Lesson 94 CR

Hemodynamic and Biophysics

  • Pressure Gradients

    • Analyzes the pressure gradients determining blood flow between the heart chambers and large blood vessels.

    • Blood flow is dictated by the pressure gradient from higher to lower pressure.

  • Flow Definition

    • Refers to the volume of fluid (blood/air) passing through a vessel per unit time (e.g., 2/min).

    • This flow is essential for ensuring oxygen delivery, nutrient supply, and waste removal.

  • Ohm's Law in Circulation

    • Flow (Q) can be described by the formula: Q = P/R

      • Where:

        • P = pressure gradient

        • R = resistance

  • Resistance and Flow

    • Flow is inversely related to resistance, impacting blood flow.

    • According to Poiseuille's Law, resistance is affected by the radius of the vessel:

      • Flow ∝ 1/radius

  • Capacity or Compliance

    • Compliance is defined as the ability of a hollow organ to stretch/expand in response to pressure changes.

    • Compliance (C) formula: C = ΔP/ΔV

      • ↑ Compliance implies a change in volume with less change in pressure.

  • Oxygen Consumption

    • Considers the metabolic needs of different systems.

  • Pulmonary vs. Systemic Circulation

    • Pulmonary circulation maintains lower pressure with same flow as systemic circulation.

    • Systemic circulation has higher pressure levels at equivalent flow rates to facilitate blood transport to the body.

    • Pulmonary circulation prevents increased hydraulic pressure in long capillaries, which avoids interstitial edema.

  • Ventricular Systole

    • Represents ventricular contraction, resulting in pressure increase.

    • Blood is directed to lungs (pulmonary) or the body (systemic) depending on circulation pathway.

Pulmonary and Systemic Circulation Insights

  • Circulatory System Dynamics

    • The closed nature of the circulatory system underlines its stability.

    • Any increase in pulmonary vascular resistance seeks to maintain systemic pressure despite similar compliance to the systemic circulation.

  • Systemic Circulation Mechanics

    • Blood pressure (BP) is higher in systemic circulation compared to pulmonary circulation.

    • The left ventricle generates a higher pressure than the aorta during systole due to the biomechanics of the arterial side.

  • Windkessel Phenomenon

    • This mechanism guarantees steady blood flow through compliance and stretch of arteries during the cardiac cycle.

    • In systole, arteries distend; during diastole, they recoil, maintaining blood flow even when the heart is not actively pumping.

  • Impact of Arterial Elasticity

    • Loss of elasticity in arteries leads to high blood pressure and reduced ability to maintain steady flow.

    • A rigid artery cannot expand adequately during systole, compromising subsequent diastolic flow and causing decreased coronary perfusion.

Clinical Cases Affecting Circulation

  • Hypoxia and Pulmonary Hypertension

    • Conditions like altitude change blood flow due to decreased oxygen partial pressure causing pulmonary hypertension.

    • Specific diseases like brisket disease in cattle and heartworm in pets exemplify the impact of high hydrostatic pressure.

  • Systemic Circulatory Factors

    • Blood pressure can be expressed by BP = CO x Total peripheral resistance (using Ohm’s Law).

Exercise Physiology in Circulation

  • Effects of Moderate Exercise

    • During exercise, sympathetic tone increases, inducing vasoconstriction and influencing blood flow.

    • Hypertension in animals often correlates with chronic kidney disease, affecting preload.

    • Exercise requires increased oxygen for ATP production, enhancing cardiac output (Co) through stroke volume and heart rate.

  • 3 Mechanisms Supporting Stroke Volume

    1. Increased filling volume due to sympathetic stimulation enhances stroke volume capacity.

    2. Catecholamines effect on beta-adrenoceptors leads to vasoconstriction and mobilization of blood from the splanchnic circulation.

    3. The Frank-Starling mechanism ensures that more blood received leads to more ejected.

Blood Flow Redistribution During Exercise

  • Redistribution Mechanisms

    • During exercise, vasodilation occurs in skeletal muscle while splenic vasoconstriction redistributes blood flow.

    • Systemic blood pressure remains stable during submaximal exercise, influenced by baroreceptor reflexes.

  • Pulmonary Factors in Exercise

    • Exercise induces increased pulmonary artery pressure, paradoxically reducing pulmonary vascular resistance and enhancing capillary recruitment.

Heartworms and Circulatory Impact

  • Dirofilaria immitis Infection

    • Adult heartworms primarily inhabit pulmonary arteries, leading to afterload increase on the right side of the heart.

    • The duration of infection correlates with eventual heart failure and vascular lung disease.

  • Pathophysiological Effects

    • Infection causes vascular damage, inflammation, and increased resistance influencing the right heart, exacerbating pulmonary hypertension.