Vascular Physiology

Physiology Lecture title: vascular physiology Date: 3 \ 1 \2022 B. At low blood flow velocity (or low shear rate) with increased vessel diameters or with low flow, such as 1.downstream from an obstruction, 2.in diastole, 3.circulatory shock blood viscosity increase Low blood flow velocity state permits increased molecular interactions to occur between red cells and between plasma proteins and red cells. This can cause red cells to stick together and form chains of several cells (rouleax formation) within the microcirculation, which increases the blood viscosity. (4) Diameter of blood vessel (FahraeusLindqvist Effect) The Fahraeus-Lindqvist effect: 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 state “The RBC 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). A plasma cell-free layer reduces viscosity and resistance to flow To illustrate this phenomenon, in vivo measurements of blood viscosity made in dog hind limbs by Whittaker and Winton (1933) showed that at a given arterial blood hematocrit, the relative viscosity of blood flowing in the hindlimb is much lower than predicted from in vitro experiments (compare figure at right with previous figure that used a viscometer Sheet Note 1 Decrease in diameter  low viscosity  true when Diameter of the blood vessel is between 0.01 and 0.3mm Diameter --- viscosity or Diameter --- viscosity true when Diameter of the blood vessel is between 0.01 and 0.3mm **after 0.5 mm there is no effect or relation between diameter and viscosity The methods used to measure the blood flow are: Electro-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.  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. Venous 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 that part. Sheet Note 2 Electro-magnetic flow meters. We take an artery from an experimental animal surgically and put it in a magnetic field ,we consider blood flow here as an electrical current … so the electrical current changes in a magnetic field. Ultrasonic Doppler Flowmeter The most common method used in hospitals. Doppler: using ultrasound to measure blood flow.(echo). Venous occlusion plethysmography We take a patient arm and put it in a well-locked tube, and wrap a rupper-like material around the arm, when blood flow into arm by an artery ,it(the blood) can not return by vein and trapped in the arm so increase the volume of the arm. Resistance & Blood flow 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. Effective perfusion pressure, which is (the mean intraluminal pressure at the arterial end minus the mean intraluminal 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 < venule < veins < heart (atria) Sheet Note 3 • • Blood inter an organ from arterial side and supply it, then it leaves through venous side Effective perfusion pressure  for individual organ 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, lowest level of arterial pressure during ventricular cycle A. 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) Sheet Note 4 Pulsatile : the difference between systolic and diastolic pressure. Systole Systole pressure Diastolic pressure in distensible\not rigid blood vessel (aorta), the relationship between flow and perfusion pressure (the curve) is exponential. Critical dosing 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. The blood flow ceases when the blood pressure falls below the critical closing pressure because: Certain amount of intramural pressure is essentially required to push the RBC (with average diameter of 7.5μm through the capillaries (average diameter 5μm). Transmural pressure= intramural pressure – surrounding pressure 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. 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 510 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. Sheet note #5 Critical dosing pressure = 20 mm Hg On sympathetic stimulation , vasoconstriction occurs so the critical closing pressure increase up to 60 mm Hg.( you need more pressure to open and start blood to flow ) On sympathetic inhibition , full vasodilation occurs so the critical closing pressure decrease to 0 mm Hg Sympathetic stimulation and other vasoconstrictors ►vasoconstriction (decrease cross sectional area) ►alter the flow because (flow= area X velocity). Thus, inhibition of sympathetic activity ►greatly dilates the vessels ►increase the blood flow twofold or more. 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. B. Flow is inversely proportion 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 where R = resistance or total peripheral resistance, ∆P: pressure difference, Q: flow 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 Sheet note #6 Vasodilation  decrease pressure. Vasoconstriction  decrease flow and diameter. Arterioles are the major determinant for resistance. Plasma has low viscosity and low critical closing pressure than whole blood. 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 is equal to the rate of blood pumping by the heart (that is, it is equal to the cardiac output). Total resistance of the body: In the adult human being 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). 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). 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. Sheet note #7 Pulmonary circulation resistance is 7 times less than systemic circulation. * Arteries in pulmonary circulation is not very thick, because it is close to the heart & to exchange gases. * Arteries in systemic circulation is thicker than that in pulmonary circulation. Resistances in parallel or series The vascular anatomy of the entire body is comprised in-parallel vascular components. for an 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 vascular a. 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. The total resistance of this parallel arrangement expressed by the following equation: This demonstrates two important principles regarding the parallel arrangement of blood vessels: 1. The total resistance of a network of parallel vessels is less than the resistance of the vessel having the lowest resistance. 2. 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. 3. Because many parallel blood vessels, however, make it easier for blood to flow through the circuit as each parallel vessel provides another pathway, or conductance, for blood flow. General characteristic of resistance in parallel  a parallel arrangement of vessels greatly reduces resistance to blood flow.  Each artery in parallel receives a fraction of the total blood flow.  In each parallel artery, the pressure is the same. b. 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 venioles and veins 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.  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 Using this empirical model‫مثال تجريبي‬, we can see that 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 1. the arteriolar segment, which has the highest relative resistance, has the greatest effect on total resistance. 2. 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. 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: Poiseuille-Hagen formula: Flow (Conductance) inversely related to viscosity Flow (Conductance) directly related to (radius)4 because diameter = 2 radius then 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, Sheet note #8 According to Poiseuille-Hagen formula the most effective factor on flow is the radius which is inversely proportional to resistance but directly proportional to flow. Relation between resistance and radius: The internal diameters of the arterioles (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 to turn off almost completely the blood flow to the tissue or at the other extreme to cause a vast increase in flow so i. The internal diameters to change tremendously, often as much as fourfold ii. 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 Conditions in the Cardiovascular System Deviate from the Assumptions of Poiseuille’s Law 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. Compliance (or Capacitance or 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 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. The term compliance is used to describe how easily a chamber of the heart, or the lumen of a blood vessel expands when it is filled with a volume of blood. 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) 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 important characteristics stand out. First, The slope of the curve at any given pressure and volume (dV/dP) represents the compliance of the vessel at that pressure and volume. 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). 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 Second A. Ventricular compliance: As the ventricle fills with blood and its volume increases, the pressure within the ventricular chamber passively increases. The relationship is not linear, particularly at higher volumes, because the compliance of the ventricular wall decreases ("stiffness" increases) the more the ventricular wall is stretched. The compliance of the ventricle is determined by the structural properties of the cardiac muscle (e.g., muscle fibers and their orientation, and connective tissue) as well as by 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 B. Vessel compliance for an artery and vein): 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. 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. The volume-pressure curves of the normal arterial and venous system 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 pressure is 100 mm Hg, but when it filled with only 400 milliliters of blood, the pressure falls to zero. In the entire systemic 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. Sheet note #9 donating blood (400 ml blood + 100 ml anticoagulant) does not affect a person’s daily life so much because vein’s compliance is high, the fall in pressure is is between 3-5 mm Hg . Laplace Law According to Laplace law for hollow vessels a. hollow tube (as vessels) P=u .T/r or T=P. r / u b. hollow sphere (as ventricle or alveoli) P=2u .T/ r or T=P. r/2 u Transmural pressure (P): dyne/cm2 acting to distending force on the wall of hollow distending vessel amount to the difference between the internal and external pressure on the vessel wall, acts perpendicular to wall and tending to push the wall outward is 1) directly proportion to tension (T) dyne/cm acting on the vessel circumference as its collapsing force (collapsing vessel) is an inward force on the vessel wall counteracting the transmural pressure 2) inversely proportional to principal radii (r) (cm) 3) directly proportion to Wall thickness (u) 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