Notes on Blood Vessels and Hemodynamics

Structural anatomy of blood vessels

• The walls have three tunics (layers): tunica intima, tunica media, and tunica adventitia.
• Internal details:
  • Endothelium lining the lumen.
  • Basement membrane.
  • Lamina propria located in the tunica intima.
  • Internal elastic membrane at the boundary between intima and media.
  • External elastic membrane at the boundary between media and adventitia.
• Vasa vasorum: small blood vessels that supply blood to the wall of larger blood vessels.
• Figure reference: Histology of a Blood Vessel (illustrated layers: tunica intima, media, adventitia; elastic membranes; endothelium; basement membrane).

Blood vessel layers and landmarks (as seen in histology)

• Tunica intima: innermost layer, includes endothelium and basement membrane.
• Tunica media: smooth muscle layer responsible for vasoconstriction and vasodilation.
• Tunica adventitia: outer connective tissue layer; contains nerves and vasa vasorum in large vessels.
• External elastic membrane and internal elastic membrane demarcate boundaries around the tunica media.

Blood Flow: laminar vs turbulent flow

• Laminar flow:
  • Streamlined flow through long, smooth-walled tubes of equal diameter.
  • Fluid near the outer wall experiences greater resistance and flows slower; central flow is fastest.
• Turbulent flow:
  • Occurs at constrictions, sharp turns, or rough surfaces.
  • Caused by many small crosswise currents; occurs when flow rate exceeds a critical velocity.
  • Common in the heart near valve regions; partially responsible for heart sounds.

Principles of blood flow

• Blood flow is directly proportional to pressure differences and inversely proportional to resistance:
  $\text{Flow} = \frac{P<em>1 - P</em>2}{R}$
• Resistance to flow ( R) is:
  • Directly proportional to vessel length (l) and blood viscosity ((\eta) or v).
  • Inversely proportional to vessel diameter (D) to the fourth power:
    $R = \frac{128\, \eta\, l}{\pi \; D^4}$
• Note: Greater diameter reduces resistance, increasing flow.

Poiseuille’s Law: flow through a tube

• When combining resistance with flow, the law becomes:
  $\text{Flow} = \frac{\pi \,(P<em>1 - P</em>2)\, D^4}{128\, \eta\, l}$
• Key implication:
  • The diameter term is raised to the fourth power, so small changes in diameter dramatically affect flow.
• Example consequence:
  • If the diameter is halved, then
  • Flow decreases by a factor of 16 (since (D^4) goes to ((D/2)^4 = D^4/16)) and resistance increases 16-fold.
• Short-term regulation of tissue blood flow is achieved by changing the radius of arterioles (resistance vessels).
• Practical takeaway: Dilating or constricting arterioles can redirect blood to different parts of the body.

Blood flow and pressure in the heart

• Cardiac output (CO): the amount of blood pumped by the heart per minute.
• The heart pumps blood at high pressure; as blood travels farther from the heart, pressure falls.
• By the time blood reaches the veins, pressure is low.
• Example: At rest, CO is approximately $\text{CO} \approx 5\ \text{L/min}$.
• Conceptual summary: Blood leaving the heart is high pressure; blood returning to the heart is low pressure.

Blood pressure (BP) basics

• BP is the force that blood exerts against vessel walls, measured in mmHg.
• Measurement method: Auscultatory method using a sphygmomanometer.
• Turbulent flow during measurement creates vibrations in the blood and surrounding tissues that can be heard with a stethoscope.

Control of blood flow in tissues

• Two main homeostatic mechanisms:
  • Local (intrinsic) control
  • Extrinsic (nervous and hormonal) control
• Local control is achieved via relaxation and contraction of precapillary sphincters.

Local control of blood flow

• Local metabolic factors promoting vasodilation (increased blood flow with higher tissue activity):
  • ↑ CO2, ↑ temperature, ↑ nitric oxide, ↓ O2.
  • Blood flow increases when tissue metabolism increases; when metabolism decreases, these factors reverse, causing vasoconstriction and reduced flow.
• Local vasoactive substances:
  • Vasodilators: histamine, bradykinin, prostacyclins.
  • Vasoconstrictors: endothelin-1, leukotrienes, thromboxane-A2.
• Myogenic control:
  • Passive stretch of vessels (e.g., surge in blood flow) causes vasoconstriction.
  • Decreased stretch (e.g., sudden drop in flow) causes vasodilation.

Extrinsic control of blood flow

• Important for minute-to-minute regulation; involves nervous and hormonal systems.
• Nervous control (autonomic):
  • Primarily sympathetic vasomotor fibers controlling vasoconstriction.
  • Rapid response (within 1–30 seconds).
  • Vasomotor center located at the lower pons and upper medulla oblongata.
• Hormonal control:
  • Noradrenaline and adrenaline cause vasoconstriction.
  • Adrenaline can act on ß2 receptors to cause vasodilation.
  • Angiotensin II and antidiuretic hormone (ADH) are vasoconstrictors.
  • Atrial natriuretic peptide (ANP) is a vasodilator.

Summary (key takeaways)

• Laminar vs. turbulent flow distinction and implications for circulatory dynamics.
• Blood flow is governed by resistance, viscosity, vessel length, and diameter via Poiseuille’s Law.
• Blood pressure measurement relies on detecting turbulent flow-induced sounds (auscultation).
• Tissue blood flow is regulated by local, nervous, and hormonal mechanisms, with precapillary sphincters playing a critical role in local control.

References

• Seeley’s Anatomy & Physiology (12th Ed), van Putte, Regan, Russo (2020) for content and images.