41. Functional characteristics of blood vessels. Hemodynamic principles – characteristics of the vessels and the blood. Hemodynamic indices. Volume and linear velocity of the blood flow through the various parts of vascular system and factors determining them.

Principles of Hemodynamics.

Hemodynamics is the study of the physical principles governing blood flow in the cardiovascular system. It describes the relationship between pressure and flow in the blood circulation around the body. Blood, like any fluid, moves from areas of high pressure to areas of low pressure. The pressure difference between the arteries (high pressure) and the veins (low pressure) is called the perfusion pressure and drives blood flow throughout the body.

Pressure-Flow Relationship

The relationship between pressure, flow, and resistance in the circulatory system can be understood using Ohm's Law, which was originally formulated to describe electrical circuits. In this context:

• Blood flow (Q) is analogous to electrical current.
• Pressure difference (ΔP) is analogous to voltage.
• Vascular resistance (R) is analogous to electrical resistance.

Just as in an electrical circuit, blood flow is directly proportional to the pressure difference and inversely proportional to resistance. Therefore:

• Increased pressure difference leads to increased blood flow. For example, during ventricular ejection, blood flows from the left ventricle to the aorta because ventricular pressure is higher than aortic pressure.
• Increased resistance leads to decreased blood flow. For example, vasoconstriction of arterioles increases resistance and reduces blood flow to a particular organ or tissue.