Hemodynamics or hæmodynamics is the dynamics human anatomy and physiology marieb hoehn 10th edition pdf blood flow. Blood is a non-Newtonian fluid, best studied using rheology rather than hydrodynamics. Blood vessels are not rigid tubes, so classic hydrodynamics and fluids mechanics based on the use of classical viscometers are not capable of explaining hemodynamics.
The study of the blood flow is called hemodynamics. The study of the properties of the blood flow is called hemorheology. Blood is composed of plasma and formed elements. Normal blood plasma behaves like a Newtonian fluid at physiological rates of shear. The osmotic pressure of solution is determined by the number of particles present and by the temperature.
C it has an osmotic pressure of 2. The osmotic pressure of the plasma affects the mechanics of the circulation in several ways. An alteration of the osmotic pressure difference across the membrane of a blood cell causes a shift of water and a change of cell volume. The changes in shape and flexibility affect the mechanical properties of whole blood. The red blood cell is highly flexible and biconcave in shape.
Its membrane has a Young’s modulus in the region of 106 Pa. Deformation in red blood cells is induced by shear stress. Where a is the particle radius, ρp, ρf are the respectively particle and fluid density μ is the fluid viscosity, g is the gravitational acceleration. We have looked at blood flow and blood composition. Before we look at the main issue, hemodilution, let us take a brief history into the use of blood. Its therapeutic use is not a modern phenomenon. Hemodilution is the dilution of the concentration of red blood cells and plasma constituents by partially substituting the blood with colloids or crystalloids.
It is a strategy to avoid exposure of patients to the hazards of homologous blood transfusions. Hemodilution can be normovolemic which, as we said, implies the dilution of normal blood constituents by the use of expanders. Therefore, blood lost by the patient during surgery is not actually lost by the patient, for this volume is purified and redirected into the patient. Here, instead of simultaneously exchanging the patient’s blood as in ANH, the hypervolemic technique is carried out by using acute preoperative volume expansion without any blood removal. In choosing a fluid, however, it must be assured that when mixed the remaining blood behaves in the microcirculation as in the original blood fluid, retaining all its properties of viscosity. In presenting what volume of ANH should be applied one study suggests a mathematical model of ANH which calculates the maximum possible RCM savings using ANH, given the patients weight Hi and Hm. See below for a glossary of the terms used.
To maintain the normovolemia, the withdrawal of autologous blood must be simultaneously replaced by a suitable hemodilute. A colloid is a fluid containing particles that are large enough to exert an oncotic pressure across the micro-vascular membrane. From the equation above it is clear that the volume of blood removed during the ANH to the Hm is the same as the BLs. How much blood is to be removed is usually based on the weight, not the volume. The model assumes that the hemodilute value is equal to the Hm prior to surgery, therefore, the re-transfusion of blood obtained by hemodilution must begin when SBL begins.
If ANH is used as long as SBL does not exceed BLH there will not be any need for blood transfusion. We can conclude from the foregoing that H should therefore not exceed s. Where RCMi is the red cell mass that would have to be administered using homologous blood to maintain the Hm if ANH is not used and blood loss equals BLH. A range of Hi and Hm was evaluated to understand conditions where hemodilution is necessary to benefit the patient. The result of the model calculations are presented in a table given in the appendix for a range of Hi from 0. 50 with ANH performed to minimum hematocrits from 0. 40, if the Hm is assumed to be 0.
For example, if Hi is 0. 30 or less it is not possible to save a red cell mass equivalent to two units of homologous PRBC even if the patient is hemodiluted to an Hm of 0. That is because from the RCM equation the patient RCM falls short from the equation giving above. 40 one must remove at least 7. 5 units of blood during ANH, resulting in an Hm of 0. 20 to save two units equivalence. Basically, the model considered above is designed to predict the maximum RCM that can save ANH.
In summary, the efficacy of ANH has been described mathematically by means of measurements of surgical blood loss and blood volume flow measurement. This form of analysis permits accurate estimation of the potential efficiency of the techniques and shows the application of measurement in the medical field. The heart is the driver of the circulatory system, pumping blood through rhythmic contraction and relaxation. Blood being pumped out of the heart first enters the aorta, the largest artery of the body.