Summary-Factors regulating Venous Return
Venous return – major determinant in cardiac output based on the fact that we need blood in
order to fill ventricles with blood, in order to enlarge the volume of ventricle during diastole so
you maintain stroke volume.
Factors that affect capacity of blood to return back to heart after it goes through the arterial
Some are Volume related (how much blood you have within the system and if you enlarge the
blood vol, SHORTERM: it would have more reabsorption than filtration that would increase in
fluid coming into the venous system which would increase venous return. Kidney plays
important role in fluid balance.
The thing that kidneys do also have major influence OVER A LONGER TERM.
Direct link between the amount of blood with the venous return.
2. Maintaining pressure gradient also influences venous return. Lower limbs, the skeletal
muscles pump to compress the veins and squeeze blood towards the heart, that works in
conjection with the valves in the vein which prenvent backflow of the blood. So when you
squeeze the veins within the muscle, the blood only has one direction which is towards the
The sympathetic nervous system constrict the veins which is called VENOCONSTRICTION.
Veins are capacittant vessels they like to stretch passively out and hold large volume of blood,
You need to able to minimize that vol of blood, the sympathetic nerves help with constricting by
tightening the walls of the veins so they do less passive stretching, they stay at smaller
diameter which causes increase in pressure in the veins, and helps the blood move back to the
Influences that affect what’s going on the heart.
Influence Respiration: change in vol of thoracic cavity due to inspiration which creates short
term negative pressure which helps stimulate more flow to the chest cavity. Small component
---More veins in the body compared to arteries- Cardiac output= since we need to have blood supply to various tissues and organs in the body
to maintain oxygen and also care about cardiac output because it is one of the two major
regulators of MAP.
Venous retun is a determinant of cardiac out ; cardiac out put is a determinant of MAP
MAP is a #1 homeostatic regulator in the body.
Body can regulate MAP, because there is a very sensitive feedback system which allows it to
detect MAP, and to influence the parameters that make up MAP.
We use baroreceptors (sensory nerves found)
Baroreceptors found in 2 locations both on arterial side, and very close promixity to the heart.
1.Baroreceptors have arch of aorta called Aortic bodies.
2. Baroreceptors found at the carotid sinus called Carotid bodies
*both are similar locations for chemoreceptors (control for respiration)*
Both locations are used for detection of gasses, the baroreceptor stretch reflex. They are
different nerves .
Baro receptors are specialized to determine DEGREE OF STRECH IN THE WALL OR AORTA
They have specilaized nerve endings that are integrated in the wall of the blood vessels, so they
are able to detect any change in the stretch of the wall.
They are not sensing the blood itself but whats happening in the wall of blood (smooth muscle
cell layers). Barorecpeotrs activated by strech When the wall of the blood vessel was strectehd .
When there is more pressure inside the blood vessel forcing outwards, trying to expand to wall
These baroreceptors will become depolarized and fire action potentials. They send these
action potential up through the axons then they will synapse in the brain at the cardiovascular control center. This cardiovascular control center is located within the brain stem within
medulla oblongata, very close proximity to the respiratory control center.
Input from the baroreceptor is one of the major inputs that comes to the cardiovascular
center (NOT THE ONLY ONE there are OTHERS)
Cardiovascular Center receives all the inputs that are coming to it from various places and
decideds what to do with those signals,.
In order to make a change , and have an effect, one of two ways: It can either do
(i)SNS (sympathetic nerves)
(ii)PNS (Parasympathetic nerves)
Activation of the cardiovascular center by the baroreceptors causes higher activation of the PNS
and at the same time, it reduces the activation of the SNS.
The cardiovascular control center uses both outputs at the same time, but is always controlling
them in the opposite way that’s good since they have opposite effects.
Pathway from the
baroreceptor to the cardiovascular control center is called afferent pathway and negative
feed back loop.
Cardiovascular center is the integrating center
SNS and PNS are the outputs
Examples of Baroreceptor Responses
Middle is NORMAL blood pressure. Left is lower blood pressure, right is higher blood pressure.
Under normal MAP, the wall of the aorta is subject to an outwards pressure, and that causes a
certain level of activation of baroreceptors. This is an important thing . Under resting conditions, what you consider, to be normal blood pressure, the baroreceptors are
being activated by the stretch.
Each line= action potential
Frequency of action potential determines how the nerves are sending the information. The code
it by frequency.
This frequency of action potentials that is caused (elicited) by MAP, results in some level of
parasympathetic activation. And some level of sympathetic.
Under resting conditions, MAP is normal, there is some PNS and SNS activation. You are in
the middle ground.
Level of PNS and SNS is going to help determine the heart rate, the contractility of the heart,
and also some other vascular effects.
If aortic pressure drops, that means there will be less blood within the aorta pushing outwards, a
less of stretching influence,
The baroreceptors are getting