11_ Viscous Effects and Blood Flow in the CVS

Cardiovascular System (CFR) Overview

Viscosity: Define and explain its impact on fluid pressure along cylindrical tubes, including how it can affect overall cardiovascular function.
Haematocrit Ratio: Discuss viscosity's effect and its relation to red blood cell (RBC) distribution, emphasizing the importance of maintaining optimal haematocrit levels for healthy circulation.
Blood Velocity Profile: Explain how viscosity influences blood velocity within vessels, especially how it creates a parabolic flow profile in larger vessels.
Poiseuille’s Law: Write and discuss the equation and its implications in the cardiovascular system, particularly how it relates to vascular resistance and changes in vessel diameters.
Blood Pressure Variation: Use Poiseuille’s Equation to describe blood pressure variations in major cardiovascular system (CVS) vessels, including factors that can manipulate these pressure gradients.

Definition: Viscosity is defined as the measure of a fluid's resistance to flow and deformation. It can be compared to friction in solids, determining how easily a fluid can move and how it behaves under different flow conditions.
SI Units: Viscosity (𝜂) is measured in Pascal seconds (Pa.s) or Poise (1 Pa.s = 10 Poise).
Example Values:
- Water at 20°C: 𝜂 = 1 x 10^-3 Pa.s
- Blood: 𝜂 ~ 5 x 10^-3 Pa.s, noting that blood's viscosity is significantly higher than water due to the presence of cells and proteins, which can alter flow dynamics.

Blood viscosity is directly proportional to the haematocrit ratio, which is the proportion of blood volume occupied by red blood cells. Higher haematocrit results in increased viscosity, thereby affecting blood flow rates and overall hemodynamic parameters.

Polycythaemia Vera: This condition leads to increased RBC production, which elevates blood viscosity, resulting in significant circulation issues like thrombosis and risk of emboli. Understanding the management of this condition is crucial for patient care.
Anaemia: In contrast, reduced RBC mass can lead to lower blood viscosity but may impair oxygen transport capabilities and create a range of symptoms including fatigue and weakness due to inadequate oxygen delivery.
Temperature Impact: A decrease in blood temperature, from a normal physiological range (37°C) to 0°C, can result in an increase in blood viscosity by 2-3 times, which severely impacts circulation and heat distribution within the body.

Bernoulli's Principle: This principle explains how viscosity affects blood velocity profiles, typically leading to a parabolic flow pattern where the highest flow occurs at the center of the vessel, and the lowest flow (or stagnation) happens at the vessel walls, influenced by shear stress and resistance.
Red Blood Cell Distribution: The viscous effects can cause red blood cells to accumulate towards the center of the flow in larger vessels, which results in an uneven distribution and affects overall oxygen delivery effectiveness, especially in diseases that cause altered flow profiles.

Smaller Vessels: The effect of haematocrit varies with vessel size, where high haematocrit in main arteries contrasts with lower haematocrit levels in smaller branching vessels; this difference influences overall blood flow dynamics and pressure regulation, thus affecting tissue perfusion.

Flow in Tubes: The volume flow rate (Q) is related to the pressure difference (∆P) over resistance (R). Poiseuille’s Law describes the flow through a cylindrical tube.
Formula:[ R = \frac{8 \eta L}{\pi r^4} ]Rearranged to calculate flow rate Q:[ Q = \frac{\Delta P \pi r^4}{8\eta L} ]
Note: Flow rate Q is significantly affected by the vessel radius (r^4 relationship), and it varies inversely with viscosity and vessel length, underscoring the importance of vascular health in regulating blood flow.

Conditions such as atherosclerosis can lead to the formation of atheromas and other blockages, which increase resistance drastically; this results in the heart working much harder (up to 81 times more) if the radius of the vessel reduces to a third of its original value.

The majority of pressure is lost in arterioles and capillaries due to drastic reductions in vessel radius and an increase in surface area in these smaller vessels, crucial for exchange of gases and nutrients.

Skeletal Muscle Pumps: These pumps aid in vein compression during muscle contraction, moving blood toward the heart. One-way valves prevent backflow, ensuring efficient return of venous blood to the heart, which is essential for maintaining systemic circulation.

Post-operative cardiac patients may need to calculate arterial radius or length using blood flow rates and pressure drops while applying Poiseuille’s Law.
Consider the implications of maintaining pressure gradients in vessels with constriction, which leads to increased pressure differences that can impact vascular health severely.

Explore the effects of decreased vessel length or reduced red blood cell counts on blood flow resistance.
Analyze the impact of arterial constrictions on heart workload and overall patient cardiovascular health.
Solve practical problems including determining pressure differences in vessels and assessing how viscosity affects blood flow in varying clinical settings, particularly during surgical and medical interventions.