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Effects of changes in arteriolar resistance in volume of blood in the veins
Arteriolar resistance affects venous pressure & volume; increase in resistance will decrease volume/Ptm
Define Vo, slope of delta V/delta P, and unstressed volume
Vo: point where veins don't have any pressure
Slope (compliance): measure of "floppiness"
Unstressed volume: volume of blood that would remain in the veins at Ptm = 0
Effects of changes in sympathetic stimulation of venous smooth muscle on the volume of blood in the veins.
Sympathetic stimulation --> (effect through alpha 1 receptors) decreases unstressed volume (but no discernible change in venous resistance/flow due to the small amount of smooth muscle or compliance) and causes decreased venous volume as blood volume is transferred to arteries.

Stepwise progression of effects: NE released from nerve terminals --> binds alpha receptors in smooth uncle --> increases active tension --> decreases unstressed volume (no change in compliance (slope))

decreased volume of blood if no change in Ptm or increase P if no change in volume.
Chief mechanism to withdraw blood from the veins of the internal organs.

Clinical relevance: administration of vasodilators (nitroglycerin) to a patient with suspected angina is given because relaxation of venous smooth muscle (opposite of SANS) --> increase venous volume and decrease pressure --> decreased flow to the heart thereby reducing size of the ventricle and wall stress --> decreases total internal work and O2 requirement --> allows demand to decrease relative to supply
Effects of right atrial pressure and venous return
Venous return (Q) = delta P/ R

delta P = P (peripheral veins) - P (right atrium) / R (veins)

P (peripheral veins) = Ptm = total venous volue - Vo = Stressed volume
See venous return curves!l

Pmc = stressed total blood volume/total compliance of system

The venous return curve shifts when Pmc shifts, due to
- increased total blood volume --> increased Pmc
- sympathetic stimulation --> increased stressed volume, increased Pmc

change peripheral resistance (arteriolar constriction-SANS activation)

Changes in arteriolar resistance affect venous return without affecting Pmc

Simultaneous sympathetic stimulation of veins and arterioles result in complex combinations of effects- lithe venous effects are generally predominant

**side note: Pmc not affected by PVR due to relative differences in volume between veins and arteries (veins ~ 4x greater volume than arterioles) consequently SANS activation on venous system predominates.

P (right atrium) = decreased rich atrial P (increased delta P) --> increased venous return (increases pressure differential driving movement of blood out of the venous system)

R(veins) = venous resistance is minimal and changes little, more due to other factors like muscle contraction.
Effects of gravity, skeletal muscle activity, and intrathoracic pressure on venous return
Gravity: weight of column of blood from the heart to the veins below it is added to the pressure in those veins by 10cm = 7.5mmHg. Due to the added Ptm, volume increases --> decreases venous return and CO. Baroreceptors compensate, but if mean arterial pressure (MAP) falls to abruptly and brain perfusion is inadequate --> fainting to automatically put the body in recumbent state

Skeletal muscle: contracting skeletal muscles compresses veins and in conjunction with venous one-way valves permits movement of blood back into the heart.

Negative intrathoracic Pressure: during inspiration, intrathoracic P = negative, which includes all parts of the thoracic cavity. Consequently, P(right atrium) decreases and allows venous return to increase during inspiration.