12 Comprehensive Notes on Blood Viscosity, Heart Mechanics, and Work Output

Overview of blood viscosity:
Plasma in blood has a viscosity of ~1.6 (with water as a reference of 1).
Adding red blood cells increases viscosity, making blood stickier.
Hematocrit ratio: The proportion of blood that is made up of red blood cells.
Effects of viscosity on blood flow:
Blood flow is not uniform; it moves fastest at the center of blood vessels.
Boundary condition: Blood near the edges moves slowly or not at all.
Average fluid velocity is half the maximum velocity due to the gradient.
Fossil's equation:
Key factors: flow rate and resistance to flow.
If viscosity (eta) and pressure are constant, length of the blood vessel and radius are the crucial factors affecting flow.
The radius affects flow to the power of four, making it a significant multiplier.
For narrower blood vessels (due to plaque), flow rate may decrease drastically (e.g., narrowing radius by a third reduces flow rate to 1/81).
Correlation with heart work:
Increased flow resistance means the heart has to work harder to maintain circulation, increasing the risk of heart failure.
Today's lecture outline:
Mechanical work done by the heart as a pump.
Application of Laplace's law to blood vessels, examining tension and pressure.

The heart operates as a force pump, creating pressure that forces blood through vessels.
Each heart contraction pumps approximately 80 ml of blood into systemic and pulmonary circulation.
Work is defined as pressure × volume, where pressure is in pascals and volume is in cubic meters.
To calculate work done by the heart in a cardiac cycle:
Determine average pressure in the left ventricle (about 100 mmHg).
Understand that average pressure for the right ventricle is much lower (around 15 mmHg).
Equations:
Work = pressure × volume
Power output can be determined using power = work/time, converting all units as needed.

Pressure conversion:
1 mmHg = 33.3 N/m²; work depends on both pressure and volume.
Remember to convert ml to cubic meters.
Final work calculations yield values like approximately 1.1 joules per cardiac cycle.
Power can be computed as work per time, considering heart rate.

Only about 10% of the heart's work is on contractions; most is maintaining its shape against blood pressure.
High blood pressure leads to increased work for the heart, risking enlargement and heart failure due to inefficient energy usage.

Blood vessels must maintain a balance between internal pressure and external forces; this is known as transferral pressure.
Key equations:
Transmural pressure keeps blood vessels open, ensuring blood flow.
Tension depends on wall tension/ radius, crucial for understanding blood vessel mechanics.
Significant differences in transmural pressure between the aorta and capillaries based on size and wall tension.

Recap of vital concepts regarding the heart's pumping mechanism, work output, and blood flow physics.
Importance of understanding how these principles apply to health conditions such as hypertension.