Clase 22 Fisiología Circulatoria - Presión,Flujo y resistencia (IG:@doctor.paiva)

Introduction to Circulatory Physiology

Overview of Circulatory Function

• The class introduces circulatory physiology, focusing on pressure, flow, and resistance.

• Key topics include the interrelation between pressure, flow, and resistance in blood circulation.

Blood Circulation Basics

• The circulatory system transports blood to tissues along with nutrients and waste products; it consists of two main circuits: pulmonary (minor) and systemic (major).

• Arteries carry high-pressure blood (e.g., aorta), branching into arterioles that regulate blood flow. Capillaries are sites for nutrient exchange.

Venous System as Blood Reservoir

• Veins serve as a major reservoir for blood, containing approximately 64% of total blood volume.

• Distribution of blood volume: 7% in arterioles/capillaries, 13% in arteries, 7% in the heart, and 9% in lungs.

Vascular Structure and Pressure Dynamics

Cross-sectional Area of Vessels

• Comparison of cross-sectional areas shows that smaller vascular structures like capillaries have significantly larger total surface area than larger vessels like the aorta.

Heart Functionality

• The heart has four chambers; left ventricular contraction expels blood at a systolic pressure of 120 mmHg and diastolic pressure of 80 mmHg.

Blood Pressure Variations

• Blood returning to the right atrium has low pressure (~0 mmHg); capillary pressures range from 35 mmHg (arterial end) to 10 mmHg (venous end).

Principles Governing Blood Flow

Basic Principles of Circulatory Function

• Three fundamental principles govern circulatory function; first is that tissue needs dictate local blood flow.

Tissue Demand Response

• Increased metabolic activity in tissues leads to vasodilation, enhancing local blood flow by up to 20–30 times resting levels due to increased carbon dioxide levels.

Role of Carbon Dioxide

Understanding Cardiac Output and Blood Flow Regulation

Principles of Cardiac Function

• The basic principle of cardiac function is that cardiac output equals the sum of local blood flows from various tissues, each regulated independently.

• Venous return is crucial; the heart responds to increased venous return by expelling more blood into arteries, a concept linked to Frank-Starling's mechanism.

• Blood pressure regulation operates independently from local blood flow control, involving special regulatory mechanisms including nervous reflexes and hormonal influences.

Factors Influencing Blood Flow

• Blood flow through a vessel is determined by two main factors: the pressure gradient across the vessel and vascular resistance.

• Resistance impedes blood flow and can be influenced by factors such as vessel size; smaller vessels exhibit greater resistance.

Understanding Vascular Resistance

• Atheromatous plaques in vessels create additional resistance, obstructing normal blood flow due to their presence in the endothelium.

• Resistance also arises from friction between flowing blood and the vascular endothelium, affecting overall circulation efficiency.

The Relationship Between Pressure, Flow, and Resistance

• According to Ohm's law for fluid dynamics, blood flow (Q) is directly proportional to pressure difference (ΔP) and inversely proportional to resistance (R).

• Normal resting adult blood flow averages about 5 liters per minute, equating cardiac output with total systemic circulation.

Types of Blood Flow

Laminar vs. Turbulent Flow

• Laminar flow occurs in smooth vessels where blood moves uniformly; velocity is highest at the center compared to edges due to minimal resistance.

Characteristics of Turbulent Flow

• Turbulent flow arises when speed increases or when passing through obstructions; it becomes chaotic with swirling currents known as vortices.

• Turbulent flow encounters higher resistance than laminar flow due to its disordered nature which disrupts streamlined movement.

Implications for Clinical Practice

Understanding Blood Flow Dynamics

Turbulent vs. Laminar Flow

• Turbulent flow is characterized by slower movement and requires greater pressure, facing more resistance compared to laminar flow.

• The law of the fourth power indicates that small changes in vessel diameter significantly affect blood conductance; for example, doubling the diameter increases flow 16 times.

• A vessel's diameter raised to the fourth power illustrates how a minor increase can lead to substantial increases in blood flow during vasodilation.

Resistance and Conductance

• Approximately two-thirds of systemic resistance to blood flow arises from arterial resistance in small arteries due to their muscular structure capable of significant vasodilation and constriction.

• The law of the fourth power explains that increased vessel diameter leads to higher conductance because blood near the walls moves slowly due to adherence, while central blood flows rapidly.

Hematocrit and Blood Viscosity

• Blood viscosity affects resistance and flow; higher viscosity results in lower flow rates, similar to how thicker liquids like yogurt are harder to draw through a straw than water.

• Blood viscosity is three times greater than that of water, primarily due to a high number of erythrocytes (red blood cells), which transport oxygen.

Erythrocyte Concentration Impact

• Hematocrit measures the proportion of blood volume made up by red blood cells: approximately 42% for men and 38% for women, with variations based on altitude or health conditions.

• Individuals living at high altitudes produce more erythropoietin due to prolonged hypoxia, leading to increased red blood cell production and consequently higher blood viscosity.

Clinical Implications of Hematocrit Levels

• Conditions such as polycythemia arise when hematocrit levels exceed normal ranges (60%-70%), often seen in individuals living at high altitudes or those with chronic respiratory issues.