Hemodynamics
Blood flow ensures the transportation of nutrients, hormones, metabolic waste products, oxygen, and carbon dioxide throughout the body to maintain cell-level metabolism, the regulation of the pH, osmotic pressure and temperature of the whole body, and the protection from microbial and mechanical harm.
The presence of these formed elements and their interaction with plasma molecules are the main reasons why blood differs so much from ideal Newtonian fluids.
A change in plasma osmotic pressure alters the hematocrit, that is, the volume concentration of red cells in the whole blood by redistributing water between the intravascular and extravascular spaces.
Its membrane has a Young's modulus in the region of 106 Pa. Deformation in red blood cells is induced by shear stress.
It is true that in a steady state flow of a viscous fluid through a rigid spherical body immersed in the fluid, where we assume the inertia is negligible in such a flow, it is believed that the downward gravitational force of the particle is balanced by the viscous drag force.
[citation needed] On the other hand, hypervolemic hemodilution (HVH) uses acute preoperative volume expansion without any blood removal.
[citation needed] To maintain the normovolemia, the withdrawal of autologous blood must be simultaneously replaced by a suitable hemodilute.
Ideally, this is achieved by isovolemia exchange transfusion of a plasma substitute with a colloid osmotic pressure (OP).
We can conclude from the foregoing that H should therefore not exceed s. The difference between the BLH and the BLs therefore is the incremental surgical blood loss (BLi) possible when using ANH.
[citation needed] The model used assumes ANH used for a 70 kg patient with an estimated blood volume of 70 ml/kg (4900 ml).
[10][11] The result of the model calculations are presented in a table given in the appendix for a range of Hi from 0.30 to 0.50 with ANH performed to minimum hematocrits from 0.30 to 0.15.
[citation needed] 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 0.15.
The model here is designed to allow doctors to determine where ANH may be beneficial for a patient based on their knowledge of the Hi, the potential for SBL, and an estimate of the Hm.
To apply these result to any body weight, any of the values BLs, BLH and ANHH or PRBC given in the table need to be multiplied by the factor we will call T Basically, the model considered above is designed to predict the maximum RCM that can save ANH.
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 capillaries connect to venules, and the blood then travels back through the network of veins to the venae cavae into the right heart.
The micro-circulation — the arterioles, capillaries, and venules —constitutes most of the area of the vascular system and is the site of the transfer of O2, glucose, and enzyme substrates into the cells.
The venous system returns the de-oxygenated blood to the right heart where it is pumped into the lungs to become oxygenated and CO2 and other gaseous wastes exchanged and expelled during breathing.
The Reynolds number (denoted NR or Re) is a relationship that helps determine the behavior of a fluid in a tube, in this case blood in the vessel.
The equation for this dimensionless relationship is written as:[16] The Reynolds number is directly proportional to the velocity and diameter of the tube.
[16] Due to its smaller radius and lowest velocity compared to other vessels, the Reynolds number at the capillaries is very low, resulting in laminar instead of turbulent flow.
[18] There are many ways to measure blood flow velocity, like videocapillary microscoping with frame-to-frame analysis, or laser Doppler anemometry.
The high resistance observed in the arterioles, which factor largely in the ∆P is a result of a smaller radius of about 30 μm.
The shear stress at the wall that is associated with blood flow through an artery depends on the artery size and geometry and can range between 0.5 and 4 Pa.[29] Under normal conditions, to avoid atherogenesis, thrombosis, smooth muscle proliferation and endothelial apoptosis, shear stress maintains its magnitude and direction within an acceptable range.
In some cases occurring due to blood hammer, shear stress reaches larger values.
[30] Veins are described as the "capacitance vessels" of the body because over 70% of the blood volume resides in the venous system.
Mean blood pressure drops over the whole circulation, although most of the fall occurs along the small arteries and arterioles.
Compared to other smaller vessels in the body, the artery has a much bigger diameter (4 mm), therefore the resistance is low.
[27] Pulmonary Artery Wedge Pressure can show if there is congestive heart failure, mitral and aortic valve disorders, hypervolemia, shunts, or cardiac tamponade.
[37] Noninvasive hemodynamic monitoring of eye fundus vessels can be performed by Laser Doppler holography, with near infrared light.
