Mechanical Forces Acting on Blood Vessel

• Vascular medicine is the study of blood vessels and how they work.

Mechanical Forces Acting on Blood Vessel

• Energy from the hearts pumping action is on the vessel wall in 3 forms.
• Blood pressure is the force per unit area, in a direction perpendicular to the surface.
• The force tends to distend the blood vessel and is measured in mm Hg.
• 1 mm Hg is equal to 0.133 kPa, and Pascal (Pa) is Newton/m².
• Tensile stress is force per unit area (Newton/m2), resulting from attraction between molecules in the vessel wall.
• This counterbalances the distending effect of pressure.
• Shear stress is tangential drag force made by blood moving across the endothelial surface.
• The shear stress is 20-40 dyne/cm² in large arteries.
• Over extended time, changes in shear stress produce marked changes in gene expression in endothelial cells.
• Endothelial cells and extra-cellular matrix (ECM) transmit pressure and shear stress to regulate the function of vascular smooth muscle cells through changing gene expression.

Poiseuille Law: Vascular Resistance

• States the relation between flow in a tube (F), viscosity of the fluid (n), length of the tube (L), and radius (r).
• Flow (amount of blood flow crossing the ventricle/unit time) is the same as cardiac output, which amounts to blood pumped into the aorta/minute.
• Flow is directly proportional to effective perfusion pressure/“pressure gradient (ΔΡ)” (P₁ — Р2).
• Flow is inversely proportional to resistance of the vessel to blood flow (R), with the relation F=ΔP÷R
• Resistance to blood flow depends on vessel radius (r), blood viscosity (n), and vessel length (L).
• Radius has the relation R ∝ 1/r⁴.
• Viscosity has a relation R ∝ n.
• Length has the relation R∝L.
• Radius, mainly in arterioles, dictates regulation of resistance.
• A 20% increase in radius leads to a 50% decrease in resistance, while flow is doubled.
• Viscosity relies mainly on hematocrit value.
• Polycythemia causes increase in hematocrit and in resistance to blood flow.
• The increase in plasma proteins, such as immunoglobulins, has a minor effect on viscosity and blood flow.
• Elasticity of blood vessels has effect on pressure, increased pressure distends vessel, increases radius, and decreases resistance.
• The same pressure gradient results in higher flow in elastic vessels than rigid ones.
• Total peripheral resistance (TPR) in systemic circulation calculates as CO = [mean arterial pressure (MAP) - right atrial pressure or central venous pressure (CVP)]/TPR.
• TPR is derived from the formula TPR = MAP – CVP / CO = (90 – 0) / 5 = 18 mmHg/L/min.
• Resistance of pulmonary circulation (PulR) calculates as PulR = [mean pulmonary pressure (MPP)- left atrial pressure (LAP)]/COP = (15 – 8) / 5 = 1.4 mmHg/L/min.

Velocity of Blood Flow

• Velocity is inversely proportional to vascular cross-sectional area.
• The aorta cross-sectional area is 2.5 cm², with a velocity of 0.5 m/sec.
• Total cross-sectional area of capillaries is 2500 cm² (1000 times that of the aorta), with a velocity = 0.5 mm/sec (1/1000 of that of the aorta).

Types of Blood Flow Inside Vessels

• Two types of blood flow inside the vessels include laminar and turbulent.
• Laminar (streamline) flow is a normal flow in vessels, occurring in layers and is silent.
• Velocity is stationary in the thin layer of blood in contact with the wall and is highest at the center of the vessel.
• Turbulent flow is flow in varying directions, resulting in eddy currents and sound (murmurs), it has a greater resistance than laminar flow.
• Laminar flow changes to turbulent flow when:
  • There is increased blood flow to a certain critical velocity.
  • The blood has decreased viscosity (anemia).
  • There is sudden change in vessel diameter.
  • The flow makes sharp turn in direction.
  • The blood passes over the rough area of the blood vessel.

Vascular Compliance

• C = ΔV/ΔP, representing the ratio between change in blood volume and in pressure.
• Is depicted by the graph slope, compliance is higher at lower volumes and vessels become stiffer and harder to distend at higher values of volume.
• Total systemic arterial compliance is dictated by the aorta and major branches.
• Decreased arterial compliance due to increased sympathetic tone or atherosclerosis which increases the change of pressure for same arterial blood volume..
• Arterial compliance:
  • Converts intermittent flow in the aorta to continuous flow in peripheral vessels.
  • Recoil of arteries during diastole pushes blood forward.
  • Minimizes increase in systolic pressure and decrease in diastolic pressure.
  • Cardiac work = pressure x volume.
  • Low compliance requires greater pressure and work for the same volume (CO).

Venous Compliance

• Veins are 24 times compliant as arteries.
• Allows accommodation of greater volumes versus arteries for the same pressure, or reservoir vessels.
• Volume and pressure relate as veins are normally partially collapsed at low volumes.
• In this state, large increases in volume result in very little increase in pressure and very high compliance.
• Veins turn cylindrical without stretching their walls.
• At higher volume, further volume increases result in marked pressure increases and low compliance.
• Venous compliance is dictated by venous tone.
• Increased venous tone (sympathetic) decreases venous compliance, shifting the volume/pressure relationship down and to the right.
• 84% of blood is in the systemic circulation/16% is in the heart and lungs.
  • 64% of total blood volume is in the veins.
  • 13%, arteries.
  • 7%, arterioles and capillaries.
  • 7%, heart.
  • 9%, pulmonary vessels.

Law of La Place

• States the relation between wall tension, distending pressure, and radius of hollow viscus.
• In a sphere: P = 2T/r.
• In a cylinder as blood vessel: P = T/r.
• In a thin-walled asymmetric viscus: P = T (1/r₁ + 1/r₂).
• Significance:
• Determines tension generated in vessel wall, to balance the distending effect of pressure.
• Calculates the pressure generated during ventricular contraction based on the level of wall tension.
• Applications:
  • Thin-walled capillaries withstand high pressure, the tension being small relative to small radius.
  • Dilated ventricles (increased radius) in heart diseases generate more contraction tension to generate the same intraventricular pressure (needed normally to open semilunar valve since P = T/r).

Pressure in Various Parts of the Circulation

• Arterial pressure (Aorta): 120/80 mm Hg, with a mean of 90 mm Hg.
• Systemic capillaries: 35 mm Hg at the arteriolar end and 15 mm Hg at the venous end. The average "functional” capillary pressure is 25 mm Hg, allowing sufficient exchange across capillary wall.
• Systemic veins: 8-12 mm Hg in small peripheral veins and 5-6 mm Hg in large abdominal veins.
• Pulmonary arterial pressure: 25/8 mm Hg, with a mean of 16 mm Hg.
• Mean pulmonary capillary pressure: is only 10 mm Hg. A low value prevents filtration of fluid from pulmonary capillaries into lung alveoli.

Arterial Blood Pressure

• Systolic pressure= maximum pressure during systole in the aorta and large arteries, average 120 mmHg (90-140 mmHg).
• Diastolic pressure= minimum pressure during diastole, average 80 mmHg (60-90 mmHg).
• Pulse pressure= difference between systolic pressure and diastolic pressure, average 30-50 mmHg.
• Mean arterial pressure (MAP)=average pressure throughout cardiac cycle, average 90 mmHg.
• MAP = Diastolic pressure + 1/3 Pulse pressure = 90 mmHg.
  • MAP is closer to Diastolic pressure because systole is shorter than diastole.
  • Increased HR leads to a shorter cardiac cycle, shorter diastolic time, and increased MAP (near to mean true arithmetic).

Factors That Determine ABP

• MAP – Central venous pressure (CVP) = cardiac output (CO) × TPR.
• МАР = (CO × TPR) + CVP.
• MABP = (SV × HR × TPR) + CVP.
• Stroke volume (SV): Increased SV leads to increased Systolic pressure > diastolic pressure, leading to increased pulse pressure.
• Heart rate (HR): Increased HR leads to decreased diastolic time for ABP drop, leading to diastolic pressure > Systolic pressure, leading to decreased pulse pressure.
• Total peripheral resistance: Increased TPR leads to increased diastolic pressure, which is greater than Systolic pressure, leading to decreased pulse pressure.
• Arterial compliance: Decreased compliance due to atherosclerosis leads to increased systolic pressure due to decreased ability to expand and accommodate SV. -Decreases diastolic pressure is due to decreased recoil and increased pulse pressure. Central venous pressure: Increased CVP leads to increased МАР.

Physiologic Variations

• ABP increases with age due to loss of vessel elasticity.
• Sex:
• Before menopause, females have less ABP than males of the same age.
• After menopause, ABP increases in females due to hormonal changes.
• Race: races may have higher ABP & incidence of hypertension
• Emotions: Stress leads to increased sympathetic activation which can increase ABP
• Exercise:
  • Dynamic exercise (alternating contractions and relaxations ): Moderate increases in systolic pressure occurs.
  • Diastolic pressure decreases or is unchanged.
  • Static exercise (continuous constant muscle contraction): Marked increases in systolic and diastolic pressures.
• Circadian rhythm relates ABP to normal persons working at daytime, in early morning ABP reaches peak sympathetic. -ABP touches highs at midnight due to lowest sympathetic levels. -Variations between 15 and 25 mmHg -Occurs opposite night-workers
• Gravity: measuring Each 1 cm below measurement of the heart increases ABP by 0.77 mmHg and each 1cm above is lowered ABP by 0.77.
• Respiration:
  • "Traube-Hering waves": ABP fluctuates with the respiratory cycle, decreasing during inspiration and increasing during expiration.
  • Variation range from 4-6 mmHg during resting breathing.
  • Can reach 20 mmHg during deep breathing, which happens because during inspiration, the pulmonary vascular bed expands leading to a reversed SV, CO, ABP.
• Vasomotor waves (Mayer waves): oscillation ABP / 10 sec. range from 10 to 40 mmHg (or ever more) caused by "reflex oscillation" of one or more neural control mechanisms.

Functions of Veins

• Veins serve as blood reservoirs (capacity vessels): hold significant blood amounts with small pressure changes - 200 ml increase only increase 1mmGH pressure.
• Large compliant venous system. Specific blood reservoirs are cutaneous veins, large abdominal, hepatic, and spleen.
• Transport Vessels: with little resistance, faster than capillary flow, but slower than arterial flow (25% of velocity in aorta)

Venous Pressures

• Central Venous Pressure (CVP)- represents a Right Atrial Pressure with a Normal value of 0-2 mm Hg which balances the VR & heart pumping ability - Main filling force for ventricular, gradient difference for VR = 6 - 8 mmHg. -Also Indexes the blood volume- decreases by losing blood and increases it due to right-sided Heart Failure (↑CVP).

Peripheral venous pressure

• Determined from two factors (Sympathetic stimulation): BV Volume and Venous Compliance, which happens due to heart failure(CO decreasing increase BV and pressure ) . Vice Versa with Hemmorage, increased sympathetic stimulation tones the veins and increases its venous pressures.
• Effects of gravity alter venous pressure in upright positions, by applying 0.77 mg for cm height which causes varicose veins, whereas the head the pressure diminishes causing neck vein collapes