Blood Flow & Blood Pressure Notes

Blood Flow & Blood Pressure

  • Cardiovascular System Overview
    • Comprised of heart and blood vessels, responsible for blood circulation.

Learning Outcomes

  • Understand the relationship between blood volume and blood flow velocity.
  • Discuss regulation of blood flow through intrinsic and extrinsic factors.
  • Differentiate between Blood Pressure (BP) and Mean Arterial Pressure (MAP).
  • Identify factors affecting MAP and Total Peripheral Resistance (TPR).
  • Discuss blood pressure regulation and its clinical implications (e.g., circulatory shock).

Distribution of Blood Volume in Circulation

  • Blood Volume:
    • 2/3 of blood volume resides in venous vasculature.
    • Variation in blood volume between arteries and veins can occur based on:
    • Total blood volume
    • Cardiac output
    • Vascular resistance and compliance
    • Intravascular pressures
  • Effects of Activity:
    • Cardiac output and vascular resistance change with physical activity, causing redistribution of blood flow and pressure.

Capillary Exchange

  • Definition: Movement of substances between blood and interstitial fluid.
  • Methods of Exchange:
    1. Diffusion: Based on concentration gradients, substances cross capillary walls via clefts or sinusoids.
    2. Transcytosis: Vesicular transport of large, lipid-insoluble molecules (e.g., hormones).
    3. Bulk Flow: Driven by pressure gradients (i.e., Net Filtration Pressure = net hydrostatic pressure - net osmotic pressure).

Blood Volume Regulation

  • Functions:
    • Regulated by kidneys through filtered fluid reabsorption.
    • Edema occurs with excess interstitial fluid accumulation.

Velocity of Blood Flow

  • Inversely Related to Cross-Sectional Area:
    • As cross-sectional area increases, velocity decreases; highest in arteries, lowest in capillaries (facilitating exchange).
    • Velocity unit: cm/sec.

Poiseuille’s Law on Blood Flow

  • Factors Affecting Blood Flow:
    • Directly related to pressure difference across a vessel.
    • Inversely related to vascular resistance.

Regulation of Blood Flow

  • Ensures adequate supply based on tissue metabolism and maintains capillary pressure limits.
  • Blood flow can increase 7-8 times from vasodilation and sphincter relaxation.

Capillary Bed Structure

  • Precapillary Sphincter: Valve regulating blood flow into capillaries.
  • Thoroughfare Channel: Direct route from arterioles to venules, lacking smooth muscle.

Types of Blood Flow Regulation

  1. Intrinsic Regulation: Local tissue mechanisms adjusting smooth muscle action.
  2. Extrinsic Regulation: Controlled by sympathetic nervous system and endocrine influences.

Intrinsic Regulation Mechanisms

  • By Tissue Metabolites: Tissues can modulate blood flow based on metabolic needs via vasodilator substance production.

Hyperemia

  • Two Types:
    1. Active Hyperemia: Activity-induced blood flow increase; returns to normal post-activity.
    2. Reactive Hyperemia: Blood flow increase following a blockage.

Raynaud’s Phenomenon

  • Characterized by vasospasms leading to ischemia and discoloration in extremities, triggered by cold or stress.

Intrinsic Regulation Components

  1. Myogenic Regulation: Maintains stable blood flow despite arterial pressure changes via vasodilation/constriction.
  2. Endothelial Factors: Release of vasoactive substances impacting smooth muscle activity.

Regulation by Temperature

  • Heat: Causes vasodilation and increased blood flow.
  • Cold: Causes vasoconstriction and reduced blood flow.

Neural & Hormonal Regulation

  • Sympathetic Activity & Hormones:
    • Influence vascular smooth muscle leading to varying blood flow responses. Examples include:
    • ADH and Angiotensin II: Cause vasoconstriction.
    • Parasympathetic: Promotes vasodilation in certain tissues.

Blood Pressure (BP) & Mean Arterial Pressure (MAP)

  • BP: Hydrostatic pressure from blood against vessel walls. Calculated from systolic and diastolic pressures (Pulse = Systolic - Diastolic).
  • MAP: Average arterial pressure indicating blood flow to organs. Calculated as MAP = Diastolic + 1/3(Pulse Pressure).

Factors Affecting Blood Pressure

  • Determined by cardiac output, total blood volume, and vascular resistance.

Measuring Blood Pressure

  • Tools: Sphygmomanometer and stethoscope.
  • Korotkoff Sounds: Five phases that help determine systolic and diastolic pressures during measurement.

Total Peripheral Resistance (TPR)

  • Definition: The resistance offered by systemic blood vessels, influenced by:
    • Arteriolar radius: Affects resistance significantly.
    • Blood viscosity: Higher viscosity increases resistance.

Regulation of Blood Pressure Overview

  1. Neural Regulation: Baroreceptor and chemoreceptor reflexes for immediate changes.
  2. Hormonal Regulation: RAA system and other hormones for long-term changes.
  3. Autoregulation: Local feedback systems for tissue-level control.

Baroreceptor Reflex Mechanism

  • Function: Negative feedback loop responding to blood pressure changes via sympathetic/parasympathetic modulation.

Physiological Changes During Shock

  • Shock involves decreased MAP, triggering compensatory mechanisms such as increased sympathetic activity, hormonal responses, and auto-transfusion to restore blood flow.

Summary

  • Understanding blood flow and pressure regulation, including physiological responses and regulatory mechanisms, is crucial in treating circulatory issues such as shock.

References

  • Tortora, G. J. & Derrickson B. H. 2011. Principles of Anatomy and Physiology. New York: Wiley.
  • Marieb, E. N. 2006. Human Anatomy & Physiology. California: Pearson/Benjamin Cummings.
  • Marieb, E. N. 2009. Essentials of Human Anatomy & Physiology. San Francisco: Pearson/Benjamin Cummings.