Flashcards in L62 – Venous Return Deck (59):
How is compliance of a vessel calculated?
Volume change/ pressure change
How much blood volume is contained in veins?
75% of blood volume
What gives veins compliance?
thin wall - easy to stretch
smooth muscle wall - easy to stretch when relaxed
What does venous pressure refer to?
pressure in peripheral veins
(~ 10-15 mmHg)
What does CENTRAL venous pressure refer to?
pressure in venae cavae
(similar to right atrial pressure: ~ 0-5 mmHg)
What is Mean circulatory filling pressure?
pressure in circulation when blood volume has re-distributed around the circulatory system
Flow = 0
What 2 factors influence Mean circulatory filling pressure (MCFP)?
overall blood volume
average compliance of whole circulatory system
What is Venous return? Unit?
flow of venous blood back to heart
What graph can represent venous return?
vascular function curve
plot venous return vs. right atrial pressure
How is mean circulatory filling pressure obtained from Vascular function curve?
Intersection with RAP axis = mean circulatory filling pressure (7 mmHg)
When Venous return = 0; flow = 0; Right Atrial Pressure = VP
What is the relationship between venous return and right atrial pressure **when RAP >0?
Venous return is inversely proportional to right atrial pressure
Directly proportional to pressure gradient from veins → right atrium
What is the equation for flow when RAP >0?
venous return/flow ∝ VP – RAP
What is the relationship between venous return and right atrial pressure **when RAP < 0?
Vascular function curve levels off
since vein collapses at RAP <0, the decreased radius of vein counteracts the increase in pressure gradient
What is difference in the flow equations when RAP is larger than 0 and smaller than 0?
When RAP <0:
flow ∝ (VP – RAP)r^4
When RAP >0:
flow ∝ (VP - RAP)
Explain why at RAP>0, flow ∝ VP - RAP (pressure gradient) and not (VP – RAP)r^4?
Because when RAP > 0, the resistance of vein is very low becomes negligible
Only when RAP <0 that veins collapse and flow is hugely affected (hence the vascular function curve levels off)
What is the difference between cardiac and vascular function curves?
Cardiac function curve shows relationship between RAP and cardiac output
Vascular function curve shows relationship between RAP and venous return
What is Starling's law?
Venous return = cardiac output (maintained under different conditions)
At what pressure does the heart operate at in the cardiac and vascular function curve?
Intersection point between cardiac and vascular function curves
Cardiac output = venous return
What 5 factors influence venous return?
Sympathetic nerve activity > venomotor tone
Skeletal muscle pump
(Effect of breathing )
What are the net effects of Increase venomotor tone on Mean Circulatory Filling Pressure and venous return and cardiac output?
Increase venous return
How does Increase Venomotor tone change the vascular function curve?
Parallel shift (upward) in the vascular function curve
Explain how Increase venomotor tone changes the vascular function curve? *venous pressure, return, CO*
Venomotor tone > contract longitudinal SM in vein walls > decrease compliance (not radius)
Increase Mean Circulatory Filling Pressure (vol/compliance)
Increase in venous pressure = increase venous return = heterometric increase in force = increase CO
What are the net effects of Increase Blood Volume on Mean Circulatory Filling Pressure, venous return and cardiac output?
Increase Venous return
How does Increase Blood Volume change the vascular function curve?
Parallel shift (upward) in the vascular function curve
Explain how Increase Blood volume changes the vascular function curve? *venous pressure, return, CO*
Increase blood volume > Increase MCFP (vol/compliance)
Veins have the highest compliance > Increase venous pressure and pressure grad. > Increase venous return
>> heterometric increase in force > increase SV thus CO
Does increase blood volume change the CARDIAC function curve?
What are the net effects of Increase Peripheral resistance on Mean Circulatory Filling Pressure, venous return and cardiac output?
Decrease venous return
How does Increase PERIPHERAL RESISTANCE change the vascular function curve?
decreased slope of vascular function curve, same MCFP (x-axis intercept)
Explain how Increase Peripheral Resistance changes Venous return?
Increase total peripheral resistance (2/3 = pre-capillary resistance; 1/3 = post)
>> Changes distribution of blood between arteries and veins ***DURING blood flow*** >> less blood flow from artery to vein
↓ venous pressure = ↓ VP-RAP gradient = ↓ venous return
Explain how Increase Peripheral Resistance changes MCFP?
Decrease in venous return
But When flow stops, blood redistributes** again to equalize pressure > MCFP unchanged
Change in flow RATE does not impact MCFP
Explain how Increase Peripheral Resistance changes Arterial pressure?
Increase pre- and post-capillary resistance = less blood can flow from artery to vein >> Increase in arterial pressure
Explain how Decrease peripheral resistance changes the vascular function curve?
Increase slope of vascular function curve, MCFP unchanged
What are the net effects of Skeletal Muscle Pump on venous return?
Depends on rhythmic or continuous skeletal muscle pump:
Rhythmic = Increase venous return
Continuous/ maintained = Reduce venous return
Explain how rhythmic contractions increases venous return?
- Contraction → ↑ venous pressure (valves prevent backflow) → “pulse” of venous return
- Relaxation → vein expands and re-fills with blood from arteries
Explain how maintained contractions changes venous return?
maintained compression → ↑ vascular resistance → hinders flow from artery to vein → reduces venous return
Explain how skeletal muscle contraction raises intravessel pressure?
Muscle contraction → increased pressure inside muscle
↑ Pout compresses blood vessels → ↑ Pin
How does breathing change venous return?
Inspiration > negative thoracic pressure and increase abdominal pressure (diaphragm flattens)
Increase pressure gradient between abdominal and thoracic vena cava
Increase venous return
Explain the physiological splitting of S2 during inspiration?
Inspriation > negative thoracic pressure > Increase pressure gradient between vein and thorax
> increase venous return > requires more time to pump blood out
> Difference in closure of 2 semilunar valves increase
> Accentuated split of S2
What pressure does gravitational force contribute to?
gravitational force contributes to total pressure inside vessels
Changes Transmural pressure
What is the transmural pressure equation?
Pin - Pout
= (hydrostatic pressure + gravitational pressure) – tissue pressure
Gravitation force on blood increases with ____?
increases with vertical
distance below the heart
What are the total pressure inside vessels at heart level and at feet level?
At heart level: Pin = hydrostatic pressure
At feet level: Pin = hydrostatic pressure + gravitational pressure
What is the effect of gravity on systemic circulation in supine position?
horizontal posture: effects of gravity on the systemic circulation are small
Does gravity change blood pressure gradient?
Gravitational pressure adds onto both arterial and venous pressure >>
P gradient unchanged
Explain how gravity changes venous return?
Increase net transmural pressure at lower limb (gravity effect largest)
> distention of lower limb vessels > increase radius of vein > more fluid accumulation in lower limb
> decrease venous return
Explain how gravity changes CO and BP?
Lower venous return > Lower Stroke volume > Lower CO
Lower systolic P and higher diastolic P >> stroke volume/ pulse pressure is smaller >> Lower BP
What receptors detect drop in BP and what is the reflex correction?
Baroreceptors detect the drop in BP >> produce reflex correction of mean BP to normal:
1. Peripheral vasoconstriction → ↑ TPR
2. ↑ heart rate and force → ↑ CO
Supine position: How does BP change?
1) Larger venous return > larger cardiac output > higher systolic pressure
2) Lower peripheral resistance > lower diastolic pressure
Compare the mean BP in supine and upright position?
Both the same mean BP
Compare the CO, Venous return, systolic P, diastolic P, TPR between upright and supine?
Completely opposite to each other
Upright = Lower venous return, Lower CO, Lower systolic pressure
Higher TPR, Higher Diastolic P
How does increase in blood volume change the cardiac function curve?
No change in cardiac function curve but shift up of vascular function curve:
Intersection point in higher = higher CO
Increase filling of heart → heterometric increase in force → ↑ stroke volume = ↑ cardiac output
How does increase contractility of heart change Vascular function curve?
How does increase contractility of heart change Cardiac function curve?
Upward shift in cardiac function curve
meaning the same CO can be achieved at lower right atrial pressure
Explain how increase contractility changes the venous return?
Increase SV but constant Venous Return > decrease in residual volume of heart
> Decrease RAP , thus increase P gradient (VP-RAP)
> Increase venous return
What determines contractility?
Rate of force development determined by intracellular [Ca2+]
How does Increase TPR change cardiac and vascular function curve?
Decreases the slope of BOTH curves
Intersection point of both curves shifts directly downwards
Explain how increase TPR causes change in vascular function curve?
Increase TPR > decrease venous pressure and return > decrease pressure gradient (VP-RAP)
Explain how increase TPR causes change in cardiac function curve?
Increase arterial pressure > decrease ventricular-arterial gradient > Decrease SV and CO