SI

cardiovascular system III

Overview of Stroke Value and Blood Flow

  • Stroke Volume (SV): The amount of blood pumped by the heart per beat, typically around 70-75 milliliters.

  • Cardiac Output (CO): Calculated as SV multiplied by Heart Rate (HR). Roughly, CO is about 5 liters per minute on average.

  • Importance of 5 Liters: Indicates that the total blood volume circulates through the heart every minute, facilitating systemic blood flow.

Measurement of Stroke Volume

  • Calculation: Stroke Volume = End Diastolic Volume (EDV) - End Systolic Volume (ESV).

    • EDV: Volume of blood in ventricle at the end of filling (diastole).

    • ESV: Volume of blood left in ventricle after contraction (systole).

Regulation of Heart Function

1. Mechanisms of Control

  • Intrinsic Controls: Factors within the heart, such as wall stretch and filling patterns.

  • Extrinsic Controls: Factors outside the heart, including:

    • Sympathetic Nervous System: Increases heart rate and force of contraction.

    • Parasympathetic Nervous System: Decreases heart rate, primarily through the vagus nerve.

2. SA Node Activity

  • The SA Node: The natural pacemaker of the heart, typically sets the heart rate at around 100 beats per minute.

    • Effects of Vagal Influence: If the vagus nerve is severed, resting heart rate could rise closer to 100 bpm.

3. Hormonal Influences

  • Epinephrine and Norepinephrine: Released during stressful situations, increasing both heart rate and contractility.

  • Glucagon: Increases blood glucose levels, also influencing cardiac function indirectly.

Factors Affecting Stroke Volume

  • Preload: Initial stretching of cardiac muscle fibers prior to contraction, related to the volume of blood filling the heart.

    • Increased Preload: Leads to greater stroke volume due to enhanced contractility (Starling's Law).

  • Afterload: The pressure against which the heart must work to eject blood; higher afterload reduces stroke volume.

  • Contractility: The inherent strength of myocardial contraction; increased contractility results in lower ESV and higher stroke volume.

Blood Pressure Dynamics

1. Blood Pressure Basics

  • Blood pressure gradient must exist for blood to flow, flowing from high to low pressure zones in the cardiovascular system.

  • Typical Pressures: Aorta at 120 mmHg while the right side of the heart is approximately 0 mmHg.

2. Regulation of Blood Pressure

  • Mean Arterial Pressure (MAP): Must be adequate to maintain organ perfusion; usually around 70-100 mmHg.

    • Calculation of MAP: MAP = Diastolic Pressure + (1/3) Pulse Pressure

3. Resistance Factors

  • Total Peripheral Resistance (TPR): Influenced by blood vessel diameter, blood viscosity, and vessel length.

    • Vasoconstriction vs. Vasodilation: Diameter changes significantly alter resistance and thus blood flow.

Vascular Overview

1. Types of Blood Vessels

  • Arteries: Carry blood away from the heart, typically with thicker walls.

  • Veins: Return blood to the heart and possess valves to prevent backflow due to lower pressure.

  • Capillaries: Sites of gas and nutrient exchange, with varying permeability based on type.

    • Continuous Capillaries: Tight junctions, minimal permeability.

    • Fenestrated Capillaries: Allow for greater permeability, found in kidneys.

    • Sinusoids: High permeability, allowing cells to pass, found in liver and bone marrow.

2. Blood Flow Control

  • Precapillary Sphincters: Regulate blood flow into capillary beds based on local tissue demands.

  • Active Hyperemia: Increased blood flow in response to heightened metabolic activity in tissues.

  • Reactive Hyperemia: Increased blood flow following a period of restricted blood supply.

3. Cardiac Output During Exercise

  • Increased Demand: During physical activity, cardiac output rises due to increases in heart rate and stroke volume.

  • Redistribution of Blood Flow: Blood is directed away from non-essential organs to working muscles.