Hemodynamics PDF

 Outlines Relationship between flow, pressure and resistance and other physical property of blood circulation. Blood flow: types, regulation Blood vessels: types Arterial Blood Pressure & regulation Microcirculation – Trans-capillary exchange – Regulat...

 Outlines Relationship between flow, pressure and resistance and other physical property of blood circulation. Blood flow: types, regulation Blood vessels: types Arterial Blood Pressure & regulation Microcirculation – Trans-capillary exchange – Regulation Characteristics of the venous system 24/09/2024 2  Objectives At the end of this session the students will be able to: State the relationship among blood flow, blood pressure, and vascular resistance. Describe blood flow types & its regulation. Identify characteristics of blood circulation in various vascular segments. Discuss about arterial blood pressure & its regulation. Describe microcirculation and characteristics of venous system.  HEMODYNAMICS Hemodynamics is the study of the relationship between flow, pressure, resistance and other physical principles of blood circulation. It addresses the properties of both blood and blood vessels. Poiseuille and Hagen described factors governing non-pulsatile flow of a homogeneous viscous fluid through a rigid cylindrical tube with constant circular cross-section. Rate of flow, Q, is given by: L length Q = Pr4 /8L R radius /8 proportionality constants P pressure 24/09/2024  Viscosity of the fluid 4  HEMODYNAMICS… Though blood is a non-homogeneous and has a pulsatile flow through distensible vessels, the law on flow through rigid tubes: – Is a good approximation to the flow of blood through the vascular system. – Facilitates our understanding on hemodynamics of the in vivo circulation. Therefore, the relation of geometry of vascular net work, nature of blood (viscosity) , mean BP, can be explained using Poiseuille’s law of laminar flow: Q = Pr4 /8L Blood flow (Q) is volume of blood that passes a given point in the circulation per time. 24/09/2024 6  Methods of Estimating Blood Flow 1. Electromagnetic Flow Metre 2. Indicator Dilution Method 3. Venous Occlusion Plethysmography Venous Occlusion Plethysmography is the method used for recording the curve of filling or enlargement of organs. Conventional methods used to estimate flow in the extremities It include air or water-filled plethysmographs. Principle: the flow to part of the extremity measured by venous occlusion plethysmography is equal to the arterial inflow. 7 Venous Occlusion Plethysmography In estimating blood flow to the extremity, the part to be studied is enclosed in a plethysmograph (oncometer) connected to a float or piston recorder to which is attached a writing lever (Fig...). Air-filled plethysmograph When measurement is to be made, for example using an air-filled plethysmograph, a pneumatic cuff is placed around the part of extremity proximal to the plethysmograph and quickly inflated to 70-80 mmHg. This pressure is maintained for 15-20 seconds so that an inflow curve (Fig..) is recorded on a moving drum. The initial swelling rate (tangent to inflow curve) measures flow to the enclosed part of the arm. 8 Application of Plethysmography 1. Assessment of:  Intermittent claudication  Vascular impotence  Diabetic angiopathy 2. As a research tool on blood flow 10  Resistance to Flow (r) This is an impediment to flow provided by a vessel or circulatory bed. Resistance to BF is: – Directly proportional to the length of the vessel and viscosity of blood. – Inversely proportional to the 4th power of radius of the vessel. R=P/Q, analogous to Ohm’s law =P÷Pr4 ÷8L Resistance progressively increase from aorta to arterioles (due to length &radius) = 8L/ r4  The r/ship of radius & R shows, organ blood flow is effectively regulated by small changes in the caliber of the arterioles. 24/09/2024 11  Resistance to Flow (r)  Variations in arteriolar diameter have such a pronounced effect on systemic arterial pressure. Using (R=P/Q) relationship one can assess resistance in different vascular beds. E.g. TPR  Aortic pressure  rt. Atrial pressure Cardiac output  Mean aortic pressure = 100mmHg  Rt. Atrial pressure = 0mmHg  Cardiac output =100mL/sec. – Total peripheral resistance=100/100 =1 (mmHg/ml/sec) or PRU – Normal TPR range from 1-1.4 PRU  Resistance to Flow (r) E.g. Total pulmonary vascular resistance – Mean pulmonary arterial pressure averages 16 mm Hg and – The mean left atrial pressure averages 2 mm Hg – 16-2 mm Hg/100ml/sec0.14 PRU Analogous to electrical circuits, vascular resistances may be arranged in series or parallel.  If in series, resistances are additive. – E.g Renal vasculature where peritubular capillaries are in series with glomerular capillaries), 24/09/2024 13  Resistance to Flow (r)  When in parallel (like in pulmonary and systemic circulations) they are additive as reciprocals.  1/RT= 1/R1+1/R2+1/R3, etc. i.e. in a parallel circuit, RT is less than any of the individual R terms. Majority of vascular resistances of organs and tissues in the body are in parallel. This provides a significant advantage leading to greater flow. 24/09/2024 14 24/09/2024 15  Velocity of Blood Flow Velocity (v) of blood flow is distance moved by RBCs per unit time. Velocity (V) is proportional to flow (Q) divided by the area of the conduit (A): V = Q ÷ Cross-sectional area (A=r2) = (Pr4 ÷8L)÷r2 V = Pr2 /8L Velocity (v) of blood flow is directly proportional to: – The pressure difference (P) and – Diameter of the vessels,  Therefore, the average velocity of the blood is high in the aorta (30-40cm/sec), declines steadily in the smaller vessels, and is lowest in the capillaries (0.3mm/sec). But inversely related to the viscosity of blood and length of the blood vessel. 24/09/2024 16 24/09/2024 17  VISCOSITY This is friction of molecules in the moving stream of fluid. It depends on: – Concentration of suspended medium (e.g. RBC in blood). Viscosity becomes high in polycythemia, and low in anemia. In anemia, decrease in O2-carrying ability of the blood. – But the improved blood flow due to decrease in viscosity partially compensates for this. 24/09/2024 18  Types of Blood Flow 1. Laminar or streamline blood flow Flow of blood along a uniform tube at a fixed rate. Laminar flow form infinite number of concentric laminae or layers sliding upon one another are seen. Innermost or axial layer has the highest velocity and the outermost layer touching the lining of the tube remains slow. Laminar flow is silent occurs throughout the normal CVS, except in the heart. 19  Types of Blood Flow 2. Turbulent Blood Flow Blood flow indifferent directions in the vascular or heart chamber. Flow continues to be laminar (streamline) as long as the product of velocity and diameter does not exceed a critical value. It becomes turbulent when the critical value is exceeded. This value is determined by the Reynolds number Re. It represents the ratio of inertia to viscous forces. Re = ρVD/η, where V= mean velocity D=diameter of tube η= viscosity ρ=density of fluid Laminar flow becomes turbulent when Re exceeds 2000. 24/09/2024 20  Types of Blood Flow Turbulent flow forms whirl. Turbulent flow rarely occurs in the normal human circulation. It is not a feature of peripheral arterial or venous circulation. It may occur in the atria, ventricles and aorta under conditions of severe exercise. 24/09/2024 21  Types of Blood Flow Turbulent blood flow produces sound. Re may reach 5000 to 12,000; in such condition it is associated with murmurs (bruits). – E.g. bruits heard over arteries constricted by atherosclerotic plaques and – The sounds of Korotkoff heard when measuring blood pressure 24/09/2024 22  Regulation of Blood Flow The vessels involved in regulation of the rate of tissue blood flow throughout the body are mainly the arterioles. Altered demands of flow are met by adjusting vessel calibre which is the acute tissue blood flow control mechanisms. Vessel calibre is adjusted by two general mechanisms: 1. Local regulation 2. Central regulation  Centrally through the nervous system & hormones, and  Locally by the conditions in tissues surrounding the blood vessels & blood vessel it self. 24/09/2024 23  Regulation of Blood Flow  The relative importance of these two control mechanisms varies in different tissues. – In some areas of the body, such as the skin and splanchnic regions, neural regulation of blood flow predominates. – Whereas in other regions, such as the heart and brain, this mechanism plays only a minor role. – In skeletal muscle both mechanisms interact. 24/09/2024 24  1. Intrinsic or local regulation of BF This is the capacity of tissues to regulate their own blood flow. a) Myogenic Hypothesis – BF stretch of blood vessels constriction to normalize (decrease) blood flow. – BF tension of blood vessel relaxation to normalize (increase) BF. b) Metabolic Hypothesis – The metabolic changes that produce vasodilation include, in most tissues, decreases in O2 tension and pH. – MR orO2 production of vasodilator metabolites. Examples: 1) O2, H+; cause relaxation of blood vessels, 2) Adenosine…dilatation of arterioles 24/09/2024 25  1. Intrinsic or local regulation cont’d 3) CO2 moderate vasodilatation in most tissues but marked vasodilatation in brain. 4) K+ Inhibition of smooth muscle contraction vasodilatation. 5) Lactate vasodilatation through increasing [H+]. 6)Ca2+Smooth muscle contraction vasoconstriction 7) Carbon monoxide: produced from heme found in cardiovascular tissues produces local vasodilatation. 26  1. Intrinsic or local regulation cont’d 8) Endothelial products  Nitric oxide (NO): endothelium-derived relaxing factor (EDRF); it is synthesized from arginine in the presence of the enzyme NO synthase. Three isoforms are identified: – NOS1,found in nervous system, – NOS 2 found in microphages and other immune cells, and – NOS 3 found in endothelial cells. Play a key role in vasodilatation 24/09/2024 27  1. Intrinsic or local regulation cont’d  Bradykinin: vasodilator Endothelins Endothelin 1: one of the most potent constrictors; primarily a local paracrine regulator of vascular tone. Endothelin 2: produced primarily in kidneys and intestine Endothelin 3: found in high concentration in brain; also found in kidney and GIT. 24/09/2024 28 29  1. Intrinsic or local regulation cont’d c) Tissue Pressure Hypothesis Perfusion pressure blood volume net transfer of fluid from intravascular to extra-vascular space tissue pressure (turgor) compression on thin-walled blood vessels blood flow to tissues. 24/09/2024 30  2. Extrinsic (Remote) Regulation A) Neural Regulation ANS receiving impulses from higher centres to change blood flow and arterial pressure through changing vessel calibre. Nor-epinephrine as transmitter of sympathetic NS causes an increase in the tone of arterioles acting on an α- adrenergic receptor on smooth muscle cells. The fibers to the resistance vessels regulate tissue blood flow and the fibers to the venous capacitance vessels vary the volume of blood "stored" in the veins. 24/09/2024 31 24/09/2024 32  2. Extrinsic (Remote) Regulation cont’d B) Hormonal Regulation 1) Catecholamines: EP & NEP, like nor-adrenaline released from sympathetic nerves, through alpha adrenergic receptors cause vaso-constriction. Many arteriolar smooth muscle cells possess beta- adrenergic receptors as well as alpha- adrenergic receptors. Epinephrine causes the muscle cells to relax rather than contract through beta-adrenergic receptors. In most vascular beds, the existence of beta adrenergic receptors on vascular smooth muscle are greatly outnumbered by the alpha-adrenergic receptors. 33  2. Extrinsic (Remote) Regulation cont’d The arterioles in skeletal muscle have a large number of beta adrenergic receptors. Circulating epinephrine usually causes vasodilatation in this vascular bed. 2. Angiotensin II constricts most arterioles. This peptide is part of the renin-angiotensin system. 3.Vasopressin, when present at high plasma concentrations, causes arteriolar constriction 4. Atrial natriuretic peptide is the hormone secreted by the atria and is a potent vasodilator on arterioles like the afferent arteriole where it increases GFR. 24/09/2024 34  Balance b/n intrinsic and extrinsic regulation of BF In some tissues, the effects of the extrinsic and intrinsic mechanisms are fixed; in other tissues, the ratio is changeable. This is important for directing BF to areas of greater need. 1) Brain and Heart The brain and heart are vital structures with a limited tolerance for a reduced blood supply. Intrinsic flow-regulating mechanisms are dominant to regulate blood flow to these area. 2) Skin Blood flow to the skin is mainly controlled by the extrinsic (neural) control system. Local control has some effect. 24/09/2024 35  Balance b/n intrinsic and extrinsic regulation of BF 3) skeletal muscle  The extrinsic and intrinsic mechanisms interact. – In resting skeletal muscle, neural control (vasoconstrictor tone) is dominant. – Start or anticipation of exercise such as running, activation of sympathetic dilator system  blood flow in the leg muscles. – After the onset of exercise, the intrinsic flow-regulating mechanism assumes control, and vasodilatation occurs in the active muscles because of the local increase in metabolites. 24/09/2024 36 37 38  Blood vessels: Classification 1. Elastic vessels:  Example: Aorta, big arteries  Pressure storing components  High ability of recoiling 2. Resistance vessels Example: small arteries and arterioles  High muscular component  Develop high resistance – Small changes in their caliber cause large changes in the total peripheral resistance.  Regulate blood flow 3. Exchange vessels: Example, capillaries made up of endothelium and basement membrane.  3 types: continuous, discontinuous and fenestrated capillaries. 24/09/2024 39  Blood vessels: Classification 4. Capacitance vessels (big to small veins)  Very high capacity of distension – Veins are eight times more distensible than the artery of comparable size.  Low resistance system  Can accommodate large volume of blood (65% of blood volume)  Have valves that assure blood return to the heart 24/09/2024 40  ARTERIES AORTA Contain large amount of elastin expansion and recoil pressure storage. A velocity of 30-40 cm/sec at aorta