Vascular Functional Organization Arterial Resistance & Venous Capacitance Richard P. Wyeth Gentleman’s Review Daniel Martingano OMS I

Total Cross Sectional Diameter and Pressures Total cross sectional areas are greatest within capillary beds circuits systemic (~3000 cm 2 ) pulmonary (~4000 cm 2 ) Pressure varies inversely with total cross sectional area Pressure is low within systemic and pulmonary capillary beds

Cardiovascular Parameters At each branching point the combined cross sectional area of daughter vessels is greater than parent vessel through systemic (peripheral) and pulmonary circuits. The radius for each subsequent generation is not ½ of previous. Anastomses are exceptional.

Composition and Size Vascular: Resistance is principally a function of vascular smooth muscle (VSM >>EF = CF) Compliance is principally a function of elastic fibers (EF >> VSM) Capacitance is principally a function of elastic fibers (EF>VSM)

Arteries– Functional Summary Physical Properties –Arteries have greater tunica media –Arteries are more elastic Arteries carry blood away from the heart Arteries maintain vascular resistance –A function primarily of small arteries and arterioles

Veins – Functional Summary Venules have small effect on vascular resistance Veins carry blood to heart –Veins mediate venous return Veins provide a reservoir and are capacitive. –A function of primarily of small veins and venules

Arteries v. Veins

Total Area and Pressures Vessel ~XS area (cm 2 ) ~Mean P (mmHg) proximal - distal aorta2.597 to 90 Small arteries2090 to 80 arterioles4080 to 40 capillaries to 15 venules25015 to 7 small veins80 7 to 4 venae cavae8<4

Pressure in Blood Vessels P effecting blood flow within vasculature is of three types: –Driving Pressure along the long axis of the vessel:  x (x 1 – x 2 ) work of the heart – Transmural Pressure:  r (r 1 – r 2 )- function of the vessel wall and interstium –Hydrostatic Pressure:  h (h 1 – h 2 ) function of gravity

Mean Circulatory Filling Pressure If the heart stops (no pump) Pressures equilibrate throughout the body = mean circulatory filling pressure

Mean Circulatory Filling Pressure Approximates Mean Circulatory Filling Pressure MCFP represents both the pulmonary and systemic pressures v. MSFP represents the pressure equilibrated only in the systemic circulation. Pulmonary circulation has little capacitance –Little resistance (~ 12% max) – Small reserve volume(~10% max) MCFP ~ MSVP

Mean Circulatory Filling Pressure and Right Atrial Pressure MSFP is a force pushing peripheral blood to right heart Greater positive difference between mean systemic filling pressure and right atrial pressure (RAP) = greater venous return Difference between these two pressures is called pressure gradient for venous return

The Converse – Resistance to Venous Return Principally due to venous resistance RVR is important because high venous capacitance and great compliance allows distension with little increase in pressure As mean RA pressure (rAP) increases VR decreases Conversely low capacitance of arteries and arterioles (~30% of the capacitance of veins)  small increases in volume raise pressure greatly (~30X) in arteries and thus overcome resistance

Factors Affecting Mean Circulatory Filling Pressure Factors Affecting MSFP –Change in circulating volume –Change in vascular compliace Arterial Venous –Changes in systemic vascular resistance does not alter MSFP

How RVR Effects Venous Return and Cardiac Output VR is inversely related to RVR –As RVR decreases VR increases –As RVR increases VR decreases

MSFP Effects Venous Return Factors may change resistance to venous return Constriction or dilation of arteries and veins Constriction or dilation of arterioles Vasoconstrictors like epinephrine, norepinephrine, angiotensin increase MSFP and resistance to VR Vasodilators act conversely (decrease RVR)

Energetics of Blood Flow (CO) Blood within the circulation has a fixed content of energy (E t ): –Potential Energy (Blood Pressure; E p ) –Kinetic Energy (Blood Flow; E k ) E t = Total E of system E k = kinetic energy (mass velocity) E p = potential energy (pressure) Conservation of energy. (E t ) is constant.

Summary What is circulatory filling pressure (MCFP)? If the heart stops (no pump) Pressures equilibrate throughout the body = MCFP it is approximately equal to MSFP. Why approximately? MSFP excludes the lung capillary filling pressure. The lungs add little to MSFP- Normally.

Summary What about pulmonary hypertension? Increased RAP, CVP. MSFP may be increased. Depends on nature of the pulmonary vasculature and how much compliance lost. Why do peripheral veins of the leg have a high incidence of varicosity? The central venous circulation is surrounded by tissue that exerts pressure inward on the vessel; Peripheral veins do not - venous stasis can generate thrombosis (blood clots if it doesn’t move in circulation

Summary Arteries and veins are unique in their compositions –a reflection of their specific roles within the circulatory system as they deliver blood to and receive blood from capillaries As blood flows from higher to lower resistance, arteries transport blood under high pressure. –Arteries require strong vascular walls with well developed musculature and significant elastic tissue to maintain resistance. –Blood flows rapidly in arteries which assists in maintaining resistance and thus blood pressure.

Summary In the absence of shunts (anastomoses), arteries arborize into arterioles. –Arterioles are the direct control conduits for blood into capillaries and the primary determinates for vascular resistance. Capillaries are not muscular and one cell layer in thickness. –They provide the exchange surface for gasses, H 2 O, nutrients, electrolytes, and hormones between blood and interstitium.

Summary Venules are found on the distal end of capillaries with which they are contiguous and collect blood from these capillaries. –They are weakly muscular and may thus influence capillary pressure. Venules dearborize to veins. –Veins are somewhat muscular, and quite elastic. – Veins transport blood at low velocity and pressure providing a blood reservoir as they are capacitance vessels –able to increase significantly in volume with small increases in pressure.

Summary As such resistance to venous return, set by right atrial pressure, decreases venous return as it increases. Conversely Mean systemic filling pressure increases (if right atrial pressure are not increased) results in an increased venous return.

Summary Pressures effecting blood flow within vasculature are of three types –driving pressure - the direct work of the heart –transmural pressure - a function of the vessel wall and interstium –hydrostatic pressure - gravitational action on the blood mass. Blood vessels increase and decrease progressively in cross sectional geometry from arteries to veins affecting vascular resistance and consequently pressure. Local capillary vascular pressure is proportionate to the resistance that precedes and follows a particular capillary bed.