Lesson 94-biophysics and CVR

Systems Overview

Speaker: Dr. Hector ZerpaInstitution: St. George's University SCHOOL OF VETERINARY MEDICINE, Grenada, West Indies

By the end of this session, students will be able to:

Analyze pressure gradients determining blood flow between heart chambers and large blood vessels.
Describe consequences of changing resistance in pulmonary and systemic circulation blood flow.
Integrate circulatory system's resistance, flow, and pressures during moderate exercise.
Integrate circulatory system's resistance, flow, and pressures in dogs suffering from heartworms.

Pressure Gradient: Blood flows from areas of higher pressure to areas of lower pressure, a fundamental principle that drives circulation and ensures that tissues receive oxygen and nutrients.
Resistance: Determined by the formula Flow = 1/radius^4 (according to Poiseuille's law). This indicates that even small changes in vessel radius can lead to significant changes in blood flow resistance.
Compliance: Refers to the capacity of blood vessels to expand and contract as needed, influencing blood flow dynamics.
Oxygen Consumption: Required to meet the metabolic needs of tissues throughout the body, highlighting the importance of efficient circulatory function.

Characterized by lower mean blood pressure compared to systemic circulation, which is crucial to prevent excess hydraulic pressure that could lead to lung edema and other complications.
Systole: During this phase, the right ventricle generates pressure greater than that in the pulmonary artery, allowing blood to flow to the lungs for oxygenation.
It is essential to maintain lower pulmonary vascular resistance to keep pulmonary pressure manageable, despite similar cardiac output to systemic circulation.
Cardiac Output (CO) Formula: CO = Stroke volume x Heart rate, an essential formula for understanding blood flow in both pulmonary and systemic circuits.

Features higher mean blood pressure than pulmonary circulation, ensuring adequate blood flow to all body parts.
Systole: In this phase, the left ventricle must generate pressure greater than that of the aorta to facilitate systemic circulation.
The system relies on a biomechanical phenomenon known as the Windkessel effect, ensuring a steady flow of blood despite the pulsatile nature of the heartbeat.

Systemic and pulmonary arteries act as conduits that distribute blood to various capillary beds across the body.
Arterioles play a crucial role by maintaining high resistance, effectively regulating blood flow distribution according to tissue demands.
Large arteries, such as the aorta, contain elastin, making them compliant and distensible, allowing them to absorb and smooth out the initial surge of blood ejected from the heart during systole.

Steady Blood Flow: Elastic walls dampen the pulsatile flow from the heart. Distention during ventricular contraction (systole) and recoil during relaxation (diastole) promote smooth blood flow.
Reduced Heart Work: The elastic properties of large arteries reduce the workload on the heart by accommodating the varying volumes during the cardiac cycle.

Hypoxia-Induced Pulmonary Hypertension: Common in cattle at high altitudes, leading to increased vascular resistance due to vasoconstriction in response to low oxygen levels.
Ascites Syndrome in Broilers: This condition in poultry is also associated with pulmonary hypertension often stemming from environmental factors.
Heartworm Disease in Dogs and Cats: A serious condition that affects circulation, caused by Dirofilaria immitis, leading to serious pulmonary and cardiovascular complications.

Blood pressure is closely correlated with cardiac output and total peripheral resistance, as explained by Ohm's law.
Common causes of elevated blood pressure include increased sympathetic tone leading to widespread vasoconstriction and chronic kidney disease in dogs/cats, which promotes fluid retention and increased cardiac output.
The renin-angiotensin-aldosterone system plays a significant role in adjusting vascular resistance, further complicating systemic circulation dynamics.

The formula for cardiac output (CO = Stroke volume x Heart rate) serves as a key reference point in understanding how the body meets increased demands during physical activity.
Stroke Volume Maintenance Mechanisms: Increased contractility mediated by sympathetic nervous activity (via catecholamines affecting beta-1 adrenoceptors), enhanced venous return through venoconstriction, and skeletal muscle activity help maintain stroke volume.
The Frank-Starling Mechanism also contributes significantly to stroke volume, enabling the heart to pump efficiently based on the volume of blood returned to it.

At Rest: Blood flow distribution shifts due to splanchnic vasoconstriction (decreasing flow to digestive organs) and skeletal muscle vasodilation during physical exercise, ensuring that active muscles receive adequate blood supply.
Blood Pressure Changes: Blood pressure remains stable during moderate intensity exercise but can significantly increase during maximal efforts as the cardiovascular system adapts to heightened demands.

Increased pulmonary arterial pressure during exercise leads to decreased pulmonary vascular resistance and enhanced vascular conductance, promoting better oxygenation and nutrient delivery. The recruitment and distension of pulmonary capillaries also increases total blood volume in the lung.

Caused by the parasitic worm Dirofilaria immitis, which predominantly inhabits the pulmonary arteries of dogs.
Mosquitoes serve as vectors, transmitting microfilariae to dogs through their bites; the lifecycle is complex and involves multiple larval stages before mature worms take residence in the pulmonary artery.

The severity of damage is usually contingent on the number of worms present, duration of the infection, and the host's response to the parasites.
Initial vascular damage, initiated by immature worms, triggers inflammation, leading to potential right heart failure if left untreated.

Analyze the pressure gradients critical for blood flow within the cardiovascular system.
Identify implications of changes in vascular resistance across pulmonary and systemic circulation.
Integrate and comprehend the cardiovascular metrics of resistance, flow, and pressure during moderate exercise, along with pathologies such as heartworm disease that can affect these dynamics.