Basic concepts and laws of the cardiovascular system

  • CNS regulates the cardiovascular system, so these laws are valid only if we consider the circulatory system as independent.
  • Blood flow rate depends on the needs of tissues, if the tissue needs more blood –> local blood flow needs to be increase –> vasodilation.
  • Different vessels differ for contained blood volume and cross-sectional area (CSA).
  • Capillaries, Heart (7%) – Arteries & arterioles (13%) – Veins (85%)
  • Cross-sectional area: veins the smaller, capillaries the biggest
  • FLOW IS CONSTANT (100ml/sec), CROSS-SECTIONAL AREA CHANGES
  • FLOW RATE = product between velocity and cross-sectional area. F = A * v
  • Largest the area = smaller the velocity
  • Examples
    • If flow is constant and CSA changes = velocity changes
    • If flow is constant and CSA changes (1.0 –> 0.25) = velocity x 4
  • Blood pressure is not constant over time, change based on systole/diastole
  • Pulsatility decreases from arteries to capillaries (steady-state – disappear)
  • Cardiac output (CO): volume of blood pumped by heart, the sum of all local flows, HR increases CO
  • Accommodation of tissues needs: vasodilation –> increases blood flow / vasoconstriction –> decreases blood flow / CNS
  • Arterial blood pressure is independent of local blood flow and/or cardiac output
    • arteries can independently regulate pressure and flow rate by
      • increasing pumping force (contractility)
      • venous vasoconstriction –> increase preload = more blood to the heart
      • arterial vasoconstriction –> more blood storage in upper/bigger arteries
    • IF BLOOD VOLUME INCREASES –> PRESSURE INCREASES
  • Regulation mechanisms: Pressure (P), Flow (F), Resistance (R)
  • FLOW
    • depends on pressure difference/gradient and vascular resistance (antagonism of flow). Pressure gradient of systemic circ = 100 mmHg , pulmonary circ = 14mmHg
    • Ohm’s Law, F = (Pin – Pout)/R –> F = ∆P/R
    • Higher pressure gradient –> higher flow
    • BUT Higher resistance –> lower flow
    • Negative ∆P –> reverse flow
    • Resistance decrease pressure gradient trough Shear stress: stress of friction of blood elements against the vascular –> energy dissipation –> decrease pressure
    • Laminar flow: parallel layers with no contact, internal velocity higher than close to walls. Small vessels.
    • Turbolent flow: crosswise, large vessels.
  • PRESSURE
    • Force exerted by blood against vessel wall
    • P = F/A
    • Main pressure drop occurs in arterioles with a progressive switch from pulsatile to steady state. P = 0 in right heart
  •  RESISTANCE
    • Impediment of blood flow (friction) in a vessel. Higher resistance –> lower flow
    • Depends on geometrical parameters (length, radius) and blood viscosity. Resistance increases with length/viscosity and decreases with radius
    • Increasing the radius, decreases the resistance –> flow increases
  • Other regulatory mechanisms
    • Vascular distensibility: ability of vessels to store more blood. Veins more distensible than arteries
    • Compliance: ratio between increase in volume and increase in pressure C = ∆V/∆P
    • Veins more compliant than arteries
    • Greater the compliance –> smaller velocity