Lecture 6: Pressure and Blood Flow

Lecture 6: Pressure, Force and Elasticity

• Lecturer: Dr. Isabel Hwang, Senior Lecturer
• Department: Division of Education, School of Biomedical Sciences, Faculty of Medicine, CUHK
• Email: [email protected]
• Office number: 3943 6795

Lecture Outline

• Flow rate equation
• Types of blood flow (laminar vs. turbulent)
• Pulmonary and systemic circuits
• Poiseuille's law
• Factors affecting resistance (R) to blood flow
• Blood pressure and its importance (arterial)
• Compliance and elastance of arteries and veins
• Physiological significance of elastic recoil in elastic artery during cardiac cycle
• Factors affecting venous return (VR)

Introduction

• Blood flow rate through tissues controlled by tissue demand for nutrients and oxygen (e.g., during exercise)
• Cardiovascular system (heart, blood vessels, and blood) constantly maintains pressure gradient driving blood to organs.

Blood flow through a blood vessel

• Determined by two factors:
  • Pressure difference (gradient) between vessel ends
  • Resistance to blood flow (friction)

Driving force of blood flow

• Ohm's law (flow rate equation): F = ΔP / R
  • ΔP: pressure gradient created by cardiac contractions. Modified by homeostatic control through hormones, nervous systems, etc.

The flow rate equation

• Flow (F) is directly proportional to pressure difference (ΔP) and inversely proportional to resistance (R)
  • F = ΔP/R
  • ΔP is measured in mmHg
  • R is measured in mmHg/mL/min or mmHg/L/min
• Bulk flow: movement of fluid or gases from high to low pressure

Pressure gradients

• ΔP is the difference in pressure between relevant points within the vascular system—not absolute pressure.
• Examples:
  • P1 = 100 mmHg, P2 = 10 mmHg, ΔP = 90 mmHg
  • P1 = 500 mmHg, P2 = 410 mmHg, ΔP = 90 mmHg.
• Positive pressure gradient needed to drive blood flow

Importance of blood pressure

• Adequate blood pressure is essential for maintaining and driving blood flow.
• Critical organs like the brain and heart rely on steady blood supply to function.
• Reduced blood flow to organs results in reduced glucose and oxygen delivery, causing potential organ damage (e.g., stroke, heart attack)
• Atherosclerosis: thickening or hardening of arteries due to plaque buildup in the inner lining

Two types of blood flow (laminar vs. turbulent)

• Laminar flow: smooth, streamlined flow through long, smooth vessels. Velocity is greater in the center of blood vessel.
• Turbulent flow: disrupted, non-streamlined flow due to high velocity or obstructions. Audible sound (bruit) in damaged/blocked blood vessels.

Turbulence in leaky or stenotic (narrowed) cardiac valves

• Laminar flow—quiet
• Turbulent flow—murmur (in diseased valves)

The cardiovascular system

• Closed-loop system: blood contained within vascular system (systemic and pulmonary circuits).

Pulmonary and systemic circuits

• Pulmonary circuit: right ventricle → lungs → left atrium

• Systemic circuit: left ventricle → peripheral organs/tissues → right atrium

• In both circuits, the blood vessels carrying blood away from the heart are called arteries.

• Those carrying blood to the heart are called veins.

Pressure gradients

• Pulmonary circuit: ≈15 mmHg
• Systemic circuit: ~ 85mmHg

Resistance in the CV system

• Poiseuille's law: R = (8Ln) / (πr^4)
  • L = vessel length
  • n = fluid viscosity
  • r = vessel internal radius
• Resistance increased by smaller radius (inversely related like 1/r^4), and length.

Effect of tube radius on flow

• Decreasing radius two-fold increases resistance sixteen-fold.
• If ΔP constant, flow decreases sixteen-fold.

Resistance in the CV system

• Factors affecting resistance:
  • Radius of vessel (r): largest contributor. Resistance increased by constriction and atherosclerosis, decreased by relaxation and dilation
  • Length of vessel (L): total number of vessels. Resistance increases with more vessels, Obesity increases vascular length
  • Viscosity of fluid (η): Blood viscosity determined by amount of red blood cells, proteins, and temperature

Five types of blood vessels in the vascular system

• Arteries, arterioles, capillaries, venules, veins

Resistance in the CV system

• Effect of arteriolar radius on resistance and blood flow: vasoconstriction decreases radius, increasing resistance vasodilation increases radius, decreasing resistance

Example: what happens to resistance (R) if smooth muscle cells in the arteriole contract?

• Contraction causes vasoconstriction, reducing radius and increasing resistance to blood flow.

Resistance in the CV system

• Total peripheral resistance (TPR): combined resistance within the systemic circuit, especially arterioles. Resistance across network varies. Arteries supply tissues in parallel circuits.

Arteries and veins

• Arteries have low compliance (high elastance, recoil) helping smooth blood flow and maintaining pressure.
• Veins have high compliance ( low elastance), stretching readily.

Venous return

• Return of blood to right atrium via veins
• Improves end-diastolic volume, stroke volume, cardiac output
• Dependent on:
  • Blood volume and venous pressure
  • Skeletal muscle pumps
  • Pressure drop during inhalation
  • Venoconstriction (sympathetic stimulation)

The skeletal muscle pump and one-way valves

• Skeletal muscle contractions and one-way valves improve venous return.

Venous return cannot be facilitated by skeletal muscle pump alone

• Without valves the flow is both directions when muscle contracts.

The respiratory pump

• Pressure differences during breathing
• Creates upward "sucking" effect, pulling blood toward the heart.

Real-life example: physiological significance of modified veins compliance

• Stress (sympathetic activation) leads to venoconstriction, reducing venous compliance, improving venous return, increasing venous pressure, and increasing blood flow to the right atrium, increasing end-diastolic volume, stroke volume, cardiac output, mean arterial pressure.

Factors affecting venous pressure and mean arterial pressure (MAP)

• Elaborated in later lecture.

If we just consider the systemic circuit

• F = ΔP/R
• ΔP = F x R
• MAP = CO x TPR

Homeostatic regulation of MAP

• Requires heart, blood vessels, kidneys
• Supervised by the brain
• Changes in one variable are compensated by others

Short-term and long-term regulation of MAP

• Fast response through cardiovascular system
• Slow response through kidneys (fluid excretion)
• Blood pressure fluctuations: counteract by cardiovascular and renal system

The baroreceptor reflex

• Short-term regulation of MAP
• Located in carotid arteries and aortic arch and walls of large neck and thoracic arteries
• Integrated into medulla oblongata in the brainstem
• Negative feedback—functions through negative feedback—change in one variable is compensated for by other variables

Explore the principles of pressure, force, and elasticity in the cardiovascular system in this detailed quiz. Focus on blood flow dynamics, resistance factors, and the physiological significance of arterial compliance and elastic recoil. Perfect for students of biomedical sciences.