Chapter 5: Venous Blood Pressure, etc

mechanical factors affecting CVP and venous return

1. gravity

2. skeletal muscle pump

3. respiratory pump

gravity

acts on vascular volume, causing blood to accumulate in lower extremities; reduces RV filling pressure (preload) and SV via Frank-Starling Mechanism

• reclined = ~uniform distribution of blood volume → gravity acts when person goes from recline→vertical

• venous pressures in feet can reach ~90 mmHg

orthostatic/postural hypotension

if systemic arterial pressure falls by more than 20 mmHG upon standing

skeletal muscle pump

• veins contain 1-way valves that allow blood to return to the heart → deep veins surrounded by muscles that cause compression that open/close valves

• rhythmic contractions enhance venous return to the heart → increase SV and CO

• “like rungs on a ladder” → no backflow

varicose veins

damage to the 1-way valves causes blood to pool in veins → swelling and venous damage (twisting)

• treatment: avoid standing for long periods, exercise, weight loss, repair vein damage, treat skin where varicose is visible

respiratory pump

increasing the rate and depth of breathing → increases venous return and cardiac output

factors increasing CVP (central venous pressure)

• increased total blood volume

• increased muscle pump

• increased respiratory pump

• valsalva breathing

• reclining or squatting

• DECREASED venous compliance, SVR (systemic vascular resistance), and CO

valsalva breathing

exhaling forcefully against a closed airway

venous return

all systemic flow returning to the heart

venous return equation

(mean aortic pressure - right atrial pressure) / systemic vascular resistance

• under steady-state conditions, venous return = CO when averaged over time

• blood flow through entire systemic circulation (CO + VR) depends on cardiac and systemic vascular function

guyton plots

systemic vascular and cardiac function curves

interdependence of CO and VR

1. return supports output

2. output creates return

return supports output

CO is dependent on preload → preload is determined by filling pressure (CVP)

• inotropy modifies this relationship

output creates return

the heart translocates blood from veins to arteries

• high HRs → CVP falls, sometimes less than 0 mmHG

• low HRs → increased CVP