mL/sec Therefore, the resistance of the entire systemic circulation, called the total peripheral resistance, is about 100/100, or 1 PRU (peripheral resistance unit = mmHg/mL/sec = mmHg. Sec/mL). Blood flow: a new equation Flow and resistance: in the pulmonary circulation ❑ Pulmonary circulation In the pulmonary system, the mean pulmonary arterial pressure averages 16 mm Hg and the mean left atrial pressure averages 2 mm Hg, giving a net pressure difference of 14 mm. Therefore, when the cardiac output is normal at about 100 ml/sec, the total pulmonary vascular resistance calculates to be about 0.14 PRU (about one seventh that in the systemic circulation).  Vasoconstriction will decrease the flow & increase the resistance Blood flow: a new equation Flow and resistance: local resistance ❑ Local resistance If the pressure difference between two points is 1 mm Hg the flow is 1 ml/ sec The resistance is said to be 1 peripheral resistance unit(PRU) = mmHg. Sec/mL In conditions in which all the blood vessels throughout the body become strongly constricted, the total peripheral resistance occasionally rises to as high as 4 PRU. Conversely, when the vessels become greatly dilate, the resistance can fall to as little as 0.2 PRU.  Concept 2: Resistance: More details Resistance: More details The vascular anatomy of the entire body is comprised in-parallel vascular components. For some individual organ is comprised in-series vascular components. Because the vascular bed do have resistance, so the resistance are both in-series and in parallel  Resistance: More details Parallel resistance Blood leaves the heart through the aorta from which it is distributed to major organs by large arteries, each of which originates from the aorta. Therefore, these major distributing arteries (e.g., carotid, brachial, superior mesenteric, renal, iliac) are in parallel with each other. A further means that the vascular networks of most individual organs are in parallel with other organ networks. For example, the circulations of the head, arms, gastrointestinal systems, kidneys, and legs are all parallel circulations. There are some exceptions, notably the gastrointestinal and hepatic circulations, which are partly in-series because the venous drainage from the intestines become the hepatic portal vein, which supplies most of the blood flow to the liver. Resistance: More details Parallel resistance The total resistance of this parallel arrangement expressed by the equation above ❑ This demonstrates two important principles regarding the parallel arrangement of blood vessels: The total resistance of a network of parallel vessels is less than the resistance of the vessel having the lowest resistance. When there are many parallel vessels, changing the resistance of a small number of these vessels will have little effect on total resistance for the segment. The presence of many parallel blood vessels makes it easier for blood to flow through the circuit as each parallel vessel provides another pathway, or conductance, for blood flow. Hence, if you cut blood flow from one pathway or entry, the blood will enter from another pathway  Resistance: More details Parallel resistance ❑ General characteristic of resistance in parallel ❶ A parallel arrangement of vessels greatly reduces resistance to blood flow. ❷ Each artery in parallel receives a fraction of the total blood flow. ❸ In each parallel artery, the pressure is the same.  In series resistance, the resistance from artery the vein will increase (unlike the parallel resistance) But why do ARTERIOLE have the greatest resistance ? -no change in blood flow - as they are in the periphery and have the greatest effect on the SNS -pressure decreases (as I can’t move blood from one area to another unless I have a difference in -As they have the greatest and thickest smooth muscle content pressure -greatest collagen content Resistance: More details Series resistance A small artery gives rises into two daughter branches (arterioles) The arterioles with arterioles branches are in parallel to each other. The small artery with arteriole is in-series The resistance in arteriole is the same as that of the other arteriole in parallel The arterioles give rise to capillaries branches are in parallel to each other. The capillary with capillary branches is in parallel to each other. The arteriole with the capillary is in-series The resistance in capillary is the same as that of the other capillary in parallel This is also true for venules and veins ARTERIOLES, you have the greatest resistance in all types of blood vessels Hence, we can conclude that the total resistance (we have talked about which includes all the blood vessels resistance ) can be summarized as one type of resistance, which is total total peripheral resistance (which is for the peripheral Arterioles with the greatest resistance)  Resistance: More details Series resistance Therefore, each of the vascular segments are in-series to each other, although within the segment there are parallel vessels. Furthermore, each vascular segment will have a segmental resistance value (Rx) that is determined by the length and radius of each of the vessels For a series resistance network, the total resistance (RT) equals the sum of the individual resistances.  Resistance: More details Series resistance ❑ The largest proportion of resistance in this series contributed by the arterioles. Assume, RA = 20, Ra = 50, Rc = 20, Rv = 6, RV = 4 Therefore, RT = 20 + 50 +20 +6 + 4 = 100 Doubling RV from 4 to 8 increases RT from 100 to 104, a 4% increase. Doubling Ra from 50 to 100 increases RT from 100 to 150, a 50% increase. ❑ If this were done for each of the segment, we would find that: The arteriolar segment, which has the highest relative resistance, has the greatest effect on total resistance. As a group, changes in diameter (and therefore resistance) of small arteries and arterioles have the greatest influence on vascular resistance because these two vessel segments comprise about 70% of the total resistance in most organs.  Resistance: More details Series resistance ❑ Each blood vessel (e.g., the largest artery) or set of blood vessels (e.g., all of the capillaries) in series receives the same total blood flow. Thus, blood flow through the largest artery = total blood flow through all of the arterioles = total blood flow through all of the capillaries and so on. ❑ As blood flows through the series of blood vessels, the pressure decreases, and the greatest decrease is in arterioles because it has the highest resistance for the body Pressure will decrease; from the arteries to the veins will decrease this pressure difference will allow the blood flow from the arteries into the veins -due to effective perfusion pressure (as we have said, we need a pressure difference to move the blood from one region to another)  Resistance: More details Poiseuille-Hagen formula Flow (Conductance) inversely related to viscosity Flow (Conductance) directly related to (radius)4  Resistance: More details Poiseuille-Hagen formula When three vessels with relative diameters of 1, 2, and 4 but with the same pressure difference of 100 mm Hg between the two ends of the vessels. Although the diameters of these vessels increase only fourfold, the respective flows are 1, 16, and 256 ml/min, which is a 256-fold increase in flow. Thus, the conductance of the vessel increases in proportion to the fourth power of the diameter Resistance: More details Poiseuille-Hagen formula The internal diameters of the arterioles is (4 to 25 micrometers) Because arterioles have a strong vascular walls with heavy muscular supply this allow them to respond strongly to nervous signals or local tissue chemical signals, and to represent major part of the total systemic resistance to blood flow and to turn off almost completely the blood flow to the tissue or at the other extreme to cause a vast increase in flow so the internal diameters to change tremendously, often as much as fourfold and the blood flow of more than 100-fold According to the law of radius to power four slight changes in radius will strongly affect the resistance Resistance: More details Exceptions for the poiseuille-Hagen formula Despite the usefulness of Poiseuille’s law, it is worthwhile to examine the ways the cardiovascular system does not strictly meet the criteria necessary to apply the law: ❑ First, the cardiovascular system is composed of tapering‫ﻣﺗﻧﺎﻗﺻﺔ‬, branching, elastic tubes, rather than rigid tubes of constant diameter. These conditions, however, cause only small deviations from Poiseuille’s law. ❑ Second, application of Poiseuille’s law requires that flow be steady rather than pulsatile, yet the contractions of the heart cause cyclical alterations in both pressure and flow. Despite this, Poiseuille’s law gives a good estimate of the relationship between pressure and flow averaged over time.  Concept 3: Compliance and elasticity Compliance and Elasticity Compliance is the ability of a vessel or hollow organs (such as heart campers or alveoli) to distend and increase volume with increasing transmural pressure Compliance the tendency of a vessel or hollow organ to resist recoil toward its original dimensions on application of a distending or compressing force Elastance (reciprocal to compliance) Elastance= 1/compliance Elastance is a tendency of a vessel or hollow organs (such as heart campers or alveoli) to recoil toward its original dimensions upon removal of a distending force.  This is because as the pressure on the walls of the blood vessel increase the resistance of Compliance and Elasticity the vessels against the blood will also increase The term compliance is used to describe how easily a chamber of the heart, or the lumen of a blood vessel expands or stretches when it is filled with a volume of blood..‫ﺗﺴﺘﻌﻤﻞ ﻟﺘﻮﺻﯿﻒ ﻣﺎھﯿﺔ ﺳﮭﻮﻟﺔ ﺗﻮﺳﻊ اﻟﻮﻋﺎء او اﻟﻘﻠﺐ ﻋﻨﺪ ﻣﻼه ﺑﺎﻟﺪم‬ The term elasticity is used to describe how resistance a chamber of the heart, or the lumen of a blood vessel to stretches when it is filled with a volume of blood. ‫ﻣﻼه ﺑﺎﻟﺪم‬. ‫ﺗﺴﺘﻌﻤﻞ ﻟﺘﻮﺻﯿﻒ ﻣﻘﺪار اﻟﻤﻘﺎوﻣﺔ اﻟﺘﻲ ﻧﻮاﺟﮭﮭﺎ ﻟﺘﻮﺳﻊ اﻟﻮﻋﺎء او اﻟﻘﻠﺐ ﻋﻨﺪ‬ At start, the elasticity (resistance) of the blood vessel wall is low hence the size of the blood vessel will increase till a certain point where the resistance is high, and there is no longer change or At start there is Increase in pressure causes an increase in volume increase in the size of the blood vessel Till you reach a point where pressure increases BUT volume stays the same The compliance will decrease with the stretching of the blood vessel Compliance and Elasticity ❑ Physically, compliance (C) is defined as the change in volume (ΔV) divided by the change in pressure (ΔP). C= ∆V/∆P C = capacitance, or compliance (mL/mm Hg) ΔV = the change in volume (mL) ΔP = the change in pressure (mmHg) Compliance and Elasticity Compliance of blood vessels The volume-pressure relationship (i.e., compliance) for an artery and vein If an isolated segment of an artery is cannulated and the volume of fluid within the segment is slowly increased while measuring pressure, a plot of volume versus pressure can be generated. Two most important characteristic stand out 1–slope 2-ventricular compliance  Compliance and Elasticity Compliance of blood vessels ❑ The slope of the curve at any given pressure and volume (dV/dP) 1. Represents the compliance of the vessel at that pressure and volume. 2. Decreases as the vessel expands (i.e., at higher volumes and pressures there is a larger change in pressure for a given change in volume (i.e., heart or the vessels become "stiffer" at higher pressures and volumes). 3. Is nonlinear. ❑ Compliance is a fundamental property of a tissue; however, the compliance can be modified by histological changes in the tissue. This occurs, in the heart and vessels specially arteries.  Compliance and Elasticity Compliance of blood vessels The slope, which represents the compliance at a given pressure the venous compliance is much higher than arterial compliance at low pressures For example, at pressure 15 mmHg, the compliance of a vein is about 10 to 20-times greater than an artery. Therefore, veins can accommodate a large change in blood volume with only a small change in pressure. ‫ﻧﺤﺘﺎج اﻟﻰ ﺿﻐﻂ ﻋﺎﻟﻰ ﻟﺘﻮﺳﯿﻊ اﻟﺸﺮاﯾﯿﻦ وﻟﻜﻦ ﻟﻀﻐﻂ ﻗﻠﯿﻞ ﻟﺘﻮﺳﯿﻊ اﻻوردة‬  Arteries have a higher percentage of collagen and smooth muscle than veins —— hence the veins can accommodate greater blood amount Compliance and Elasticity than arteries —— hence as well, the vein resistance(veinioles ) is much less than Compliance of blood vessels the arteries resist(arterioles ) Hence their (veins) named capacitance vessels At high pressure for example 80 mmHg, volume, and compliance for both artery and vein decreases (i.e., vessels become "stiffer" at higher pressures and volumes) This characteristic makes veins suitable for use as arterial by-pass grafts. When the arterial system of the average adult person (including all the large arteries, small arteries, and arterioles) filled with about 700 milliliters of blood, the mean arterial At low blood pressure, the veins due to their thin pressure is 100 mm Hg, but when it filled with only 400 walls (unlike arteries with very thick walls )can’t stand the blood pressure milliliters of blood, the pressure falls to zero. …..But at high blood pressure, the veins can expand , increase in size & act like arteries Compliance and Elasticity Compliance of blood vessels In the entire venous system, the volume normally ranges from 2000 to 3500 milliliters, and a change of several hundred milliliters in this volume is required to change the venous pressure only 3 to 5 mm Hg. This requirement mainly explains why as much as one-half liter of blood can be transfused into a healthy person in only a few minutes without greatly altering the function of the circulation Vasoconstriction by chemical or sympathetic stimulation (vascular smooth muscle contraction), which increases vascular tone will decrease compliance. Vasodilation by chemical or sympathetic inhibition (vascular smooth muscle relaxation), which decrease vascular tone will increase compliance. Another example of changing compliance is reduced aortic compliance with age or disease (e.g., arteriosclerosis). When this occurs, there is a qualitatively similar downward shift in the compliance curve for the aorta.  Arteries have a higher percentage of collagen and smooth muscle than veins —— hence the veins can accommodate greater blood amount than arteries Compliance and Elasticity —— hence as well, the vein resistance(veinioles ) is much less than the arteries resist(arterioles ) Compliance of ventricles Hence their (veins) named capacitance vessels As the ventricle fills with blood and its volume increases, the pressure within the ventricular chamber passively increases. The relationship is not linear ❑The compliance of the ventricle is determined by ① the structural properties of the cardiac muscle. (e.g., muscle fibers and their orientation, and connective tissue) ② the state of ventricular contraction and relaxation, for example, in ventricular hypertrophy the ventricular compliance is decreased in heart failure the ventricular compliance is increased  Compliance and Elasticity Laplace law ❑ According to Laplace law for hollow vessels Hollow tube (as vessels) P=T/r Hollow sphere (as ventricle or alveoli) P=2T/ r Transmural pressure (P) is the difference between the internal and external pressure on the vessel wall Transmural pressure acts perpendicular to wall and tending to push the wall outward Tension (T) is an inward force on the vessel wall that counteracts the transmural pressure, it acts on the vessel circumference as its collapsing force ❖ Transmural pressure is 1) directly proportion to tension (T) dyne/cm 2) inversely proportional to principal radii (r) (cm) 3) directly proportion to Wall thickness (u)  Compliance and Elasticity Laplace law ❑ Tension (T) on the wall is the sum of I. a passive tension owing to the vessel wall elasticity II. an active tension due to the tone and contraction of the smooth muscles on the wall ▪ In aorta, the tension at normal pressure is 170,000 dyne/cm ▪ In inferior vena cava, the tension is about 21,000 dyne/cm ▪ In the capillary the tension is about 16 dyne/cm  Compliance and Elasticity Vascular distensibility ❑ Means how easily a vessels is stretches Vascular distensibility normally is express as relative change in arterial volume (Δ𝑉⁄original volume) for a change in pressure (Δ𝑃), in accordance with the following formula: Distensibility = {Δ𝑉} / {𝛥𝑃 X original V} or {∆V/∆P. V} or {(Δ𝑉⁄𝑉)}. {(Δ𝑃)} That is, if 1 mm Hg causes a vessel that originally contained 10 millimeters of blood to increase its volume by 1 milliliter, the distensibility would be 0.1 per mm Hg, or 10 percent per mm Hg Compliance = ∆V/∆P. V/V = {∆V/∆P. V}. V= distensibility. Volume VP3  Compliance and Elasticity Vascular distensibility So: Highly distensible vessel/ slight volume means less compliance less distensible vessel/ high volume means more compliance ❑ Compliance and distensibility are quite different. Distensibility is the ability of vessel to stretch (distend) Distensibility is a marker of the mechanical load of the arterial wall. Compliance is the ability of vessel to stretch and hold volume Compliance is a marker of the artery's ability to store volume and buffer pressure changes that occur during the cardiac cycle. The compliance of a systemic vein is about 24 times that of its corresponding artery because it is about 8 times as distensible and has a volume about 3 times as great (8 × 3 = 24) VP3 Thank you! As you shine on your medical journey, remember that the pursuit of knowledge and the practice of compassion are intertwined. Never lose sight of the fact that medicine is not just a profession, it is a lifelong commitment to the well-being of others, but don’t lose yourself while doing that! «ِ‫ »ﻓَﺄَﻣﱠﺎ اﻟ ﱠﺰﺑَ ُﺪ ﻓَﯿَ ْﺬھَﺐُ ُﺟﻔَﺎ ًء ۖ َوأَﻣﱠﺎ ﻣَﺎ ﯾَﻨﻔَ ُﻊ اﻟﻨﱠﺎسَ ﻓَﯿَ ْﻤﻜُﺚُ ﻓِﻲ ْاﻷَرْ ض‬:‫وﺗﺬﻛﺮوا ﻗﻮﻟﮫ ﺗﻌﺎﻟﻰ‬