Venous Physiology and Hemodynamics 2025

CHAPTER 6 VENOUS PHYSIOLOGY

• Author: Ann Marie Kupinski

• Editors: Andrew Stalter BS, RVT, RDMS; Tara Bartholomay MS, RT(R), RDMS, RVT

OBJECTIVES

• Hemodynamic Factors Affecting Venous Blood Flow

• Components of Hydrostatic Pressure

• Forces Leading to Edema Formation

• Changes in Venous System at Rest and Exercise

• Types of Varicose Veins

TERMS TO KNOW

• Edema: Excessive accumulation of fluid in cells, tissues, or cavities.

• Hydrostatic Pressure: Pressure within the vascular system due to weight of blood column.

• Transmural Pressure: Pressure exerted by the walls of a vessel.

• Valvular Insufficiency: Abnormal retrograde flow across a venous valve.

VENOUS CAPACITANCE

• Veins as capacitance vessels: act as reservoirs for blood.

• Hold about two-thirds of total blood volume.

• Distended veins: 3-4 times larger than corresponding arteries.

• Paired veins enhance blood-holding capability.

VENOUS RESISTANCE

• Vein Shape and Resistance:

  • Distended, circular veins: Low resistance to flow.

  • Partially empty, elliptical veins: High resistance.

• Shape changes accommodate increases in blood flow without elevating pressure gradients.

HYDROSTATICS

• Hydrostatic pressure influenced by fluid weight:

  • Formula: Hydrostatic Pressure = ρ × g × h

    • ρ = blood density

    • g = gravity

    • h = height of blood column

• Reference point: Right atrium.

HYDROSTATICS: Effects of Position

• Lying flat: Negligible hydrostatic pressure.

• Standing increases venous pressure, particularly at the ankle.

• Exercise mitigates pressure somewhat, enhancing capillary blood flow.

BLOOD CLOTS AND DVT

• Taller and obese men: ≈ 2.57 times higher risk of venous thromboembolism.

• Longer distance for blood flow may relate to increased clotting risks.

PRESSURE-VOLUME RELATIONSHIPS

• Transmural Pressure:

  • High transmural pressure indicates a distended vein.

  • Low transmural pressure indicates a collapsed vein.

• Veins can handle large volume changes with minor pressure changes.

• Position affects transmural pressure (supine vs. standing).

COMPRESSION STOCKINGS

• Designed to enhance lower leg blood flow and reduce swelling.

• Used for conditions like varicose veins and lymphoedema.

• Post-DVT recovery: Prevent post-thrombotic syndrome.

EDEMA

• Sign of increased venous pressure.

• Factors influencing fluid movement:

  • Intracapillary pressure

  • Interstitial osmotic pressure

  • Reabsorption forces (interstitial pressure, capillary osmotic pressure)

• Standing increases capillary pressure, contributing to fluid loss.

• Compression stockings can decrease intracapillary pressure, limiting edema.

VENOUS DYNAMICS AT REST

• Cardiac Influence:

  • Upper extremity veins: More pulsatile, closer to the heart.

  • Lower extremity veins: Less pulsatile, more phasic.

  • Congestive heart failure increases pulsatility in lower veins.

VENOUS DYNAMICS WITH THROMBOSIS

• DVT elevates venous pressure and resistance.

• Normal phasic flow may be reduced or continuous.

VENOUS DYNAMICS WITH EXERCISE

• Calf Muscle Pump: Aids blood return against hydrostatic pressure.

• Contracting muscles push blood up; relaxing draws it back.

• All calf veins contribute, with valves preventing reflux.

VARICOSE VEINS

• Primary Varicose Veins:

  • Develop without DVT; caused by valve incompetence.

• Secondary Varicose Veins:

  • Result from valve damage due to DVT.

VENOUS STASIS ULCERS

• Result from increased venous pressure leading to capillary distention.

• Also involves leakage of plasma proteins into tissues.

• Insufficient oxygen transfer results in tissue ischemia and ulceration.

PREGNANCY AND VARICOSE VEINS

• Pregnancy doesn't cause varicose veins but can exacerbate factors.

• Enlarged uterus compresses IVC & iliac veins leading to increased pressure.

• Hormonal factors increase venous compliance.

REFERENCES

• Kupinski, A. & Schifrin, G. The Vascular System. Philadelphia, PA: Wolters Kluwer Health, 2022, Chapter 6, pp. 63-69.