Biophysical Consideration for Circulation PDF

Cardiovascular physiology Dr.AhlamKadhim Lecture: 4 Biophysical consideration for circulation Objective: Demonstration of the following 1- Blood flow. 2- Velocity. 3- Low of Laplace. 4- Resistance and capacitance vessels 5- Starling forces. Blood flow: Blood flow me...

Cardiovascular physiology Dr.AhlamKadhim Lecture: 4 Biophysical consideration for circulation Objective: Demonstration of the following 1- Blood flow. 2- Velocity. 3- Low of Laplace. 4- Resistance and capacitance vessels 5- Starling forces. Blood flow: Blood flow means the quantity of blood that passes a given point in the circulation in a given period of time. Blood flow is expressed in milliliters per minute or liters per minute, but it can be expressed in milliliters per second or in any other units of flow and time. The overall blood flow in the total circulation of an adult person at rest is about 5000 ml/min. Types of blood flow: 1-Laminar blood flow : When blood flows at a steady rate through a long, smooth blood vessel, it flows in streamlines, with each layer of blood remaining the same distance from the vessel wall. This type of flow is called laminar flow or streamline flow,figure-1 Figure-1 Laminar blood flow 2- Turbulent flow, which is blood flowing in all directions in the vessel and in the form of eddies. The rate of blood flow becomes too great, when it passes by an obstruction in a vessel, when it makes a sharp turn. 1  Cardiovascular physiology Dr.AhlamKadhim The tendency for turbulent flow increases in direct proportion to the velocity of blood flow, the diameter of the blood vessel, and the density of the blood and is inversely proportional to the viscosity of the blood, in accordance with the following equation: Re=v.d.p/ η where Re is Reynolds' number and it predicts the tendency for turbulence to occur, ν is the mean velocity of blood flow (in centimeters/second), d is the vessel diameter (in centimeters), ρ is density, and η is the viscosity (in poise).Figure-2 Figure-2 laminar & turbulent blood flow Ohm's law : Blood flow through a blood vessel is determined by two factors: (1) pressure difference of the blood between the two ends of the vessel, also sometimes called "pressure gradient" along the vessel. (2) The impediment to blood flow through the vessel, which is called vascular resistance. Blood Flow (Q) = ∆ Pressure (P) / Resistance (R). Poiseuille's equation: vascular resistance (R) means impediment to blood flow through the vessel.The following equation is called Poiseuille's equation. R = 8ηL/ π r4 η η= Viscosity of blood. L = Length of vessel. r = radius of vessel. π =22/7 From this equation factors affecting resistance are: 1- Viscosity of blood, and length of the vessel. Resistance is directly proportional to them. 2-Vessel radius: It is inversely proportional to the forth power of the vessel radius. The unit of resistance is pressure unit divided by flow unit which is expressed in R unit. For example, when the mean aortic pressure is 90 mmHg and the left ventricular output is 90 ml/s, the total peripheral resistant is 90 mmHg/90 ml/s = 1 R unit. Blood viscosity The blood viscosity depends on: 2 Cardiovascular physiology Dr.AhlamKadhim A- Hematocrit: It is the percentage of red blood cells volume. The viscosity of blood increases as the hematocrit increases. B- Plasma protein concentration. Increased plasma protein cause increase viscosity. But this effect is less than the effect of hematocrit. the viscosity of the plasma is about 1.8 times as viscosity of water. Whole blood is 3 -4 times as viscous of water. Blood velocity: It is displacement of blood per unit of time (eg. cm/s). The blood velocity (V) is proportional to blood flow (Q) divided by the area of the conduit (A). Velocity (cm / s) = Flow (ml /s) / Area (cm²). V = Q/ A The average velocity of fluid movement at any point in a system is inversely proportionate to the total sectional area at that point. Therefore, the average velocity of the blood is higher in aorta (small cross sectional area) and lower in the capillaries (large cross sectional area), which have 1000 times the total cross-section area of the aorta. Laplace law :In a vessel containing a gas or a liquid under constant pressure, such as a balloon or blood vessel, the outward force exerted by the pressure stretches the wall of the vessel, creating tension. This tension exerts an inward force that balances the outward force of the pressure, so that the system comes to equilibrium, figure-1. Laplace law states that tension in the wall of a cylinder (T) is equal to the product of the transmural pressure (P) and the radius (r) divided by the wall thickness (w): T = Pr/w The transmural pressure is the pressure inside the cylinder minus the pressure outside the cylinder, but because tissue pressure in the body is low, it can generally be ignored and P equated to the pressure inside the viscus. In a thin-walled viscus, wall thickness (w) is very small and it too can be ignored, but it becomes a significant factor in vessels such as arteries. 3 Cardiovascular physiology Dr.AhlamKadhim Figure-1Relation between distending pressure (P) and wall tension (T) in hollow viscous. Resistance and capacitance vessels the veins are an important blood reservoir. Normally, they are partially collapsed and oval in cross-section. A large amount of blood can be added to the venous system before the veins become distended to the point where further increments in volume produce a large rise in venous pressure. The veins are therefore called capacitance vessels. The small arteries and arterioles are referred to as resistance vessels because they are the principal site of the peripheral resistance. Exchange Across Capillary Walls and Starling forces: Materials are exchanged across capillary walls through different mechanisms, depending on the molecular size and lipid solubility of the substance and on the location of the capillary. Mechanisms of exchange include simple diffusion, transcytosis, and mediated transport. Most small solutes, whether lipid- or water-soluble, move across capillary walls by simple diffusion. Small lipid-soluble solutes diffuse across the plasma membranes of endothelial cells, whereas small water-soluble solutes diffuse through the water-filled pores between endothelial cells. The direction of diffusion depends on the electrochemical gradient for a specific substance. Because nutrients and oxygen tend to be plentiful in the blood and are needed by cells, these substances diffuse from blood to tissue. Waste products and carbon dioxide, in contrast, are produced by the cells and, therefore, are more abundant in the tissues. Consequently, these substances diffuse from tissue to blood. Proteins, are too large to pass through water-filled pores, so they cannot pass through endothelial cells. Bulk Flow Across Capillary Walls Because capillary walls are freely permeable to water and small solutes, fluid can move from blood to interstitial fluid (called filtration) or from interstitial fluid to blood (called absorption) based on pressure gradients. The rate of filtration at any point along a capillary depends on a balance of forces called Starling forces, the four primary forces that determine whether fluid will move out of the blood into the interstitial fluid or in the opposite direction are: 1- The capillary hydrostatic pressure (Pc): It tends to force fluid outward the capillary membrane. 2- The interstitial hydrostatic pressure (Pi): It tends to force fluid inward through capillary membrane. 4 Cardiovascular physiology Dr.AhlamKadhim 3- The plasma colloid osmotic pressure (c): It tends to force fluid inward through the capillary membrane as the presence of a nonpermeating solute exerts an osmotic pressure that tends to draw water to the side where it is present in greater concentration. The primary nonpermeating solutes in plasma are proteins, 4-The interstitial colloid osmotic pressure(i). Fluid movement = K (Pc + лi) – (Pi + лc) If the result is positive, there will be a net fluid filtration across the capillaries to the interstitial spaces. If it is negative, there will be a fluid absorption from the interstitial spaces into the capillaries.Figure-2 Normally fluid moves into the interstitial space at the arteriolar end of the capillary and into the capillary at the venular end. Figure-2 Starling forces 5