Arterial Physiology

Arterial Physiology: Comprehensive Study Notes

Fundamental Concepts of Energy and Flow

• Kinetic Energy (Energy of Motion):

  • Defined as the energy of work or motion.

  • In the vascular system, it is partially represented by the velocity of blood flow.

• Potential Energy (Stored/Resting Energy):

  • Defined as stored or resting energy.

  • In the vascular system, this is primarily represented by intravascular pressure.

• Total Energy in the Vascular System:

  • Composed of kinetic energy, potential energy (pressure), gravitational energy, and hydrostatic pressure.

  • Gravitational energy and hydrostatic pressure are components of the total energy but are not components of kinetic energy.

  • They tend to cancel each other out when expressed in relation to a specific reference point.

• Blood Movement:

  • Blood always moves from one point to the next due to a pressure or energy gradient.

  • The lowest energy/pressure in the vascular system is typically found in the right atrium.

  • When moving farther from the reference point of the right atrium, the hydrostatic pressure increases.

• Volume Flow Measurement:

  • Units for measuring flow volume include mL/s, cL/min, and L/min.

  • m/s is a unit of velocity, not volume flow.

Fluid Dynamics and Resistance

• Laminar Flow:

  • Characterized by liquid traveling smoothly in parallel layers.

  • The layers of blood cells at the center of the vessel move the fastest.

  • The layers of cells at the wall of the vessels do not move (due to friction).

  • The differences in velocities between layers are primarily due to friction.

  • The velocity at the center of the vessels is the highest, not half the mean velocity.

• Plug Flow:

  • Typically demonstrated at the entrance of a vessel where all blood cells move at nearly the same velocity.

• Viscosity:

  • The property of a fluid that resists the force tending to cause it to flow.

  • In the human body, the major component of the blood influencing viscosity is hematocrit.

• Inertia:

  • The tendency of a body at rest to stay at rest or a body in motion to stay in motion.

  • Inertia and viscosity are two key components in the vascular system that contribute to resistance.

• Poiseuille's Law:

  • States that the volume flow ($Q$) of a liquid flowing through a vessel is directly proportional to the pressure of the liquid ($riangle P$) and the fourth power of the radius ($r^4$), and is inversely proportional to the viscosity ($extstyle{ extstyle ext{η}}$) of the liquid and the length ($L$) of the vessel.

  • Expressed as: $Q = rac{ extstyle{ extstyle ext{π ΔP r^4}}}{ extstyle{ extstyle ext{8 η L}}}$

  • Changes in the radius of vessels most significantly affect resistance in the vascular system due to its fourth-power relationship in Poiseuille's Law. A decrease in vessel radius is a prominent cause of abnormal energy losses in pathologies like obstruction and stenosis.

• Ohm's Law Analogy in Vascular System:

  • The law stating that the current through two points is directly proportional to the potential difference across the two points and inversely proportional to the resistance between them.

  • In the vascular system, pressure gradient represents the potential difference or voltage.

• Resistance Types:

  • Viscous Resistance: Occurs in long, narrow vessels due to friction between layers of blood and the vessel wall.

  • Inertial Resistance: Occurs at bifurcations, curves, or stenoses when blood flow direction or velocity changes abruptly.

• Vessels in Parallel vs. Series:

  • When vessels are arranged in parallel, the total resistance of the system is lower compared to when vessels are in series.

• Turbulent Flow:

  • Requires greater pressure to move blood compared to laminar flow due to increased energy losses.

  • Factors that cause turbulent flow include:

    • Increased blood velocity.

    • Vessel narrowing (stenosis).

    • Changes in blood density (e.g., anemia).

    • Atherosclerotic disease causing significant stenosis.

  • Turbulence typically occurs when the Reynolds number exceeds approximately $2000$.

• Bernoulli's Principle:

  • States that as the velocity of a fluid increases, its pressure decreases.

  • In the circulatory system, this principle explains why pressure drops across a stenosis where velocity is elevated.

Arterial System Characteristics

• Cross-sectional Area and Velocity:

  • The cross-sectional area of vessels increases from the aorta, moving through the arteries, then arterioles, and finally the capillaries.

  • Consequently, blood velocity decreases as blood flows from the aorta to the capillaries, inversely related to the increasing cross-sectional area (assuming relatively constant volume flow).

  • If the volume of blood or flow remains the same, a decrease in the area of a vessel leads to an increase in the velocity of blood.

• Highest Pressure:

  • The highest pressure in the vascular system (approximately $120 ext{ mm Hg}$) is found in the left ventricle during systole.

• Hydraulic Filter Function (Windkessel Effect):

  • The arterial system, composed of elastic arteries and high-resistance arterioles, acts as a hydraulic filter.

  • Its primary function is to convert the intermittent, pulsatile output of the heart into a steady flow through the capillaries.

  • This conversion occurs during diastole through the elastic recoil of the arteries, which maintains blood flow when the heart is not contracting.

• Control of Arterial Resistance:

  • Resistance in the arterial system is controlled:

    • Locally: By conditions at the tissue bed (e.g., metabolic demands, local chemical signals).

    • Centrally: By the nervous system, specifically through the contraction and relaxation of smooth muscle cells in the media of arterioles.

  • When norepinephrine is released by the sympathetic nervous system, it triggers the contraction of smooth muscle cells in arterioles, leading to vasoconstriction and increased resistance.

• Effects of Vessel Stiffening with Age:

  • As a vessel wall stiffens with age, this results in an increase in systolic pressure as well as an increase in pulse pressure (the difference between systolic and diastolic pressure).

• Flow Systems and Narrowing:

  • In high flow systems, significant drops in pressure and flow occur with less severe narrowing compared to low flow systems.

High- and Low-Resistance Vascular Beds

• Low-Resistance Vascular Beds:

  • Characterized by continuous antegrade flow (forward flow throughout the cardiac cycle).

  • This flow profile indicates high metabolic demands from the tissues supplied.

  • The flow profile typically does not display alternating antegrade/retrograde flow.

  • Examples include the Internal Carotid Artery (ICA), Renal Artery, Hepatic Artery, and Splenic Artery.

• High-Resistance Vascular Beds:

  • Characterized by pulsatile flow that may exhibit two to three phases (triphasic).

  • Displays alternating antegrade/retrograde (reversed) flow, especially during diastole, in resting arteries.

  • The flow profile is primarily due to vasoconstriction of arterioles.

  • Examples include the Femoral Artery, Brachial Artery, and other peripheral resting arteries.

Collateral Vessels and Peripheral Arterial Occlusive Disease

• Collateral Vessels:

  • These are preexisting pathways for blood flow that can develop or enlarge to bypass an obstruction.

  • The resistance in collaterals is generally fixed.

  • Vasodilator drugs have little to no effect on collaterals because they are typically already maximally dilated to compensate for reduced flow.

  • Midzone collaterals are small intramuscular branches.

• Peripheral Arterial Occlusive Disease (PAOD):

  • A significant indicator of PAOD is when occluded arteries cannot increase blood supply to meet heightened demand (e.g., during exercise).

  • This leads to a drop in pressure distal to the obstruction as the blood struggles to travel through narrowed segments, causing symptoms like claudication.

Here is a practice test based on your study guide:

Practice Test: Arterial Physiology

1. Fundamental Concepts of Energy and Flow
   a. What two forms of energy primarily constitute the total energy in the vascular system, excluding gravitational and hydrostatic pressure?
   b. Where is the lowest energy/pressure typically found in the vascular system?
   c. Blood always moves from one point to the next due to what driving force?
   d. Identify which of the following is not a unit for measuring volume flow: mL/s, cL/min, L/min, m/s.

2. Fluid Dynamics and Resistance
   a. Describe the characteristics of laminar flow in terms of blood cell movement and velocity distribution within a vessel.
   b. What is plug flow, and where is it typically observed?
   c. Which major component of blood primarily influences its viscosity in the human body?
   d. Besides viscosity, what two other key components contribute to resistance in the vascular system?
   e. State Poiseuille's Law and identify the variable within the law that most significantly impacts vascular resistance. Explain why.
   f. In the vascular system, what concept is analogous to the 'potential difference' or 'voltage' in Ohm's Law?
   g. Differentiate between viscous resistance and inertial resistance, providing examples of where each might occur.
   h. How does the total resistance of a system of vessels arranged in parallel compare to the total resistance when the same vessels are arranged in series?
   i. List at least three factors that can cause turbulent blood flow. At approximately what Reynolds number does turbulence typically occur?
   j. Explain Bernoulli's Principle as it applies to blood flow through a stenotic vessel.

3. Arterial System Characteristics
   a. Describe the trend in cross-sectional area and blood velocity as blood flows from the aorta to the capillaries.
   b. Where is the highest pressure in the vascular system found, and at what approximate value (in mm Hg) during systole?
   c. Explain the primary function of the arterial system as a 'hydraulic filter' (Windkessel Effect).
   d. How is arterial resistance controlled both locally at the tissue bed and centrally by the nervous system?
   e. What effect does the release of norepinephrine by the sympathetic nervous system have on arterioles and vascular resistance?
   f. How does vessel stiffening with age impact systolic pressure and pulse pressure?

4. High- and Low-Resistance Vascular Beds
   a. Describe the characteristic flow profile of a low-resistance vascular bed and provide two examples of arteries that typically exhibit this profile.
   b. Describe the characteristic flow profile of a high-resistance vascular bed, including its phases, and provide two examples of arteries that typically exhibit this profile.

5. Collateral Vessels and Peripheral Arterial Occlusive Disease
   a. What are collateral vessels, and what is their typical response to vasodilator drugs?
   b. What is a significant indicator of Peripheral Arterial Occlusive